advantage of wjm over punch press

• 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

• 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

• 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

• 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

• 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

• 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

• 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

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

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

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

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

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

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

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?

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

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

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

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

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?

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

 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

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

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

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.