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Red wheel does not touch aluminium beam
Red wheel does not touch aluminium beam

I was checking the POM wheels of my printer, since I have a minor banding issue with my prints.
I noticed that the wheel marked in red never touches the beam no matter how tight the eccentric nut on the right green wheel is tightened.

Things I have already tried:

  1. Replacing all three POM wheels
  2. Tightening eccentric nut on the right wheel
  3. Check if the z-axis beams are square
  4. Making sure the holes on the bracket and on the gantry are square/centered

Any ideas on what the root cause the problem could be? I am thinking the easiest solution would be to add a second eccentric nut to the offending wheel?

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    $\begingroup$ You'll want to loosen the screws between the bracket and the rail, then adjust the eccentric nut, then tighten it back up $\endgroup$
    – towe
    Commented Jul 10, 2023 at 10:43
  • $\begingroup$ @towe So I eventually figured that was the best approach. I noticed the most symmetrical load on the POM was achieved when the gantry was leveled relative to the top horizontal beam. However the bottom POM wheel always seem to have less grip on the beam relative to the top one. I guess this is by design? $\endgroup$
    – Adam Smith
    Commented Jul 10, 2023 at 11:12
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    $\begingroup$ I think that's just a result of gravity applying a torque to the bracket. If you want to alleviate that too, you can push the outer wheels against the frame with the X-rail loose and then tighten the nut. But it won't really matter in practice $\endgroup$
    – towe
    Commented Jul 10, 2023 at 12:51
  • $\begingroup$ @towe Good idea! If I keep having the banding issues after I exclude other factors I will revisit the z axis wheels and try that. $\endgroup$
    – Adam Smith
    Commented Jul 10, 2023 at 14:06

1 Answer 1

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Your right (undriven) side Z carriage is out of square, and is deflecting the gantry.

Imagine the left side Z carriage wasn't attched to the gantry at all, just on its own. The only way one of the two wheels without the eccentric nut, on the outer side, could be loose, is if the wheel with the eccentric nut, on the inner side, were not tight.

So start by disassembling the system and attaching the gantry only on the driven-side carriage, i.e. in a cantilever configuration. Do this without the X carriage attached so gravity isn't fighting it, and ensure it's square to the frame and that the eccentric nut is tightened enough there's no play. You should be able to rotate any one of the wheels with manual force while holding another still, but if you don't hold one still, rotating any wheel by hand should move the carriage and cause all three wheels to turn.

It's possible that the bolts holding the gantry to the carriage don't have it aligned square; if it's off, you'll have to take it off the top to adjust. These bolts need to be really tight or they'll tend to self-loosen and throw everything out of wack again.

Once you have the driven-side carriage bolted to the gantry tightly square, it's time to do the undriven side. Start by adjusting the eccentric nut to get the carriage snug, and only then bolt it to the gantry.

Confirm that it moves all the way up and down the Z axis without binding. If the Z extrusions are not parallel or if either of them is bent, you'll have problems and you need to go back and fix that before connecting the undriven side to the gantry again.

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