Limitations of waterjet cutting, Part 2

Hole size

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.)


Lead in / lead out

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