I'm in the middle of my MK-I build and I've run into a problem with one of my stepper motor couplers. A little background first:

From the start, I decided to go with 3/8" 4-turn ACME thread for my z-axis. At the same time I ordered my motors, I ordered a custom ACME thread coupler from dumpstercnc.com based on the spec sheet/line drawings from where I ordered the motors (web-tronics.com), which shows an 8mm shaft. (See attached pics of what the coupler looks like - these are dumpstercnc photos, not my exact setup)

Now, during the build, I discovered the motor shaft is not 8mm, it's 6mm, which means my coupler is way too big to fit on the motor shaft. So, to make up the difference, I wrapped a couple turns of electrical tape around the shaft to make up the difference and then mounted the coupler onto that. It fits nice and snug, but even with it cranked down tight, I get slippage between the tape and the coupler.

So my question is this: short of buying another whole new coupler, does anyone have any good ideas on how to take up the 2mm difference and also prevent the motor shaft from slipping?

A few notes: The motor shaft is round - no flats. These couplers are made of Durlin, so a hose clamp probably won't work any better than the built in screws. Also, I don't want to glue the shaft to the coupler in case I need to swap the motor out sometime in the future.

Any suggestions would be greatly appreciated!

Thanks,
Nunny Attached files

 

nunny, forget the electrical tape. It will never work with this stuff. Do you have thin aluminum that you can cut a strip off and rap it around the shaft and then insert it? About 1mm thick or less?

Yoram

 

Or you can get a brass or steel tube that is 6mm ID and 8mm OD. You'll have to heat the brass and press it on the shaft. This will give the best connection between the coupler and the motor. Anything else is a bandaid and will fail.

 

If you can't find the steel or brass tube Thewz talks about, the next best thing would be to get some .005" (.2mm) or less shim material (a thin feeler gauge would work) and wrap it around the shaft multiple times to make up the difference.
Third option (failing the prior two) would be to wrap the shaft smoothly with aluminum foil.

 

drill it and put a roll pin in it maybe

 

Man, you guys are good... all excellent suggestions!

Thewz, I think yours might work best, and I think I can improve on that idea... I've got a friend who's a machinist and I'll ask him to machine me something out of aluminum or steel with a 6mm ID/8mm OD. With something so small, he'll be sure to have some scrap to make it out of.

I'll let you know how it turns out.

Thanks all for great ideas and fast responses. I'm hoping to make more progress on my build this weekend.

Nunny

 

Cool, nothing like having a buddy in a machine shop....all of mine are techie type or mechanics....no machinists...

 

Nunny - I'd suggest asking your buddy to slit that tube open after he makes it, so it will squeeze open or closed as required to fit your shaft. Otherwise, you might be hunting for a 0.001" oversize reamer or something.

 

My buddy whipped up something for me this weekend in less than 10 minutes. I didn't see Dorsal's slit suggestion until now, but my buddy put the slit in there on his own for the same reasons Dorsal said. It fits perfectly! Initial testing shows no slippage. Thanks again all for the help!

Nunny

 

Not a bad idea randy. Not sure how feasible it is now that I have it all put together :? , but if I need to take it apart sometime, I think I'll definitely do that.

 

Cool! Glad it worked out.