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Limitations of waterjet cutting, Part 1

Like any tool, waterjet cutting has limitations. The good news is that the limitations of waterjet cutting are easy to understand. This makes it possible to develop your design around those limitations (which is what this article series is about).


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Absence of depth control
This is a limitation for many people, but there are design techniques that can help you compensate, as you will see.
In my experience, it is almost never practical to use a waterjet to engrave or cut only part way through a material. For one thing, it is difficult to get accurate depth control. Another problem is that the area which is removed is limited to the thickness of the waterjet cutting stream, typically 0.04 inches or 1 mm.

Thus, for a part which requires marking or etching, we would typically use laser engraving or printing to apply the design.
A design which would require a slot or pocket feature would be better done though modifying the design (see the technique described in our stacking article) or, as a last resort, via a secondary operation such as milling.

Kerf width
Kerf refers to the material removed by a cutting implement. For example, imagine that you are cutting a 2 foot board in half with a hand saw. If you were to join the finished halves back together, you will find that the total length is less than 2 feet due to the material removed by the saw. The amount of material removed and turned to sawdust would be roughly the width of the saw.
The width of the waterjet cutting stream is typically 0.04 inches or 1 mm. The less common microjet waterjet cutting heads have a smaller nozzle and stream width of 0.02 inches (0.5 mm).
The shape of the waterjet stream means that inside corners cut on the waterjet cannot be perfectly sharp. For some applications, this is not significant, but if you want to fit a part with a sharp outside corner into a waterjet cut sharp inside corner perfectly (for example, when using the tab and slot technique) it is best to make the design with a rounded corner that “overcuts" the corner by a little bit. For an 0.04 inch wide waterjet stream, I would recommend designing for an inside corner radius of slightly larger than the stream radius: 0.025 or even 0.03 inches (0.05 or 0.06 inch diameter).


Illustration : Diagram showing a close up of an inside corner which is unreachable due to the width of the waterjet
cutting stream.This corner will be slightly rounded in the final part.


Illustration : If you must remove the material from an inside corner, you should
create a cut line which cuts beyond the corner.

Similarly, slots narrower than the kerf width of the waterjet stream cannot be cut. If you have a narrow slot which leads to a larger open area, this means that the larger area cannot be reached by the waterjet cutting stream either, as shown in the diagram below.


Illustration : When you have an area too narrow for the cutting stream,
there is no way to reach larger connected areas.

Compensating for kerf

Most waterjet cutters, including Big Blue Saw, compensate for the kerf as best as we can in order to produce a part that’s as close to your CAD design as possible. In other words, you don’t have to worry about making your drawing larger in order for the part size to come out correct. So, for instance, if you send us a CAD drawing with a 3 inch diameter design, your final part will come out to be 3 inches in diameter, not 2.96 inches. (This is a change for many people who are used to working with most laser cutter shops, where they typically do not compensate for kerf. When doing laser cutting at Big Blue Saw, we also compensate for kerf in order to keep it consistent with the waterjet cutting process.)