Bob La Londe wrote:

I broke a drive coupler on my littlest machine the yesterday day. It was a
pretty simple piece of plastic. Basically a split clamp threaded on one end
to go on the drive screw, and slip fit on the other end to go on the motor.
I hated the idea of waiting for one to come in, but I didn't have an acme
tap the right size to make one. I used your suggestion to try the old slot
cut bolt method with a piece of the acme rod I had left over from
retrofitting the machine. It didn't work very well. So I tried something
else. I ground four flats on the rod so that it look more like a square
thread file than a piece of threaded rod. That worked. I twisted the heck
out of it, but I made a replacement split clamp coupler out of a piece of
aluminum that way. It took a lot of back and forth twisting, but it worked,
and it's a very nice fit on the drive rod. I bet it would work great in
acetal plastic for making drive nuts.



I actually milled my acme rod to give it a profile similar to a tap, a
sharp cutting edge, with chip clearance ahead of it, and good support
behind it. You also need to grind in some taper so you are starting out
with a light cut and progressively cutting deeper. If you have a torch
and Kasenit you could do a quick hardening and touch up the edges with a
stone before use. I recall an issue of HSM had an article on making
taps. When tapping softer plastics, freezing them before tapping helps,
as does re-freezing and a second finish pass to clean up.