mech world

advantage of wjm over punch press

2008-12-12T02:14:00.000-08:00

Lower cost per piece for short runs
Place holes closer to the materials edge
Fast turn-around
Minimal setup
Thick materials are fine
Brittle materials are no problem.
Hard materials are easy.
Some stamping houses are using waterjets for fast turn-around, or for low quantity / prototyping work. Waterjets make a great complimentary tool for punch presses and the like because they offer a wider range of capability for similar parts. For high production of thin sheet-metal, the stamp will be more p rofitable in many cases, but for short runs, difficult material, thick material, and many other similar but different applications, waterjets have their place.

advantage of wjm over milling

2008-12-12T02:12:00.000-08:00

There is only one tool to qualify on an abrasivejet
Setup and Fixturing typically involves placing the material on the table with an abrasivejet
Cleanup is much faster with an abrasivejet
Programming is easier and faster
Machine virtually any material, including:
brittle materials
pre hardened materials
otherwise difficult materials such as Titanium, Hastalloy, Inconel, SS 304, hardened tool steel.
Waterjets are used a lot for complimenting or replacing milling operations. They are used for roughing out parts prior to milling, for replacing milling entirely, or for providing secondary machining on parts that just came off the mill. For this reason, many traditional machine shops are adding waterjet capability to provide a competitive edge.

advantages of wjm over flame cutting

2008-12-12T02:07:00.000-08:00

Abrasivejets provide a much nicer edge finish
Abrasivejets don't heat the part
Abrasivejets can cut virtually any material
Abrasivejets are more precise
Flame cutting is typically faster
Flame cutting is typically cheaper, if you can use it.
Waterjets would make a great compliment to a flame cutting where more precision or higher quality is required, or for parts where heating is not good, or where there is a need to cut a wider range of materials.

Do pre-machining, and save your other tools from having to do so much work

advantages of wjm over plasma cutting

2008-12-12T02:03:00.000-08:00

Abrasivejets provide a nicer edge finish
Abrasivejets don't heat the part
Abrasivejets can cut virtually any material
Abrasivejets are more precise
Plasma is typically faster
Waterjets would make a great compliment to a plasma shop where more precision or higher quality is required, or for parts where heating is not good, or where there is a need to cut a wider range of materials.

Modern machines are relatively clean and quiet.

advantage of wjm over EDM

2008-12-12T01:56:00.000-08:00

Abrasive jets are much faster than EDM.
Abrasive Jets machine a wider variety of materials (virtually any material).
Uniformity of material is not very important to an Abrasivejet.
Abrasive jets make their own pierce holes.
Abrasive jets do not heat the surface of what they machine.
Abrasive jets are capable of ignoring material aberrations that would cause wire EDM to lose flushing.
Abrasive Jet machining is useful for creating start holes for wire insertion later on. (a mill could do the job, but only after spotting the hole, changing tools to drill a pilot, then changing tools again to drill out the hole).
New technology allows Abrasive jets to obtain tolerances of up to +/-.003" (0.075mm) or better (I have personally done some +/-.001" (0.025mm) work, but that's the exception, not the norm, and only on certain shapes and materials.)
No heat affected Zone with Abrasive jets.
Abrasive jets require less setup.
Make bigger parts.

Many EDM shops are also buying waterjets. Waterjets can be considered to be like super-fast EDM machines with less precision. This means that many parts of the same catagory that an EDM would do can be done faster and cheaper on an abrasivejet, if the tolerances are not extreme.

Wire EDM fixturing in an abrasivejet machining center. This makes precision fixturing possible. It also allows for pre-machining on the abrasive jet to release stresses in the material, and then use the exact same fixturing on the EDM to do secondary operations and final cutting to extreme tolerance.

advantage of wjm over lasers

2008-12-12T01:53:00.000-08:00

Abrasive waterjets can machine many materials that lasers cannot. (Reflective materials in particular, such as Aluminum and Copper.
Uniformity of material is not very important to an Abrasivejet.
Abrasive jets do not heat your part. Thus there is no thermal distortion or hardening of the material.
Precision abrasive jet machines can obtain about the same or higher tolerances than lasers (especially as thickness increases).
Your capital equipment costs for water jet are generally much lower than that for a laser. I.e. for the price of a laser, you can purchase several abrasivejet-machining centers.
Abrasive jets can machine thicker materials. How thick you can cut is a function of how long you are willing to wait. 2" (50mm) steel and 3" (76mm) aluminum is quite common. I heard of people doing up to 10" (250mm) steel, and 24" (600mm) thick glass with high horsepower systems. Once you get over 2" (50mm) thick it is very difficult to get precision, however. Lasers seem to have a maximum of 0.5" (12mm) - 0.75" (19mm).
Abrasive jets are safer. No burnt fingers, no noxious fumes, and no fires. (You still have to keep those fingers out of the beam.)
Abrasive jets are more environmentally friendly.
Maintenance on the abrasive jet nozzle is simpler than that of a laser, though probably just as frequent.
Abrasive jets are capable of similar tolerances on thin parts, and better on parts thicker than .5"
Abrasive jets do not loose much "focus" when cutting over uneven surfaces.
While lasers are often faster on thinner materials...
it may be cheaper and faster to simply buy two or three abrasive jet machining centers to do the same work
you can stack materials, so you are cutting multiple thin parts simultaneously.
you can run additional cutting heads in parallel on a single machine
Modern Abrasive jets are typically much easier to operate and maintain than lasers, which means that every employee in your shop can be quickly trained to run one!
Abrasivejets don't create "scaley" edges, which makes it easier to make a high quality weld
Many shops that have lasers also have waterjets, as they are complimentary tools. Where one leaves off, the other picks up.

advantages of water jet machining

2008-12-12T01:50:00.000-08:00

Cheaper than other processes.
Cut virtually any material:
pre hardened steel
mild steel
exotics like Titanium, Inconel, Hastalloy
gummy 304 stainless
(most steels cut at the same speed, whether hardened, or not.)
Copper, Brass, Aluminum: They are a cinch!
brittle materials like glass, ceramic, quartz, stone.
laminates
flammable materials
Cut thin stuff, or thick stuff
Make all sorts of shapes with only one tool.
Cut wide range of thickness’ to reasonable tolerance up to 2” (50mm) thick
Up to 5” (127mm) or thicker where tolerance not important, or in soft materials.
No Heat Generated / No heat affected zones - this is cold cutting!
No mechanical stresses
Cut virtually any shape:
Fast Setup:
Only one tool to qualify / No tool changes required
Fast turn around on the machine. Make a part, then 2 minutes be making a completely different part from a completely different material.
Leaves a satin smooth finish, thus reducing secondary operations
Clean cutting process without gasses or oils
Makes its own start holes
Narrow kerf removes only a small amount of material.
Your "scrap" metal is easier to recycle or re-use (no oily chips!)
Modern systems are now very easy to learn.
You can trade off tolerance vs speed from feature to feature on your part.
Can easily switch between high production, and single piece production, on the same machine, with no extra effort.
Are very safe. (No, they don't explode, thanks to the nearly incompressible property of water.)
Draw the part / cut the part. It is that easy! Everyone in your shop can learn to use it quickly.
No "scaley" edges, which makes it easier to make a high quality weld
Machine composite materials, or materials where dissimilar materials are glued together
Machine stacks of thin parts all at once.

factors to deciding cutting speed

2008-12-12T01:44:00.000-08:00

Material being cut

Hardness: Generally speaking, harder materials cut slower than soft materials. However, there are a lot of exceptions to this. For example, granite, which is quite hard, cuts significantly faster than Copper, which is quite soft. This is because the granite easily breaks up because it is brittle. It is also interesting to note that hardened tool steel cuts almost as quickly as mild steel. (Though "absolute black" granite, which is tough as nails, actually cuts a bit slower than copper.)

Thickness: The thicker the material, the slower the cut. For example, a part that might take 1 minute in 1/8" (3mm) steel, might take a half hour in 2" (50mm) thick steel, and maybe 20 hours in 10 inch (250mm) thick steel.

Geometry of the part

It is necessary to slow the cutting down in order to navigate sharp corners and curves. It also takes additional time to pierce the material. Therefore, parts with lots of holes and sharp corners will cut much slower than simpler shapes.
Desired Result

If you want a high tolerance part and / or a smooth surface finish, then the part will take longer to make. Note that you can make some areas of a part high tolerance and other areas fast, so you can mix and match to get the optimal balance between cutting speed and final part quality.
Software controlling the motion

This is probably one of the most overlooked aspects of abrasivejet machining by novice users. You would not think that software would have much to do with the speed of cutting. In fact, this is (mostly) true if all you are doing is cutting in a straight line. However, as soon as you introduce any complexity to the part, such as a corner, there is great opportunity for software to optimize the cutting speed.

Power at the nozzle (pressure and water flow rate)

The more horsepower at the nozzle, the faster it can cut. How much horsepower makes it to the nozzle is a function of the pressure and the orifice that it is being squeezed through. (Note: do not confuse "horsepower of the motor" with "horsepower at the nozzle". It is the power that actually makes it to the nozzle that is most important. Having a big motor makes no difference, if the power all goes into wasted heat!)

Simply put, the higher the pressure, the faster the cut. The more water you flow, the faster the cut. Unfortunately, as the pressure increases, so does the cost and maintenance, so this is not as simple as it seems.

A good way to learn more about how pressure and jewel size effect cutting rates, and to calculate "nozzle horsepower" is to run the Abrasivejet Feed Rate Calculator, which you can download from this web site by clicking here.

Quantity and Quality of abrasive used

Type of abrasive: In the industry, most machines run 80 mesh garnet for abrasive. However, it is possible to cut slightly faster with harder abrasives, but the harder abrasives also cause the mixing tube on the nozzle to wear rapidly. So, pretty much everyone uses garnet. It is worth mentioning that not all garnet is the same. There are big variations between purity, hardness, sharpness, etc, that can also effect the cutting speed, accuracy, reliability, and operating cost.

Quality of abrasive: Typically, abrasivejets consume between 0.5 and 1 Lb (0.25 and 0.5Kg) of abrasive per minute. There is a sweet spot for every nozzle size and pressure as to what amount of abrasive flow rate will cut the fastest, and what amount will cut the cheapest.

source of errors during machinig

2008-12-12T01:39:00.000-08:00

Around curves

As the jet makes its way around a radius, the jet lag causes a tapering effect. Therefore it is necessary to slow the jet down, and let the tail catch up with the head. (And / or tilt the cutting head to compensate)

Inside corners

As the jet enters the corner, the traverse speed must slow down to allow the jets tail to catch up. Otherwise the tail lag will cause the corner to "blow out" a little.
As the jet exits the corner, the feed rate must not be increased too quickly, otherwise the jet will kick back and damage the part.

Feed rate

When the jet slows down, its kerf width grows slightly.

Acceleration / Jerk

Any sudden movement (like a change in feed rate) will cause a slight blemish as well. Thus for highest precision it is necessary to control the acceleration as well as feed rate, and even Jerk ("Jerk" is a change in acceleration.).

Nozzle Focus

Some nozzles produce more taper than others. Longer nozzles usually produce less taper. Smaller diameter nozzles also produce less taper. Holding the nozzle close to the work piece produces less taper as well. (And, of course, it is possible to tilt the cutting head to elliminate the taper in most cases.)

Speed of cutting

Typically, the slower the cutting, the higher the tolerance. This is because as the cutting is slowed down, the surface finish improves, and the taper begins to disappear, and the jet exhibits less lag. However in some cases it is possible to slow the cutting down so much that tolerances begin to get worse due to reverse taper, unless the head is tilted.

Active taper compensation

Some newer machines now have the option of tilting the cutting head against the taper. This can be used to virtually eliminate the taper, or to purposely add taper into a part. The big advantage to active taper compensation is that taper can be reduced without having to slow the cutting down. ("Taper" is when the edge of the part is not 100% perpendicular.) I have an entire page dedicated to this topic elsewhere in this web site. If you want to go there now, click here.

Kerf width

Kerf width, which is the width of the cutting beam, determines how sharp of an inside corner you can make. About the smallest practical abrasivejet nozzle will give you a kerf width of .015" (0.38mm) in diameter. Higher horsepower machines require larger nozzles, due to the amount of water and abrasive that they flow through.
Some waterjet (water only) nozzles have very fine kerf widths (like .003" / 0.076mm). Likewise, it is possible to make ultra-small abrasivejet nozzles, but they can be problematic.

Kerf width is typically compensated for by the controller by specifying a "tool offset", where the jet is moved 1/2 of its diameter away from the edge of the part when it cuts.

Consistency of Pump Pressure

Variations in waterjet pump pressure can cause marks on the final part. It is important that the pump pressure vary as little as possible while machining is in progress to prevent these. (This becomes an issue only when looking for better than +-.005" (0.125mm) tolerances, however). Typically it is older Intensifier type pumps that exhibit this problem. Some newer intensifiers, and as far as I know all crankshaft driven pumps have smoother pressure delivery, and this is usually not an issue.

Operator experience

Abrasive jets are capable of anywhere from +-0.02" to +-0.001" (0.5mm - 0.025mm) depending on the above factors. What distinguishes one machine from another is how easy those tolerances are obtained. If you had a nozzle attached to any X, Y table capable of positioning to +-.001" (0.025mm), then, in theory, in 0.5" (13mm) thick steel, you could perhaps machine +-0.002" (0.05mm) or so. This is given either software to compensate for jet behavior, and/or an experienced operator tweaking the machine through trial and error. I have personally been able to produce parts in the slightly better than +-0.001" (0.025mm) range on an OMAX 2652, which as far as I know is the most precise machine on the market (other than an OMAX 2626xp), but that usually requires cutting the part once, measuring the error, then cutting it again, and is only possible on certain materials and geometries.

Buying a machine? Look at, and measure parts that come off the machine. Measure the first part, then cut the same part at different locations on the table to get an idea of repeatability. Ideally, have the seller do so while you watch, to prevent cheating. (One way to cheat is to slow the cutting way down, and another is to simply use a different machine - It happens.) Also, don't forget to check out the buyers guide which you can link to from the home page of this web site, or the waterjet equipment manufacturers listing page.

obtainable tolerance with wjm & ajm

2008-12-12T01:36:00.000-08:00

It is important to have a machine with good precision to get precision parts, but there are many other factors that are just as important. A precise machine starts with a precise table, but it is the control of the jet that brings the precision to the part. A key factor in precision is software - not hardware. This is also true for cutting speed. Good software can increase cutting speeds dramatically. This is because it is only through sophisticated software that the machine can compensate for a "floppy tool" made from a stream of water, air, and abrasive.
Obtainable tolerances vary greatly from manufacturer to manufacturer. Most of this variation comes from differences in controller technology, and some of the variation comes from machine construction. Recently, there have been significant advances in the control of the process allowing for higher tolerances. A machine from 1990 may be capable of tolerances of 0.060"-.010" (1.5mm-0.25mm) Today, some machines are capable of making some parts +/- 0.001" (0.025mm), or even better in special circumstances (though +/-0.002" is perhaps more realistic).

When purchasing a machine, be sure measure parts that come off the machine you are going to buy. Some manufactures stretch the truth a bit when quoting tolerances, or they quote the positioning accuracy of the mechanics of the machine, which does not necessarily translate into the cutting accuracy in the final parts. The reality of it is that Manufactures of abrasive jet equipment are in a tough spot when trying to advertise obtainable tolerances because of these and other factors:

Material to machine

Harder materials typically exhibit less taper, and taper is a big factor in determining what kind of tolerances you can hold. It is possible to compensate for taper by adjusting the cutting speed, and/or tilting the cutting head opposite of the taper direction.

Material thickness

As the material gets thicker, it becomes more difficult to control the behavior of the jet as it exits out the bottom. This will cause blow-out in the corners, and taper around curves. Materials thinner than 1/8" (3mm) tend to exhibit the most taper (which is perhaps the opposite of what you might expect.), and with thicker materials, the controller must be quite sophisticated in order to get decent cuts around complex geometry.

Accuracy of table

Obviously, the more precise you can position the jet, the more precise you can machine the part. Generally speaking, though, it is much easier to find precise tables, than it is to find machines that can make precise parts. (More on why this is in "control of the abrasivejet" below.)

Stability of table

Vibrations between the motion system and the material, poor velocity control, and other sudden variances in conditions can cause blemishes in the part (often called "witness marks")
The hardware that is out there varies greatly in stability and susceptibility to vibrations. If the cutting head vibrates relative to the part you are cutting, then your part can be ugly.

Control of the abrasive jet

Because your cutting tool is basically a beam of water, it acts like a "floppy tool". The jet lags between where it first enters your material and where it exits.

life of cutting nozzle

2008-12-12T01:33:00.000-08:00

How long will a mixing tube last?

A "worn" mixing tube is like a worn tool bit: It is difficult to say when a mixing tube is fully worn, but as it wears, it becomes a less effective cutting tool. (although once it starts to go bad, the wear rate accelerates). For precision work, a new mixing tube performs better than a used one. How long a mixing tube will last depends on a number of factors, including the sales person that you talk to. Numbers from 20 to 80 hours are fairly typical, although it is possible that they may wear faster, or last longer, depending on circumstances.

So what's the real cost?

When looking at costs such as mixing tubes and jewels that are expensive wear parts, consider the "total cost of operation", and compare it with the productivity of the machine. When you make such a comparison you will quickly see that an abrasivejet will probably be the most profitable machine tool in your shop - by far. Consider that your operating cost of the machine will vary between $20 and $35 per hour, but for "typical" jobs you will earn between $60 and $150 per hour, with $120/hour being quite typical.

Price varies considerably depending on regional factors such as competitors and local markets. Reasearch this carefully when looking to purchase a machine.

When pricing the work, it is often more sensible to price based on a "per part" price, instead of "per hour". Often profits can be maximized this way, and it is possible to then realize the benifits of faster cutting machines, and/or machines with multiple nozzles.

advantage of abrassive jet machining

2008-12-12T01:25:00.000-08:00

Extremely fast setup and programming

No tool changes required, so there is no need to program tool changes or physically qualify multiple tools. For some systems, programming simply involves drawing the part. If you customer gives you that drawing on disk, half the battle is won.
This means that (for some machines) you can make good money off single part and low volume production!

Very little fixturing for most parts

Flat material can be positioned by laying it on the table and putting a couple of 10 lb weights on it. Tiny parts might require tabs, or other fixturing. At any rate, fixturing is typically not any big deal - though it is important to secure the material in the X, Y, and Z directions.

Machine virtually any 2D shape (and some 3D stuff)

Including tight inside radii, Make a carburetor flange with holes drilled and everything. Some exotic machines are capable of 3d machining, (robot arms, (x,y) machines with lathe axis, and (x,y)-(u,v) axis machines). (3D machining is especially tricky, however, due to issues regarding control of a "floppy tool". For this reason, 3D machining is reserved strictly for specialty applications.). In other words, abrasivejets are exceptional at 2D machining, but limited in 3D capability.Very low side forces during the machining

This means you can machine a part with walls as thin as .01" (0.25 mm) without them blowing out. This is one of the factors that make fixturing is so easy. Also, low side forces allow for close nesting of parts, and maximum material usage.

Almost No heat generated on your part

Machine without hardening the material, generating poisonous fumes, recasting, or warping. You can machine parts that have already been heat treated with only a tiny, tiny decrease in speed. On piercing 2" (50mm) thick steel, temperatures may get as high as 120 degrees F (50 C), but otherwise machining is done at room temperature.

Aerospace companies use abrasivejets a lot because of this.

No start hole required

Wire EDM, eat your heart out. Start holes are only required for impossible to pierce materials. (Some poorly bonded laminates are sometimes the exception. In which case pre-drilling or other special methods may be employed)

Machine thick stuff

This is one huge advantage Abrasive jets have over lasers.
While most money will probably be made in thickness' under 1" (25mm) for steel, It is common to also machine up to 4" (100mm). How thick it is possible to cut is dictated by the time it takes. Cutting speed is a function of thickness, and a part twice as thick will take more than twice as long. People make low tolerance parts and roughing out metal up to 5-10" thick (125mm-250mm), but those people are very patient, and probably have no other way to do it. Typically, most money is made on parts 2" (50mm) thick or thinner.
Environmentally friendly

As long as you are not machining a material that is hazardous, the spent abrasive and waste material become suitable for land fill. The red color of garnet abrasive also looks nice in your garden. If you are machining lots of lead or other hazardous materials, you will still need to dispose of your waste appropriately, and recycle your water. Keep in mind, however, that very little metal is actually removed in the cutting process. This keeps the environmental impact relatively low, even if you do machine the occasional hazardous material.

In most areas, excess water is simply drained to the sewer. In some areas, some water treatment may be necessary prior to draining to sewere. In a few areas, a "closed loop" system that recycles the water may be required.

The pumps do use a considerable amount of electricity, though, so there is some additional environmental (and cost) impact due to this.

Your clippings are valuable

When machining or roughing out expensive materials such as titanium, your scrap still has value. This is because you get chunks, not chips. You can also get more parts from the same material because of the abrasive jets low kerf width.

There is only 1 tool

There is no need to qualify multiple tools, or deal with programming tool changes. Programming, Setup and Clean up time is reduced significantly, meaning you make more money because you can turn more parts faster.

components of water jet/abrasive jet machine

2008-12-12T01:20:00.001-08:00

A typical abrasivejet machining center is made up of the following components:
High pressure water starts at the pump, and is delivered through special high pressure plumbing to the nozzle. At the nozzle, abrasive is (typically) introduced, and as the abrasive/water mixture exits, cutting is performed. Once the jet has exited the nozzle, the energy is dissipated into the catch tank, which is usually full of water and debris from previous cuts. The motion of the cutting head is typically handled by an X / Y axis structure. Control of the motion is typically done via a computer following the lines and arcs from a CAD drawing.

As with automobiles, there are loads of other accessories and options, such as automatic tank clean-out systems, water recyclers, special tilting heads, fixturing, or motorized Z axis, etc., but the above compromise the basic system, and pretty much everything you need for making most 2 dimensional parts.

water jet machining and abrasive jet machining comparision

2008-12-12T01:18:00.000-08:00

Cost comparison:

Complete water jet nozzle assemblies cost around $500.00 - $1000.00 (US), while abrasive jet nozzles cost from $800 - $2000. The abrasive nozzle also requires support hardware for abrasive feed which can cost anywhere from $500 to $2,000 (These numbers are not precise - for exact pricing, contact a waterjet supplier or waterjet equipment manufacturer.) Cost of operation is much higher for the Abrasive jet because of mixing tube wear, and abrasive consumption.

Limitations to water only nozzles:

Typically, the only problems that arise with a water only nozzle will be with the jewel (the orifice with the tiny hole that the water squirts through).

Jewels can crack, plug, or form deposits on them. Cracking and plugging happens as a result of dirty inlet water, and is typically avoided with proper filtration. Deposits accumulate gradually as a result of minerals in the water. Depending on your water supply, slightly fancier filtering may be necessary. Jewels are easily replaced in about 2 - 10 min., and are typically cheap ($5-$50). There are also diamond orifices for sale for $200.00 and up, which can last longer in many applications. Which is better, will depend on your exact needs.

Limitations of Abrasive Jet nozzles:

Despite their simple design, abrasive jet nozzles can be troublesome at times. There are many designs, but they share the same problems:

Short life of an expensive wear part: The mixing tube. Like I said, the abrasive jet can cut through just about anything - including itself. This will be a large part of your operating cost. (more on operating cost later)

Occasional plugging of mixing tube: Usually caused by dirt or large particles in abrasive. (This used to be a big problem with abrasivejet nozzles, but not so much any more.)

Wear, misalignment, and damage to the jewel.

which nozzle is best for my material?

2008-12-12T01:04:00.000-08:00

Water Jet Nozzle:

Soft rubber
Foam
Extremely thin stuff like Foil
Carpet
Paper and cardboard
Soft Gasket material
Candy bars
Diapers
Soft, or thin wood
...All sorts of other soft stuff

AbrasiveJet Nozzle:

Hardened tool steel
Titanium
Aluminum
Hard Rubber
Stone
Inconel
Hastalloy
Copper
Exotic materials
Hard, or thick Wood
Glass (even bullet proof!)
Marble
Plastic
Nylon
Graphite
Many ceramics
Carbon Fiber
Composites
mild steel
Stainless Steel
Kevlar
Granite
Mixed materials
Brass
In Fact, there are very few materials that abrasivejets can't cut!

waterjet machining-background

2008-12-12T00:49:00.000-08:00

Waterjets (or abrasivejets) are fast, flexible, reasonably precise, and in the last few years have become friendly and easy to use. They use the technology of high pressure water being squirted through a small hole to concentrate an extreme amount of energy in a small area to cut stuff.

"A machine shop without a waterjet, is like a carpenter without a hammer - Sure the carpenter can use the back of his crow bar to hammer in nails, but there is a better way..."
You have already heard the terms "Waterjet" and "Abrasive jet". It is important to understand that Abrasive jets are not the same thing as water jets, although they are nearly the same. Water Jet technology has been around since the early 1970s or so, and abrasive jets extended the concept about 10 years later by adding abrasive to the mix.

Both technologies use the principle of pressurizing water to extremely high pressures, and allowing the water to escape through a very small opening (typically called the "orifice" or "jewel"). The restriction of the tiny orifice creates high pressure and a high velocity beam, much like putting your finger over the end of a garden hose.
Water jets use the beam of water exiting the orifice (or jewel) to cut soft stuff like diapers, candy bars, and thin soft wood, but are not effective for cutting harder materials.

The inlet water is typically pressurized between 20,000 and 60,000 Pounds Per Square Inch (PSI). (Or 1300 - 6200 "bar" if you prefer metric). This is forced through a tiny hole in the jewel, which is typically 0.007" to 0.020" in diameter (0.18 - 0.4mm) This creates a very high velocity beam of water!

Abrasive jets use that same beam of water to accelerate abrasive particles to speeds fast enough to cut through much harder materials:

(top): A diagram of an abrasive jet. Notice that it is just like a water jet with more stuff underneath the jewel. The high velocity water exiting the jewel creates a vacuum which pulls abrasive from the abrasive line, which then mixes with the water in the mixing tube to form a high velocity beam of abrasives.

(bottom): An actual photograph of the same nozzle, with the guard removed, cutting out some parts.

People often incorrectly use the word "waterjet" when they really mean "abrasivejet". Also, people sometimes say "abrasivejet", "abrasive waterjet", or "AWJ", which mean the same thing. Don't worry. If you accidentally call an "abrasivejet" a "waterjet". Nobody will laugh at you, as it is fairly common to do so. Likewise, their are multiple spellings for the terms "water-jet", "waterjet", "water jet", etc. Any of these variations is ok to use, though perhaps "waterjet" and "abrasivejet" are the most common.

Above: On the top is a typical waterjet nozzle. On the bottom is an abrasivejet nozzle.

waterjet machining-introduction

2008-12-12T00:41:00.000-08:00

Abrasive jet and Water Jet technologies have been around for years. Waterjet cutting has been a specialty technology used in a wide variety of industries since about 1970. Around 1993, big advances in the technology were introduced that have caused this technology to become very popular for machine shops. There are now a lot of companies making a lot of money by replacing and complementing conventional machining with water jet cutting methods.

Over the last 10 years, abrasivejet machining has taken off like wildfire. Thousands of job-shops have sprung up around the world.

Why are so many people suddenly buying abrasive waterjet machine tools? Because:

They are quick to program (make money on short runs.)
They are quick to set up, and offer quick turn-around on the machine.
They complement existing tools, used for either primary or secondary operations.
They make parts quickly out of virtually any material.
They do not heat your material.
All sorts of intricate shapes are easy to make.
They are money making machines.

how to format programs in cnc

2008-12-08T00:31:00.000-08:00

the CNC control will execute a CNC program in sequential order exactly as it is written. All commands necessary to make the machine do the required operations must be included in the CNC program in the proper order. And of course, part of learning how to program is understanding the program structure a CNC machine requires.

To this point, you have been exposed to several features and programming functions related to programming. With all the new ideas and commands introduced, you may be getting somewhat confused trying to keep them all straight. You may be worried about how you're going to memorize all of this.

One of the main reasons to strictly format CNC programs has to do with making it easy to write your first few programs. When writing your first program, the related commands will by no means be memorized. However, if you have good example formats to go by, writing your first few programs will be much easier.

We relate this to driving a car. It is unlikely that any driver can recite from memory all road signs used to direct traffic. However, when a driver sees a road sign, it is quite likely the driver will recognize its meaning. In the same way, it is unlikely that even an experience CNC programmer could recite every word used with CNC programming. But when even a relative newcomer to CNC sees a command, it is likely its meaning will be remembered. One of our intentions with program formatting is to keep you from having to memorize all commands needed for programming. Instead, you will be looking at an example and simply recollecting the function of each command.

A second reason for strict program formatting is consistency. Once you have a format that works, use it. If you use the same format (or structure) for all programs you write, you will be able to repeat past successes. If all programmers in you company use the same format for a given CNC machine, each programmer will be easily able to work on another's program.

The third (and most important) reason for strictly formatting programs is related to multi tool jobs. Almost all CNC machining center and turning center programs require that more than one tool in the program. For this kind of program, there will be MANY times when it will be necessary to rerun only one tool in the program a second, third, or fourth time.

Say for example, you have a machining center program that uses ten tools. After running a workpiece, you determine that the fifth tool in the program did not go quite deep enough. After fixing the problem (changing tool offset or Z position in program), you will need to run the fifth tool again. However, you would NOT want to run the entire program just to get to tool number five. Doing so would be a waste of time and may actually cause unwanted problems with workpiece accuracy and finish. Instead, you will want to be able to run ONLY tool five a second time.

To do so will require that ALL information necessary to get the machine running (just like at the beginning of the program) is included at the beginning of tool five. If the programmer makes certain assumptions related to modal information from a previous tool, it may not be possible to run tool number five by itself.

Here is an example of a time when the programmer must include some redundant information at the beginning of a tool in order to give the ability to rerun the tool. In our previous ten tool example, we still wish to run tool number five a second time. Say that tools four and five both run at 500 RPM. Say the last tool in the program (tool number ten) runs at 1500 RPM. Spindle speed is modal. The programmer may decide to leave out the S500 word at the beginning of tool five, expecting it to carry over from tool four. After running the entire program, it is determined that tool number five did not go deep enough. The operator fixes the problem and intends to run only tool number five. In this case, tool number five will start at the same spindle speed as the last tool in the program (1500 RPM), not 500 RPM! This is but one time when redundant information must be programmed from tool to tool in order to give the capability to rerun tools.

At the beginning of each tool, the programmer MUST include all information necessary to begin the tool, even if it means including some redundant information. In essence, the programmer must treat each tool as a mini-program that can run separate from the rest of the program. When you think about it, this actually simplifies the programming task, allowing the programmer to break a seemingly complicated multi-tool program into smaller and easier to handle pieces. Each tool makes up one piece of the program.

The four kinds of program format

For machines that have the ability to perform operations with several tools, there are four kinds of program format: program start-up format, tool ending format, tool start-up format, and program ending format. The programmer will begin every program with program start-up format. At the completion of program start-up format, the tool will be ready to begin cutting. At this point, the programmer will program the cutting operations with the first tool. When finished cutting, the programmer will follow the format to end the tool (tool ending format). Then tool start-up format to begin the second tool. The programmer will then toggle among cutting information, tool ending format and tool start-up format until the finished cutting with the last tool. At this point, the programmer will follow the format to end the program.

For an example of the four kinds of program format, refer to the program given during our discussion of tool length compensation (key concept number four). This program uses two tools and follows the strict format we are now discussing. Let's determine what commands are related to each kind of format.

The first four commands (beginning with the program number) makeup the program start-up format. At the completion of line N015, the tool is ready to begin machining. Lines N020 and N025 makeup the cutting commands for the first tool. (In line N030, the feedrate should be considered part of program start-up format.) Lines N030 and N035 form the tool ending format. Lines N040 through N055 are tool startup format. (In line N060, the feedrate should be considered part of tool startup format.) Lines N060 and N065 are the cutting commands for the second tool. And lines N070 and N075 are program ending format.

By breaking up the program in this manner, you should be able to see just how much of the program is nothing more than program format that can be copied from one program to another. Of course, certain word values like spindle speeds, feedrates, axis positions, and tool station and offset numbers will change based on the program you are currently writing. But the basic structure can be copied, keeping you from leaving out important information. Note that there are only four commands that do any cutting in this program. The bulk of the program is just format.

How do you come up with program format information for your machine?

The best way is to take an example program that is currently running successfully and break it up in the manner shown above. When doing this, analyze just what each tool is doing to determine the various types of format. Ensure that each tool contains all information needed to run independently.

If you are working from scratch with a new CNC machines and have no examples to go by, contact your machine tool builder to gain an understanding of how your programs should be formatted. You may also find example programs given within your machine tool builder's programming manual.

types of compensations in cnc

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Now let's discuss the compensation types for the two most popular forms of CNC machine tools, machining centers and turning centers. Keep in mind that while the actual use of these functions vary dramatically from one machine to the next, the basic reasoning behind each compensation type remains remarkably similar. With an understanding of why the compensation type is required, and with an elementary understanding of how it is applied to one specific control, you should be able to adapt to any variations that you come across.

Tool length compensation

This machining center compensation type allows the programmer to forget about each tool's length as the program is written. Instead of having to know the exact length of each tool and tediously calculating Z axis positions based on the tool's length, the programmer simply instates tool length compensation on each tool's first Z axis approach movement to the workpiece.

At the machine during setup, the operator will input the tool length compensation value for each tool in the corresponding offset. This, of course, means the tool length compensation value must first be measured.

If tool length compensation is used wisely, the tool length compensation value can be measured off line (in a tool length measurement gage) to minimize setup time. With this method, the tool length compensation value is simply the length of the tool.

Many CNC controls allow the length of the tool to be used as the offset value. One popular command to instate tool length compensation is G43. Within the G43 command, the programmer includes an H word that specifies the number of the offset containing the tool's length value. Here is an example program that utilizes tool length compensation with two tools. The program simply drills two holes (one with each tool). Notice that tool length compensation is being instated in lines N015 and N055.

Program: O0001 (Program number); N005 T01 M06 (Place tool number one in the spindle); N010 G54 G90 S400 M03 T02 (Select coordinate system, absolute mode, start spindle CW at 400 RPM, get tool number two ready); N013 G00 X1.0 Y1.0 (Rapid to first XY position); N015 G43 H01 Z.1 M08 (Instate tool length compensation on first Z move, turn on coolant); N020 G01 Z-1.5 F4. (Drill hole); N025 G00 Z.1 M09 (Rapid out of hole, turn off coolant); N030 G91 G28 Z0 M19 (Return to tool change position, orient spindle); N035 M01 (Optional stop) N040 T02 M06 (Place tool number two in spindle); N045 G54 G90 S400 M03 T01 (Select coordinate system, absolute mode, start spindle at 400 RPM, get tool number one ready); N050 G00 X2. Y1. (Rapid to first XY position); N055 G43 H02 Z.1 M08 (Instate tool length compensation on tool's first Z move, turn on coolant); N060 G01 Z-1.2 F5.5 (Drill hole); N065 G00 Z.1 M08 (Rapid out of hole, turn off coolant); N070 G91 G28 Z0 M19 (Return to tool change position, orient spindle); N075 M30 (End of program)

As stated, this feature varies dramatically in use from one control model to the next. You must reference your control manufacturer's programming manual to learn more about how tool length compensation applies to your particular machining center. Sizing with tool length compensation

In the marksman analogy, we said that the marksman would not know for sure whether the initial sight adjustment is perfectly correct until the first shot is fired. In similar fashion, the CNC operator will not know for certain whether the tool length compensation value is perfectly correct until the first workpiece is machined. Say for example, the tool length measurement was made incorrectly. During the measurement, the operator finds the tool to be 6.5372 in long. But the actual tool length is 6.5355 in. In this case, the tool would machine slightly shallower in Z that it is supposed to. After machining, the minor depth change can be made by adjusting the offset, NOT THE PROGRAM.

In some cases, even if the tool length value is measured perfectly, other problems may cause the tool not to machine to the proper depth. If, for example, the workpiece or setup is quite weak, tool pressure may cause the workpiece to tend to push away from the tool doing the machining.

For critical surfaces or when tool pressure is unpredictable, the operator can even trial cut the workpiece under the influence of an offset slightly LARGER than the measured value to ensure that some excess stock will be left. After machining, the operator can measure the surface to determine precisely how much offset change is necessary to machine the workpiece to size.

Cutter radius compensation

Just as tool length compensation allows the machining center programmer to forget about the tool's length, so does cutter radius compensation allow the programmer to forget about the cutter's radius as contours are programmed. While it may be obvious, let us point out that cutter radius compensation is ONLY used for milling cutters and only when milling on the periphery of the cutter. You would NEVER consider using cutter radius compensation for a drill, tap, reamer, or other hole machining tool.

Reasons for cutter radius compensation

Let's begin by discussing four reasons why cutter radius compensation is not only required, but also very helpful to the CNC user.

Program coordinates are easier to calculate

Without cutter radius compensation, machining center programmers must program the centerline path of all milling cutters. An example program using this technique was shown during our discussion of motion types (key concept number three). When programming centerline path, the programmer must know the precise diameter of the milling cutter and calculate program movements based on the tool's centerline path.

With cutter radius compensation, the programmer can program the coordinates of the work surface, NOT the tool's centerline path. This eliminates the need for many calculations.

Keep in mind that we are now talking about manual programming. If you have a CAM (computer aided manufacturing) system, your CAM system can probably generate centerline path just as easily as work surface path.

Range of cutter sizes

Say you do program centerline path for a given workpiece contour and do not use cutter radius compensation. Say you have programmed based on a one inch diameter tool. But when the job is to be run, you find that your company does not have any one inch end mills. Say the closest you have is a 0.875 in cutter. In this case, the entire cutter path would have to be changed in the program to match the new cutter size. With cutter radius compensation, handling this problem is as simple as changing an offset value.

Easy sizing

As with tool length compensation, the operator can use the cutter radius compensation offset to help with sizing. If the contour is not coming out to size (possibly due to tool pressure), an offset can be changed to allow for the imperfection.

Roughing and finishing

This is also a manual programming related reason for using cutter radius compensation. If contours must be rough and finish milled, cutter radius compensation allows the programmer to used the same programmed coordinates needed to finish mill the workpiece to rough mill the workpiece. This keeps the programmer from having to calculate to sets of milling coordinates (one for roughing and one for finishing). To leave stock for finishing during the rough milling, the operator will simply make the cutter radius compensation offset value slightly larger than the cutter's actual size. This will keep the cutter away from the surface being milled and leave the desired finishing stock.

How to program cutter radius compensation

The usage of cutter radius compensation does vary from one control to the next. Additionally, each control will have a set of strict rules that specify how cutter radius compensation is instated, used, and cancelled. Here we just show the basics of how it is programmed and give an example for how it is used on one popular control model. You must refer to your CNC control manufacturer's manual for more on your particular control.

Most controls use three G codes with cutter radius compensation. G41 is used to instate a cutter left condition (climb milling with a right hand cutter). G42 is used to instate a cutter right condition (conventional milling). G40 is used to cancel cutter radius compensation. Additionally, many controls use a D word to specify the offset number used with cutter radius compensation.

To determine whether to use G41 or G42, simply look in the direction the cutter is moving during machining and ask yourself if the cutter is on the left or right side of the surface being machined. If on the left, use G41. If on the right, use G42. Figure 4.6 shows some examples that should help you understand how to determine whether to use G41 or G42 to instate. Figure 4.6 - Drawings show how to determine whether to use G41 or G42 to instate cutter radius compensation.

Once cutter radius compensation is properly instated, it the cutter will be kept on the left side or right side (depending on whether G41 or G42 is used to instate) of all surfaces until the G40 command to cancel compensation.

Dimensional tool (wear) offsets

This compensation type applies only to turning centers. When setting up tools, it is infeasible to expect the setup person to perfectly set each tool into position. It is likely that some minor positioning problem will exist. And even if all tools could be perfectly positioned, as any single point turning or boring tool begins cutting, it will begin to wear. As a turning or boring tool wears, the tool wear will affect the size of the workpiece being machined.

For these reasons, and to allow easy sizing of turned workpieces, dimensional tool offsets are required (also called simply tool offsets). Tool offsets are instated as part of a four digit T word. The first two digits command the tool station number and the second two digits command the offset number to be instated. The command T0101, for example, rotates the turret to station number one and instates offset number one. It is wise to always make the number of the primary offset used with a tool the same as the tool station number.

When a tool offset is instated, the control actually shifts the entire coordinate system by the amount of the offset. It will be as if the operator could actually move the tool in the turret by the amount of the offset.

Each dimensional offset has two values, one for X and one for Z. The operator will have control of what the tool does in both axes as the workpiece is being machined. Here's an example that should help you understand how dimensional tool offsets work. Say you have written a program to use tool number one (with offset number one) to turn a three inch diameter. After machining the three inch diameter, it is measured and found to be 3.005 in. That is, the workpiece is 0.005 in oversize. In this case, the X value of offset number one will be reduced by 0.005 in. When the program is run again, tool number one will machine the workpiece 0.005 smaller.

How to guarantee your first workpiece comes out on size

If working on an engine lathe, manually turning a precise diameter, you would first allow the tool to skim cut to find out exactly where the tool is located. After skim cutting, you can determine precisely how much to turn the crank or handle to make the tool turn the desired diameter.

In the same way, you can use dimensional tool offsets to ensure that any tool will not violate the workpiece on its first cut. Outside diameter turning tools, for example, could be offset slightly plus in X to ensure that some excess stock is left. Inside diameter bored holes could be offset slightly minus in X for the same purpose. In either case, the first time the tool is run, the operator can rest assured that the workpiece will come out with some excess finishing stock (it will NOT be scrapped). After machining the first time, the surface can be measured. The operator will then adjust the corresponding offset accordingly and re-machine with the tool This time the surface will be machined perfectly to size. Using this technique on each tool in the program will almost guarantee that the first workpiece will not be scrapped.

Tool nose radius compensation

This turning center compensation type is very similar to cutter radius compensation. In fact the same three G codes are used. G41 instates tool nose radius compensation in a tool left condition. G42 instates with a tool right condition. G40 cancels tool nose radius compensation. For this reason, minimize our discussion of tool nose radius compensation to avoid repeating information. Just as cutter radius compensation allows the programmer to program work surface coordinates (not allowing for tool radius), so does tool nose radius compensation.

To determine G41 or G42, simply look in the direction the tool is moving during the cut and ask yourself which side of the workpiece the tool is on. If the tool is on the left, use G41 (this would be the case when boring toward the chuck). If the tool is on the right, use G42 (turning toward the chuck). Once you determine which to use, include the proper G code in the tool's first approach to the workpiece. Once tool nose radius compensation is instated, it remains in effect until cancelled.

Keep in mind that the tool nose radius is quite small (usually 1/64, 1/32, 3/64, or 1/16 in), meaning the deviation from the work surface will also be quite small. It is possible that if you are only chamfering corners to break sharp edges, you may not need tool nose radius compensation. However, if the surfaces being machined are critical (Morse taper, for example), you must compensate for the radius of the tool. Also, you should only need tool nose radius compensation when finishing. You should not use it for roughing operations.

Other types of compensation

The compensation types shown have been for machining centers and turning centers. But all forms of CNC equipment have some form of compensation to allow for unpredictable situations. Here are some other brief examples.

CNC Wire EDM machines have two kinds of compensation. One, called wire offset works in a very similar way to cutter radius compensation to keep the wire centerline away from the work surface by the wire radius plus the overburn amount. It is also used to help make trim (finishing) passes using the same series of motion coordinates.

The second form of compensation for wire EDM machines is taper cutting. For machining the clearance angle needed with dies and form tools, the programmer can easily specify the direction of the taper (left or right) and the angle desired. The operator fills in some offsets to tell the control the position of the upper guide relative to the workpiece and the control does the rest.

Laser cutting machines also have a feature like cutter radius compensation to keep the laser the radius of the laser beam away from the surface being machined. CNC press breaks have a form of compensation to allow for bend allowances based on the workpiece material and thickness. Generally speaking, if the CNC user is faced with any unpredictable situations during programming, it is likely that the CNC control manufacturer has come up with a form of compensation to deal with the problem.

what is offset?

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All forms of compensation work with offsets. You can think of CNC offsets as like memories on an electronic calculator. If your calculator has memories, you know you can store a constant value into each memory for use during a calculation. This keeps you from having to enter the number over and over again with redundant calculations.

Like the memories of an electronic calculator, offsets in the CNC control are storage locations into which numerical values can be placed. Just as the value within the memory of a calculator has no meaning until referenced by its user within a calculation, neither does the value within an offset of the CNC control have any meaning until it is referenced by a CNC program.

From the marksman analogy, you can think of the values stored in CNC offsets as like the amount of adjustment required on the sight of the rifle necessary to compensate for the distance to the target. Keep in mind that the rifle only needed adjustment for one purpose, to adjust for the distance to the target. With most CNC machine tools, there is a need for at least one offset per tool.

Reasons for tool offsets

Offsets can be used for several purposes depending on the style of machine tool and type of compensation being used. Here are some of the more common applications for offsets.

To specify tool each tool's length

For machining center applications, it would be very difficult for the programmer to predict the precise length of each tool used in the program. For this reason, the feature tool length compensation allows the programmer to ignore each tool's length as the program is written. At the time of setup, the setup person measures the length of each tool and inputs the tool length value into the corresponding offset.

To specify the radius of the cutting tool

When milling on the periphery of the cutter (contour milling), it can be cumbersome and difficult for the programmer to program the cutter's path based on the size of the milling cutter being used. Also, if the cutter size must change (possibly due to re-sharpening), it would be infeasible to change the program based on the new cutter size. For this reason, the feature cutter radius compensation allows the programmer to ignore the cutter size as the program is written. The setup person inputs the size of each milling cutter into its corresponding tool offset. In similar fashion, turning centers have a feature called tool nose radius compensation. With this feature, an offset is used to specify the radius of the very tip of the turning or boring tool.

To assign program zero

Machining centers that have fixture offsets (also called coordinate system shifting) allow the user to specify the position of the program zero point within offsets, keeping the assignment of program zero separate from the program. In similar fashion many turning centers allow the assignment of program zero with offsets (this feature is commonly called geometry offsets).

To allow sizing on turning centers

Tool offsets are used on all turning centers to allow the operator to hold size with tools used within their programs. This allows the operator to adjust for imperfections with tool placement during setup. It also allows the operator to adjust the tool's movements to allow for tool wear during each tool's life.

Organizing offsets

With some CNC controls, the organization of offsets is very obvious. Some machining center controls, for example, automatically make the offset number correspond to the tool station number. With this kind of machine, when tool station number one is commanded, the control will automatically invoke offset number one. Within offset number one, the operator can store a tool length value as well as a tool radius value.

Unfortunately, not all controls make it this simple. In many controls, each offset contains only one value and the offset number has no real relationship to the tool station number. In this case, the programmer must cautiously organize which offset/s are used with each tool.

For example, the tool length compensation offset numbers can be made the same as tool station numbers. Cutter radius compensation offset numbers can be made equal to the tool station number PLUS a constant value larger than the number of tools the machine can hold. If the machine can hold 25 tools, tool station number one could be made to use offset number one to store its length compensation value and offset number thirty-one could be used to store its cutter radius compensation value. With this method of offset organization, the programmer and operator are constantly in sync.

The offset table on most turning centers incorporate at least two values per offset. Generally speaking, the programmer will instate the offset number corresponding to the tool station number for each tool offset used. That is, tool number one will use (only) offset number one, tool two will use offset two, and so on. Additionally, most turning center offset tables allow the user to enter data related to the tool's radius (for tool nose radius compensation). Typically the radius (R column of the offset table) and the tool type (the T column of the offset table) can be specified within the turning center's offset table.

example program with all 3 basic motion

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In this particular example, we are milling around the outside of a workpiece contour. Notice that we are using a one inch diameter endmill for machining the contour and we are programming the very center of the end mill. Later, during key concept number four, we will discuss a way to actually program the workpiece contour (not the cutter centerline path). While you may not understand all commands given in this program, concentrate on understanding what is happening in the motion commands (G00, G01, and G02/G03). With study, you should be able to see what is happening. Messages in parentheses are provided to document what is happening in each command.

Program

O0002 (Program number)

N005 G54 G90 S350 M03 (Select coordinate system, absolute mode, and start spindle CW at 350 RPM)

N010 G00 X-.625 Y-.25 (Rapid to point 1)

N015 G43 H01 Z-.25 (Instate tool length compensation, rapid tool down to work surface)

N020 G01 X5.25 F3.5 (Machine in straight motion to point 2)

N025 G03 X6.25 Y.75 R1.0 (CCW circular motion to point 3)

N030 G01 Y3.25 (Machine in straight motion to point 4)

N035 G03 X5.25 Y4.25 R1.0 (CCW circular motion to point 5)

N040 G01 X.75 (Machine in straight motion to point 6)

N045 G03 X-.25 Y3.25 R1.0 (CCW circular motion to point 7)

N050 G01 Y.75 ((Machine in straight motion to point 8)

N055 G03 X.75 Y-.25 R1.0 (CCW circular motion to point 9)

N060 G00 Z.1 (Rapid away from workpiece in Z)

N065 G91 G28 Z0 (Go to the machine's reference point in Z)

N070 M30 (End of program)

Keep in mind that CNC controls do vary with regard to limitations with motion types. For example, some controls have strict rules governing how much of a full circle you are allowed to make within one circular command. Some require directional vectors for circular motion commands instead of allowing the R word. Some even incorporate automatic corner rounding and chamfering, minimizing the number of motion commands that must be given. Though you must be prepared for variations, and you must reference your control manufacturer's programming manual to find out more about your machine's motion commands, at least this presentation has shown you the basics of motion commands and you should be able to adapt to your particular machine and control with relative ease.

basic motion types of cnc

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While your particular CNC machine may have more motion types (depending on your application), let's concentrate on becoming familiar with the three most common types of motion. These three motion types are available on almost all forms of CNC equipment. After briefly introducing each type of motion, we'll show an example program that stresses the use of all three.

These motion types share two things in common. First, they are all modal. This means they remain in effect until changed. If for example, several motions of the same kind are to be given consecutively, the corresponding G code need only be specified in the first command. Second, the END POINT of the motion is specified in each motion command. The current position of the machine will be taken as the starting point.

Rapid motion (also called positioning)

This motion type (as the name implies) is used to command motion at the machine's fastest possible rate. It is used to minimize non-productive time during the machining cycle. Common uses for rapid motion include positioning the tool to and from cutting positions, moving to clear clamps and other obstructions, and in general, any non-cutting motion during the program.

You must check in the machine tool builder's manual to determine a machine's rapid rate. Usually this rate is extremely fast (some machines boast rapid rates of well over 1000 IPM!), meaning the operator must be cautious when verifying programs during rapid motion commands. Fortunately, there is a way for the operator to override the rapid rate during program verification.

The command almost all CNC machines use to command rapid motion is G00. Within the G00 Command, the end point for the motion is given. Control manufacturers vary with regard to what actually happens if more than one axis is included in the rapid motion command. With most controls, the machine will move as fast as possible in all axes commanded. In this case, one axis will probably reach its destination point before the other/s. With this kind of rapid command, straight line movement will NOT occur during rapid and the programmer must be very careful if there are obstructions to avoid. With other controls, straight line motion will occur, even during rapid motion commands.

Straight line motion (also called linear interpolation)

This motion type allows the programmer to command perfectly straight line movements as discussed earlier during our discussion of linear interpolation. This motion type also allows the programmer to specify the motion rate (feedrate) to be used during the movement. Straight line motion can be used any time a straight cutting movement is required, including when drilling, turning a straight diameter, face or taper, and when milling straight surfaces. The method by which feedrate is programmed varies from one machine type to the next. Generally speaking, machining centers only allow the feedrate to be specific in per minute format (inches or millimeters per minute). Turning centers also allow feedrate to be specified in per revolution format (inches or millimeters per revolution).

A G01 word is commonly used to specify straight line motion. Within the G01, the programmer will include the desired end point in each axis.

Circular motion (also called circular interpolation)

This motion type causes the machine to make movements in the form of a circular path. As discussed earlier during our presentation of circular interpolation, this motion type is used to generate radii during machining. All feedrate related points made during our discussion of straight line motion still apply.

Two G codes are used with circular motion. G02 is commonly used to specify clockwise motion while G03 is used to specify counter clockwise motion. To evaluate which to use, you simply view the movement from the same perspective the machine will view the motion. For example, if making a circular motion in XY on a machining center, simply view the motion from the spindle's vantage point. If making a circular motion in XZ on a turning center, simply view the motion from above the spindle. In most cases, this is as simple as viewing the print from above.

Additionally, circular motion requires that, by one means or another, the programmer specifies the radius of the arc to be generated. With newer CNC controls this is handled by a simple "R" word. The R word within the circular command simply tells the control the radius of the arc being commanded. With older controls, directional vectors (specified by I, J, and K) tell the control the location of the arc's center point. Since controls vary with regard to how directional vectors are programmed, and since the R word is becoming more and more popular for radius designation, our examples will show the use of the R word. If you wish to learn more about directional vectors, you must reference your control manufacturer's manual.

interpolation understanding

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CNC control manufacturers try to make it as easy as possible to make movement commands within the program. For those styles of motion that are commonly needed, they give the CNC user interpolation types.

Understanding interpolation

Say for example, you wish to move only one linear axis in a command. Say you wish to move the X axis to a position one inch to the right of program zero. In this case, the command X1. would be given (assuming the absolute mode is instated). The machine would move along a perfectly straight line during this movement (since only one axis is moving). Now let's say you wish to include a Y axis movement to a position one inch above program zero in Y (with the X movement). We'll say you are trying to machine a tapered or chamfered surface of your workpiece in this command. For the control to move along a perfectly straight line to get to the programmed end point, it must perfectly synchronize the X and Y axis movements. Also, if machining is to occur during the motion, a motion rate (feedrate) must also be specified. This requires linear interpolation.

During linear interpolation commands, the control will precisely and automatically calculate a series of very tiny single axis departures, keeping the tool as close to the programmed linear path as possible. With today's CNC machine tools, it will appear that the machine is forming a perfectly straight line motion. However, Figure 3.1 shows what the CNC control is actually doing during linear interpolation. Figure 3.1 - Actual motion generated with linear interpolation. Notice the series of very tiny single axis movements. The step size is equal to the machine's resolution, usually 0.0001 in or 0.001 mm.

In similar fashion, many applications for CNC machine tools require that the machine be able to form circular motions. Applications for circular motions include forming radii on turned workpieces between faces and turns and milling radii on contours of machining center workpieces. This kind of motion requires circular interpolation. As with linear interpolation, the control will do its best to generate as close to a circular path as possible.

Other interpolation types

Depending on the machine's application, you may find that you have other interpolation types available. Again, CNC control manufacturers try to make it as easy as possible to program their controls. If an application requires a special kind of movement, the control manufacturer can give the applicable interpolation type. For example, many machining center users perform thread milling operations on their machines. During thread milling, the machine must move in a circular manner along two axes (usually X and Y) at the same time a third axis (usually Z) moves in a linear manner. This allows the helix of the thread to be properly machined. This motion resembles a spiraling motion (though the radius of the spiral remains constant).

Knowing that their customers need this type of motion for thread milling, CNC machining center control manufacturers offer the feature helical interpolation. With this feature, the user can easily command the motions necessary for thread milling.

programmable functions of cnc

2008-12-08T00:14:00.000-08:00

The programmer must also know what functions of the CNC machine are programmable (as well as the commands related to programmable functions). With low cost CNC equipment, often times many machine functions must be manually activated. With some CNC milling machines, for example, about the only programmable function is axis motion. Just about everything else may have to be activated by the operator. With this type of machine, the spindle speed and direction, coolant and tool changes may have to be activated manually by the operator.

With full blown CNC equipment, on the other hand, almost everything is programmable and the operator may only be required to load and remove workpieces. Once the cycle is activated, the operator may be freed to do other company functions.

Reference the machine tool builder's manual to find out what functions of your machine are programmable. To give you some examples of how many programmable functions are handled, here is a list a few of the most common programmable functions along with their related programming words.

Spindle control

An "S" word is used to specify the spindle speed (in RPM for machining centers). An M03 is used to turn the spindle on in a clockwise (forward) manner. M04 turns the spindle on in a counter clockwise manner. M05 turns the spindle off. Note that turning centers also have a feature called constant surface speed which allows spindle speed to also be specified in surface feet per minute (or meters per minute)

Automatic tool changer (machining center)

A "T" word is used to tell the machine which tool station is to be placed in the spindle. On most machines, an M06 tells the machine to actually make the tool change. Tool change (on turning centers) A four digit "T" word is used to command tool changes on most turning centers. The first two digits of the T word specify the turret station number and the second two digits specify the offset number to be used with the tool. T0101, for example specifies tool station number one with offset number one.

Coolant control

M08 is used to turn on flood coolant. If available M07 is used to turn on mist coolant. M09 turns off the coolant.

Automatic pallet changer

An M60 command is commonly used to make pallet changes.

Other programmable features to look into

An M60 command is commonly used to make pallet changes.

As stated, programmable functions will vary dramatically from one machine to the next. The actual programming commands needed will also vary from builder to builder. Be sure to check the M codes list (miscellaneous functions) given in the machine tool builder's manual to find out more about what other functions may be programmable on your particular machine. M codes are commonly used by the machine tool builder to give the user programmable ON/OFF switches for machine functions. In any case, you must know what you have available for activating within your CNC programs.

For turning centers, for example, you may find that the tailstock and tailstock quill is programmable. The chuck jaw open and close may be programmable. If the machine has more than one spindle range, commonly the spindle range selection is programmable. And if the machine has a bar feeder, it will be programmable. You may even find that your machine's chip conveyor can be turned on and off through programmed commands. All of this, of course, is important information to the CNC programmer.

direction of motin(axis) in cnc

2008-12-08T00:10:00.000-08:00

The CNC programmer MUST know the programmable motion directions (axes) available for the CNC machine tool. The axes names will vary from one machine tool type to the next. They are always referred to with a letter address. Common axis names are X, Y, Z, U, V, and W for linear axes and A, C, and C for rotary axes. However, the beginning programmer should confirm these axis designations and directions (plus and minus) in the machine tool builder's manual since not all machine tool builders conform to the axis names we show.

As discussed in key concept number one, whenever a programmer wishes to command movement in one or more axes, the letter address corresponding to the moving axes as well as the destination in each axis are specified. X3.5, for example tells the machine to move the X axis to a position of 3.5 inches from the program zero point in X (assuming the absolute mode of programming is used.

The reference point for each axis

Most CNC machines utilize a very accurate position along each axis as a starting point or reference point for the axis. Some control manufacturers call this position the zero return position. Others call it the grid zero position. Yet others call it the home position. Regardless of what it is called, the reference position is required by many controls to give the control an accurate point of reference. CNC controls that utilize a reference point for each axis require that the machine be manually sent to its reference point in each axis as part of the power up procedure. Once this is completed, the control will be in sync with the machine's position.

know yourr cnc machine

2008-12-08T00:07:00.000-08:00

From a programmer's standpoint, as you begin to learn about any new CNC machine, you should concentrate on four basic areas. First, you should understand the machine's most basic components. Second, you should become comfortable with your machine's directions of motion (axes). Third, you should become familiar with any accessories equipped with the machine. And fourth, you should find out what programmable functions are included with the machine and learn how they are programmed.

Machine components

While you do not have to be a machine designer to work with CNC equipment, it is important to know how your CNC machine is constructed. Understanding your machine's construction will help you to gauge the limits of what is possible with your machine. Just as the race car driver should understand the basics of suspension systems, breaking systems, and the workings of internal combustion engines (among other things) in order to get the most out of a given car, so must the CNC programmer understand the basic workings of the CNC machine in order to get the most from the CNC machine tool.

For a universal style slant bed turning center, for example, the programmer should know the most basic machine components, including bed, way system, headstock & spindle, turret construction, tailstock, and work holding device. Information regarding the machine's construction including assembly drawings is usually published right in the machine tool builder's manual. As you read the machine tool builder's manual, here are some of the machine capacity and construction questions to which you should find answers.

* What is the machine's maximum RPM?

* How many spindle ranges does the machine have (and what are the cut-off points for each range?

* What is the spindle and axis drive motor horsepower?

* What is the maximum travel distance in each axis?

* How many tools can the machine hold?

* What way construction does the machine incorporate (usually square ways, dovetail, and/or linear bearing ways)?

* What is the machine's rapid rate (fastest traverse rate)?

* What is the machine's fastest cutting feedrate?

These are but a few of the questions you should be asking yourself as you begin working with any new CNC machine. Truly, the more you know about your machine's capacity and construction, the easier it will be to get comfortable with the machine.

basic machining practice with cnc

2008-12-08T00:05:00.000-08:00

Many forms of CNC machines are designed to enhance or replace what is currently being done with more conventional machines. The first goal of any CNC beginner should be to understand the basic machining practice that goes into using the CNC machine tool. The more the beginning CNC user knows about basic machining practice, the easier it will be to adapt to CNC.

Think of it this way. If you already know basic machining practice as it relates to the CNC machine you will be working with, you already know what it is you want the machine to do. It will be a relatively simple matter of learning how to tell the CNC machine what it is you want it to do (learning to program). This is why machinists make the best CNC programmers, operators, and setup personnel. Machinists already know what it is the machine will be doing. It will be a relatively simple matter of adapting what they already know to the CNC machine.

For example, a beginner to CNC turning centers should understand the basic machining practice related to turning operations like rough and finish turning, rough and finish boring, grooving, threading, and necking. Since this form of CNC machine can perform multiple operations in a single program (as many CNC machines can), the beginner should also know the basics of how to process workpieces machined by turning so a sequence of machining operations can be developed for workpieces to be machined.

This point cannot be overstressed. Trying to learn about a particular CNC machine without understanding the basic machining practice related to the machine would be like trying to learn how to fly an airplane without understanding the basics of aerodynamics and flight. Just as a beginning pilot will be in for a great number of problems without understanding aerodynamics, so is the beginning CNC user have difficulty learning how to utilize CNC equipment without an understanding of basic machining practice.

program makeup and programmable functions

2008-12-08T00:01:00.000-08:00

As stated programs are made up of commands and commands are made up of word. Each word has a letter address and a numerical value. The letter address tells the control the word type. CNC control manufacturers do vary with regard to how they determine word names (letter addresses) and their meanings. The beginning CNC programmer must reference the control manufacturer's programming manual to determine the word names and meanings. Here is a brief list of some of the word types and their common letter address specifications.

* O - Program number (Used for program identification)

* N - Sequence number (Used for line identification)

* G - Preparatory function

* X - X axis designation

* Y - Y axis designation

* Z - Z axis designation

* R - Radius designation

* F - Feedrate designation

* S - Spindle speed designation

* H - Tool length offset designation

* D - Tool radius offset designation

* T - Tool Designation

* M - Miscellaneous function (See below)

As you can see, many of the letter addresses are chosen in a rather logical manner (T for tool, S for spindle, F for feedrate, etc.). A few require memorizing.

There are two letter addresses (G and M) which allow special functions to be designated. The preparatory function (G) specifies is commonly used to set modes. We already introduced absolute mode, specified by G90 and incremental mode, specified by G91. These are but two of the preparatory functions used. You must reference your control manufacturer's manual to find the list of preparatory functions for your particular machine.

Like preparatory functions, miscellaneous functions (M words) allow a variety of special functions. Miscellaneous functions are typically used as programmable switches (like spindle on/off, coolant on/off, and so on). They are also used to allow programming of many other programmable functions of the CNC machine tool.

To a beginner, all of this may seem like CNC programming requires a great deal of memorization. But rest assured that there are only about 30-40 different words used with CNC programming. If you can think of learning CNC manual programming as like learning a foreign language that has only 40 words, it shouldn't seem too difficult.

Decimal point programming

Certain letter addresses (CNC words) allow the specification of real numbers (numbers that require portions of a whole number). Examples include X axis designator (X), Y axis designator (Y), and radius designator (R). Almost all current model CNC controls allow a decimal point to be used within the specification of each letter address requiring real numbers. For example, X3.0625 can be used to specify a position along the X axis.

On the other hand, some letter addresses are used to specify integer numbers. Examples include the spindle speed designator (S), the tool station designator (T), sequence numbers (N), preparatory functions (G), and miscellaneous functions (M). For these word types, most controls do NOT allow a decimal point to be used. The beginning programmer must reference the CNC control manufacturer's programming manual to find out which words allow the use of a decimal point.

Other programmable functions

All but the very simplest CNC machines have programmable functions other than just axis motion. With today's full blown CNC equipment, almost everything about the machine is programmable. CNC machining centers, for example, allow the spindle speed and direction, coolant, tool changing, and many other functions of the machine to be programmed. In similar fashion, CNC turning centers allow spindle speed and direction, coolant, turret index, and tailstock to be programmed. And all forms of CNC equipment will have their own set of programmable functions. Additionally, certain accessories like probing systems, tool length measuring systems, pallet changers, and adaptive control systems may also be available that require programming considerations.

The list of programmable functions will vary dramatically from one machine to the next, and the user must learn these programmable functions for each CNC machine to be used. In key concept number two, we will take a closer look at what is typically programmable on different forms of CNC machine tools.

cnc program

2008-12-07T23:58:00.000-08:00

Almost all current CNC controls use a word address format for programming. (The only exceptions to this are certain conversational controls.) By word address format, we mean that the CNC program is made up of sentence-like commands. Each command is made up of CNC words. Each CNC word has a letter address and a numerical value. The letter address (X, Y, Z, etc.) tells the control the kind of word and the numerical value tells the control the value of the word. Used like words and sentences in the English language, words in a CNC command tell the CNC machine what it is we wish to do at the present time.

One very good analogy to what happens in a CNC program is found in any set of step by step instructions. Say for example, you have some visitors coming in from out of town to visit your company. You need to write down instructions to get from the local airport to your company. To do so, you must first be able to visualize the path from the airport to your company. You will then, in sequential order, write down one instruction at a time. The person following your instructions will perform the first step and then go on to the next until he or she reaches your facility.

In similar manner, a manual CNC programmer must be able to visualize the machining operations that are to be performed during the execution of the program. Then, in step by step order, the programmer will give a set of commands that makes the machine behave accordingly.

Though slightly off the subject at hand, we wish to make a strong point about visualization. Just as the person developing travel directions MUST be able to visualize the path taken, so MUST the CNC programmer be able to visualize the movements the CNC machine will be making BEFORE a program can be successfully developed. Without this visualization ability, the programmer will not be able to develop the movements in the program correctly. This is one reason why machinists make the best CNC users. An experienced machinist should be able to easily visualize any machining operation taking place.

Just as each concise travel instruction will be made up of one sentence, so will each instruction given within a CNC program be made up of one command. Just as the travel instruction sentence is made up of words (in English), so is the CNC command made up of CNC words (in CNC language).

The person following your set of travel instructions will execute them explicitly. If you make a mistake with your set of instructions, the person will get lost on the way to your company. In similar fashion, the CNC machine will execute a CNC program explicitly. If there is a mistake in the program, the CNC machine will not behave correctly.

Program:

O0001 (Program number)

N005 G54 G90 S400 M03 (Select coordinate system, absolute mode, and turn spindle on CW at 400 RPM)

N010 G00 X1. Y1. (Rapid to XY location of first hole)

N015 G43 H01 Z.1 M08 (Instate tool length compensation, rapid in Z to clearance position above surface to drill, turn on coolant)

N020 G01 Z-1.25 F3.5 (Feed into first hole at 3.5 inches per minute)

N025 G00 Z.1 (Rapid back out of hole) N030 X2. (Rapid to second hole)

N035 G01 Z-1.25 (Feed into second hole)

N040 G00 Z.1 M09 (Rapid out of second hole, turn off coolant)

N045 G91 G28 Z0 (Return to reference position in Z)

N050 M30 (End of program command)

While the words and commands in this program probably do not make much sense to you (yet), remember that we are stressing the sequential order by which the CNC program will be executed. The control will first read, interpret and execute the very first command in the program. Only then will it go on to the next command. Read, interpret, execute. Then on to the next command. The control will continue to execute the program in sequential order for the balance of the program. Again, notice the similarity to giving any set of step by step instructions.

assigning program zero

2008-12-07T23:56:00.000-08:00

Keep in mind that the CNC control must be told the location of the program zero point by one means or another. How this is done varies dramatically from one CNC machine and control to another. One (older) method is to assign program zero in the program. With this method, the programmer tells the control how far it is from the program zero point to the starting position of the machine. This is commonly done with a G92 (or G50) command at least at the beginning of the program and possibly at the beginning of each tool.

Another, newer and better way to assign program zero is through some form of offset. Commonly machining center control manufacturers call offsets used to assign program zero fixture offsets. Turning center manufacturers commonly call offsets used to assign program zero for each tool geometry offsets. More on how program zero can be assigned will be presented during key concept number four.

Other points about axis motion

To this point, our primary concern has been to show you how to determine the end point of each motion command. As you have seen, doing this requires an understanding of the rectangular coordinate system. However, there are other concerns about how a motion will take place. Fore example, the type of motion (rapid, straight line, circular, etc.), and motion rate (feedrate), will also be of concern to the programmer. We'll discuss these other considerations during key concept number three.

absolute versus incremental motion

2008-12-07T23:53:00.000-08:00

All discussions to this point assume that the absolute mode of programming is used. The most common CNC word used to designate the absolute mode is G90. In the absolute mode, the end points for all motions will be specified from the program zero point. For beginners, this is usually the best and easiest method of specifying end points for motion commands. However, there is another way of specifying end points for axis motion.

In the incremental mode (commonly specified by G91), end points for motions are specified from the tool's current position, not from program zero. With this method of commanding motion, the programmer must always be asking "How far should I move the tool?" While there are times when the incremental mode can be very helpful, generally speaking, this is the more cumbersome and difficult method of specifying motion and beginners should concentrate on using the absolute mode.

Be careful when making motion commands. Beginners have the tendency to think incrementally. If working in the absolute mode (as beginners should), the programmer should always be asking "To what position should the tool be moved?" This position is relative to program zero, NOT from the tools current position.

Aside from making it very easy to determine the current position for any command, another benefit of working in the absolute mode has to do with mistakes made during motion commands. In the absolute mode, if a motion mistake is made in one command of the program, only one movement will be incorrect. On the other hand, if a mistake is made during incremental movements, all motions from the point of the mistake will also be incorrect.

cnc motion control

2008-12-07T23:51:00.000-08:00

The most basic function of any CNC machine is automatic, precise, and consistent motion control. Rather than applying completely mechanical devices to cause motion as is required on most conventional machine tools, CNC machines allow motion control in a revolutionary manner. All forms of CNC equipment have two or more directions of motion, called axes. These axes can be precisely and automatically positioned along their lengths of travel. The two most common axis types are linear (driven along a straight path) and rotary (driven along a circular path).

Instead of causing motion by turning cranks and handwheels as is required on conventional machine tools, CNC machines allow motions to be commanded through programmed commands. Generally speaking, the motion type (rapid, linear, and circular), the axes to move, the amount of motion and the motion rate (feedrate) are programmable with almost all CNC machine tools.

Accurate positioning is accomplished by the operator counting the number of revolutions made on the handwheel plus the graduations on the dial. The drive motor is rotated a corresponding amount, which in turn drives the ball screw, causing linear motion of the axis. A feedback device confirms that the proper amount of ball screw revolutions have occurred.

A CNC command executed within the control (commonly through a program) tells the drive motor to rotate a precise number of times. The rotation of the drive motor in turn rotates the ball screw. And the ball screw causes drives the linear axis. A feedback device at the opposite end of the ball screw allows the control to confirm that the commanded number of rotations has taken place.

Though a rather crude analogy, the same basic linear motion can be found on a common table vise. As you rotate the vise crank, you rotate a lead screw that, in turn, drives the movable jaw on the vise. By comparison, a linear axis on a CNC machine tool is extremely precise. The number of revolutions of the axis drive motor precisely controls the amount of linear motion along the axis.

How axis motion is commanded - understanding coordinate systems It would be infeasible for the CNC user to cause axis motion by trying to tell each axis drive motor how many times to rotate in order to command a given linear motion amount. (This would be like having to figure out how many turns of the handle on a table vise will cause the movable jaw to move exactly one inch!) Instead, all CNC controls allow axis motion to be commanded in a much simpler and more logical way by utilizing some form of coordinate system. The two most popular coordinate systems used with CNC machines are the rectangular coordinate system and the polar coordinate system. By far, the most popular of these two is the rectangular coordinate system, and we'll use it for all discussions made during this presentation.

One very common application for the rectangular coordinate system is graphing. Almost everyone has had to make or interpret a graph. Since the need to utilize graphs is so commonplace, and since it closely resembles what is required to cause axis motion on a CNC machine, let's review the basics of graphing.

As with any two dimensional graph, this graph has two base lines. Each base line is used to represent something. What the base line represents is broken into increments. Also, each base line has limits. In our productivity example, the horizontal base line is being used to represent time. For this base line, the time increment is in months. Remember this base line has limits - it starts at January and end with December. The vertical base line is representing productivity. Productivity is broken into ten percent increments and starts at zero percent productivity and ends with one hundred percent productivity.

The person making the graph would look up the company's productivity for January of last year and at the productivity position on the graph for January, a point is plotted. This would then be repeated for February, March, and each month of the year. Once all points are plotted, a line or curve can be drawn through each of the points to make it more clear as to how the company did last year.

Let's take what we now know about graphs and relate it to CNC axis motion. Instead of plotting theoretical points to represent conceptual ideas, the CNC programmer is going to be plotting physical end points for axis motions. Each linear axis of the machine tool can be thought of as like a base line of the graph. Like graph base lines, axes are broken into increments. But instead of being broken into increments of conceptual ideas like time and productivity, each linear axis of a CNC machine's rectangular coordinate system is broken into increments of measurement. In the inch mode, the smallest increment is usually 0.0001 inch. In the metric mode, the smallest increment is 0.001 millimeter. (By the way, for rotary axes the increment is 0.001 degrees.)

Just like the graph, each axis within the CNC machine's coordinate system must start somewhere. With the graph, the horizontal baseline started at January and the vertical base line started at zero percent productivity. This place where the vertical and horizontal base lines come together is called the origin point of the graph. For CNC purposes, this origin point is commonly called the program zero point (also called work zero, part zero, and program origin).

For this example, the two axes we happen to be showing are labeled as X and Y but keep in mine that program zero can be applied to any axis. Though the names of each axes will change from one CNC machine type to another (other common names include Z, A, B, C, U, V, and W), this example should work nicely to show you how axis motion can be commanded.

The program zero point establishes the point of reference for motion commands in a CNC program. This allows the programmer to specify movements from a common location. If program zero is chosen wisely, usually coordinates needed for the program can be taken directly from the print.

With this technique, if the programmer wishes the tool to be sent to a position one inch to the right of the program zero point, X1.0 is commanded. If the programmer wishes the tool to move to a position one inch above the program zero point, Y1.0 is commanded. The control will automatically determine how many times to rotate each axis drive motor and ball screw to make the axis reach the commanded destination point. This lets the programmer command axis motion in a very logical manner.

With the examples given so far, all points happened to be up and to the right of the program zero point. This area up and to the right of the program zero point is called a quadrant (in this case, quadrant number one). It is not uncommon on CNC machines that end points needed within the program fall in other quadrants. When this happens, at least one of the coordinates must be specified as minus. .

fundamentals of cnc

2008-12-07T23:48:00.000-08:00

While the specific intention and application for CNC machines vary from one machine type to another, all forms of CNC have common benefits. Though the thrust of this presentation is to teach you CNC usage, it helps to understand why these sophisticated machines have become so popular. Here are but a few of the more important benefits offered by CNC equipment.

The first benefit offered by all forms of CNC machine tools is improved automation. The operator intervention related to producing workpieces can be reduced or eliminated. Many CNC machines can run unattended during their entire machining cycle, freeing the operator to do other tasks. This gives the CNC user several side benefits including reduced operator fatigue, fewer mistakes caused by human error, and consistent and predictable machining time for each workpiece. Since the machine will be running under program control, the skill level required of the CNC operator (related to basic machining practice) is also reduced as compared to a machinist producing workpieces with conventional machine tools.

The second major benefit of CNC technology is consistent and accurate workpieces. Today's CNC machines boast almost unbelievable accuracy and repeatability specifications. This means that once a program is verified, two, ten, or one thousand identical workpieces can be easily produced with precision and consistency.

A third benefit offered by most forms of CNC machine tools is flexibility. Since these machines are run from programs, running a different workpiece is almost as easy as loading a different program. Once a program has been verified and executed for one production run, it can be easily recalled the next time the workpiece is to be run. This leads to yet another benefit, fast change-overs. Since these machines are very easy to setup and run, and since programs can be easily loaded, they allow very short setup time. This is imperative with today's Just-In-Time product requirements

basic definations of cnc

2008-12-07T23:37:00.000-08:00

code ...........To create programmable sets of instructions for a CNC machine.

computer numerical control ..........A type of programmable automation, directed by mathematical data, which uses microcomputers to carry out various machining operations.

downtime ...............Unproductive blocks of time during which operations cease to function, normally due to mechanical problems or a lack of materials.

drill .............A machining tool used to penetrate the surface of a workpiece and make a round hole.

electrical circuitry .............A closed path that an electric current follows, usually through devices and wires.

end mill ...........A thin, tall mill cutter with cutting edges that wind up the sides. Both the bottom and side of the end mill are used during milling operations. End mills resemble drills.

face mill ...........A flat mill cutter with multiple cutting teeth surrounding the tool. The bottom of the face mill is primarily used during milling operations.

finishing ............Final operations performed for obtaining desired tolerance and/or surface finish.

fixed automation .........A process using mechanized machinery to perform fixed and repetitive operations in order to produce a high volume of similar parts.

grinder .............A machine that uses an abrasive to wear away at the surface of a workpiece.

hardware ..........The physical components of a CNC machine.

interface ..........The control panel and displays with which the operator interacts with the machine.

job changeover ........The time it takes to switch from one part to another.

lathe .........A tool commonly used to machine cylindrical forms. It is generally considered the backbone of the machine shop.

machine control unit .........A small, powerful computer that controls and operates a CNC machine.

magazine ...........A arrangement of multiple tools that allows a CNC machine to rapidly change from one machining operation to the next.

mill ..........A machining tool used to either horizontally or vertically remove metal from the surface of a workpiece.

mylar tape .............A thin, yet strong polyester film that was used to transmit programs to numerically controlled machines.

paper tape .........A way of transmitting programs to numerically controlled machines. This method is all but extinct.

part program ...........A series of numerical instructions used by a CNC machine to perform the necessary sequence of operations to machine a specific workpiece.

prototype ..........The original test model of a product.

punch presses ..........A machine that uses force to either cut or form a workpiece.

punching ...........Using force to cut or form a workpiece.

repeatable ........The ability to position workpieces in the same place part after part.

stamping ..........Forming metal with the use of dies and punches.

three-axis curvature data .........Information that describes the motion and position of an object using three-dimensional data.

turret press .........A CNC punch press that contains several tools.

welder .........A device used to join two pieces of metal together through the application of heat.

cnc introduction

2008-12-07T23:31:00.000-08:00

Today, computer numerical control (CNC) machines are found almost everywhere, from small job shops in rural communities to Fortune 500 companies in large urban areas. Truly, there is hardly a facet of manufacturing that is not in some way touched by what these innovative machine tools can do.

Everyone involved in the manufacturing environment should be well aware of what is possible with these sophisticated machine tools. The design engineer, for example, must possess enough knowledge of CNC to perfect dimensioning and tolerancing techniques for workpieces to be machined on CNC machines. The tool engineer must understand CNC in order to design fixtures and cutting tools for use with CNC machines. Quality control people should understand the CNC machine tools used within their company in order to plan quality control and statistical process control accordingly. Production control personnel should be abreast of their company's CNC technology in order to make realistic production schedules. Managers, foremen, and team leaders should understand CNC well enough to communicate intelligently with fellow workers. And, it goes without saying that CNC programmers, setup people, operators, and others working directly with the CNC equipment must have an extremely good understanding of CNC.

principle of working of steam turbine

2008-12-03T02:52:00.000-08:00

“A steam turbine is a prime mover that derives its energy of rotation due to conversion of the heat energy of steam into kinetic energy as it expands through a series of nozzles mounted on the casing or the fixed blades.”
Water is converted to steam by application of heat in the boiler, which makes the steam at specified pressure and temperature. To convert the steam’s energy into work, it must go through a thermodynamic cycle that combines expansion compression, heat input, and heat rejection. The most efficient thermodynamic cycle for an ideal fluid is Carnot cycle. It consists of an isothermal heat input, isentropic expansion, isothermal heat rejection, and an isentropic compression. Regardless of the combination, the efficiency of the cycle, assuming constant mass flow is based on the difference in the enthalpy and between the beginning and end of the cycle.

• 1 to 2: Isentropic expansion
• 2 to 3: Isothermal heat rejection
• 3 to 4: Isentropic compression
• 4 to 1: Isothermal heat supply

Steam can be used as the working fluid in the Carnot Cycle. But its properties adversely impact its usefulness. In this case the steam expansion process takes place completely in the moisture region. This requires compression of a vapour/moisture mixture to return to the cycle’s starting point. Moisture is an expansion process imposes large mechanical efficiency losses. Also, vapour compression is inefficient and consumes relatively large amounts of power.
To avoid a two-phase vapour compression process, turbines are based on the Rankine cycle. It is similar to the Carnot Cycle, except that the initial pressure of the steam is raised and the condensation process that accompanies heat rejection continues until the liquid saturation point is reached. At the end of the cycle, then, condensate is simply pumped back to the boiler to begin the cycle. The role of the steam turbine is to expand the steam from high pressure and temperature to lower pressure and temperature.

Rankine cycle is a heat engine with vapor power cycle. The common working fluid is water. The cycle consists of four processes:

1 to 2: Isentropic expansion
(Steam turbine)
2 to 3: Isobaric heat rejection
(Condenser)
3 to 4: Isentropic
compression (Pump)
4 to 1: Isobaric heat supply
(Boiler)

Several things can be done to steam to improve the Rankine Cycle efficiency. Raise initial steam condition and reduce the amount of moisture near the end of expansion stage. The first is accomplished by superheating the steam before it does any work. The second involves re- heating steam to near initial-conditions after it is partially expanded by directing it back to the heat source, then completing the expansion. In converting the thermal energy of steam into mechanical energy turbines takes advantage of this facts- as it expands or drops in pressure, through a small nozzle or opening, it accelerates and forms a high-speed jet. Directing this momentum in a rotating blade provides mechanical energy.

maintanance and inspection of screw compressor

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The installation should be inspected regularly at intervals that will be determined by the severity of operating conditions. For convenience, suggested maintenance routine may be segregated into two separate but interdependent classifications as follows:

I] OPERATION INSPECTIONS
An accurate operational inspection system is the best means of detecting the need for maintenance work.

a) Continuous log or graph of all pressure and temperature readings should be kept. For the first few weeks of operational break-in, 4 readings per 8 hour shift are recommended. After that, the readings could be reduced to a suggested minimum of two per shift. Any rapid changes in consecutive readings would indicate possible malfunction and should be investigated immediately. Any gradual but consistent change not related to normal ambient or process variations should also be investigated.
b) Any change in the characteristic should or increase in vibration of the unit should be investigated.
c) Oil and water leaks should be repaired when first observed.
d) The water flow from jacket, after cooler and oil cooler should be observed when each reading is taken.
e) Check lube oil level in reservoir.
f) Make all checks recommended for the main driver.

II] PERIODIC INSPECTION
a) Every 500 – 700 hours: Check operation of alarms and controls.
b) Every 1000 – 1500 hours: Drain oil sample from reservoir. Check oil for oxidation, contamination and water. Frequency of oil change depends upon this analysis.
c) Every 7500 – 8500 hours (Annual):

Inspect rotors through inlet and discharge nozzles. Some polish areas on the rotor lobes are to be expected, due to float and relative shift of the rotors during start up and shutdown. Excessive rubbing between rotors or wear on radial and end sealing strips will require readjustment or possible replacement of the bearings.
Remove discharge end cover and inspect timing gears. There should be even wear pattern on gear teeth.
Measure thrust bearing end clearance with a dial indicator. Measure dial bearing clearance with a dial indicator mounted through thrust bearing housing drain opening.
Loosen inlet end cover and visually inspect radial bearings.
Remove compressor jacket cover plates and inspect for sediment or calcium deposits. Clean if necessary.
Remove coupling guards and inspect couplings for broken shims.
Recheck alignment at all couplings.
Check and reset, if necessary, all temperature and pressure gauges and pickups.
Inspect inside of silencers for deterioration of foreign materials.
Inspect water side of after cooler and oil cooler for foreign material or calcium deposits and clean if necessary.

d) Every 15000 hours (2 years):

Inspect per paragraph 2c.
Remove coupling hub at inlet end cover. Measure inlet radial bearing clearances with a dial indicator mounted adjacent to the bearing.

e) Every 30000 hours (4 years):

Inspect per paragraph 2d.
Remove and inspect timing gears, thrust bearings, radial bearings and seals.
With radial bearings in place, indicate shaft runout at all diameters of each shaft with a dial indicator.
If condition dictate, remove the rotors from the housing, clean and inspect the rotors and housing bores.

shut down procedure of screw compressor

2008-11-14T03:08:00.000-08:00

When unit is to be taken off the stream and shutdown, the following procedure should be followed:
1. Gradually open bypass to reduce discharge pressure to ½ normal pressure. Operate unit at this condition for approx. 20 minutes or until the gas discharge temperature levels off.
2. Stop unit. If unit starts turning backwards, immediately close the block valve.
3. Allow lube oil to circulate for 10 minutes after the unit is stopped to properly cool the rotors.
4. If unit is to be shutdown for an extended period in a freezing environment, drain all water from oil coolers, compressor jackets and piping.
5. If unit is shutdown for an extended period of time, following maintenance program should be followed to keep unit ready for service:

Open compressor inlet casing drains to allow condensate to drain out Circulate lube oil for a minimum of one hour once a week and rotate unit several revolutions.For extended down periods, remove silencers and piping from compressor inlet and discharge and spray a light film of lubricating oil on rotor surfaces and machines internal housing surfaces once every two weeks to prevent rusting If the unit shuts down instantly under full load (power failure, emergency shutdown), following procedure is to be followed if unit is to be started:

1. Let the cooling water flow through the oil cooler and compressor
2. Circulate the lube oil for one hour and rotate the compressor drive shaft ¾ turn by hand every 10 minutes
3. Turn until over by hand three revolutions. The unit is ready to restart when it turns normal with no tight areas.
4. Restart unit in normal manner. Operate at approx. ½ normal pressure for 15 min before applying rated pressure

screw compressor-normal operation

2008-11-14T03:05:00.000-08:00

Some conditions are subject to slight variations as listed below:

1. Process gas pressures and temperatures - changes in process requirements may cause a variation in operating temperatures and pressures. System temperatures will vary directly with incoming gas temperatures. Temperature differential between inlet and discharge nozzles should remain fairly constant. Pressures will be dependent upon and affected by incoming gas and barometric conditions.

2. Bearing temperatures - bearing temperatures will vary slightly with ambient temperature and with lube oil temperature. Higher lube oil temperatures will produce proportionately higher bearing temperatures. A variation of 6 C to 11 C in bearing o o temperatures is of no consequence.
A running plot of all temperatures and pressures is advisable. Log all gauge pressure and temperature readings at initial start after unit has reached normal operating conditions. Use these initial readings as a guide and check frequently as recommended in “operational inspection”

Any rapid or gradual change, not due to process conditions, may indicate a possible malfunction.

screw compressor-normal start procedure

2008-11-14T03:03:00.000-08:00

After all the precautionary measures have been taken, the unit can be started as follows:

1. Turn on the seal buffering gas (if applicable)
2. Start the main lube oil pump for prefabrication. Prelube compressor approx. One hour at initial start and for 10 minutes after each shutdown and at each subsequent start. Start pressure gauges and sight glasses for oil flow.
3. Turn on the cooling water so that a steady continuous supply flows into the unit
4. Check all valves external to the machine for proper adjustment
5. Bar unit over using strap wrench.
6. Make sure process shut off valve and bypass valve is open. Start the driver in accordance with the manufacturer’s instructions and bring unit up to speed. Gradually choke down on bypass valve and bring unit up to pressure over a period of approx. 15 min.
7. Check all lube oil pressure gauges and sight flow indicators to see that the machine is being properly lubricated. Also observe temp. Gauges.
8. Observe the action of the machine to make sure there is no undue vibration or noise.
9. Check auxiliary equipment. Be sure the unit is not operating beyond the rating stamped on the nameplate
10. In the event of an emergency shutdown while unit is operating, do not attempt to restart compressor until it bars over freely.

screw compressor-initial start procedure

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1. Run driver up to speed without load as follows:

disconnect coupling between driver and gear
pump driver and check rotation
run driver up to speed, checking for undue noise and vibration

2. Run driver and gear.

disconnect coupling between gear and compressor
check alignment between driver and gear
connect coupling between driver and gear
start lube oil pump and check pressures and flows to gear
pump driver and check gear rotation
run driver and gear up to speed, checking for undue noise and vibration

3. Run in compressor on gas as follows:

Install temporary wire screens at the joint between the compressor and inlet silencer to catch any weld berries, slag or dirt remaining in the pipe. Screen information is found at the back of this section. Remove screen before putting unit into service.
Bar compressor over by hand to check for freeness. If any metal- rubbing-on metal sounds are detected in the compressor, determine the cause and correct the same before starting. Use strap wrench to bar compressor.
Check alignment between the gear and the compressor. Couple compressor to gear.
Turn on buffering gas and/or other sealing fluid to compressor.
Start main oil pump and check flows and pressures to bearings. Oil temperature should be 16 C to 27 C before starting unit.
Make sure process shutoff valve and bypass valve are open.
Run driver for a period just long enough to bring the unit up to approx. ¼ speed, trip off driver and observe the unit while it coasts to a stop to make sure there is no undue vibration or noise. Bar unit over again to check for freeness.
Start driver and bring unit up to speed. Do not operate for more than two minutes with zero discharge pressure.
Gradually close down on the bypass valve and bring unit up to 50% of discharge temperature over a period of 15 minutes. (if applicable) Run for 30 minutes at above condition and observe operation of unit and auxiliary equipment. Check for any unusual vibration or noise.
Stop driver and let unit coast to a stop.
Bar unit over to check for freeness.
Check inlet screen and clean if necessary
Continue to operate unit for 30 minute interval until inlet screen is no longer picking up dirt
Always turn injection water off immediately before shutting off main driver. (if applicable)

screw compressor-pre start procedure

2008-11-14T02:52:00.000-08:00

Before starting the unit for the first time, and always after a major overhaul or repair,check the following:
1. Be sure the intake, all oil and air piping and internal parts of the unit are free from dirt and moisture.
2. Remove paint and preservative coating, if used, from all moving parts. (See instructions for removing preservatives)
3. Extreme care must be taken to prevent foreign material such as nuts, bolts, tools etc from remaining in or dropping into any of the piping or compressors.
4. See that all bolts and nuts are secure. Some may have loosened in shipment.
5. Check alignment
6. Lubrication- in preparation for start up, flush the lube oil system and adjust the lube oil pressure switches. Check all switch settings in accordance with the lube / seal oil console section.
7. See that all valve internal to the unit are properly adjusted.
8. Check all other auxiliary equipment including safety alarm system to see that it is in proper order
9. Check compressor side inlet drain for condensation at initial start and after a shutdown period.

flexible element coupling

2008-11-04T05:46:00.000-08:00

1. Flexible elements may be straight sided, contoured, or convoluted. Acceptable arrangements include a single or element, multiple series elements, or multiple parallel elements (elements packs) at each end of the spacer.

2. The flexible elements shall be positively secured to adjacent parts of the coupling by splines, bolts, or welds. Alternative methods shall be used only when approved by the purchaser.

3. When the flexible elements of a coupling in intermediate-or-high-speed service are combined in a factory-assembled disk pack, the coupling spacer shall be removable without disturbance of the factory assembly of the elements (disks).

4. Unless otherwise specified, the vendor shall treat the coupled equipment as an infinite mass when calculating the ANF. The ANFs of the coupling shall not fall within 10 percent of any of the following speeds or ranges:

a. Any speed within the range extending from the minimum allowable speed to the maximum continuos speed.
b. Tow times any speed within the range specified in item a.
c. Any other speed or exciting frequency specified by the purchaser. (For some applications these restrictions may preclude a practical coupling design. In this case the vendor and the purchaser should investigate and agree upon means of relaxing these criteria).

5 If the coupling must operate within a close-fitting enclosed coupling guard, the purchaser will furnish details of the guard for the vendor to inspect. The vendor shall determine and so advise if cooling is required and, if necessary, shall recommend a cooling system for the coupling.

6 When a tapered hub is specified for one or both ends of the coupling, the vendor shall furnish spacer shims to adjust the spacer gap. The shims shall provide a variance of +1 1/6 inch (1/8 inch total) [+ _ 3.2 millimeters (6.4 millimeters total)] in couplings for shafts with a nominal diameter of less than 4 inches (102 millimeters). In coupling

trim-balance holes

2008-11-04T05:42:00.000-08:00

When specified, tapped holes shall be provided in the coupling for trim balancing. The number, size, depth, and location of such holes shall be agreed upon by the purchaser and the vendor. The optimum location for keyed hubs is generally on the outboard faces of the hubs, midway between the inside and outside diameters of the hub barrel. The optimum location for keyless (hydraulically fitted) hubs is generally on the coupling flanges, between the bolt holes of the flange.

Note: Because of eccentricity of the shaft end or incompletely filled keyways trim balancing the rotor after the coupling hub has been mounted may be advisable. The practice normally precludes moving the hub to another rotor, unless balance is achieved by using balance holes. When balance holes are used the hub can always be returned to its original state of balance by removing the weights inserted into the holes.

Materials

1. Except as required by the data sheets or this standard, materials of construction shall be the vendor’s standard for the specified operating conditions.

2. Material shall be identified in the proposal with their applicable ASTM, AISI, ASME or SAE numbers, including the material grade. When no such designation is available the vendor’s material specification, giving physical properties, chemical composition, and test requirements shall be included in the proposal.

3. Neither copper nor copper alloys (excluding Monel and precipitation-hardened stainless steels) shall be used for coupling parts.

4. All metallic components of the coupling shall be made from high-quality material manufactured by hot rolling, cold finishing, or foreign and shall be appropriately heat treated. Hubs and sleeves shall be made of alloy steel. Flexing elements in flexible-element couplings shall be of corrosion-resistant material or shall be suitable coated. The vendor shall state the nature of the coating and how it must be applied. The purchaser will specify whether all other parts shall be made from corrosion resistant material or suitably coated.

5. The purchaser will specify any corrosive agents present in the environment, including constituents that may cause stress corrosion cracking.

6. All fasteners shall be of heat-treated alloy steel, SAE J 429, Grade 5, or better. The threads shall be American Standard unified fine thread series. Materials shall be corrosion resistant to the specified environment. If plated bolts are required, they shall be treated properly to avoid cracking caused by hydrogen embrittlement. The quality of the nuts shall be at least equal to that of the bolts.

balancing options in coupling

2008-11-04T05:38:00.000-08:00

Unless otherwise specified, couplings shall be component balanced. Each component, such as the hubs, sleeves, flexible elements, shims, spacer, and adapter plates, shall be balanced individually. All machining of components, except for the keyways of single-key hubs, shall be completed before balancing. Two-plane balancing is preferred, but single-plane balancing may be used for components with a short axial length. Each component shall be balanced so that the level of residual unbalance for each plane does not exceed the greatest value determined by the following expressions:

U = 4W/N

U = 0.0008W

U = 0.01

In SI Units,

U = 6350W/N

U = 50.8W

U = 7.2

Where:

U = Residual unbalance, in ounce-inches
(gram Millimeters)

W = Weight of the component, in pounds (kilograms),
apportioned to the balance planes so that the sum
of the weight apportionment for both planes equals
the total weight of the component.

N = Maximum continuous operating speed,
In revolutions per minute.

1 When specifiedm residual unbalance checks shall be performed on components of couplings with maximum continuous speeds of 5000 revolutions per minute or greater. These residual unbalance checks shall be performed after balancing is complete and before the component is removed from the balancing machine.
2 Unless otherwise specified, couplings balanced in accordance with the above shall be assembled and the balance verified. The residual unbalance for the randomly assembled coupling shall not exceed the greatest value determined by the followin expressions:

U = 40W/N

U = 0.008W

U = 0.1

In SI Units,

U = 63,500W/N

U = 0.8W

U = 72.0

Where:

U = Residual unbalance, in ounce-inches
(gram Millimeters)

W = Weight of the coupling, in pounds (kilograms),
apportioned to the balance planes at the two coupling
hubs so that the sum of the weight apportionment
equals the total weight of the coupling.

N = Maximum continuous operating speed,
In revolutions per minute.

Couplings that fail to meet these criteria shall be balance corrected by repeating the component balance, not by trim balancing the assembly.

Sample plot and calculation of Resiudal Unbalance

3 When specified, an assembly balance shall be performed, and the components shall be match marked. The coupling shall then be match marked and two-place balanced, with corrections being made only to the component or subassembly that was not periously balanced. The final residual unbalance of the assembled coupling in each of the two correction planes shall not exceed the greatest value determined by the following expressions:

U = 4W/N

U = 0.0008W

U = 0.01

In SI Units,

U = 6350W/N

U = 50.8W

U = 7.2

Where:
U = Residual unbalance, in ounce-inches
(gram Millimeters)
W = Weight of the component, in pounds (kilograms),
apportioned to the balance planes so that the sum
of the weight apportionment for both planes equals
the total weight of the component.
N = Maximum continuous operating speed,
In revolutions per minute.

4 When specified, the coupling shall be checked after the assembly balance to ensure that the assembly balance can be repeated. The coupling shall be disaaembled to the same extent required for normal field disaaembly and remounted on the balance fixture or fixtures. The unbalance of thr reasseambled coupling shall then be measured on the balancing machine, and the residual unbalance shall not exceed the greatest value determined by the following expressions.

U = 40W/N

U = 0.008W

U = 0.1

In SI Units,

U = 63,500W/N

U = 0.8W

U = 72.0

Note: Assembly balancing corrects for overall coupling unbalance caused by eccentricities of the pilot fits that are used to center components during assembly. However, assembly balancing may prohibited subsequent interchange of duplicate coupling components and may require that the entire coupling be maintained as a unit, except for the bolts and nuts.

basic design of coupling

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1. TORQUE

The vendor shall furnish couplings with the maximum practical service factor, considering torque transmitted by the coupling, overhung movement, speed, and lubrication. Unless otherwise specified, the coupling, coupling-to-shaft junctures, and machinery shafting shall be capable of continuos operation at a torque determined by equation given in Chapter 3 using an experience factor of 1.75. the purchaser will specify the expected magnitude, nature and number of occurrences of transients to which the coupling will be subjected in service. The coupling, coupling-to-shaft juncture and shafting shall be capable of transmitting 115 percent of the purchaser-specified maximum transient torque without damage. The vendor shall state the coupling design without damage. The vendor shall state the coupling design rating (in horsepower per 100 revolutions per minute). The coupling size selection shall be submitted to the purchase of approval.

Note: Should reasonable attempts to achieve the specified experience factor fail to result in a coupling weight and subsequent overhung moment commensurate with the requirement for rotor dynamics of the connected machines, a lower factor may be selected by mutual agreement of the purchaser and the vendor. The selected value shall not be less than 1.25.

2. CRITERIA FOR CONTINOUS OPEATION

The coupling design shall permit continuos operation at the maximum continuos torque level with at least 125 percent of the purchaser-specified maximum steady-state for transient axial displacement (whichever is larger) occurring simultaneously with 125 percent of the purchaser specified maximum angular misalignment and 125 percent of the purchase-specified maximum parallel offset. (the maximum changes in misalignment and parallel offset are normally experienced during start-p of a machinery train).

3. SPACER

All couplings shall be if the spacer type. The spacer shall be of sufficient length to allow removal of coupling hubs and to allow for maintenance of adjacent bearings and seals without removal of the shaft or disturbance of the equipment alignment. The minimum spacer length shall therefore correspond to a between-shaft-ends dimension of 18 inches (457 millimeters), unless otherwise specified by the purchaser.

4. HUBTYPE

The purchaser will specify whether integral or removable hubs are to be used.

5. DRILL JIG

Unless otherwise specified, when the coupling is to be used with integrally flanged shaft ends, a drill jig (or template) shall be used to locate the flange holes. The purchaser will specify whether he or the coupling vendor is to furnish the jig (or template).

6. CORROSIVE ENVIRONMENT

When the purchaser specifies that the coupling is to operate in a corrosive environment, either oil mist or an inert-gas purge may be necessary. The vendor shall advise the purchaser when material limitations demand such protection for the coupling.

7. MOMENT SIMULATOR/ADAPTER PLATE

7.1 When specified, a moment simulator shall be supplied. The purchaser will supply the vendor with the measurement of the distance from the end of the shaft to the first bearing. The simulator shall also serve as an adapter plate.

7.2 The vendor shall supply an adapter plate for the price end of the coupling to allow uncoupled operation of the driver. The adapter plate shall be designed so that the late can rigidly bolted to the sleeve and centered on the hub by a pilot fit.

8. Removable Hubs

8.1. Removable coupling hubs shall be secured to the shaft by means of interference fit. The degree of interference will be specified by the purchaser and is subject to approval by the vendor. The choice of non-keyed (tapered-bore, hydraulically fitted) or keyed (tapered-or-straight-bore) hubs will be specified by the purchaser.

8.2. The surface finish of the hub bore shall be 125 micro-inches (3.2 micrometers)
arithmetic roughness (R ) or better.

8.3. The eccentricity of the hub bore, whether straight or tapered, shall not exceed 0.0002 inch (5.1 micrometers) TIR for bores less than or equal to 4 inches (102 millimeters) in diameter and shall not exceed 0.0005 inch (12.7 micrometers) TIR for hub bores greater than 4 inches in diameter, Eccentricity measurements shall be made before any keyways are cut.

8.4. The following guidelines are recommended for hub-to-shaft fits:

a) The interference fit for straight-bore keyed hubs shall be from 0.0005 or 0.00075 inch per inch of bore diameter. Shaft sizes and coupling bores shall conform to AGMA 9002.
b) The interference fit for tapered-bore keyed hubs shall be at least 0.001 inch per inch of bore diameter. The inspection procedure shall be in accordance with AGMA 9002.
c) The interference fit for tapered-bore hydraulically fitted hubs shall be at least 0.0015 inch per inch of bore diameter. The inspection procedure shall be in accordance with AGMA 9002.

For tapered-bore hubs and keyed hubs, please refer the API standard 671.

9. COMPONENET FIT TOLERANCES

9.1 Components of intermediate and high-speed couplings shall be centered by means of piloted fits. The eccentricity of these fits shall not exceed 0.001 inch TIR per foot of diameter or 0.0005 inch TIR, whichever is greater. Fits that tighten under centrifugal loading are preferred. The fit clearance shall range from a loose fit of 0.001 inch to an interference fit, with actual fit determined by balancing equipment.

9.2 The face run-out of mating faces (except for flexible elements) shall not exceed 0.001 inch TIR, whichever is greater.

9.3 Radial indicator surfaces used to align equipment shall be concentric to the hub bore within 0.001 inch TIR per foot of diameter or 0.001 inch TIR whichever is greater.

9.4 For hubs, pilot fits at fear coupling teeth shall be concentric to the bore within 0.001 inch TIR per foot of diameter or 0.0005 inch TIR, whichever is greater.

9.5 For sleeves, pilot fits at gear coupling teeth shall be concentric within 0.001 inch TIR per foot of diameter or 0.001 inch TIR, whichever is greater. Pilot fits shall be round within 0.002 inch TIR per foot of diameter or 0.0015 inch TIR, whichever is greater.

10. BOLTING CONSIDERATIONS

10.1 Bolting between coupling components shall be designed to transmit the required torque without dependence on flange-face friction.

10.2 Bolts for piloted flanges shall have a diametrical clearance of from 0 to 0.0005 inch (127 micrometers) in the bolt holes.

10.3 Bolts for low-speed couplings with nonpiloted flanges shall be of the body-bound style and shall be through-fitted into line-reamed holes, with the assembly’s diametrical clearance ranging from 0 to 0.0015 inch (38 micrometers).

10.4 Self-locking nuts shall be used. Lock washers shall not be used.

Note: The self-locking feature of nust may lose effectiveness with each removal of the nut. The coupling vendor shall recommended the interval at which nuts should be replaced.

10.5 the coupling vendor shall specify the required bolts torque, and shall state whether this value is for dry or lubrictaed applications.

10.6 the bolt shall be held within dimensional tolerances sufficient to permit both interchange within the same sset of bolts and substitution of spare bolts withut affecting coupling integrity and balance by more than 1 percent.

11. ELECTRICAL INSULATION

When specified, the coupling shall be electrically insulated.

12. MACHINGING

All coupling parts, except for flexible disks, shall be machined all over to minimise inherent unbalance.

coupling selection criteria

2008-11-04T05:17:00.000-08:00

Usually couplings are supplied as part of any new equipment. Instead of having to select a new coupling, one is faced only with the need to replace an old one, or part of an old one. Assuming that the equipment manufacturer selected the right coupling type and size, couplings generate few problems. There are cases, however, when wither the coupling does not live up to expectations or when a new piece of equipment is purchased without a driver and a coupling must be selected. The process is not simple because there is no application for which only one type of coupling would work. The best approach is to let an application engineer from a coupling manufacturer make the selection. Today most manufacturers make more than one type of coupling and can objectively recommend the best one for the application.

Choosing a coupling of the correct size is very important. To do this one must know not only the required power and speed, but also the severity of service the coupling must accommodate. A correction factor, or service factor, must be applied.

Coupling manufacturers rate their couplings in horsepower per 100 r/min. for instance, if a pump required 50 hp (37.3kW) at 1750 r/min, it needs a coupling that can handle 2.86 hp (2.13k W) at 100 r/min. This is correct only if the pump is centrifugal and is driven by an electric motor. In this case the service factor is 1. If we have a double-acting reciprocating pump driven by an internal combustion engine, we have to use a service factor of 2.0 + 1.0 = 3.0 for a gear coupling and 2.0 + 0.5= 2.5 for an elastomer coupling, according to one manufacturer. As a result, we must choose a gear coupling that can handle 8.58 hp (6.4 kW) at 100 r/min or an elastomer coupling that can handle 7.15 hp (5.3 kW) at 100 r/min. it seems that we can choose a smaller coupling if we choose the elastomer type.

However, the elastomer coupling will be about 8 ¾ in (222 mm) in diameter, while the gear coupling will be only half that size! If size is not important, price can be the next selection criterion. But the price of the coupling alone is not a good guide; one should consider the total cost including maintenance, replacement parts, lost production, etc.

Although couplings represent a small percentage of the total cost of a piece of machinery, they can cause as much, if not more, trouble than the rest of the equipment if they are not properly selected. Buying an inadequate size or type of coupling will never be economical in the long run. Maintenance personals are frequently faced with the problem of replacing a worn-out or broken coupling. After the cause of the failure has been determined, careful consideration should be given to the type size, and style of the coupling that will used as a replacement. Whenever possible, it should satisfy all the needs of the drive.

Proper selection as to type of coupling is the first step of good maintenance. A well chosen coupling will operate with low cross loading of the connected shafts, have lower power absorption, induce no harmful vibrations or resonance into the system, and have negligible maintenance costs. The primary consideration in selecting the correct type of coupling, as well as its size and style are:

1. Type of driving and driven equipment
2. Torsion characteristics
3. Minimum and maximum torque
4. Normal and maximum rotating speeds
5. Shaft sizes
6. Span or distance between shaft ends
7. Changes in span due to thermal growth, racking of the bases, or axial movements of
the connected shafts during operation.
8. Equipment position (horizontal, inclined or vertical)
9. Ambient conditions (dry, wet, corrosive, dust, or grit)
10. Bearing locations
11. Cost (initial coupling price, installation, maintenance, and replacement)

The coupling should be conservatively selected for torque involved. Consideration must be given to all peak loads and shock loads encountered in normal service. If the coupling is to operate at high speeds, it should be dynamically balanced. Special coupling modifications dictated by the connected equipment should be made. If any doubt exists as to proper type or size of coupling to use, it is recommended that the manufacturer be consulted. Most manufacturers are usually qualified to make recommendations and assist in the coupling procurement.

GENERAL SELECTION CRITERIA:

TORQUE

The vendor shall furnish couplings with the maximum practical service factor, considering torque transmitted by the coupling, overhung movement, speed, and lubrication. Unless otherwise specified, the coupling, coupling-to-shaft junctures, and machinery shafting shall be capable of continuos operation at a torque determined by following equation (I) or (II), using an experience factor of 1.75. The purchaser will specify the expected magnitude, nature, and number of occurrences of transients to which the coupling will be subjected in service. The coupling, coupling-to-shaft juncture and shafting shall be capable of transmitting 115 percent of the purchaser-specified maximum transient torque without damage. The vendor shall state the coupling design rating (in horsepower per 100 revolutions per minute). The coupling size selection shall be submitted to the purchaser of approval.

Ts=
63,025 X Pnormal X SF
-------------------------- - Eq. (I)
Nnormal

In SI Units,

Ts=
9549 X Pnormal X SF
------------------------- - Eq. (II)
Nnormal

Where:

Ts = torque used to make the coupling selection, in inch-pounds (joules)

Pnormal = input power required by the driven machine at the specified normal operating point, in horsepower (kilowatts)

Nnormal = speed corresponding to the normal power, in revolution per minute

SF = experience factor derived from various modes of off-design operation that may result from such factors as a change in the density of the pumped fluid (molecular weight, temperature or pressure variation), unequal load sharing, fouling, and driver output at maximum conditions.

Note: Should reasonable attempts to achieve the specified experience factor fail to result in a coupling weight and subsequent overhung moment commensurate with the requirement for rotor dynamics of the connected machines, a lower factor may be selected by mutual agreement of the purchaser and the vendor. The selected value shall not be less than 1.25.

Working Mechanism of a Centrifugal Pump

2008-10-24T02:34:00.000-07:00

Working Mechanism of a Centrifugal Pump

A centrifugal pump is one of the simplest pieces of equipment in any process plant. Its purpose is to convert energy of a prime mover (a electric motor or turbine) first into velocity or kinetic energy and then into pressure energy of a fluid that is being pumped. The energy changes occur by virtue of two main parts of the pump, the impeller and the volute or diffuser. The impeller is the rotating part that converts driver energy into the kinetic energy. The volute or diffuser is the stationary part that converts the kinetic energy into pressure energy.

Note: All of the forms of energy involved in a liquid flow system are expressed in terms of feet of liquid i.e. head.

Generation of Centrifugal Force

The process liquid enters the suction nozzle and then into eye (center) of a revolving device known as an impeller. When the impeller rotates, it spins the liquid sitting in the cavities between the vanes outward and provides centrifugal acceleration. As liquid leaves the eye of the impeller a low-pressure area is created causing more liquid to flow toward the inlet. Because the impeller blades are curved, the fluid is pushed in a tangential and radial direction by the centrifugal force. This force acting inside the pump is the same one that keeps water inside a bucket that is rotating at the end of a string.

Conversion of Kinetic Energy to Pressure Energy

The key idea is that the energy created by the centrifugal force is kinetic energy . The amount of energy given to the liquid is proportional to the velocity at the edge or vane tip of the impeller. The faster the impeller revolves or the bigger the impeller is, then the higher will be the velocity of the liquid at the vane tip and the greater the energy imparted to the liquid.This kinetic energy of a liquid coming out of an impeller is harnessed by creating a resistance to the flow. The first resistance is created by the pump volute (casing) that catches the liquid and slows it down. In the discharge nozzle, the liquid further decelerates and its velocity is converted to pressure according to Bernoulli’s principle.

Introduction to Centrifugal Pumps

2008-10-24T02:20:00.000-07:00

Introduction

The operating manual of any centrifugal pump often starts with a general statement,“Your centrifugal pump will give you completely trouble free and satisfactory service only on the condition that it is installed and operated with due care and is properly maintained.” Despite all the care in operation and maintenance, engineers often face the statement “the pump has failed i.e. it can no longer be kept in service”. Inability to deliver the desired flow and head is just one of the most common conditions for taking a pump out of service. There are other many conditions in which a pump, despite suffering no loss in flow or head, is considered to have failed and has to be pulled out of service as soon as possible. These include seal related problems (leakages, loss of flushing, cooling, quenching systems, etc), pump and motor bearings related problems (loss of lubrication, cooling, contamination of oil, abnormal noise, etc), leakages from pump casing, very high noise and vibration levels, or driver (motor or turbine) related problems.

The list of pump failure conditions mentioned above is neither exhaustive nor are the conditions mutually exclusive. Often the root causes of failure are the same but the symptoms are different. A little care when first symptoms of a problem appear can save the pumps from permanent failures. Thus the most important task in such situations is to find out whether the pump has failed mechanically or if there is some process deficiency, or both. Many times when the pumps are sent to the workshop, the maintenance people do not find anything wrong on disassembling it. Thus the decision to pull a pump out of service for maintenance / repair should be made after a detailed analysis of the symptoms and root causes of the pump failure. Also, in case of any mechanical failure or physical damage of pump internals, the operating engineer should be able to relate the failure to the process unit’s operating problems.

Any operating engineer, who typically has a chemical engineering background and who desires to protect his pumps from frequent failures must develop not only a good understanding of the process but also thorough knowledge of the mechanics of the pump. Effective troubleshooting requires an ability to observe changes in performance over time, and in the event of a failure, the capacity to thoroughly investigate the cause of the failure and take measures to prevent the problem fro m re-occurring.

labyrinth piston compressor

2008-10-18T04:14:00.000-07:00

LABYRINTH PISTON

These are vertical type reciprocating. In this type of compressor, rider rings and piston rings are not used as in case of horizontal type. In labyrinth piston compressors, an extremely large number of throttling points provide the sealing effect around pistons and piston rods. No contact seals are used. Piston is having labyrinth type piece at the centre called skirt. Cylinder is also
having serration like labyrinths on its inside surface. Piston is not in direct contact with the cylinder and close clearance is maintained in between both. This increases the durability, reliability and availability of the compressor along with its economic operation.

These machines are very popular in the service where total dry operation is required as in case of polypropylene and polyethylene plant. This is unique application where lubricants are not allowed in the cylinders, which is true in case of oxygen compressor, where safety is the most important. It is also employed for applications where the process gas is heavily contaminated with the impurities.

Piston and piston rod are guided by the crosshead and the guide bearing which are located in the oil lubricated crankcase. The oil is supplied by the crankshaft driven lube oil pump
The distance piece separates the gas compressing section from the oil lubricated crankcase. Where process gas can be permitted in the distance piece, then open distance piece type compressors are used. When strict separation of cylinder from the crankcase is essential and at the same time no ambient air is also allowed in the distance piece, then Nitrogen purge is provided. Crankshaft is provided with mechanical seal to prevent the gas leaking in to the atmosphere.

ADVANTAGES:
1. Reliable operation
2. Safe for operation , environment
3. Economical
4. High availability
5. Less floor area is required.
6. As rider rings, piston rings and lubrication is not required the valve life is more.

classification of reciprocating compressor

2008-10-18T04:03:00.000-07:00

Reciprocating Compressors can be classified as follows based on

I. Cylinder lubrication:
1. Lubricated type & Non-lubricated type
2. Dry running piston rings
3. Ringless or labyrinth type.

II. Cylinder cooling:
1. Air cooled
2. Water cooled
III Cylinder loading
1. Single acting
2. Double acting
IV Cylinder arrangement,
1. Vertical inline or V-type
2. Horizontal opposed balanced.

CYLINDER LUBRICATION:

Lubricated Compressors :
Generally big reciprocating compressor cylinders are lubricated to avoid wear and tear of liner, piston rings, rider rings and stuffing box. Lubricants are injected in drops and are lost with the process gas.

Lubrication of cylinder reduces wear of parts, enhances life of parts and also reduces gas discharge temperature. It is necessary for the lubricant to be compatible with the process gas and down stream system. Generally recip compressors above 100 KW are lubricated type.

Non-Lubricated Type :
There are many services in which oil in any form in the compressed gas, is not acceptable such as instrument air compressor and some process gas compressors. In such services oil is not injected in the cylinders, instead wear parts are made of soft material with low co-efficient of friction such as PTFE,CFT etc. The wear rate of stuffing box packing, rider rings piston rings and cylinder/liner surface may be comparatively more.

Process requirements and gas to be compressed dictate whether to use a compressor with lubricated or non-lubricated cylinder. Some chemical processes do not permit use of lubricant in the system, due to quality problems, catalyst-poising etc.

Following factors must be considered,
1. Non-lubricated compressors cost more than lubricated compressors
2. Non-lubricated compressors require more power
3. Non-lubricated compressors require more maintenance, labyrinth compressors
being exception.

TYPES OF COOLING:

Air Cooled type :
This type of compressors has fins cast as part of the cylinder to dissipate some of the heat generated by the compression of the gas. In some compressors vanes are cast as part of the flywheel or sheave to act as fan to help remove the heat from the cylinder surface. This type of cylinder cooling is used in small portable compressors.

Water-cooled type:
Water-cooled compressors are most common in industry. It is impossible to sustain cooling with air in big compressors where heat generation is very high. Water jackets are cast as part of the cylinder. Water is circulated in the cylinder jacket from external source.

In some case the cylinder jacket temperature is to be maintained few degrees higher than the process gas temperature to avoid condensation during standstill, this is possible with water-cooled cylinders by maintaining the water temperature with use of heaters.

TYPES OF CYLINDER LOADING:

Single acting
In single acting cylinder compression takes place only on one side of the piston and valves are installed only on that side.

Double acting
In double acting cylinder compression takes place on both sides of the piston. When one side is in compression, the other is in suction.

CYLINDER ARRANGEMENT:

Horizontal Type:
This type of arrangement is most common in industries. Multi cylinder reciprocating compressors with horizontal cylinders are very often designed as balanced opposed type. Balanced opposed frame is characterized by adjacent pair of crank 180 ° out of phase and separated by crank webs only. With this configuration inertia forces are balanced. The balanced-opposed design is separable frame.

Vertical Inline or V-type:
Vertical in line or V-type are used for small or moderate compression ratio and duty. Normally this type of compressors cylinder arrangement are non-lubricated type and occupy less space.

Labyrinth piston compressors
These are vertical type. In this type of compressor, rider rings and piston rings are not used as in case of horizontal type. Piston is having labyrinth type piece at the centre called skirt. Cylinder is also having serration like labyrinths on its inside surface. Piston is not in direct contact with the cylinder and close clearance is maintained in between both. These machines are very popular in the service where total dry operation is required as in case of polypropylene and polyethylene plant.

multistaging with inter cooling

2008-10-18T04:01:00.000-07:00

It is not always desirable or possible to achieve the required rise in pressure in a single compression stage. In multistaging gas discharge from the first stage is cooled in the inter cooler up to suction temperature of the first stage before going to the second stage. This is called as perfect intercooling.

Advantages of multistaging:

1. Good volumetric efficiency as compression is done in more than one stage and hence compression ratio is controlled.
2. Lower discharge temperature and hence selection of material of construction for cylinder and its components and results in smaller size of subsequent stages.
3. Reduced work of compression, as due to intercooling, compression is closer to isothermal (gives rise to minimum work of compression). This results in to saving of power and smaller sizes of subsequent stages.
4. Limits pressure differential. This reduces excess strains in the frame.

schematic of reciprocating compressor

2008-10-18T03:54:00.000-07:00

Suction:
When the piston moves towards BDC (backward stroke), the pressure within cylinder on the top of the piston drops below suction pressure in the header, thereby forcing suction valve to open and allows gas in the cylinder.

Compression:
When the piston moves from BDC towards TDC (forward stroke) pressure on the top side of the piston start increasing, thereby closing the suction valve.

Discharge:
As the piston approaches TDC, the discharge valve opens as the pressure inside the cylinder on top side of the piston is higher than that in the discharge pipeline. Thus gas is discharged in to the header.

Expansion:
During backward stroke the trapped gas (between piston & cylinder end cover) called clearance volume, expands.

thermodynamic Process

2008-10-18T03:52:00.000-07:00

Compression of gas is a thermodynamic process, and can be done in many ways; some of them are listed below.

Isobaric Process – a process wherein pressure remains constant.
Isothermal Process – a process in which there os no change in temperature.
Isentropic Process – a process in which there is no change in entropy.
Adiabatic Process – a process during which there is no external exchange of heat. PVk = Constant
Polytropic Process – a process for perfect gases, follows the law PVn = Constant n=1 for isothermal process.

basic gas laws

2008-10-18T03:46:00.000-07:00

First Law of Thermodynamics: Energy is neither created nor destroyed, but can be converted from one form to another form.

Second Law of Thermodynamics: Energy, which exists at various levels, is available for use only if it can move from a higher to a lower level.

Ideal or Perfect Gas Laws

Gases may consist of one specific gas or may be constituent of number of gases. An ideal or perfect gas is one, which obeys below mentioned Boyle’s Law and Charle’s Law. Perfect gases do not exist in practice.

Boyle’s Law – states that at a constant temperature, the volume of an ideal gas decreases with increase in pressure.

P1V1 = P2V2 = CONSTANT

Charle’s Law – states that at constant pressure, the volume of an ideal gas will increase as the temperature increases.

V1/T1=V2/T2=CONSTANT

Amonton’s Law – states that at constant volume, the pressure of an ideal gas will increase as the temperature increases.

P1/T1=P2/T2=CONSTANT

Avagadro’s Law – states that equal volume of gas under same condition of temperature and pressure contains same number of molecules.

balance quality for rigid rotors

2008-10-14T05:16:00.000-07:00

Based on present experience, the ISO has categorized the normally available rigid rotors into various groups and have suggested the balancing levels for them in ISO-1940 specifications. The following table gives the level to which a rotor should be balanced.

QUALITY
GRADE 'G' ROTOR TYPES - GENERAL EXAMPLES
G 4000 Crankshaft drives of required mounted slow marine diesel engines with uneven number of cylinders.
G 1600 Crankshaft drives of rigidly mounted on large two cycle engines.
G 630 Crankshaft drives of rigidly mounted large four-cycle engines. Crankshaft drives of elastically mounted marine diesel engines.
G 250 Crankshaft drives of rigidly mounted last four cylinder diesel engines.
G 100 Crankshaft drives of fast diesel engines with six and more cylinder complete engines (gasoline or diesel) for cars, trucks and locomotives.
G 40 Car wheels, wheel rims, wheel sets, drive shaft; crankshaft drives of elastically mounted fast four-cycle engines (gasoline or diesel) with six and more cylinders; crankshaft drives for engines of cars, trucks and locomotive.
G 16 Drive shafts (propeller shafts, cardan shaft) with special requirements; parts of crushing machinery; parts of agriculture machinery. Individual component of engines (gasoline or diesel for cars trucks and locomotive; crankshaft drives of engine with six and more cylinders under special requirements.
G 6.3 Parts of process plant machines. Marine main turbine gears (merchant service), centrifugal drums, fans, assembled air craft gas turbine rotors, fly wheels, pump impellers, machine tool and general machinery parts, normal electrical armatures, individual components of engines under special requirements.
G 2.5 Gas and steam turbines including marine main turbines (merchant service), rigid turbo- generator rotors, turbo-compressors, machine tools drives medium and large electrical armatures with special requirements, small electrical armatures, and turbine driven pumps.
G 1 Tape recorder and photograph (gramophone) drives, grinding machine drives, small electrical armatures with special requirements.
G 0.4 Spindles, discs, and armature of precision grinders, gyroscopes.

what if the balancing fails

2008-10-12T23:57:00.000-07:00

As pointed out earlier it should be precisely confirm unbalance is the only problem before implementing the balancing, as there are many different problems which are having similar vibration characteristics to unbalance. Yet, after the confirmation, all attempts to balance have failed, the next recommended step is to check the rotor for repeatability of unbalance readings. We have to simply stop the rotor and remove all trial weights and balance correction weights and rerun the rotor to the condition it was in when original readings were taken. Compare readings taken in this run with the original unbalance readings. They should be the same. These exercise may be done several times to verify the repeatability of the unbalance readings. If the two sets of readings differ significantly in amplitude and/or phase, this may be due to one of the following factors.

1. The actual configuration or shape of the rotor may not have been stabilized when the first set of original unbalance readings was taken. The rotor may have had a temporary 'sag', which has disappeared now that it has had an opportunity to to run.

2. The unbalance condition of the rotor may be changing from one run to the next. For example, a rotor that is loose on the shaft may assume a slightly different position on the shaft each time it is started and stopped. In addition, fans with hollow blades or hollow shafts may have accumulated dirt or water that changes location each time the rotor is started or stopped.

3. If nothing of the above observed, the rotor should be thoroughly examined for possible cracks on the shaft or rotor. If a repeatability check reveals that the rotor is repeating the unbalance readings from run to run some other problem is usually the reason and a more thorough analysis should be carried out to determine the cause.

shop balancing

2008-10-12T23:52:00.000-07:00

Shop balancing is always preferred in case of new machines and in case of running machines where high degree of balancing is required. Moreover, some machines such as totally enclosed motors, pumps, compressors and others not easy to balance in-situ because extensive disassembly is required to gain access to the rotor for adding or removing balance weight. In these instances, the machine is disassembled and rotor is balanced on a balancing machine.
In-situ balancing will generally produce better results in terms of vibration. However, balancing on a balancing machine will generally produce a better balance. To clarify, the balancing machine is better at measuring and correcting for unbalance, especially two plane or dynamic unbalance. It can not compensate for field installation factors. Two plane balancing on a balancing machine usually produces better results than two plane balancing in the field because the compounding field installation factors like bearing clearances, support stiffness and resonance response, additional components attached to the rotor like coupling, keys, fasteners etc. and alignment are not present. The primary advantage of balancing machine operations is that the unbalance effect is directly measured. The disturbing factors that can cause 1xrpm vibration are not present, so unbalance can be evaluated without these other complications. Balancing machine operations are not so severely troubled by cross effect because they have very flexible supports or they employ plane separation technique. Balancing can be done by either of the methods described earlier depending upon type of rotor, type of unbalance, normal operating speed of the rotor, etc. Some basics of the balancing machines are explained in the coming sections.

balancing mthods

2008-10-12T23:47:00.000-07:00

Before actually attempting to balance a rotor, there are few preliminary factors that need consideration. The most important include:

1. Determining the type of unbalance: This is important in deciding whether a single or two plane approach will be needed. If in doubt, use a two plane approach. A single plane problem can be solved by the with a two plane approach. However, a two plane problem can not be solved with a single plane approach.

As a general guideline the length to diameter (L/D) ratio of a rotor can be useful to determine whether single plane or two plane approach should be adopted for balancing, which is described as follows:

L/D ratio Single Plane Two plane
<0.5> 1000
>0.5 rpm <> 150

2. Calculating the amount of trail weight: Great care must be taken in selecting the size of trial weights. If the weight is too small, no significant change in the unbalance readings will result and a run will have been wasted. On the other hand, if the trial weight is too large for the machine being balanced, the unbalance forces generated by the trial weight may cause extensive damage to the machine.
As a general rule, a trial weight that produces a minimum of a 30% change in amplitude and / or a 30 degree change in phase from the original unbalance readings will help insure satisfactory results. A common approach for selecting a trial weight is to use one that will produce an unbalance force equal to 5-10 % of the rotor weight supported by the bearing.

3. Correction method: Correction can be done either by weight addition or by weight removal. Weight addition is preferred than weight removal for greater accuracy. In large rotors slots for weight addition are given in pre-determined balancing planes. If weight addition is not feasible, great care should be taken when weight removal.

Balancing may be done in-situ (i.e. in installed condition) or in a balancing shop depending on certain factors like unbalance severity, type of rotor, planes of correction, rotor weight, downtime of the machine etc. The above mentioned criteria and balancing types applies to both of these. In shop balancing, measurements and calculations are done by the balancing machine software while in in-situ balancing portable vibration analyzers or polar charts may be used.

balancing types

2008-10-12T23:40:00.000-07:00

Balancing can be classified into three types depending upon the number of correction planes used for balancing:

Single plane balancing
Two plane balancing
Multi plane balancing

Single plane balancing:

As the name implies, single plane balancing is the correction of unbalance in one plane to achieve good balance. This method is always applicable for thin disks and can also be applicable to long rotors also if the unbalance is mostly in one plane. The one plane does not need be through the center of gravity. The single plane balancing corrects for static unbalance only. It can do nothing for couple unbalance. Generally "thin" overhung rotors, single impellers, motors and other long rigid rotors with unbalance vibration substantially higher at one end, are balanced with this method.

Two plane balancing:

The primary reason for doing two plane balancing is to correct for a couple unbalance or combination of static and couple unbalance, which are usually the case. This type of unbalance generates a rocking force. The correction is to apply two weights in two planes separated by some axial distance. This creates a counterrocking force; i.e. a couple unbalance is corrected by another couple. For rigid rotors, any combination of static, couple or dynamic unbalance can be corrected by two plane balancing and two planes are selected arbitrarily depending upon the accessibility and feasibility.

Multi plane balancing:

When we balance rotors externally by making corrections in planes other than where the original unbalance exists, these cause internal bending moments in the rotors. For the rotors operating above their critical speeds these internal bending moments cause deflection and change in shape of the rotors at different critical speeds as explained earlier. If unbalance is corrected in the plane where it exists, there would be no internal bending moments and the rotors would not deflect at different critical speeds. This necessitates balancing in more than two planes for rotors, which operate above their critical speeds. Rotors like high speed multistage compressors, large steam turbines, gas turbines, rolls of paper making machines etc come under this class of rotors.

balancing steps

2008-10-12T23:28:00.000-07:00

Component balancing:

For shaft rotors, which comprise of more than one component, it is vital to balance all of the major components individually before assembly. This is done because if the rotor is fully assembled, there is no way to know exactly what contribution each component part is making to the total measured unbalance vector. In addition, if a large unbalance exists in one of the major components, within the rotor, the rotor shaft may flex at this point during high speed operation and cause significant damage to the rotating and stationary parts.

Each major rotor component must be individually balanced on a precision ground mandrel (note that expanding mandrels are not acceptable for this purpose). The balance mandrel should be ground between centers to assure concentricity of all diameters throughout its length as well as to assure a good smooth surface. After grinding, the mandrel must be precision balanced. A trial bias weight may be used to raise the observed residual unbalance readout of the balancing machine. The desired balance result is such that no matter at what angular location the bias weight is added, the balance readout is always the same. In this case the residual unbalance of the precision mandrel is as close to zero as possible. The rotor component should always be mounted to the mandrel with an interference fit, never a sliding or loose fit. If the rotor component has a key fit to its shaft, than the balancing mandrel should also have a matching keyway. After each component is shrunk on its mandrel, the axial and radial runouts should be checked to ensure that the mounted impeller or hub is not cocked on its mandrel prior to component balancing. As a general rule, runout should not exceed 0.16 mm/meter of diameter.

Progressive component stack balancing (sequential balancing):

After individual balancing of all major rotor components, the rotor must be progressively stack balanced as each major component is assembled onto the rotor shaft. Progressive or stack balancing is necessary due to the deformation of components during assembly. Components with unequal stiffness in all planes, such as those with single keyways, may deform when shrunk onto the rotor shaft. For such components, considerable deformation and resultant unbalance can occur between mandrel balancing using a light shrink fit and stack balancing on the job shaft with a heavy shrink fit.

Progressive balancing is accomplished by stacking no more than two rotor components at a time onto the rotor shaft. Component axial and radial runouts should be checked against mandrel runouts, as each component is start. In general, the start component runouts should match those runouts recorded with the components on the mandrel.

As each rotor component is start into position and the runouts checked as acceptable, the rotating assembly is to be placed in the balancing machine and trim balanced (if required) as necessary to achieve the balance tolerance. Balance weight correction is to be performed only on the most recently stacked component.

After the rotor is completely stacked, trim balancing, if required at all, should be very small to meet the tolerance of permissible residual unbalance. As a general rule of thumb, the remaining residual unbalance in the rotor should not exceed two times the residual balance tolerance prior to trim balancing.

co2 gas sensor

2008-10-12T23:17:00.000-07:00

The CO2 Gas Sensor measures gaseous carbon dioxide levels in the range of 0 to 5000 ppm. This probe is great for measuring changes in CO2 levels during plant photosynthesis and respiration. With this sensor, one can easily monitor changes in CO2 levels occurring in respiration of organisms as small as crickets or beans! The CO2 Gas Sensor is easily calibrated using a calibration button. A chamber with probe attachment is included for. running controlled experiments with small plants and animals

types of Refrigeration systems

2008-10-07T03:06:00.000-07:00

Carnot Vapour compression Refrigeration cycle

(a) Schematic representation (b) T-s diagram

Processes: -

1-2: Isentropic compression from state 1 (wet vapour) to state 2 (saturated vapour)

2-3: Heat rejection (QH) in the condenser

3-4: Isentropic expansion from state 3 (saturated liquid)

4-1: Heat absorption ( QL) in the evaporator

The COP of the refrigerator,

Practical Vapour compression refrigeration cycle

Application of the first law of thermodynamics to the control volume compressor, condenser, throttle and evaporator gives

(Ws)compressor=h2-h1

QH=h2-h3

h3=h4

and QL=h1-h4

The COP of the refrigerator is given by,

In the ideal refrigeration cycle, the refrigerant leaves the evaporator as wet vapour.

In some cases the refrigerant leaves the evaporator as either saturated vapour or superheated vapour.

T-s diagram for a vapour compression refrigeration cycle when the refrigerant leaves the evaporator as (a) saturated vapour (b) superheated vapour

Gas refrigeration cycle

(a) Schematic diagram (b) T-s diagram

The simplest gas refrigeration cycle is the reversed Brayton cycle

Processes: -

1-2: isentropic compression for state 1 (atmospheric air) to state 2

2-3: energy exchange with the surrounding, air is cooled

3-4: isentropic expansion to state 4

Work obtained during the expansion process can be used to run the compressor

Work done on the compressor,

Work delivered by the expander,

The net work required= CP (T2-T1-T3+T4)

The COP of this refrigeration system is given by,

Refrigeration Cycle

2008-10-02T03:16:00.001-07:00

Mechanical refrigeration is accomplished by continuously circulating, evaporating, and condensing a fixed supply of refrigerant in a closed system. Evaporation occurs at a low temperature and low pressure while condensation occurs at a high temperature and high pressure. Thus, it is possible to transfer heat from an area of low temperature (i.e., refrigerator cabinet) to an area of high temperature (i.e., kitchen).

Referring to the illustration below, beginning the cycle at the evaporator inlet (1), the low-pressure liquid expands, absorbs heat, and evaporates, changing to a low-pressure gas at the evaporator outlet (2).

The compressor (4) pumps this gas from the evaporator through the accumulator (3), increases its pressure, and discharges the high-pressure gas to the condenser (5). The accumulator is designed to protect the compressor by preventing slugs of liquid refrigerant from passing directly into the compressor. An accumulator should be included on all systems subjected to varying load conditions or frequent compressor cycling. In the condenser, heat is removed from the gas, which then condenses and becomes a high-pressure liquid. In some systems, this high-pressure liquid drains from the condenser into a liquid storage or receiver tank (6). On other systems, both the receiver and the liquid line valve (7) are omitted.

A heat exchanger (8) between the liquid line and the suction line is also an optional item, which may or may not be included in a given system design.

Between the condenser and the evaporator an expansion device (10) is located. Immediately preceding this device is a liquid line strainer/drier (9), which prevents plugging of the valve or tube by retaining scale, dirt, and moisture. The flow of refrigerant into the evaporator is controlled by the pressure differential across the expansion device or, in the case of a thermal expansion valve, by the degree of superheat of the suction gas. Thus, the thermal expansion valve shown requires a sensor bulb located at the evaporator outlet. In any case, the flow of refrigerant into the evaporator normally increases as the evaporator load increases.

As the high-pressure liquid refrigerant enters the evaporator, it is subjected to a much lower pressure due to the suction of the compressor and the pressure drop across the expansion device. Thus, the refrigerant tends to expand and evaporate. In order to evaporate, the liquid must absorb heat from the air passing over the evaporator.

Eventually, the desired air temperature is reached and the thermostat or cold control (11) will break the electrical circuit to the compressor motor and stop the compressor.

As the temperature of the air through the evaporator rises, the thermostat or cold control remakes the electrical circuit. The compressor starts, and the cycle continues.

In addition to the accumulator, a compressor crankcase heater (12) is included on many systems. This heater prevents accumulation of refrigerant in the compressor crankcase during the non-operating periods and prevents liquid slugging or oil pumpout on startup.

Additional protection to the compressor and system is afforded by a high- and low-pressure cutout (13). This control is set to stop the compressor in the event that the system pressures rise above or fall below the design operating range.

Other controls not indicated on the basic cycle which may be part of a system include: evaporator pressure regulators, hot gas bypass regulators, electric solenoid valves, suction pressure regulators, condenser pressure regulators, low-side or high-side float refrigerant controllers, oil separators, etc.

It is extremely important to analyze completely every system and understand the intended function of each component before attempting to determine the cause of a malfunction or failure.

refrigeration-basic definations

2008-10-02T02:35:00.000-07:00

Temperature

Temperature scales are a way of describing how hot a substance is. A lump of matter contains energy. There are many forms of energy, one of them is Kinetic energy and measuring temperature is a way of measuring how furiously the molecules contained in a lump of substance are moving about. This molecular activity causes what we perceive as the temperature of an object. A refrigeration mechanic must be able to deal with temperatures in various scales. Traditionally the English system has been used (Fahrenheit degrees) and a whole series of familiar capacity measurements like Horse Power, BTU's, Tons, and PSI have been the norm. However the metric system which is supposed to be easier to work with is becoming popular in many locations. In both systems there are standard and absolute temperature scales. Try experimenting with the above temperature converter. Type a value in any one of the input boxes and click on the Convert Button. Here are several interesting values to try: -40 F, 0R, 40 F, 373 K, 21 C

Fahrenheit

The Fahrenheit temperature scale was developed by no less than Fahrenheit himself back in the early 1700's. It was based on scientifically observable occurrences such as human body temperature and melting ice. Those points were assigned arbitrary values which made sense at the time. The newly created number scale was widened for easier reading and when boiling water was measured at 212 degrees, Fahrenheit changed the value of freezing water from 30 to 32 degrees to achieve the more attractive scale of 180 degrees between water's freezing and boiling points. There are 180 degrees in 1/2 of a circle and this was a temptation too great to resist.

Celsius

In theory the Celsius scale should be much easier to work with. It is based on calling the freezing point of water zero and the boiling point of water 100. There are therefore 100 degrees between those 2 points. The Celsius temperature scale is also referred to as the "Centigrade" scale. Centigrade means "consisting of or divided into 100 degrees. I wonder what a comfortable room of 70 F would be in Celsius? If you don't happen to have a conversion calculator at your disposal you can always rely on the following 2 formulae:

Kelvin

Scientists use the Kelvin scale, which is based on the Celsius scale, but has no negative numbers. Instead of basing it's zero point on the freezing point of water, it bases it's zero point on Absolute Zero. which is the theoretical temperature where all heat has been removed from a substance. Hence any amount of heat added creates a positive temperature. Negative numbers can mess up a scientist's mathematical calculations. You will find that in refrigeration, we too must use absolute temperature scales for some things. At Absolute Zero scientists believe that molecular motion would stop.

Rankine

Rankine is the English version of an absolute temperature scale. Add 460 degrees to Fahrenheit temperatures to obtain the Rankine temperature. Input 0 degree in the Rankine box on the calculator above and you will see why.

Heat

Temperature is a qualitative measurement. Heat is a quantitative measurement. The temperature "quality" of a object describes how hot it is but not the total amount of heat it actually contains. Here's a silly example which makes clear the distinction. Let's say we have two blocks of iron. One is a mere cubic inch, the other is 10 feet cubed. We heat each of them to 150 F and you verify this with some sort of thermometer. They both have the same temperature but do they both contain the same amount of heat? When you throw the little cube in your swimming pool nothing noticeable happens to the temperature of the pool water but when you toss in the huge iron chunk the pool water can be measured to rise somewhat over time. If there was a noticeable amount of heat transfer from the large chunk of iron but not from the small chunk of iron then surely the large chunk contained more heat than the small one even though they were at the same temperature. The temperature of an object is a reflection of the kinetic energy of the atoms or molecules that make it up. Fast molecules = high kinetic energy = high temperature. On the other hand heat represents the total amount of kinetic energy in an object. Heat is measured in BTU's. Recall that 1 BTU is the amount of heat required to change the temperature of 1 Lb. of water through 1 F. So it would take 2 BTU to raise the temperature of 2 Lb. of water through 1 F. And it would take 30 BTU to raise the temperature of 3 Lb. of water by 10 F. BTU's (or their metric counterparts) Larger quantities of heat in the Imperial system are described with the term Ton. 12,000 BTU = 1 Ton. A building might have a 3 Ton Air Conditioning system which would be equivalent to 36,000 BTUH.

Specific Heat

Specific heat capacity is the amount of heat required to change temperature of a given quantity of a substance by one degree. Specific heat may be measured in Btu/lb F or kJ/kg K. Different substances have different heat holding capabilities and thermal properties. Just because 1 Lb. of water will change precisely through 1 F when 1 BTU is applied to it does not necessarily mean that the same thing will happen with 1 Lb. of copper or 1 Lb. of steel or 1 Lb. of ice cream. There is a need to be able to specify those differences and the method utilized is to compare all substances to water. Water is given a specific heat value of 1. This means that 1 BTU is required to change the temperature of 1 Lb. of water through 1 F. The specific heat of water can also be described in the metric system. The metric specific heatof water is 1 calorie per gram per degree Celsius. This value also works out to 1. In other words it would take 1 calorie of heat to raise the temperature of 1 gram of water through 1 degree Celsius. Specific heat is adimensionless quantity. It is purely a number having no unit of measurement associated with it. In Refrigeration specific heat values are used to calculate capacity requirements for refrigerating known quantities of product. For example one might need to be able to select refrigeration equipment capable of cooling 5000 Lb. of beef from 55 F to -20 F. A calculation like that must take into consideration the fact that the specific heat of a substance usually is different above and below it's freezing point.

Latent Heat

Latent Heat is the heat given up or absorbed by a substance as it changes state. It is called latent because it is not associated with a change in temperature. Each substance has a characteristic latent heat of fusion, latent heat of vapourization, latent heat of condensation and latent heat of sublimation.

Sensible Heat

Sensible Heat is associated with a temperature change, as opposed to latent heat. This is so-called because it can be sensed by humans. If the air in a building was to be heated from 60 F to 70 F only sensible heat would be involved. However, if the air in a building was to be cooled from 80 F to 70 F and humidity was to be removed from the air at the same time, then both sensible and latent heats would be involved.

Insulator

Electrical wires are coated with an insulating material so electricity stays in the conductor (wire). Thermal insulation on the other hand tries to keep heat from transferring. Thermal insulation does not stop heat transfer, it only slows down the rate of transfer. The greater the amount and quality of insulation, the greater the insulating effect and the slower is the thermal transfer. There is insulation inside cooler and freezer walls and in the perimeter walls of conditioned spaces. If fiberglass batting is being used it should be noted that the glass fibers are actually a poor insulator. It is the tiny pockets of trapped air in-between the fibers that actually are responsible for the insulating effect.

Pressure

Pressure is what occurs when a force is applied over an area. More specifically, pressure is the ratio of the force acting on a surface to the area of the surface. The equation for pressure represents this rather straightforwardly; P=F/A This equation means that Pressure equals Force divided by Area. Let's look at a couple of very simple examples. As is demonstrated in the sketches below, the same weight can exert completely different pressures depending on how much surface area it is spread out over. Note that in the Imperial System when you multiply FT by Lbs you get a unit called FT Lb. (pronounced Foot Pounds) This is a legitimate unit of pressure however. However refrigeration pressures in the Imperial System are measured in pounds per square inch not pounds per square foot. This is abbreviated to PSI. Refrigeration gauges are zeroed to 1 Atm pressure and the units are then called PSIG. (as in PSI Gauge) The calculations shown in the metric picture yield pressure units in kg/m2 (kilograms per meter squared). This is also a legitimate unit of pressure however kPa (kilopascals) are the pressure units that you will see on Metric refrigeration gauges. As with temperature, pressure has many different scales that can be used and can be described with the English system or the Metric system. We seldom deal with gravitational forces as shown in the diagram although it is an important concept to be aware of. Rather, we deal with the pressures and temperatures of gases and that is what the next section is all about.

Energy

Energy is the capacity of a system to do work where "system" refers to any physical system, not just a refrigeration system.

Enthalpy

Enthaply is the total amount of heat in one Lb. of a substance. It's units are therefore BTU/Lb. The metric counter part is kJ/Kg. (kilo joules/kilogram)

Entropy

Entropy measures the energy dispersion in a system divided by temperature. This ratio represents the tendency of energy to spread out, to diffuse, to become less concentrated in one physical location or one energetic state. That spreading out is often done by molecules because molecules above absolute zero always have energy inside of them. That's why they are incessantly speeding through space and hitting each other and rotating and vibrating in a gas or liquid. Entropy is measured in BTU per Lb. per degree change for a substance.

Mollier Charts

Mollier charts are used in designing and analyzing performance of vapour compression refrigeration systems. Each refrigerant has it's own chart which is a graph of the Enthalpy of a refrigerant during various pressures and physical states. Mollier charts are also called Pressure-Enthalpy diagrams. Pressure is shown on the verticle axis, enthalpy is on the horizontal axis. You can compare Imperial versus SI Unit Mollier Charts by clicking on the buttons below the chart.

Infrared Gas Sensors

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Infrared Gas Sensors exploit the property that many gases absorb radiation in the 2-14 micron, infrared region of the spectrum. These spectral absorbance show features which may be regarded as 'fingerprints' to identify the gases and enable their concentrations to be deduced. The sensor bodies contain an infrared source and infrared detectors inside a compact and combined gas cavity/ optical cell. The detectors have infrared band pass filters placed in front, which tune them to the specific gases to be sensed. When the specific gas enters the cavity it is registered as a change in detector signal. The magnitude of this change is related to the concentration of that gas via a simple exponential formula.By utilizing different infrared filters a range of gases can be sensed and discriminated with these devices.

In cases where spectral lines overlap, then an individual sensor may show cross sensitivities to a gas range. Infrared gas sensors are very robust devices not affected by contact with a harsh chemical environment. Any changes in ambient conditions such as temperature are compensated for in software. Their dimensions and power requirements are compatible with and complementary to pellistor gas sensors. After over thirty years of successful manufacture of pellistor-based flammable gas sensors, the range of Non-Dispersive Infra-red (NDIR) gas sensors represents the first of many diversifications into other areas of gas sensor technology.

In a molecule, absorption or emission of energy can occur in transitions between different energy levels. These transitions can be associated with changes in the vibrational energy and changes in the rotational energy of the molecule. Such internal energies are quantized, so that the molecule can exist only in certain discrete vibrational and rotational energy levels. The energy related to transitions between vibrational energy levels is equivalent to radiation in the near infra-red region of the electromagnetic spectrum. Each vibrational level is associated with a set of rotational levels, which results in several closely spaced energy levels existing within a frequency band in the infra-red spectrum of the molecule. The fundamental frequencies at which the bands exist are functions of the particular bond and the mode of vibration, e.g. stretching or bending. When a molecule is exposed to infra-red radiation with an energy equivalent to a vibrational transition, the radiation is absorbed and the molecule undergoes the transition. This absorption is used as the means to determine the amount of target gas molecules present.

The NDIR technique uses a broad-spectrum source, such as a filament lamp, to expose the gas to a wide range of infra-red frequencies. An associated detector is fitted with an optical filter such that it can only monitor the intensity of a certain narrow frequency band. This frequency band is selected to match a frequency band within the absorption spectrum of the target gas and the detector output is therefore affected by the concentration of the target gas. The frequency of radiation, for our purposes, is more often expressed in terms of its wavelength, as the two terms are directly related

Thermal Conductivity Gas Sensors

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Pellistors measure the flammability of a gas; they cannot be used to measure levels of gas above the Lower Explosive Limit (LEL), since the reducing level of oxygen will result in a fall-off of signal. However, a similar device can be used to monitor these high levels of gas. We have a range of thermal conductivity sensors, which are designed to complement the pellistor range in terms of electrical characteristics, so that they can be used in the same Wheatstone Bridge circuits. They are supplied with a compensator bead which is in a sealed enclosure of air. This enclosure acts as the thermal conductivity reference in exactly the same way as it acts as the reference for a pellistor.

Thermal conductivity measurements do not rely on the flammability of the gas, the technique can be used to analyze a whole range of gas mixtures, provided that there are only two gases present and that the two gases have significantly different thermal conductivities. Examples includes

100% Hydrogen in Air
100% Methane in Air
100% Carbon Dioxide in Air
100% Carbon Dioxide in Methane
100% Helium in Air

Thermal conductivity cannot be used for gas mixtures where the thermal conductivities of the two gases are similar. The best example of this is oxygen levels in air, as the thermal conductivities of oxygen and nitrogen are too close to give a meaningful signal.Our pellistors and thermal conductivity sensors can be obtained already packaged as complete, flameproof gas detection heads for use in fixed gas detection systems.

Combustion Turbine-Heat Recovery

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Combustion turbines generate a large volume of very hot air. The exhaust is also high in oxygen content as compared to other combustion exhaust streams, as only a small amount of oxygen is required by the combustor relative the total volume available.

Depending on how much thermal energy is required for the application, the turbine exhaust may be supplemented by a duct burner.

A duct burner is a direct fired gas burner located in the turbine exhaust stream. It has a very high efficiency due to the high inlet air temperature, and is used to boost the total available thermal energy. The turbine exhaust boosted by the duct burner is directed into the waste heat boiler, called the Heat Recovery Steam Generator, or HRSG commonly pronounced as 'HerSig'.

Turbine exhaust can also be ducted directly into hot air processes, such as kilns and material drying systems. This is the least costly first cost, as there is no boiler or steam system to purchase. Turbine exhaust can also be ducted directly into absorption chillers for large cooling loads.

The system will also include a diverter for times when waste heat is not needed. The diverter vents the turbine exhaust to atmosphere; this substantially reduces the system efficiency, as only the electric energy is being used. Single or Simple Cycle electric plants (typical of peaker plants) dump all of their turbine exhaust, as they have no thermal requirements. These plants generally use turbines with recuperators to maximize their electrical efficiency.

The higher the electrical efficiency of the turbine, the lower the available thermal energy in the exhaust. Newer turbines with recuperators, and larger sized turbines, tend to have higher efficiencies.

Combustion Turbine-Operations

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A gas turbine has a compressor to draw in and compress air; a combustor (or burner) to add fuel to heat the compressed air; and a turbine to extract power from the hot air flow. The gas turbine is an internal combustion (IC) engine employing a continuous combustion process. This differs from the intermittent combustion occurring in diesel and automotive IC engines. About 2/3rds of the shaft power produced by the turbine is used to run the compressor, leaving about 1/3rd available to turn a genset to produce electrical power.

Gas Turbine Cycles
A cycle describes what happens to air as it passes into, through, and out of the gas turbine. The cycle usually describes the relationship between the space occupied by the air in the system (called volume, V) and the pressure (P) it is under. The Brayton cycle (1876), shown in graphic form as a pressure-volume diagram, is a representation of the properties of a fixed amount of air as it passes through a gas turbine in operation. These same points are also shown in the engine schematic above.

Air is compressed from point 1 to point 2. This increases the pressure as the volume of space occupied by the air is reduced.

The air is then heated at constant pressure from 2 to 3. This heat is added by injecting fuel into the combustor and igniting it on a continuous basis.

The hot compressed air at point 3 is then allowed to expand (from point 3 to 4) reducing the pressure and temperature and increasing its volume. In the engine, this represents flow through the turbine to point 3' and then flow through the power turbine to point 4 to turn a shaft or a ship’s propeller. The Brayton cycle is completed by a process in which the volume of the air is decreased (temperature decrease) as heat is absorbed into the atmosphere.

A gas turbine that is configured and operated to closely follow the Brayton cycle is called a simple cycle gas turbine. Most aircraft gas turbines operate in a simple configuration since attention must be paid to engine weight and frontal area. However, in land or marine applications, additional equipment can be added to the simple cycle gas turbine, leading to increases in efficiency and/or the output of a unit. Three such modifications are regeneration, intercooling and reheating.

Regeneration involves the installation of a heat exchanger (recuperator) through which the turbine exhaust gases pass. The compressed air is then heated in the exhaust gas heat exchanger, before the flow enters the combustor.

If the regenerator is well designed (i.e., the heat exchanger effectiveness is high and the pressure drops are small) the efficiency will be increased over the simple cycle value. However, the relatively high cost of such a regenerator must also be taken into account. Regenerators are being used in the gas turbine engines of the M1 Abrams main battle tank of Desert Storm fame, and in experimental gas turbine automobiles. Regenerated gas turbines increase efficiency 5-6% and are even more effective in improved part-load applications.

Intercooling also involves the use of a heat exchanger. An intercooler is a heat exchanger that cools compressor gas during the compression process. For instance, if the compressor consists of a high and a low pressure unit, the intercooler could be mounted between them to cool the flow and decrease the work necessary for compression in the high pressure compressor. The cooling fluid could be atmospheric air or water (e.g., sea water in the case of a marine gas turbine). It can be shown that the output of a gas turbine is increased with a well-designed intercooler.

Reheating occurs in the turbine and is a way to increase turbine work without changing compressor work or melting the materials from which the turbine is constructed. If a gas turbine has a high pressure and a low pressure turbine at the back end of the machine, a reheater (usually another combustor) can be used to "reheat" the flow between the two turbines. This can increase efficiency by 1-3%. Reheat in a jet engine is accomplished by adding an afterburner at the turbine exhaust, thereby increasing thrust, at the expense of a greatly increased fuel consumption rate.

Combustion Turbine

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1. Air Intake Section
2. Compression Section
3. Combustion Section
4. Turbine Section
5. Exhaust Section
6. Exhaust Diffuser

Combustion turbines have been used for power generation for decades and range in size from units starting at about 1 MW to over a 100 MW. Units from 1-15 MW are generally referred to as industrial turbines, a term which differentiates them from larger utility grade turbines and smaller microturbines. Combustion turbines have relatively low installation costs, low emissions, high heat recovery, infrequent maintenance requirements, but low electric efficiency. With these traits, combustion turbines are typically used for cogeneration, as peakers, and in combined cycle configurations.

The Pellistor Catalytic Gas Detector

2008-09-22T08:29:00.000-07:00

Pellistors are miniature calorimeters used to measure the energy liberated by the burning of a combustible (flammable) gas or vapour. A pellistor consists of a coil of small- diameter platinum wire supported in a refractory bead on which is deposited a layer of catalytic material, on which the gas is burnt. The coil serves two purposes. Firstly, it is used to heat the bead electrically to its operating temperature, about 500°C, and secondly it is used to detect changes in temperature produced by the oxidation of the flammable gas. The earliest forms of catalytic gas sensors consisted solely of bare coils of platinum wire, operating at 800-1000°C. At such temperatures, platinum wire evaporates extremely quickly causing signal drifts resulting from a reduction in the wire diameter. The specification for such a sensor, which is still produced commercially, requires that the sensor has a life of 100 hours. The majority of present day devices, as stated earlier, have the coil cloaked in a porous ceramic onto which is deposited the precious metal catalyst. The enhanced catalytic activity resulting from the much larger surface area of catalyst available permits much lower operating temperatures of around 500°C, resulting in lower power drain and longer device lifetime.

The concept of the pellistor is based on the fact that the most foolproof way to determine whether a flammable gas is present in air is to test a sample by trying to burn it! A pellistor consists of a very fine coil of wire suspended between two posts. The coil is embedded in a pellet of a ceramic material, and on the surface of the pellet (or 'bead') there is a special catalyst layer.

In operation, a current is passed through the coil, which heats up the bead to a high temperature. When a flammable gas molecule comes into contact with the catalyst layer, the gas 'burns'. The reaction occurs without a flame since the level is below the Lower Explosive Limit (or LEL) of the gas. However, just as in a burning reaction, heat is released which increases the temperature of the bead. This rise in temperature causes the electrical resistance of the coil to rise. There is another bead in the circuit which is identical to the detector bead, but does not contain any catalyst

This bead will react to changes in humidity, ambient temperature etc, but will not react to flammable gas. All that is required is a comparison of the resistance of one bead against another in a Wheatstone bridge type circuit in order to obtain a meaningful signal.

gas sensor-application

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(i) Gas Leak Protection: - These are areas of industry where the possibility of flammable gas build-up is small, but the consequences of a gas escape could be catastrophic. These tend to be industries which by their nature have large volumes of gases piped around the works:

Engineering companies
Metal working plants
Research laboratories

(ii) Confined Space Entry

The most prominent application for portable gas detection instruments. The instrument is used to check the atmosphere of sewers, tanks and other vessels prior to entry for maintenance purposes. These instruments invariably are 'multi-gas'. They have 3 or even 4 sensors included in the package. Large volumes of these instruments are purchased by:

Public utilities - especially water and telecoms
Chemical and petrochemical - for entry into vessels
Cabling contractors
Piling contractors

Tunnelling contractors
Civil engineers
Landfill operators

(iii) Hazardous Area: Working Areas of industry where the build-up of flammable gas or vapour is an ever present danger. These instruments are very often the same multi-gas instruments used for confined space entry, but there are areas where single gas monitors ('explosimeters') are used. Typical industrial sectors here are:

Chemical and petrochemical industries
Oil/gas exploration
Mining

gas sensor-introduction

2008-09-22T08:19:00.000-07:00

Gas sensor has recently attracted much attention due to increasing demand of environmental monitoring and other gas detecting applications. Among different types of gas sensor, thin film gas sensor has been much of interest because of microelectronic batch- fabricated compatibility, reproducibility, and ability to form multilayer device structures. In this work, thin film based gas sensing circuit is designed for immediate applications of CO detection for environmental monitoring. Ion assisted deposition (IAD) process offers several advantages for gas sensor fabrication, including reactive deposition for gas-sensitive metal- oxide material optimization and improved thin film adhesion for better device reliability. The metal oxide layer was deposited on alumina or glass substrates. The sensors were tested with reducing gases, in the temperature range between 200 C and 350 C and the electrical change in gas sensor is detected.

Gas sensors interact with a gas to initiate the measurement of its concentration. The gas sensor then provides output to a gas instrument to display the measurements. Common gases measured by gas sensors include ammonia, aerosols, arsine, bromine, carbon dioxide, carbon monoxide, chlorine, chlorine dioxide, Diborane, dust, fluorine, germane, halocarbons or refrigerants, hydrocarbons, hydrogen, hydrogen chloride, hydrogen cyanide, hydrogen fluoride, hydrogen selenide, hydrogen sulfide, mercury vapor, nitrogen dioxide, nitrogen oxides, nitric oxide, organic solvents, oxygen, ozone, phosphine, silane, sulfur dioxide, and water vapor. Important measurement specifications to consider when looking for gas sensors include the response time, the distance, and the flow rate.

The response time is the amount of time required from the initial contact with the gas to the sensors processing of the signal. Distance is the maximum distance from the leak or gas source that the sensor can detect gases. The flow rate is the necessary flow rate of air or gas across the gas sensor to produce signal. Gas sensors can output a measurement of the gases detected in a number of ways. These include percent LEL, percent volume, trace, leakage, consumption, density, and signature or spectra. The lower explosive limit (LEL) or lower flammable limit (LFL) of a combustible gas is defined as the smallest amount of the gas that will support a self-propagating flame when mixed with air (or oxygen) and ignited. In gas-detection systems, the amount of gas present is specified in terms of % LEL: 0% LEL

being a combustible gas-free atmosphere and 100% LEL being an atmosphere in which the gas is at its lower flammable limit. The relationship between % LEL and % by volume differs from gas to gas. Also called volume percent or percent by volume, percent volume is typically only used for mixtures of liquids. Percent by volume is simply the volume of the solute divided by the sum of the volumes of the other components multiplied by 100%. Trace gas sensors given in units of concentration: ppm. Leakage is given as a flow rate like ml/min. Consumption may also be called respiration given in units of ml/L/hr. Density measurements are given in units of density: mg/m^3. A signature or spectra measurement is a spectral signature of the gases present; the output is often a chromatogram. Common outputs from gas sensors include analog voltage, pulse signals, analog currents and switch or relays. Operating parameters to consider for gas sensors include operating temperature and operating humidity.

Graphite and CFRP machining

2008-09-15T10:12:00.001-07:00

High speed machining is common practice in many workshops generating electrodes through high speed, which are then used in SEDM. Cutting speeds are so as high as the machine can, using very high speed spindles (40.000 rpm) although with little power.

Graphite is very soft (he’s the pencil mogul) yet its character is abrasive thus we recommend using diamond coated tools achieving a very high duratioon. Another problem is the dust generated and must be aspirated effectively (specially problematic in CFRP, carbon fibber reinforced plastics), otherwise one can infilter CNC, and other electrical elements causing short-circuits. Plus he has an abrasive effect which can lead to damage of the machine carriage.

Fig.(Left.) High speed milling of a graphite electrode. (Right.) Image of same. Centre Detail of introduction in an electroerosion machine.

Machining of thermo-resistant alloys

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Thermoresistant alloys are also called superalloys, two examples are nickel (Inconel 718) and cobalt (Haynes 25). Regarding their machining, they are even worse than titanium, A very high cutting machine speed for these is 80 m/min. Therefore, their machining is performed on conventional machines, and is by no means a high speed process. The term conventional does not mean a low quality machine, but rather it does not have spindle over 6000 rpm nor does it work with feeds over 4 m/min.

The added value of components manufactured in these alloys is very high and the machines used usually have a very high precision, therefore high range machines and a lot of precision. If you will allow a simile, i.e. if one could choose between a high speed car of “I want and can’t have’ or a high-end German one, which would prefer ?. Machining of superalloys was the purpose of the study using hybrid machining techniques, as shown in Fig. and will be commented in the corresponding.

Fig. Cobalt alloy machining at the Univeristy of the Basque County with plasma asistance.

Machining of titanium alloys

2008-09-15T10:08:00.001-07:00

With coated hard metal tools machining at cutting speed around 200 m/min is currently common, against the 40 m/min applied 8 years ago. Thus, it is not truly a high speed machining but a much ‘quicker machining’. Diameters of tools used are between 4 and 20 mm, therefore the milling machines do not require spindles over 6000 rpm.

Titanium alloys present several problems:

- They have very low thermal conductivity, and therefore heat concentrates in the cutting area.

- High temperatures in the contact area between tool/chip and the high chemical reactivity of the titanium alloys with most tool materials, are the main causes for the rapid crater wear.

- The low elasticity module of these alloys causes flexions in the part, particularly on thin walled parts. This causes large inaccuracies on the finish and enables machining instability

Fig.11 Aeronautic engine components (photo courtesy of Volvo Aero).

Iron casting machining in the die stamping sector

2008-09-15T10:04:00.000-07:00

In this sector HSM is located exclusively in the superfinish operation of the matrizes pursuing a specific object, i.e. reduction of maximum roughness ( Rt) of surfaces with values of 10 microns or less. As feeds can reach 5 to 10 times higher than conventional machining, it offers the possibility of increasing the number of passes to the same extent for the same finish time. The result is a better quality surface, reducing subsequent manual polishing tasks, which might imply almost 30% of the total mould manufactuer.

Machines used are gate type with 5 axes. However, they do not usually machine with the 5 axes simultaneously, but simply orientate headstock and machine after that. As they are superfinishing operations with ball-end tools and over-thicknesses of 0,2 mm we would be talking about a machining process similar to that of tempered steels regardig their physical nature. Superfinishing times with HSM are very long, 39 o 40 hours a big die. This is why it is important for the process to be highly reliable and the tool unlikely to break or wear during the operation. The iron castings used like the globular type GGG70, are easy to machine. Cutting speed reaches 400 m/min with coated hard metal tools and up to 1000 m/min with the PCBN. In Fig. 10 we can see a machine in progress and a partially mechanised boot.

Fig. Milling a die and aspect of this die prior to the operation.

Manufacture of forge dies and recovery thereof

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This sector has traditionally used shinking electrodischarge (SEDM) for the manufacture of forging dies in treated steels of approximately 40 HRC of hardness. Nevertheless, the simplicity of the cavities, wide tolerances and roughness requirements to be reached have made this one of the high speed milling star applications.

In the last eight years there has been a real migration from (SEDM) to HSM. In this sector, development times of new series are critical given the high competition between forgings, thus the reduced HSM process times is a highly important factor. In Fig. we can see two die cases, one medium-sized and the other small. Both are high speed machined in under 2 hours from treated steel (already hardened) to over 40 HRC. High speed milling also allows dies to be remachined for their recovery. Once used in the forging process some parts of the die are worn out, so welded material is added. After this process it is remilled at high speed, in this case a somewhat uneven material which co-exists with the original, i.e. additions and even over-tempered areas. The final result is a die ready for forging again.

Fig.9 Forging die of securing element and a suspension bearing.

Machining treated or tempered steel in mould sector

2008-09-15T09:58:00.001-07:00

The entry of high speed in this sector has perhaps been a decisive factor in the rapid spread of this technology, since number of users is high yet company size small. I.e. there is a very varied demand which for machine tool manufacturers represents a clear target sector and numerous. If HSM had continued being exclusively for the aeronautical sector it would have had its heyday in the second half of the 1990s. The die and mould sector means talking about numerous plastic injection companies of parts of all sizes and applications, companies dedicated to aluminium and zamac injection; and finally those dedicated to forging. There seems to have been a reciprocal effect between cutting process development and machine tool performance.

In the 1990s tools were developed enabling tempered steel cutting conditions to be increased (@ 50HRC) beyond those considered conventional. These tools were and are of submicrograin hard metal, coated in TiAlN, undoubtedly the kings of machining today, or the PCBN tools PCBN (Polycrystalline Cubic Boron Nitride). Using these tools cutting speeds can be increased 4 and 5 times. This cutting speed obliges the machine to have a spindle capable of spinning at high speed (> 15000 rpm). This spindle rotation speed, together with feeds per tooth recommended for tools implies the machine must maintain working feedrates higher than usual, i.e. greater than 5 metres/minute. Moreover Numerical Control must control axes which are interpolated with sufficient precision. Therefore a machine with very high performances is required, called “ high speed machine ”. This machine has a high speed spindle, a CNC capable of governing spatial movements at high feeds and be very rigid to achieve good precision.

Fig. Small plastic injection mould machined in HSM.

Growing industrial demand for these machines has led to rapid development of different machinery aspects and subsystems like electro-spindles, the axis drives new structures equipped with greater robustness, etc. Thus, machines with superior technology to the conventional have appeared. The new machines also open new application possibilities and substantial improvements in the process, like greater cutting stability, greater contour precision, possibility of machining on 5-axis simultaneously, etc.

Fig. Moulds machine on 3 and 5 axes machines.

n conclusion, if the ‘egg’ came first, the possible new cutting speeds in the ‘proces’, subsequently was the ‘chicken’ i.e. the ‘high speed milling machine’ equipped with features highly superior to the conventional. And at the same time, these new features enable greater process performance, opening new perspectives. Therefore, we find ourselves in a spiralling improvement process aimed at seeking ‘global machining solutions’ with greater productivity and precision, not to mention capable of generating greater added-value for the user.

The high speed milling of tempered steel moulds is centred on the finishing operation with a ball-end mills. In this phase excess material of 0.2 or 0.3 mm is eliminated. Cutting geometry is reflected in Fig. 8. Due to the complex geometry of the cavities ball-end mills whose diameter should not exceed 20 mm., must be used. If one bears in mind the slopes of the shapes to be generated vary between 0 and 90º inclination, one can conclude reaching an effective cutting speed of 300 or 400 m/min (at point A of the figure) requires head rotation speed to exceed 15,000 rpm. However, effective cutting speeds have a value of 200 to 400 m/min, not 4000 m/min as Solomon claimed (see Fig.). Therefore, the chipping process at these speeds is similar to conventional without variation of basic phenomena.

Fig. Cutting speed ratio with axial depth and slope.

In conclusion, high speed milling of hardened steels is ‘a conventional process’ from a thermophysical viewpoint, but performed on a ‘high speed machine’, which machines small chip thicknesses much quicker than the conventional one. Nevertheless, in this case we should forget numerous theories (like Solomon’s) which are found in many informative articles which may lead to confusion.

Another aspect to highlight is that it is currently becoming difficult to clearly separate a high speed machine from a conventional one for the mould world. Some industrial solutions have even appeared like spindle machines with direct coupling of motor and spindle which reach 12,000 rpm., being a cheaper and more robust option for multiple sector applications.

Machining magnesium alloys

2008-09-15T09:57:00.000-07:00

These alloys are even softer than the aluminium ones and so easier to machine. The most widely known is AZ91, which is cast and given its lightweight is used in manufacturing parts previously made from aluminium.

The main problem posed is the inflammability of the chips and problems of possible explosion of stored chips, therefore it is a question of safety. Cutting speed may be higher than for aluminium.

Machining aluminium alloys

2008-09-15T09:53:00.001-07:00

Undoubtedly this kind of machining is close to the physical high speed concept, since cutting speeds can reach a value of 2000 m/min, or higher when using milling plates whose diameters exceed 50 mm. At this speed chip generation is different from conventional, mainly because almost all the heat generated by the deformation energy inherent in the chipping process is evacuated with the same, which is highly positive for both tool and part.

We could say lightweight alloys is the most traditional field of high speed machining, known since the 1970s and applied to the fuselage component machining sector. In fact the first systematic HSM studies were in the aeronautical field.In the case of aluminium alloys, there are two different cases as per alloy type:

- Aeronautical alloys, particularly the 2000 or 7000 series, called malleable or wrougth. Easily machinable, used in component construction obtained by eliminating a large amount of chips from an initial prismatic block. A small component example is shown in Fig. 5 (left).

- Cast alloys (series 3xx), used mainly in car engines (blocks and pistons), are highly abrasive because they contain silica. The typical operation is planing and finish of a cast part very similar to the final one, with little chip volume. An example is the block shown in Fig.

Airframe component. Aluminium Aluminium engine block.

High speed machining concepts for different materials

2008-09-14T23:35:00.001-07:00

The first question is what does one understand by high speed machining. In innumerable articles reference is made to historical theories regarding temperature reduction with high speed cutting as described by Carl Salomon, who patented the process in the 1920s ( German patent #523594).

It is also common to find ballistic references consisting of firing projectiles against materials to simulate material deformation at high cutting speeds like those of Kronenberg in the 1960s.

There are also complex discussions related to chipping under different thermophysical conditions depending on the cutting speed magnitude. Each definition has its own nuances and motivation, valid within a certain application range. In almost all high speed reports, the same generalities are repeated (copying or ‘inspiration’ among authors is evident), many of which use the Solomon curves. You have to remember Solomon put forward his idea almost 15 years prior to modern cutting models, thus it was a highly intuitive invention. He performed milling experiments at speeds over 15,000 m/min. To sum up this patent put forward: “tool temperature and wear increase with cutting speed until reaching a maximum value called critical speed, from they decrease with speed. Critical speed depends on material, as shown in Fig. Thus results will be optimum when machining above this value”.

Today we know temperature and wear always increase, although they tend to stabilise when cutting speed is high enough. The McGee (1979) curve is considered more appropriate for aluminium, although it is just one more obtained by different researchers.

Solomon Curves.

Regarding other materials, it is difficult to verify whether machining steels withstands speeds of 1700 m/min well, because prior to reaching these values, the tools break, As we see in the figure, the HSS rapid steel only withstands up to 650ºC, and the hard metal tool approximately 850ºC. Forgetting the generalities which are always repeated and whose repetition sometimes only manages to this kind of ‘high speed’ term magic halo, cutting values understood today as high speed are reflected in Fig. 4, with orientative cutting speed in each case. To reach these speeds machines capable of doing so are required, since they may be limited by their construction and above all by main motion (spindle). As can be seen there is an area (marked in red) where the machine to be used is high speed, coinciding with today’s industrial offer of this type of machine. Another area (in blue) requires a very high speed machine which although extant today is still in the prototype phase.

High speed machining

2008-09-14T23:33:00.000-07:00

This machining type is contrary to the aforementioned traditional concept. Chip section is maintained, i.e. feed per tooth and immersion conditions (radial and axial depths of cut) and to a great extent are even reduced, however, cutting speed is higher than usual. How much is it increased? There is no real academic response, however, it is understood to be higher than ‘traditional’ (x10,x20,...) cutting speed. From the academic viewpoint one could say v _c is increased to the point where the thermophysics of the chipping process varies considerably in relation to that of the conventional process.

This definition implies one or several of these aspects:

- The shearing process deformation speed occurring in the primary shearing area, exceeds 10 ⁵ s ^-1.

- Almost all the heat is evacuated with the chip, the process being close to adiabatic conditions regarding the material. Almost no heat is transmited to the tool, which is optimum in preventing its degradation.

- The effect of chip direction change, which is the material kinetic momentum change, is appreciable and should be considered in the global energy balance.

With this definition we would find almost none of today’s machining processes are high speed, when almost all machine offerers claim that what their milling machines allow is. Why the paradox? Its explanation can be found in the following section. We can advance that only in easily machinable lightweight alloys of magnesium or aluminium, the chipping process differs from that of conventional speeds. However, for this fact to be evident, cutting speeds must be much higher than those applied in today’s industrial HSM processes, which must exceed a cutting speed of 2500 m/min. In the case of steels, castings, difficult-to-machine alloys, titanium, etc., even with today’s cutting speeds being considerably higher than conventional, there is no great variation in intrinsic mechanisms (viscoplastic, thermal, etc.) associated with chipping.High Speed Machining will be abbreviated here to HSM. It should be remembered HSM is recognised worldwide even as a verb (its participle is HSM’ed)

High performance machining, current sense

2008-09-14T23:28:00.000-07:00

Today the HPM (High Performance Machining) term is much more general referring to all kinds of cutting technology, substantially improving two aspects of the process:

- Productivity measured as material removal rate, i.e. the amount of material eliminated in a time unit. One should also ensure machining is done under conditions which do not lead to excessive tool wear and tear.

- Quality with regard to greater dimensional precision and less surface roughness. Figure 2 shows a precision map for a test part machined at high speed, which served to detect a non-admissible error in machining.In the last 10 years there has been a minor revolution regarding improvement in processes and an increase in knowledge thereof.

The reasons may be several:

- Materials constituting manufactured components have higher mechanical features, leading almost always to lower machinability. A good example is the growing use of titanium, nickel and cobalt alloys (heat resistant), Csi infiltrated composites, etc.

- Cutting tools have greatly improved in the last 10 years, with the appearance of new hard metal grades (sintered carbide), extrahard materials have been perfected (PCBN, and PCD). Tools tend to become ever more specialised in one application, abandoning their purpose of being applicable to many material groups.

- Tool performance improvement has contributed decisively to new coating developments, ever harder and more resistant to high temperatures. TiAIN coating applied by PVD technology ( Physical Vapour Deposition) has been extremely important.

- Detailed knowledge of processes and their modelisation has been proven to contribute to earning money, i.e. value is obtained from knowledge albeit it still limited. An example is preduction of milling stability conditions ( chatter study); after 30 years and numerous articles on chatter prediction, today there are companies and consultancies earning money with it.

- In Europe,Asia, USA and of course Spain, the machine tool and manufacture by machining sector are very important, thus research resources have been assigned by companies and administrations. Furthermore, improvement in machinery and its process concerns both machine tool builders and suppliers likewise users thereof. Western industry tends to produce components with greater added value more and more; the term high performance refers to everything which contributes to increasing this value, either because it reduces production times and costs increases manufactured quality.

Thus, today High Performance Machining is understood as everything which incorporates notable improvement with respect to traditional machining, increasing process added value both in productivity and quality. The machining term includes chipping process with defined cutting tool (milling, turning, drilling, sawing) with non-defined edge or abrasives (grinding), and even non-conventional processes (electrodischarge, ultrasounics, etc).

High Performance Machining(HPM)-concepts

2008-09-14T23:17:00.000-07:00

These two terms are not exactly synonymous and in the last decade have been taken and affected by different nuances. The exact meaning of the same is set out below, and which at times complies simply with the usual inflation of terms derived from their commercial use.

High performance machining, a traditional or classical concept

'High performance machining’ traditionally referred to that simultaneously applying great feeds and cutting depths, while maintaining cutting speeds considered conventional. Under these operating conditions, large chips arise as shown in Fig.This process should be carried out on machine tools with very powerful rigid headstocks. it is called High Removal Rate Machining, which is a more specific descriptive term than that understood by the vaguer ‘high performance’. Higher chip thicknesses than usual are generated by applying large feeds and great cutting depths. Cutting forces grow almost in direct proportion to the chipping section. Thus with this hypothesis we approach two kinds of problems: possible catastrophic tool failure ( breakage) due to high cutting forces or edge breakages at multiple points ( chipping).

Therefore, this kind of machining is only applicable to soft materials as in the case of lightweight alloys, low resistance steels or steels prior to hardening through tempering. A typical case is the turning of large crankshafts and casting rollers on large powerful heavy-duty lathes. In this case large tool inserts are used as shown in Fig.

(Left) Large turning tool inserts for large chip sections.

(Right) Large chip in turning.

pollution control systems in vehicles

2008-09-14T23:07:00.000-07:00

Controlling the menace:

There is now a growing realization in the society at large about the need to curb the automobile related pollution and though catalytic converter has captured the popular imagination as the panacea for controlling pollution, in reality there are several methods and devices that help control pollution in the automobile. Some of these act independently while others function in tandem with other systems.

Proper maintenance of the vehicle does not mean regularly topping the air, oil and water coolant levels alone. It means getting to understand the function of various components and ensuring they perform the intended purpose. Many a times inexperienced and unqualified mechanics simply disconnect the system or component without realizing its function as it does not interfere with the normal running of the vehicle thereby exposing it to slow but sure damage in the long run. Most of the devices are meant to ensure the correct air/fuel mixture and controlled exhaust emissions in the fall in this category.

In view of this it is pertinent to know the various devices and systems that are incorporated in the automobile and the role they perform. Maintaining them in working condition will help ensure that the vehicle does not exceed its emission limits

Evaporative Emission Control System:

This system is to control non-exhaust pollution originating from evaporation from the fuel tank and the float chamber of the carburetor. Evaporative emissions from the carburetor are higher due to its proximity to heat generated by the engine. The system includes a positive seal fuel tank cap, vapour vent line, a canister containing, canister purge line, vacuum single line and a purge control valve. Fuel vapours from the fuel tank and carburetor float bowl are absorbed in the carbon in the canister. When there is high vacuum in inlet manifold, vacuum single line forces purge-control valve to open and admit fuel vapours from the canister into the intake manifold, to be burnt with incoming charge.

Crankcase ventilation system:

In older model engines, vapour from the crankcase were allowed to escape to the atmosphere through a venting pipe installed on the tappet/rocker-cover/engine-block/crankcase. On all new generation engines a close crank-case ventilation system is adopted. Vapours from the crankcase are routed to the air intake hose through a metering positive crankcase ventilation valve installed on the hose to be burned along with air/fuel mixture. This PCB valve requires replacement at regular intervals of 40-50000 kms.

Exhaust Gas Re-circulation System:

Conditions of high-pressure and high-temperature exist in an engine which are conductive for the formation of oxides of nitrogen. Reduction is one of the conditions i.e. high-pressure or high-temperature, reduces formation of oxides of nitrogen. Exhaust gas recirculation helps reduction in combustion temperature in an engine. Exhaust gas are drawn into intake manifold from the exhaust manifold through an EGR valve. This valve remains closed during idle , as exhaust gases cause rough idling. At full throttle also the valve remains closed as exhaust gas recirculation is not required. Sometimes the device known as back-pressure transducer (BPT valve) is also incorporated in the system. All valves are basically to regulate the EGR to match the varying operating conditions besides bringing down the temperature to reduce the formation of oxides of nitrogen, hydro-carbons which escape through exhaust gases are also burnt with EGR

Spark Timings Control System:

The system is designed to retard full spark-advance except when the car is in high gear. In conventional engines the vacuum in the inlet manifold through a hose is utilised to operate the mechanism in the distributor for the purpose. However, lately the system has been improved to match load conditions as well as the operating temperature, for better results.

Full spark timing is retarded except when the car is in high gear and the engine is at normal operating temperatures. At all other times, the spark advance is retarded to one degree or another. This is achieved by a thermal vacuum valve, a high gear detecting switch and number of hoses. This system is called ' Transmission Controlled System ' (TCS)

In some later model cars, a system which works solely from engine coolant temperature changes has been incorporated. System includes a thermal vacuum valve, a vacuum delay valve and attendant hoses. This system is called 'Spark Timing Control System' (STC). This system performs the same function as TCS i.e., to retard full advance at times when high levels of pollutants would otherwise escape into exhaust gases.

Generally both the systems are trouble free unless wiring or hoses are mishandled or disconnected.

Catalytic Converters:

Catalytic converters are being widely used the world over This system is designed for the oxidation of pollutant gases escaping primary combustion in the engine within the exhaust system. The catalytic converter is a muffler shaped device installed into the exhaust system. The temperature for bulk gas oxidation and reduction of hydro0-carbon gases, carbon mono-oxide and nitric oxide gas is about 600 to 700 ° C. The temperature of exhaust gases in the exhaust system is lower. The catalytic converter oxidizes and reduces all the three pollutant gases at lower temperature because of catalytic chemical reaction. Catalytic converter becomes effective in the temperature range between 250 to 300 ° C. The use of catalytic converter has become wide spread as it is an effective means of controlling pollution.

The converter is filled with a monolithic substrate coated with small amounts of platinum and palladium through a catalytic action, a chemical change converts carbon monoxide and hydrocarbon into carbon monoxide and water. Such a converter is called a two-way catalytic converter.

A three-way catalytic converter is installed on cars to check pollution. Such a converter uses thin coating of platinum, palladium and rhodium over a support metal and acts on all three major constituents of exhaust pollution: hydro-carbon, carbon monoxide and oxides of nitrogen, oxidising these to water, carbon-dioxide and free hydrogen and nitrogen respectively.

Necessary oxygen required for catalytic reaction is provide to the converter by air induction system. Air is lead into the through an air-induction pump or by pulsations in the exhaust which cause air section. No regular maintenance is required for the catalytic converter system, except for periodic replacement of the air filters of induction system, is provided. Catalytic converters may also require replacements at about 80,000 kms or more. The catalysts durability is affected by engine durability. Any engine malfunction that will expose the catalysts to excessive amounts of unburnt fuel will severely overheat the catalyst and impair its efficiency and its life.

Water jet based tooling strategies for microproduction

2008-09-08T21:35:00.001-07:00

The main objective of this contribution is to present a new tooling strategy based on the application of WJ machining, which would allow relatively quick and cost effective production of prototype micro components. In the first step the tool for MEDM is produced in copper by WJ technology. Then the copper tool is used by MEDM technology to produce the final tool in tool steel, which may be further used for replication processes such as hot embossing, pressure molding and others. The complete process chain is shown in Figure.

The proposed tooling strategy offers high flexibility and cost effectiveness. Additionally, it provides more freedom and testing opportunities during the development of new micro devices. The most common used tooling strategy is direct manufacturing of the tool by micro milling. When the features of the tool are rather ribs then grooves, the tooling strategy proposed in Figure has a great advantage over micro milling tool manufacturing, which is the most common used tooling strategy. In the latter case, an end-mill with a relatively small diameter has to remove relatively big volume of the tool which is time consuming and not cost effective.

The main application field of the proposed tooling strategy is the design and development of micro-fluidic devices. Typically, these devices require a well-controlled geometry and surface roughness. With this technology these devices can be manufactured relatively fast and in a cost effective way. Therefore many new concepts and designs can be experimentally validated during the development phase in order to improve the performance of the final product. In the actual context of R&D, flexibility in the manufacturing process enables variety and innovation in the design. The proposed tooling strategy consumes most of its machining time in MEDM machining while WJ machining accounts just for a small portion of the total machining time. However, facing this sequence of different processes, WJ machining of the MEDM tool has an important influence on the final result.

Figure: The process chain of micro-fluidic channel production

On-line selection of the rough machining parameters upon the eroding surface size

2008-09-08T21:24:00.000-07:00

The material removal rate and the surface roughness increase with increased power in the gap. In this way, rough and fine machining is distinguished. When rough machining is performed, the material removal rate should be as high as possible, while the achieved surface roughness does not play an important role.

The EDM process stability is determined by the proportion of harmful discharges in the gap between a workpiece and an electrode, i.e. arc and short-circuit discharges, which not only lower the material removal rate, but also increase the electrode wear. The process is more stable in the case of lower proportion of the harmful discharges. The main cause for unstable EDM process is the contamination of the gap with discharge products. But the surface power density in the gap also affects the process stability. To achieve the highest material removal rate, the roughing setup parameters should be tuned to the eroding surface size. The eroding surface is a projection of the engaged surface of the electrode to the plane perpendicular to the machining direction as shown in Figure. This was analyticaly prooved in the In general, the engaged surface is not plane and the eroding surface size changes with the depth of machining. To select the appropriate roughing setup parameters at any machining depth, the eroding surface size has to be determined on-line.

Figure: The eroding surface is a projection of the engaged surface of the electrode to the plane perpendicular to the machining direction.

Voltage and current in the gap define electric power in the gap (P=UI). There exists the optimal set of the setup parameters' values to obtain the certain power in the gap and the discharge voltage is nearly constant at all machining regimes, thus the power in the gap depends only on the current in the gap. In the literature, the boundary surface current density is given rather then boundary surface power density and it is stated that stable EDM process is achieved when the surface current density is less than 0.1 A. The relation between the surface current density and the material removal rate Vw is presented in Figure. At constant eroding surface size A1, the material removal rate increases with increased surface current density until the boundary surface current density is reached. Higher surface current density causes unstable machining process and the material removal rate decreases. When greater eroding surface is employed (A2), the higher current is needed to reach the boundary surface current density, thus the material removal rate is higher compared to the material removal rate at eroding surface A1.

To select the appropriate roughing setup parameters when eroding surface size varies during the machining, the eroding surface size has to be determined on-line.

Figure: Material removal rate Vw versus the surface current density

For on-line detection of the eroding surface size, it is necessary to monitor the appropriate process quantities z. Proper evaluation of the process quantities is the key to gain suitable process attributes x for the determination of the eroding surface size. The process attributes are the inputs into the model for the selection of the optimal rough machining parameters(see figure)

Figure: On-line selection of the roughing setup parameters of the EDM process

Design adaptation system for machining by EDM process

2008-09-03T10:53:00.000-07:00

The EDM is still very often used, specially in tool engineering where tools for mass production are produced. The shape of the tool is a negative image of the product and it has to be easily and cheaply made. To adapt tool design, and at the same time also product design, for easier manufacture of the tool we distinguish two levels: design and manufacturing level. On the manufacturing level a manufacturing technology for tool machining is determined. There is always feedback information from manufacturing to design level to change the tool and the product design according to easier (cheaper) manufacture of the tool. The designer considers the suggestions of the technologist and together they find the best design by taking into account also the demands for the product and the tool.

A system for segmentation and determination of a proper machining process for machining each segment of the tool separately, has already been developed at the Faculty of mechanical engineering. A high speed milling (HSM) and the EDM process are considered as two machining processes for making each segment of the tool. In our work the system for adaptation of the product to easier tool manufacture with EDM process was developed. It is designed for designers to establish critical parts of the product from the point of view of machining the tool with EDM process. With these information the designer can adapt the critical parts of the product design without the tool engineer. By using the system it is possible to reduce number of information from manufacturing to design level and to reduce the time necessary to manufacture tools. In reality it is impossible to eliminate all information from manufacturing to design level or to replace the tool engineer with an expert system.

Figure: Scheme of the design adaptation system for EDM.

EDM controller

2008-09-03T10:48:00.000-07:00

EDM process is very unstable, especially when working in fine regime. Often surface damage occurs by arc discharging. Unstable working is unavoidable, so the process is forced to work in liable, but effective region of working. The process is run by operator who overlook it and make a feedback control. Automation of process is the final goal of our research.

A strategy of controlling is a very important part of stable EDM action. It is complex and operator can master it after a long period of learning and collecting his own experience. Operator's knowledge and technological knowledge together with technological receipts can be added to adaptive controller which must be able to accept both of them. We are working on development of such a controller (Fig.). All basic function of controller (identification, reasoning and control) are computer made. Identification of process is well done by computer. The same is with reasoning, based on qualitative and probability assessment, which is usually reserved for human. Operator-to-computer communication is possible and controller can be improved. While the split between human way of reasoning and reasoning algorithms, a strategy of controller is developed by artificial intelligence (learning by example). Method FORS (First Order Regression System) is used. Rules IF-THEN-ELSE can be obtain by human demonstration of process leading. By this method, operator knowledge is transformed to algorithm used by computer. Rules IF-THEN-ELSE are human understandable, too.

Figure: Adaptive control system for EDM process control

EDM small hole machining

2008-08-30T05:01:00.000-07:00

The use of EDM process is in parallel to development of new technologies. One of these new technologies is small hole drilling by EDM.

Small hole drilling (d <2mm,>h/d > 10) is a huge technological problem. By drilling problems arise with chip transport and heat deviating. Drilling tough and rigid materials on depth is also a problem. By other non-conventional processes like laser and electron beam small holes can also be made, but these processes are still to expensive for common use. EDM is the best choice for machining electric conductive materials, especially for holes of uncommon shapes.

Technological problems arise:

precision of electrode machining
electrode positioning
high dielectric liquid pressure
electrical parameters selecting
electrode guide

Figure: EDM small hole sinking design

Figure: Small hole made by EDM machining

By research carried out and by our experience we obtain technological properties of the process and directions how to control it.

Electrical discharge machining (EDM)

2008-08-27T07:05:00.000-07:00

Electrical discharge machining (EDM) is a non-conventional process of machining. the development of the EDM technology started in the forties. Since then, it is the most important machining process in tool engineering. General advantages to other machining processes are: accuracy, surface quality and the fact that hardness and stiffness of a workpiece material is not important for the material removal. The EDM has become mature technology but the researches and improvements of the process are still going on. The main reason why, is that there still does not exist a machining process, which could successfully replace the EDM.

Crater to pulse classification

By research on basic principles for material removal in EDM process we try to improve effectiveness of the process. A single discharge - an unit event of EDM process was researched (Fig. 1). The correlation between electrical parameters and surface give us important information about process.

Figure: Time dependence and surface crater of single discharge made by EDM machining

exhaust flow in an automobile-the future

2008-08-13T04:22:00.000-07:00

These days, you can’t think of exhaust system as just some crude plumbing hung on as an afterthought to pipe away air, heat, and to keep those decibels down. It’s become an integral part of the powertrain and under-car architecture critical to performance, fuel efficiency, and emissions reduction. There has already been development of low or zero emission vehicles already in the recent auto shows by major automobile manufacturers like Honda (Natural Gas vehicle) and Ford (Electric car). A oncoming development I would like to discuss about in this section is the Electronic Muffler.

As an executive with Walker, one of the major muffler makers involved in developing the concept puts it, "After the introduction of the catalytic converter in 1975, this is probably the most revolutionary technology that’s happened to exhaust systems in the entire history of the automobile."

While the idea is surprising, the basic principle isn’t hard to grasp. From a microphone and a crankshaft speed/position sensor, the computer receives input on the pattern of pressure waves (that’s what sound is, after all) the engine is emitting at its tail pipe. This data is processed using patented algorithms, which produce mirror-image pulses that are sent to speakers mounted near the exhaust outlet, creating contra-waves that cancel out the noise. In other words, the sensors trap the waveform signature of the engine, and the speakers generate anti-noise waves 180 degrees out of phase with the gas waves. This destructive interference idea is sort of like fighting fire with fire. The sound waves collide, wiping each other out. It doesn’t just mask the noise, it actually removes sound energy from the environment, and from the law of conservation of energy where the energy has to turn up someplace, all that is left is low-level heat.

Although electronic mufflers are not widely used (if ever) at present, they may be installed on vehicles in the near future. In 1989, a joint Electronic Muffler System development program was started and the University of Michigan’s Delphi study predicts that 20% of the cars produced in North America will have electronic mufflers by the turn of the century.

Well, if the electronic mufflers are really as effective as they claim to be and they were available now, we could build a perfect exhaust system using the setup described earlier with the addition of an electronic muffler then the problem of loud exhaust wouldn’t exist. But then again, by the time the electronic muffler is out in the market, technology might have other improvements of the exhaust system and we will again try to match components to produce more horsepower and attain better gas mileage (efficiency).

In conclusion, we go back to the basic analogy of the engine as a pump; the more air that can flow freely, the more horsepower that can be optimally achieved from the engine. This research paper has only dealt with how to get air OUT of the engine. It is important to note that the INFLOW of air also influences the output performance of the engine. As a matter of fact, we need the inflow of air before the outflow process starts. In brief, the inflow of air can be modified by removing the intake resonator, or even removing the entire airbox and installing a pipe with a cone-shaped filter at the end. There are many other ways to improve air inflow, but I shall not discuss about them as it would be outside he scope of this paper if I’m primarily interested in the outflow of exhaust gases.

Also, it is important to note that "horsepower" is a unit of energy over time. So the more energy it requires to do something, the less power you will get out of it. In other words, it is because motorcycles are lighter than cars that they can achieve similar if not higher horsepower. That is why race cars are stripped of the interior, air conditioning, and any other unnecessary weight. This way, there will be less weight to move, meaning less energy required and thus more power produced. That is why automotive engineers are trying to use materials of lighter weight, like plastics and carbon fiber.