trident downward z motion sounds like a grumpy walrus

[swh41]

Hi. I just built my first Trident, LDO Rev D. It is working, but somewhat rapid downward z motion (as when homing or z-tilt) sounds like a grumpy walrus. The cause is vibration of the rear lead screw. I can silence it by damping the vibration with a finger near the top of the lead screw. I don't know the exact physics of the vibration/chattering, but sure it is my fault. I failed to notice that LDO's recommended printer config did not did not default to the correct TR8x4, wondered why my prints were short, and when I finally realized the problem, in my excitement to try out the fix I failed to remember I had to edit each individual extruder config. So that next first homing went... badly and probably tweaked the lead screw and/or the POM nut.

The printer seemed to work well enough to print most of the remaining parts with it but it doesn't sound right and I'm wondering if the problem is affecting the z offset a bit.

I removed and inspected the POM nut and saw a bit of thread wear. But didn't notice obvious cracking. The lead screw seems ok, but haven't pulled the motor out to lay on a surface.

Things I've thought of:

• Replace the POM nut if I can find the same one for sale, and see if that matters.
• Try to straighten the lead screw (might do more harm than good?)
• Lubricant?
• Replace the motor assembly.
• Craft a damper to lightly hold the top of the lead screw

But before I do that, thought I'd check to see if it's obvious to the experts what the next thing is to do.

 

[the23rdwarrior]

Do you have stealthChop active on Z ?

 

[swh41]

I'm stunned. I did not have it on, don't hardly know what it is, but since you asked, I tried setting all z stealthchop_threshold to 999999 and the first home briefly made the sound but all subsequent moves are now quiet! All of the downward z motions that used to make a noise are quiet!

So the klipper docs suggest preferring "spreadCycle", but evidently spreadCycle is inducing vibration and triggering a resonance it my printer?

For completeness sake, since my first post, I had removed and inspected the rear stepper, put it on the edge of a quartz flat surface to look for bends (did not find any). Did notice a small chip in one of the threads near the top (~7 threads down), and wondered if this is enough to put the screw out of balance enough to matter. But that's the closest I got to finding anything wrong. Upon reassembly I inverted the POM nut with the long end facing down, in case that would do something, which I briefly thought had worked, but once the chamber was up to 40C, the sound was back with a vengeance.

It never would have occurred to me that the stepper config might need to change.

Thank you!

 

[the23rdwarrior]

It's not recommended to have stealthChop activated on a/b motors but it's fine on z.

Have fun!

 

[swh41]

My noise problem is finally fixed, here are all the things I did since the above:

* Activating Stealthchop: this fix lasted all day the day I made the change. It was glorious. But the next time I used the printer after it had fully cooled and reheated, the problem was back. I think any extra vibration was a trigger for the problem, the stealthchop activation reduced z stepper vibration, but wasn't really the source of the vibration/resonance causing the noise.
* Eventually decided I must have some extra tension in the Z system between the screws. I ran the bed towards the bottom saw some deflection of the screws which I had not seen when the bed was at the top. I checked the z motor mount screws to make sure they were tight, then I loosened the screws pictured below (all around), ran the bed to the bottom, and retightened. I know during the build there was a point where I had to keep things loose, shake the printer, then tighten. I remember doing it, but maybe I had the bed too high, or, perhaps more likely, in pursuit of a fix for the noise, adjusted the center Z mount screws while the bed was too high. Noise problem seemed a bit better, but still occurred.



* Noticed the Z screw itself (center rear) felt sticky near the top (the bed range for what I'd been printing) and I don't know if this was just normal consequence of printing ABS (this is my first printer that can do it properly) or residue from the nut on the screw. The kit nut is black plastic, and it isn't supposed to need lubrication, if I understand correctly. But as noted in my first post, I probably bent the screw during the build. Cleaning the top of the screw didn't seem to make a difference, but I just tried to wipe it down with IPA, I didn't remove it and dip it.
* Purchased a new Z motor/screw assembly. To my surprise, the new z motor had the brass nut. I removed the nut and used the original plastic nut, only wanting to change one thing at a time. After this change, the problem was again better, but would still make the noise often enough that I did not consider the problem fixed.
* Lastly I finally swapped out the plastic nut for the brass, lubed it, and the z noise is finally gone.

The two front Z screws are fine, still using the black plastic nut.

I'm not sure what the root cause was. I would like to have tried a brand new plastic nut, maybe I'd damaged it (didn't look damaged to me, but I don't know), but I didn't see that exact nut for sale anywhere by itself. I could still try the brass nut on the old Z screw. But this being my only nice printer, I'd rather not fiddle with it while things are good.