Waterjets can make parts to very tight tolerances and today some machines can create parts with a tolerance of as small as ± 0.001″ (0.025 mm), although ±0.002″ (0.05 mm) is perhaps more realistic in most cases. Achieving these tolerances requires an understanding of the factors that affect precision waterjet machining.
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. Over the years, progress has been made in improving the ability of waterjets to achieve high tolerances. A machine from 1990 was capable of tolerances of 0.060″ to 0.010″ (1.5 mm to 0.25 mm), between 10 and 60 times higher than today’s machines.
Positioning accuracy vs. tolerance
Positioning accuracy refers to how precisely the abrasivejet head can be positioned on the X-Y cutting table, and specifically how reproducible that positioning is. Being able to precisely position the cutting head is an important first step to achieving high tolerances, but it’s only the first step.
When purchasing a machine, be sure measure parts that come off the machine you are going to buy. Some manufacturers 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.
A machine that can position the cutting head to within 0.001″ (0.025 mm) might still produce parts accurate to only 0.005″ (0.12 mm).
What affects precision?
There are a number of factors that affect the tolerances that can be achieved on a part. Some of these are easier to control than others.
The hardness of the material affects how precisely you can machine it. Harder materials typically exhibit less taper, and taper is a big factor in determining what kind of tolerances you can hold. You can compensate for taper by adjusting the cutting speed or tilting the cutting head opposite of the taper direction. See Introduction to tilting.
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 more taper around curves. Material thickness also affects both the amount and the type of taper. Materials thinner than 1/8″ (3 mm) tend to exhibit V-shaped taper, while very thick material can show barrel-taper.
Obviously, the more precisely you can position the jet, the more precisely 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.
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), as shown in a severe case below.
Witness marks on a part
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 will be uneven. Witness marks can also be caused by poor fixturing of the material.
Control of the abrasive jet
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.
Waterjet machining has certain built-in limitations to tolerance. While you can compensate for them to some extent, they will limit the precision of your final parts. Following is a discussion of the major areas that limit precision with waterjet machining.
Also note that “precision” is a relative term–waterjets can create parts with a tolerance of as small as ± 0.001″ (0.025 mm), although ±0.002″ (0.05 mm) is perhaps more realistic in most cases.
Because the waterjet is a “floppy tool,” it tends to wander as you are machining. On straight line cutting, this translates into a “lag” as shown in this illustration:
Note that the jet lags behind the moving nozzle
This lag can usually be ignored when cutting in a straight line, but becomes critical when near a corner. As the jet approaches a corner, it becomes necessary to slow the motion down so the bottom of the jet can catch up to the top, and be perpendicular to your material. If you don’t slow down, you will have an ugly corner indeed.
If you accelerate quickly when coming out of a corner, the jet will kick back, and mar your part.
Most modern waterjet controllers compensate for this behavior automatically. The difference between manufacturers is the extent to which their software accurately models the behavior of the jet. Some software does a better job than others.
The important thing if you already own a waterjet is to make sure you take advantage of software updates offered by the manufacturer. These updates can incorporate newer models, which can improve the accuracy of your waterjet without changing any of the hardware.
Abrasive jets have a tool width that typically ranges between 0.020″ (0.5mm) and 0.040″ (1.0mm), depending on the mixing tube diameter. As you slow down to make a nice corner, the kerf grows slightly. The amount it grows is a function of how much you slowed down, while the amount you must slow down by is a function of your material thickness. In other words, the thicker the material, the harder it is to get precision in the corners.
Most controllers compensate for kerf width automatically, but don’t fully compensate for the kerf growing as a function of speed.
Kerf width depends on the nozzle you are using. With low horsepower pumps, you have a narrower kerf. With high horsepower pumps, you have a larger kerf.
Pure waterjets have a smaller kerf than abrasivjets.
Taper is the difference between the top profile of the cut verses the bottom profile. As the waterjet stream rapidly erodes the material, it may do so unevenly, resulting in an uneven edge, similar to a canyon formed by a stream. For more information on taper, see All about taper.
Taper prevents your part from having a straight edge and will affect precision. For maximum precision, you should invest in a tilting jet head to eliminate taper.
Lead-in and Lead-out
It takes time, and distance, to pierce the material before you can begin making the part. This section of the tool path is called the “lead-in” (at the beginning of the part), and a “lead-out” at the end of the part, where the cutting continues while the water pressure is reduced. The size of the lead-in and lead-out will depend on the thickness and type of material, as you need to wait for the bottom of the jet (where the jet exits the material), which will lag more in thicker materials.
Even with a lead-in and lead-out, it is difficult, and often impossible to avoid a small witness mark in your final part.
The precision of the positioning system sets a practical limit on how precisely you can make parts. If the machine positions the head to a precision of 0.001″ (0.025 mm), then that is the limit to the precision of your final part.