Accurate holes with waterjet cutting

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.

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.

Many waterjet cut parts are designed to be fastened together using bolts or pins. This comes up when using the using the stacking or t-nut construction techniques, as well as when using a waterjet cut part as a custom flange, bracket, bearing block, or control panel. When making a part which must accept a bolt or pin, you should modify your design to deal with the accuracy limitations of the waterjet process.

In short, you should plan to enlarge the hole with the right size twist drill or reamer. Since most of the material has already been removed from the hole, this is easy to do. The existing waterjet cut hole will guide the drill or reamer down through the material. It is easiest to do on a drill press, but it can even be done with a handheld power drill.

For instance, you might need a hole for a bolt with a 0.25″ (1/4″) diameter. Because the width of the waterjet stream can vary, you might end up with a hole 0.26″ or 0.24″ in diameter. If you have a 0.25″ post or bolt, the best thing to do is to specify a size of 0.24″ in diameter, then finish the hole with the appropriate drill or reamer. The exact size drill or reamer will depend upon how close of a fit you need: whether it’s an interference, close running, or free running fit. For most ordinary fastening applications, an ordinary twist drill will work.

If you don’t want to enlarge the hole with a drill, you’re going to have to specify a hole 0.262″ in diameter, and be prepared to accept that the bolt may wiggle around a bit in the hole. This is OK for many applications.

If you are unsure of the size of the bolt, pin, or post, you should double-check the diameter of the posts with a precision caliper or a micrometer.

Also, you should consider the effect of taper on your holes. For metal material that’s 3/16” (5 mm) or thicker in diameter, taper will be significant enough to interfere with most through bolts, as hole diameter on the bottom face will be smaller than the hole diameter on the top face. Again, it is probably best to undersize the original hole and finish it with a drill. If you are using low-taper waterjet cutting, this is less of a problem.

Tapped/threaded holes

For tapped holes where the amount of thread engagement is not critical, I similarly recommend undersizing the hole to just below the minor diamter of the thread, then enlarging the hole with a handheld power drill before tapping. Where you’re not concerned about the exact amount of thread engagement, you can usually get away with making the hole in your drawing exactly minor diameter of the thread, then tapping directly into the waterjet cut hole.


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