Wobbling Y axis issue

Apologies for a repost but it’s been a few weeks since I bumped - I figured I should start fresh.

My Primo has developed an odd wobble on the Y axis - it seems like it’s vibrating as it moves. The X axis doesn’t have this issue. The wobble manifests itself while moving the drag knife, resulting in odd outlines - not really a good option going forward. It had some kind of grinding issue coming from Y1 but I think I’ve gotten rid of it.

This video shows the vibration in action - it’s hard to capture on video but it’s there.

Both Y steppers rotate correctly with the belts removed. I’ve tried tightening/loosening the belts and running the stepper, but it doesn’t seem to make a difference either tighter or looser. I powered off, disconnected Y1, moved Y2 normally, then powered off again, reconnected Y1, disconnected Y2, moved Y1 normally, then powered off and reconnected Y2. No change - the vibration remains.

My plan B is to completely disassemble, re-square, and rebuild my MPCNC, so any suggestions for plan A would be most welcome.

It’s become a bit of a running joke here on the forums, but have you checked that the grub screws on the pulleys are tight? That can cause things to get out of alignment and judder like you’re seeing.

Next, I wonder if there might be a loose or intermittent connection in the stepper motor wiring, something that may have fatigued over time due to flexing while the machine is moving, or got bumped while working on the earlier grinding issue.

does it happen without the drag knife?
Does it happen if you strap a pen to it and plot?
If the answer is yes I think your on the right track looking at the motors, rails & alignment (maybe a homing switch got nudged out of place? That gave me hell for a few days once)
If the answer is no, then maybe the issue is with the drag knife?

Pulley grub screws are tight - they were Loctited in during assembly and weren’t touched thereafter. Everything worked fine for a while until this issue started - I can remove and redo them but it’d be odd if they were at fault.

The issue occurs with anything attached to the gantry - fully empty, drag knife, RT0700 router, you name it. If I power off the MPCNC and push the gantry back on the Y axis, the vibration still happens - even if I bring it close enough to home and then move it back via the controller on the unit, the vibration happens en route to home and after homing.

How would I go about checking and fixing the motors/rails/alignment? I’ve already tested the steppers, they rotate correctly when the belts are off and their plugs are all secure. Should I re-square the gantry or is there some other process I should follow?

I don’t remember the size of your MPCNC, but it looks pretty big. If the chatter only occurs in the middle, it might be because of the size.

Check that all bearings are making proper contact with the rails. If things have loosened (mine has a couple times) then it may be allowing chatter on movement. I have had the most problem with the trucks,but have also had to adjust the core bearings once, and I also found an issue with the Z bearings at that time which didn’t seem to be yet causing problems.

The inner bearings of the Y2 truck are definitely in tighter contact with the rail than the inner bearings of the Y1 truck. IIRC I had gotten them to those levels through the process of squaring the gantry. Should I be tightening Y1, loosening Y2, or some other combo?

Can you feel a difference between Y1 and Y2 when you push it by hand? Like if one has a rough spot on the rails for example. Or the center could be the culprit too. If you push both Y1 and Y2 with your hands while the core is near X min, and do the same with the core near X max, then you could perhaps feel indirectly if the gantry rail is the problem.

The motion does not require the trucks to be particularly tight – the belt is what carries the load and the rollers just carry the weight, almost no torsion or lateral load is to be expected. If anything, I might suggest the trucks are too tight, not too loose. The trucks only need to be tight enough for all bearings to be in contact.

Dumb question but does “bearings in contact” mean “bearings can no longer move freely because they are touching the rail” or “bearings can move just a little bit but sound like they’re touching the rail”?

Bearings should have enough load that all the inherent play in the bearing is taken up. So it’s more like the former. It may still be possible to turn the bearing and have it slip on the rail. It doesn’t need a high load. But they shouldn’t turn freely.

I’m redoing the bearings with the belts off. I can’t quite seem to get them fully stable but then I realized the upper bearings on one truck allowed some play. I can wiggle the Y2 truck vertically, Y1 doesn’t go anywhere.

Is that acceptable? If not should I be focusing only on getting one of them to wiggle less/more?

Video of the wiggle: Truck wiggle - Album on Imgur

After redoing all the truck bearings and redoing the belts, the wobble remains. I tried undoing all the gantry rail tension bolts and noticed something: one of the lower Y bearings doesn’t seem to move or make contact with the rail. It’s this one:

How should I be approaching this to get it to move? Tightening all the bolts for the clamps, loosening, etc.?

so I am not saying this is what you have going on. last weekend I reprinted my core and replaced it and brackets.

now when I removed, my bolts were soooo tight. The new core, they were not. So when i re-assembled, still had some play, loosed core bolts on the Brackets. I then snugged, NOT tightened the middle bolt on the brackets then tightened the other 4 bolts. That fixed mine. My original core the bolts could not move. (it was 2-3 years old, so no sweat and it was pla plus). New is pla and my printer is printing much better than then as well. Hope you get it.

I think I kinda get it except for one part - when you said you snugged the middle bolt, was this the one you meant?

Correct but the other 2 need to be loose

You can also take those clamps off, snug them and only then install them. That’s what I did.

The end result was basically a redo of the trucks, core clamps, and belt tension. I wish I knew what I did to induce it or how it sorta just came to pass, but after getting just enough pressure on each clamp and truck bearing to make a little contact, and after I erred the belts to looser, the wobble seems to be gone.

One thing I’m curious about with belt tension - has anyone ever tried tuning their belts to a guitar tuner app on their phone? I feel like it’s pretty close to tuning a guitar at the peg to just get it right, but I’m not sure if belt tension would meet a standard level of hz/khz for every build size.