While working in the machine shop the other day, curiosity got the best of me. I was thinking about how to add fasteners to my printed part, but with a minimal amount of work. Fasteners can be drilled and tapped, Helicoil-ed, or ultrasonically welded into ABS parts. If the thread is large enough, it can even be modeled on the part directly, but what about smaller fasteners?
Typically, to create a threaded hole on a 3D Printed part, we would need to perform a few operations on another machine. First, the part and the hole position would need to be located precisely, straight and true (“tramming and indicating”). Second, three tools need to be used: a center drill for the pilot hole, a tap drill of a specific size, and finally a tap. All these tools need to be used in the exact same location in order to generate an accurate screw thread profile. This is standard operating procedure with a CNC machine. However, you’re probably using a 3d printer to avoid the machine shop in the first place. You can save a few steps by inserting the tap drill size directly into the 3d print, with impressive results.
By printing parts with the tap holes already modeled in, we can eliminate several extra steps in the process, and the need of using a separate machine. Hand-tapping the precisely located holes is adequate for creating a functional thread.
The thread felt a little loose, indicating an oversize minor diameter. To remedy that, the tap hole can be made slightly undersize. With a smaller tap hole, the slop wasn’t so dramatic of a concern on the larger threads (the largest here being 1/2-13). Loose fit wasn’t a problem at all with the two tapered threads in the upper left hand side, a 1/8-27 NPT and 1/4-18 NPT.
By printing the tap holes directly into the part, we avoid several steps typically needed to insert threads into 3d printed parts, only needing threading. All of these threads were created with a hand tap, and as long as the tap goes in relatively straight, it produces quality threads without all the usual effort.