Z wobble and squaring

Hello, so I recently finished my first mpcnc build with auto squaring and the 23.5mm conduit. I’ve managed to get some drawings that look pretty good, but I’ve had some z wobble and squaring issues. On both the x and y axis I have one bearing that just barely touches the conduit or has a very small gap. I read through the forums and did a decent amount of troubleshooting before writing this. I tried twisting the conduit which definitely helped and then tightening the tension bolts as much as possible. Even with this one of the bearings on the y axis still doesn’t touch. And there’s no hope of it being square with this much tension so I thought I’d live with the small gap and let the auto squaring do it’s job. It works ok but the machine just really doesn’t like to be square, the one motor on the shorter axis will bind as it’s trying to square sometimes and if I’m drawing something that takes quite a while it will skip steps to try to come back to its natural position. I took it apart and I have all the necessary bearings and all the bearings are the correct dimensions. I measured the conduit and it was a bit smaller than 23.5 mm, its diameter varied a lot but it averaged something about 23.35 mm. This made sense because the untensioned gap between the bearing and conduit was about .58mm so divided by 2 is .29, with the difference between the conduit diameters at .15 plus a .14 room for error. I’m really not sure if this small difference could lead to this much play and maybe some other factor is at play here. Let me know if you all have any ideas before I pull it apart again. It’s really not as bad as I’m probably making it sound, it’s just a bit annoying to work with, and I just want something more polished. Since I can’t edit the cad files and move the bearing holes in slightly I was thinking of adding a few layers of electrical tape on the bearings, thoughts? I realize at this point I should have went with the stainless steel tubing instead of cheaping out, it seems more widely adopted.
In these pictures the gapped bearing is the bottom one for the y axis and In the top one for the x axis in the second picture. I should mention the middle assembly has quite a lot of slop when it is fully untensioned, not sure if this is normal.

1 Like

I think that if the tube diameter varies ‘a lot’, you’re never going to get good results. Most people are able to find conduit that is good enough but some take more than one try.

If you’ve got some high spots, you could file them down, and theoretically you could squeeze or hammer the low spots to make the tube more triangular, so it’s larger at the three spots where the bearings run - but you’d probably introduce bends.

I don’t think electrical tape will last long.

There is a little adjustment in the machine; each group of three bearings has one that is bolted through two layers of plastic. That bolt can be tightened to squeeze the plastic slightly, to adjust the fit. You might need to warm the back of the XY pieces for it to work though.

A more ‘engineered’ solution to slightly undersized tube would be to file one hole (maybe the bottom one in your first picture) slightly oversize, and then run a machine screw in through the plastic so that it pushes on the nut and holds the bearing against the tube.

1 Like

Tightening tension bolts “as much as possible” is definitely not recommended. My approach is to tighten as little as possible, see which bearings are not touching, and selectively tighten the tension bolts to just barely get all the bearings touching. Most of the bearings it should be clear which bolt is needed for which bearings, but I can point it out (my opinion anyway) if it’s unclear.

But the better solution might be to take some calipers to the store (if you can go to the store) and look for conduit that is closer to 23.5 and more uniform. Even though the conduit is cheap, it is a fair bit of work to tear everything down and rebuild, but this might still be easier and more effective than hacking a solution.

1 Like

Rotating the conduit to ensure that the bearings are rolling on the smooth areas of the conduit, and not over the stampings or the seam may help your issues. Check all 3 bearing paths of each rail.

1 Like

I brought my calipers to the store and I’ve bought a couple pieces of new conduit, turns out Home Depot has nicer conduit than Lowe’s, who would’ve thought. Ill replace the two pieces for the middle assembly and update y’all when it is done. Thanks for all the suggestions.


Keep going until you hear a ‘snap’, then back it off half a turn, right? :wink: