Friday, December 12, 2008

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.

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