Many waterjet manufacturers advertise accuracies ranging from +/-0.0005” to +/-0.0015”. It is necessary to understand that this is referring to the positional accuracy of the machine rather than the accuracy of the parts it cuts, and there is a significant difference between the two.
Many factors play into the accuracy of parts cut on a waterjet—likely more than for any other cut process—because of the fact that the waterjet stream is not rigid. You could have the most precise gantry system in the world, but you wouldn’t be happy with the parts it cut if your software and cutting head couldn’t compensate for the variables that are presented by cutting with a water stream. The most precise waterjets can only achieve their precision through this compensation. Poor compensation can result in a part that is quite accurate at the top of the part but misshaped and inaccurate at the bottom.
So, if you go out and buy a good waterjet, then you should be able to put any rookie on it and put out great work, right? After all, it’s a computerized machine! This is a common misconception. Producing tight-tolerance waterjet cutting has everything to do with having a skilled operator. The operator must know how to nest parts and create good tool paths in order to maintain stability during the cut; how to properly fixture the material so it doesn’t move or vibrate; how to inspect a part and know what adjustments to make to improve tolerances; how to troubleshoot a waterjet that’s not cutting properly; and how to maintain the machine so it continues to be precise.
Waterjets tend to be troublesome. In the same way that a rocket is propelled by a controlled explosion, waterjets cut by high-speed erosion. The same process that carves deep canyons and hollows out caves in solid rock is being harnessed through waterjet technology to do the same thing in a very controlled and accelerated manner. Fortunately, the technology already exists, and the complex mathematical models that are used to determine the machine’s movements are of little concern to the operator. However, because of the complexity of the system and the extreme forces involved, problems can at times develop that are very difficult to troubleshoot. There is always a solution if you have an operator that can take a technical approach and work through issues logically. Because of the difficulties of waterjet cutting, many shops finally view their waterjet as a beast that cannot be understood or tamed, and they no longer require very much from it.
The good news is that waterjets that are properly mastered can consistently produce accurate parts. They do live up to their hype. What kind of tolerances can we expect? Assuming that you have a good machine and a good operator, it is possible to get tolerances of +/- 0.003” or possibly even better if the part is not very large. That being said, it’s unlikely that you’ll find a waterjet shop that can actually hold tolerances that tight. At AquaJet Services, we usually don’t guarantee anything tighter than +/- 0.005”. We are comfortable holding that tolerance up to at least 1” thick or possibly even thicker, depending on the part. Above that, the tolerance increases the thicker you go. The achievable tolerance depends very much on the type and shape of the part as well as the type of material.
It is important to understand that better tolerances require slower feed rates and more setup time, which directly translates to higher costs. While +/- 0.005” is attainable, loosening the tolerance to +/- 0.010″ already reduces the price significantly. If there are no stated specifications to be met, the parts should be within +/- 0.015″.
Waterjet technology is still relatively new when compared to other machining processes, but it has been growing by leaps and bounds. The last decade has brought many improvements, and it appears that the next 10 years will likely do the same. We are excited to be involved in a growing field like this, and we plan to make sure that we stay on the cutting edge to continue providing our customers with everything that waterjet cutting can offer.