Motor Coupling

Earlier I posted about making a weak stepper motor.  I took a break from that and thought I would try working with a manufactured one first then returning to making a stronger one.  I got a NEMA17 motor and the first thing for me to figure out was how to connect the shaft to get it to turn something.

The motor shaft is 5mm in diameter.  I wanted to connect it to a 10-24 threaded rod as a worm gear.

I also picked up some 10-24 coupling nuts.

The the 5mm motor shaft is too large to go into the coupler, so I drilled halfway into the coupler from one side with a 3/16 inch bit.  3/16" is 4.76mm plus a bit more because of wobble from the drill.  Essentially this removed the threads and it looked like it could almost fit. 

In fact it was maddeningly close; less than 0.24mm is a tiny space.  I tried and tried to get it on but couldn't.  Then I had an idea!  I tossed the motor in the freezer for a while then heated the coupling on the eye of the stove with some pliers.  The kids already think I am nuts and are used to this kind of thing.  My hope was the cold would shrink the metal in the motor and the heat would expand the coupling enough to fit them together.  At first it wouldn't work; as soon as the metals touched they would equalize temperatures, but they kept just barely grabbing hold of each other and sticking for a moment before coming off.  I tried one last time and gave a light tap with a hammer (through a piece of wood--although you are not supposed to do this with stepper motors; the tolerances are very tight and can be ruined if it is bent or twisted at all) and it slipped on just a bit and held! 

For a few minutes, then popped off again.  This wasn't going to work.  My next size drill bit is 1/4" which is too large but I drilled it out anyway.  I the process I made some zinc wool.

Above is the nut drilled out to 1/4 inch on the near side.  It is still hot from drilling so I am still holding it with pliers. 

Now the hole is too large and I need a way to hold the motor shaft.  I drilled into the side with a 7/64" bit. 

Then I used a 6-32 tap to cut threads on the sides. 

And here it is with a 6-32 size nylon screw.

I drilled and tapped two holes, one for the motor and one for the rod. 

This works but the hole is so large the axis of the motor is shifted quite a bit from the axis of the threaded rod.  In the future I might try to get a drill bit in between 1/4" and 3/16", perhaps a 7/32".  7/32 inch is 5.56mm so that might work well.  A 13/64" would be even closer at 5.16mm. Also, I can cut the screws shorter so they don't stick out so much.