Discussion about alternate uses for shaft collars in @Xenon27winter reveal reminded me of a project I was fiddling with in the before times of 2019. Drilling and tapping nylon spacers for use as connectors. Picked it back up today - some pictures:
The jig doesn’t have to be all that sturdy and could surely be 3D printed in plastic. I just have a wood shop so built it.
Procedure: Fence and stop on drill press to fix position of the jig. Insert spacer and tighten screw to hold. (I also used a wood scrap to hold the part down in the jig.) Drill through with 9/64" or #29 bit. Rotate the spacer 90 degrees and tighten screw into hole to align. Drill through. Remove and tap through each axis fully to match threads across.
You could also use the same process for shaft collars – I put some spacers in this jig and did just that. I think the nylon spacers are better because their diameter means you can fit them into the corner of structure and the tapped holes align with the structure holes. Also, the threads are very much like nylock nuts and hold screws well.
I meant to make wedges to put under the jig to make 3-holed connectors – one with 3 at 120 degrees and another with 135-90-135. Started plugging away at these you see and used all the spacers I brought home. But yeah, other useful angles would be possible.
Not sure a rivet nut would hold in the nylon, as they are made to clinch onto a piece of sheet metal, but a press insert might be the ticket. They expand and wedge into the hole tighter when a screw is installed: Zoro Select Press Insert, Reverse, 8-32x5/16 L, PK50 1GUC8 | Zoro
Is this out of concern that the nylon threads would fail? They seem very sturdy when a screw is fully in one of the threaded holes. There is no give when I flex hard on the screws coming out of the top center connector in the first picture.
The press fitting looks like it would fit, just not sure if you could put more than two in.
I would expect the screws to hold reasonably in the nylon, but for higher stress, or where disassembly/reassembly might be desirable, an insert would be good.
If the press inserts were used on a single axis, it would be unlikely that you would be able to thread a screw through both. Maybe you could press in one and incrementally press in the second to get the spacing correct.
I would think tapping the holes would make it weaker. Just having a slightly smaller than 5/32 hole (my preferred hole size for standard screws) would allow for the screws to self tap themselves and create a stronger connection.
I would expect you could just use the screws to make the threads. They are quite tight though when tapping 9/64 holes. It feels like the nylon is being both cut and compressed out of the way. When you insert a screw you get the same strong resistance as nylock nuts. Those nuts have a metal portion threaded before the screw engages the unthreaded nylon. It would be a little fussy to get a screw started parallel to the hole without any guide.
It requires more effort to tap a nylon spacer than the steel of a shaft collar.
