Waterjet Accuracy

Nearly all waterjet manufacturers, as well as many waterjet cutting shops, advertise accuracies ranging from +/-.0005” to +/-.0015”. It is very necessary to point out that this is invariably referring to the positional accuracy of the machine rather than part accuracy, and there can be a massive difference between the two.

There are many factors that play into part accuracy 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 it wouldn’t do you a bit of good if your software wasn’t capable of accurately compensating for the many variables that are presented by cutting with a flexible tool. The most precise waterjets on the market are achieving that status not so much by the positional accuracy of their machines, but more through the knowledge they have of the behavior of the stream and their ability to accurately compensate. Poor compensation can result in a part that is very accurate at the top of the part but badly out of tolerance and ugly at the bottom of the part.

So, if you go out and buy a good waterjet, then you should be able to put a low-paid flunky on it and put out great work, right? After all, it’s a computerized machine! Although this mentality appears to be prevalent, it is very much of a 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 are ornery by nature. In the same way that a rocket is basically propelled by a controlled explosion, waterjet cutting is nothing more than 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 extremely accelerated manner. Fortunately the technology already exists, and the complex mathematical models that are

used to determine the machine’s movements are of no 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 is able to take a technical approach and work through issues logically. Many waterjet shops apparently do not understand this; They rather view their waterjet as a beast that cannot be understood or tamed, and eventually make peace with the monster by never requiring very much from it.

So what kind of tolerances can a waterjet produce? As you can see, there are very many variables that play into the answer to that question. Assuming that you have a good machine and a good operator, it is possible to get tolerances of about +/- .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. We generally don’t guarantee anything tighter than +/- .005” even though the parts often come out better than that. 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. Where +/- .005” is attainable, decreasing the tolerance to +/- .010” would already make a significant difference in price, and +/- .015” would generally be attainable without even trying.

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 produced some massive 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 in order to continue providing our customers with everything that waterjet cutting can offer.