Small parts must be connected, either to each other or to the sheet of material, to prevent them from falling into the waterjet tank when they are cut. This can mean that they must be broken off by working the part back and forth until it snaps off from the tab; the part is left with a vestige of the original tab. At Big Blue Saw, we offer various finishing options, like Basic Finish, to remove the tabs for you when necessary.
Below is a set of waterjet cut stainless steel parts which shows what this looks like.
Illustration : Tabbed parts with US Quarter (24.26 mm/0.955 inches diameter)
Parts cut using ordinary waterjet cutting will have a slight taper along the cut edge of the part, typically between 0.0005 inches and 0.01 inches. This is due to the waterjet cutting stream spreading out as it gets farther from the nozzle. The top face is cut accurately, and the accuracy gets worse as you get toward the bottom face.
With a stream width of 0.04 inches, you might think that this would also be the smallest accurate hole that can be made with the waterjet. But this is not so. The waterjet stream must break through the material before cutting along the cut line. In this process, called piercing, the machine sends a powerful blast of water and abrasive at the material surface in order to create the initial hole.
This initial hole is fairly irregular. Once the material is pierced, however, the cutting is much smoother. If you want a hole to be reasonably round, I recommend a diameter of no smaller than 0.1 inches. Of course, a waterjet can make hole smaller than that, but be aware that its diameter and shape will be proportionally less accurate.
In the photos below, you can see holes of various sizes cut by a waterjet in ¼ inch thick aluminum. The part is designed so that equal sized round holes are arranged in columns. You can see the irregularities on the closeups of the holes. These irregularities make up a higher fraction of the diameter as the hole size gets smaller. On the right is a pierce only hole, the smallest hole that the waterjet can make.
Illustration : Various hole sizes, arranged in columns. From left: 0.25", 0.1875",
0.125", 0.1", 0.05" and a pierce only hole approximately 0.04" diameter.
Illustration : Close up of the 3 smallest hole sizes from above.
Positioning of the waterjet stream is generally very accurate; close to 0.001 inches. However, several factors can affect the accuracy of the part, including wear on the mixing tube, vibration within the part, and marks created by lead-in and lead-out, as shown below. Thus, we typically quote +/- 0.005 inches (5/1000 of an inch) as our accuracy. This means that you should not depend upon edges of features to be more accurately placed than 0.005 inches. Holes may be up to 0.01 inches smaller or larger in diameter than your specification. (See the section on creating accurate holes later in the book.)
You will occasionally see small indentations where the waterjet stream begins and ends cutting a piece.
Illustration : Indentation at lead-in/lead-out point
Illustration : Indentation at lead-in/lead-out point
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).
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 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.
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.)