Saturday, September 09, 2006


Making gearmotor couplings from scratch for Tommelise...

The Solarbotics GM gearmotors have a sturdy but very short (5.4 mm) plastic drive shaft. Obviously, Adrian's coupling design for the Mk II would work beautifully. Unfortunately, I'd have to redesign it for the GM8 then ship the specs over to Adrian and then wait for a few weeks for them to be made on the Stratasys and shipped. I'm just not patient enough for that way of doing things, so I invented another coupling.

First, I took a length of 1/4 inch round rod and used my old Demel tool with a grinding wheel on it to carve two flats onto the end of it leaving the same width between the two, 0.196 inch as the drive shaft on the GM8 has.

Then, I took a coupling nut for the threaded rod I'm using. In my case using 3/8ths inch threaded rod is wonderful in that it is bigger than the 7 mm drive shaft. I used a standard 3/8-24 bolt to plug the bottom, filled the top with CAPA and popped in in my toaster oven at 300 degrees F.

After about ten minutes the CAPA had gone transparent and slumped, so I added more and popped it back into the oven. I had to do that once more before I had the molten CAPA level where I wanted it.

At that point I inserted the makeshift mandril that I'd made into the molten CAPA and then ran cold water over it to solidify the CAPA.

I then checked for a fit with the GM8 and as I expected the slot was too narrow. I heated the mandril with my paint stripper (which is basically a hair drier {which would have done as well} on steroids) and widened the slot slightly till the GM8 drive shaft fitted properly.

The process seems to make a good coupling in about 20 minutes for the first one for about US$0.79. :-)

Polymorph and threaded couplings for tying the motor to the rod....

That's quite a good idea, and I'm right up to that point anyway
LOL! Glad to be of assistance.

BTW, if you don't get it exactly straight, heat up the mandril and reshape the slot.
Niffy trick, maybe a locknut of some type could be tightened against the back end of the coupling to resist it slacking off when the motor is spinning the "wrong" way. Or maybe the CAPA embeded in it is already acting as a locknut?
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