Friday, December 12, 2008

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.

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