3/8" - 3/16" coupler

My repstrap design calls for a stepper motor to directly drive the threaded rod for the z-axis (like darwin).Unfortunately, I have no access to printed parts. My motor shaft is 3/16" and my threaded rod is 3/8". I was thinking about finding some way to attach them using a coupling nut and drill a hole in the threaded rod and coupling nut. My motor shaft has a hole in it that seems to be made for something like this. Is there some sort of coupling nut I can buy that has two different diameters?

Have you tried using tubing like the McWire design uses? Something like a silicone tube with a thick wall and 3/16 inner diameter (maybe a little less for tighter grip) could be stretched or possibly drilled out on the 3/8 side.

---

----
[www.binaryconstruct.com]

that's a really good idea! thanks. I should have looked at the mcwire a little more

The silicon tubing method mentioned above works well.
There are dual dimension shaft couplers available, but they are not cheap.
If you have the tools and patience you can make your own with a piece of
scrap 9/16" rod about an 1" long. Just drill a 3/8" hole in one end, a
3/16" hole in the other and a couple of small set screw holes to secure
the coupler to the shafts. The trick is getting those shaft holes exactly on center.

Another option would be turn the end down on the 3/8" shaft to 3/16" and then just use a 3/16" coupler. This will lower the amount torque load that the shaft can handle, but I think the z-axis torque is low enough to be supported by only 3/16" steel.