Help! New build, Y axis coupler on one side won't stay secured

I’m hoping this is a known situation and I’m just missing something, but I have a bad feeling maybe my coupler is shot or something.

Have my build completed and just wrapping up the wiring and initial tests. I am at the stage where you start screwing down the Longmill to the wasteboard. When I send the unit all the way back on the Y-axis, my left lead screw slips the coupler.

I have re-set it 7-8 times and it just keeps doing it. I don’t know if I’m missing something or what has happened. I could really use help from @chrismakesstuff or @andy or anyone in the community who can offer any thoughts.

Here is what I’m doing to re-do it when it comes off:

I loosen off both of the set screws on the coupler and slide it down against the motor, then I push the lead screw through the flange bearing, add a washer on the end of the lead scew and bring it forward deep in to the coupler. I use the coupler set screw on the lead screw side to secure it and then push the coupler back up the motor shaft. Once there is enough room, I use an Allen key to encourage the coupler along up the motor shaft. Once it is pressed against the washer and the housing I tighten the second set screw on the coupler. It all feels nice and tight. (Edit: I wonder if I have the coupler on backwards on the motor axel? Short of taking it off and inspecting the bore size is there any visual way to tell when it is in place?)

At that point I return to the other end of the lead screw and try and set the locking nut on the end. This part I am a bit hazy on but essentially I try and maintain tension on it against the flange bearing and tighten down the set screw. The first couple of jogs usually work but as soon as I back it against the upper limit of the Y axis the lead screw comes loose again.

As it is 2AM here I am going to leave it for tonight, but I will take the motor off and inspect the coupler in the AM. Other than that, has anyone seen this before?



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The only issue that I can address is your edit. The coupler cannot be on backwards. The smaller bore end that is supposed to be on the motor shaft is too small to go over the lead screw.

When it starts to slip, which end of the coupler is slipping? Meaning, is the couple tight to the motor shaft, but slipping on the lead screw, or the other way around?

@gwilki It is slipping off the lead screw end. It is still firmly attached to the motor.

Any ideas? So frustrating to be so close!


is something causing the set screw to hang up before it engages the lead screw? Have you tried swapping another coupler to see it has the same issue? Like just swap between motors.

I had an issue when I assembled mine similar to yours. When I pushed the coupler onto the lead screw it wasn’t going all the way into the coupler. It was a tight fit. I made sure the set screws were out of the way. I actually had to tap the coupler onto the lead screw to get it to seat.

Thanks everyone, I will remove it and tap it on and try it again and report back.

The only other thing I can offer is the set screw doesn’t actually contact the lead screw on my V1 couplers. It is a compression fitting. The V2 couplers look to be a different type. So that could possibly bring up another possibility. Could it be that the screw is not going deep enough into to the coupler itself that it’s not compressing enough to provide a tight connection. Just a thought. Jeff, can you provide a picture of the coupler?

I have a V1 unit, so forget everything that I said in my previous comment. I just deleted it. Heyward is right. The set screw does not contact the lead screw. If you are have V2 unit, it may be different. I’ve not seen one of those couplers up close and personal.

Sorry for the misinformation.

I have a V1 machine.

Will go take a picture right now. BRB.

Ok here you go. First ones are the problem side, second ones are the working side:

Left/Problem Side:

And here is the retaining screw from the other end. I think it is working fine, it appears to walk off but that is because the lead screw has slipped the coupler and pushed out:

And here are similar shots form the right side (working so far):

A second issue I meant to mention is that while I was jogging my ZX-axis, which was working nice and smoothly, the flange bearing suddenly came loose. I have re-torqued it and it seems ok now, but it popped out on the right side here:

Given that this is a V1, does the coupler advice still stand? I don’t want to damage them by doing the wrong thing.

Thanks @Heyward43 and others!


Just finishing lunch then will remove and inspect the coupler and if I can’t see anything obvious then I’ll swap it with the right side and see if the problem moves.


Not sure if I am seeing right but it looks like the flange bearing on the non-working side and possibly the working side is in backwards. They should be pushed in from the motor side. When mine are installed I can still see the flange and the washer. On the non-working side you can’t see all of the set screw. It should be fully visible. That’s what it looks like to me. Anybody else?

Looking at the last photo (back right) I can’t see the bearing/washer at all.

Doh, I’ll go check but I think you may be right. That would be an obvious problem now that you say it. I feel pretty stupid if I got that wrong. Hopefully that is it!


Pretty sure this is it, just about to walk over there. @chrismakesstuff It occurs to me that it would be trivial to add some text to the 3D prints for the legs that identifies which side the flange bearing goes on and which side the set screw goes on. When you are working with them all l aid out on the table, especially with left and right, it is an easy mistake to me (at least if you’re tired and not seeing the whole mechanical picture right away).


That would fix part of the problem but not sure if that will explain why the lead screw is coming loose.

Earlier, you said that the coupler was secure on the motor and was slipping off the lead screw. In your pictures, the coupler does not seem to be as far back on the motor shaft as it should be.
The coupler should be as far on the motor as it will go. Then thread the lead screw through the front bearing, the back lash nut, the rear bearing, the washer, then the coupler.

Maybe you’ve done all that, and it’s just the pics that are throwing me off.

If you’ve done it that way, I would suggest removing the coupler from the motor and the lead screw. Try tightening it on the other end of the lead screw just to see if it will tighten. At least at that end, you will have a better view of what is going one.

Doh that appears to be the issue. Just so I don’t get it wrong again when I take the ends off to adjust, can someone please confirm, the flange bears go on the outside of both ends of the Y axis, the motor end and the retaining nut end at the front?

And likewise, where do they go on the ZX-axis? I am on the motor side on one end but on the inside on the far end. I believe the far end should be on the outside with the retaining nut, correct?


Yes, the flanges go to the outside. That way they don’t get pulled through the hole.

Good call, H. I was concentrating so hard on the forest that I didn’t notice the trees were backwards.