Coupling

I picked up some 5/16" threaded rod couplings (7/8" long) and some 6-32 grub screws at the hardware store. The inside diameter of the threads is just slightly larger than the motor shaft. I bored out one end with a 5/16" bit to a depth of about 1/4", then drilled and tapped two 6-32 holes along the length. The grub screw for the 5/16" bore is about 1/8" from the end. I made two of these: one for the Y motor; one for the Z.

The image here is a test fit using some scrap rod. I've put the Y drive pulley on backwards to give myself more room for the coupling. The motor shaft isn't quite as snug as I'd like but it should do.

The current plan is to use the McMaster pulleys and belts. I've bored all six pulleys to 5/16". One of the Y pulleys is slightly off center. I may need to replace it.