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[Solved] UMO (+ Heated bed): Z axis sliding back when close to top

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I just assembled my UMO, with the heated bed extension.

Now that everything is finished, I'd proceed to calibrate the Z axis.  However, after stepping the Z motor upwards, when reaching the top ~ 10 mm, it always "slides back" a bit, as if the holding force of the motor would not be sufficient to keep it in place.

This only happens near the top (but still before reaching the end-stop switch which I'm about to calibrate).  A few millimeters below, everything is very stable.

Did I mount something wrong?

Edited by Guest

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It depends, if your motors are not powered, i.e. You can move the bed by hand or turn the screw, then congratulations you assembled everything right! The bed should be able to drop yo the bottom slowly when unpowered. This was different on the original bed where it would stay in place.

If you have your machine on, and use the menu to make the z move by 1 or 0.1mm then it should not fall down. So calibrate with the motor powered on and move to using the ulticontroller (or Cura if without ulticontroller)

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That's exactly my problem: the motor *is* powered.  It moves up (using the Ulticontroller), but after reaching the top range, it appears as if it is reducing the motor drive after stepping, so it slides just a few millimeters only back.

I guess it's somewhat intentional to reduce the motor drive after stepping (to reduce the power consumption and motor driver heat), but obviously, it ought to be only reduced enough to still have the bed in place. Strange to me though that this only happens in the top range (low Z coordinates). When the bed is a little below that, it doesn't slide back.

I'll see to upgrade Marlin tonight, in the hope this might change it.

Edited by Guest

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Well, as I don't have Cura running yet (first have to manage getting it to run on FreeBSD), how would I get the appropriate firmware binary without Cura?

I could start compiling my own, of course, but that's going to be yet another task.

Present firmware is 13.12, but it's the stock firmware that shipped with the UMO, so it doesn't know about the heated bed, or the second extruder.

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