When a CNC motor coupling goes bye-bye

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vederstein

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I was using my CNC'd Mini-Mill for machining a walking beam for an engine to which I'm making. Near the end of the NC program, the Y-axis motor coupling snapped. Losing an axis on a three axis system isn't good. The mill then promptly decided to mill right through the beam web. Ugh....

I went though my stash of junk, and found a Oldham coupling that I could adapt to the shaft sizes and continue the project.

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...Ved,
 
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That is a lousy thing to have happen. Lots of folks say the spiral cut couplings can introduce errors, but I haven't heard of one breaking in use until your post.

Several of my machines have this style coupling. Guess I'll be doing an inventory and making some measurements to get rid of the ones on larger machines at a minimum. Not sure if I should say thinks for the heads up or Phooey! :cool:

Take care,
Stan
 

vederstein

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If your machines are commercial units, the spiral cut coupling is probably ok. Being that my failure was on a Mini-Mill that I converted to CNC using cheap equipment, I assume I have shaft alignment issues that caused the premature failure.

Spider couplings with zero backlash do exist and I use them on the five axis gantry robots I design at my workplace. I will note that at work we no longer use spiral cut couplings at work.

The Oldham coupling I had laying around probably isn't the best because Oldhams do have some backlash, but it's what I could come up with in short order.

...Ved.
 
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If your machines are commercial units, the spiral cut coupling is probably ok. Being that my failure was on a Mini-Mill that I converted to CNC using cheap equipment, I assume I have shaft alignment issues that caused the premature failure.
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...Ved.

Greetings,

Thanks for the info. My stuff is far from commercial, the particular units with these are open builds CBeam and Lead1010 routers, about as far from commercial as you can get without being in the lowest end :) My Sherline CNC stuff uses the Sherline motor mounts and couplings and have been no problem. The other stuff has different couplings, still hobby grade machines. With the current pricing on machinery I'm thinking that I'll be satisfied with what I'm lucky enough to have and leave it at that. I just looked at the recent prices on Grizzly G4003 lathes, my goodness, should have bought one a few years back. Prices have doubled... Think I'll just enjoy my peaceful retirement and putter with what I have and can make or modify!:)
Take care,
Stan
 

quickcut

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I had that exact issue with the same type of coupling on a cnc plasma cutter. Of course it happened on a weekend and an urgent job. My solution was to use two simplex sprockets with a duplex chain connecting them. Worked well and I never replaced it again. I used 04-b2 chain 5.0mm pitch I think.
charles
 

Bentwings

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I was using my CNC'd Mini-Mill for machining a walking beam for an engine to which I'm making. Near the end of the NC program, the Y-axis motor coupling snapped. Losing an axis on a three axis system isn't good. The mill then promptly decided to mill right through the beam web. Ugh....

I went though my stash of junk, and found a Oldham coupling that I could adapt to the shaft sizes and continue the project.

I have a number of Lohan couplings of different sizes I think they are stronger at least in small sizes . I think we broke 3 small beam couplings attempting to bore them out for specific purposes they generally are 6061 or 7075 with some even 2024 aluminum but in any case the actual beam is pretty thin. Steel might be better but I like Logan couplings. You can get softer spiders if necessary but I’d use urathane in cnc they are really hard to assemble in small sizes but work well. The beam couplings and diaphragm couplings can get expensive but then destroying a one off part is a bad deal too. Personally I love the gear couplings. We use very expensive ones in the funny cars and dragsters. They early fail. They let you know there are issues coming
Byron

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...Ved,
 

kf2qd

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Those spiral couplings are more suited for drivimng an encoder, no torque no backlash. For your application the Oldham will suffice, I have them on my Sieg KX3 and they are okay, but it shows in holes. A bellows tyoe would probably work better, as long as the misalignment is under control. But any of them do need some periodic minding (like we all do that...)
 

Toymaker

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I use the same type spiral couplers on my DIY CNC mill which I've been using for the last 10 years. I've even used it as a "shaper" to cut internal splines and keyways: Internal spline on mill. I bought my couplers on eBay from a Chinese supplier, so they're nothing special. Perhaps your stepper motors are much larger than mine. Are you driving the motors without using "soft start" and stop; repetitive instantaneous loads can lead to failures.
 

Bentwings

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Those spiral couplings are more suited for drivimng an encoder, no torque no backlash. For your application the Oldham will suffice, I have them on my Sieg KX3 and they are okay, but it shows in holes. A bellows tyoe would probably work better, as long as the misalignment is under control. But any of them do need some periodic minding (like we all do that...)
we tried to re bore the end of ne only slightly larger. Boom it literally exploded. So either use as is or go to some other coupling. The beam couplings do have torque spec but don’t expect miracles . I’ve wrecked at least half a dozen in my attempt to modify my steamer. Timing. So far the best is either make a special or my best is use a shaft collar inside an old ham coupling half the drill and tap a drive hole in an arm and matching hole in the smaller shaft collar it’s hard to get exact correct sizes since the old hams dot always give the spider opening sizes it’s a wait and see game I’ll try and model as many as I can so I can order close fitting collars. The entire coupling takes up too much room on my crankshafts . I have material on the way for shaft extensions but it’s added complexity where it should not be necessary . I may still try and make special eccentric hubs but even these will be a compromise as I can’t make a thread righ to the bottom of the thread hole . It will have to break through whic I did not want if I had some new ones I mihtvexperiment with a brazed addition but it means ordering new ones and over seas shipping I haven’t looked into actual cost but I’m sure it’s not really practical .
What I have will work, it’s just not as clean as it could be . I’m still getting just endless interruptions that I didn’t expect . The estate issue just won’t go away .


Byron
 

ddmckee54

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My first CNC router was a CNC engraver that I got at salvage price. It had let the magic smoke out of it's proprietary electronics. I got a 3 axis stepper driver kit and converted it to use a version of Linux CNC that was ported to run on Win95. That'll give you an idea how long ago this was.

That engraver used those same zero backlash couplings. I also had one that broke, just like yours. And that engraver only used NEMA17 stepper motors. I went back to the manufacturer and got a replacement. Including shipping, about $100 exchanged hands and I was sent the replacement.

Imagine my surprise when the replacement arrived and it was a chunk of 1" diameter aluminum with a 1/4" hole in one end, and an 8mm hole in the other. I could have done that for probably about $1.25 including buying the aluminum. But it was a zero-backlash coupler, just like advertised.

I never went back to them for anything else.

Don
 

Bentwings

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My first CNC router was a CNC engraver that I got at salvage price. It had let the magic smoke out of it's proprietary electronics. I got a 3 axis stepper driver kit and converted it to use a version of Linux CNC that was ported to run on Win95. That'll give you an idea how long ago this was.

That engraver used those same zero backlash couplings. I also had one that broke, just like yours. And that engraver only used NEMA17 stepper motors. I went back to the manufacturer and got a replacement. Including shipping, about $100 exchanged hands and I was sent the replacement.

Imagine my surprise when the replacement arrived and it was a chunk of 1" diameter aluminum with a 1/4" hole in one end, and an 8mm hole in the other. I could have done that for probably about $1.25 including buying the aluminum. But it was a zero-backlash coupler, just like advertised.

I never went back to them for anything else.

Don
You can get all kinds of couplers from McMaster car. They even offer made to order but they are over $100 I probably have 20 couplers I’ve ordered like old ham couplers you order each end plus the spider there are soft rubber and harder urathane ones I’ve got them with different bore sizes on each end
As long as you stay within the size ranges they are ok I found a couple shaft couplers that just happen to fit between the lugs. There are no specs for this however , pure luck on my part. I just put 5 thicknesses of paper under the split surfaces and drilled them out to the shaft size needed I’ll probably drill and tap the drive lugs to lock them in place . That remains to be evaluated . Most of these are available in various cad formats too.

I’ve also ordered on Amazon but you have to watch carefully as sizeses can change unexpectedly Rutland makes many of these.
Automation direct is another supplier ive had good response from there are others that Amazon gets parts from but you may have to check them out I like two piece split collars or couplings , but it’s just my preference you can add or replace these with out further disassembly the urethane ones can take a horrible beating like a two stroke IC engine.

Old hams can be done with shaft keys too. I used these on a twin engine go kart that Seemed to last forever ever there are also stainless steel bellows couplings that work well . Very expensive however .

Byron
 
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