The Original Prusa i3 MK3 is the successor of the award-winning Original Prusa i3 MK2 3D printer.

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Trapezoidal nut too tight, Z-axis calibration fails

I have just assembled my MK3 kit, and I am having a problem. I cannot get past the calibration wizard, the furthest I can get is the homing test but after that it always seems to fail almost immediately with "Calibration failed! Check the axes and run again."

During assembly, I noticed that one of the two "trapezoidal motor nuts" fits very tightly on the screw shaft. One spins quite freely, I can flick it with my finger and it will move a few rotations up or down - but the other does not move well at all, and requires me to use my other hand to hold the screw shaft still to be able to move the nut at all. If I do not hold the screw shaft, the motor is forced to turn when I try to turn the nut. It seems that during the calibration, the motors for the Z axis try to turn, but get stuck.

If I cancel the calibration, I can force the motors to move by using the Settings -> Move axis -> Move Z - although I suppose since calibration has not passed it only moves upwards from where I manually place it, it just says "0 mm" even though it's randomly half way up the axis.

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I have also just flashed the latest 3.1.1 b198 firmware, still seeing the same problem

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So, I think I managed to get past this, for now. I used the "Move Z" setting to force the Z-axis up and down a whole heap of times, and tried running the calibrate several more times and eventually it succeeded. I had more luck by starting the test with the extruder all the way near the top (leaving it there after playing with the "Move Z" option). The second trapezoidal nut still looks tight though, although I don't want to take it apart to confirm.

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Hmm this sounds like it was a friction problem, and the moving the assembly up and down, helped spread any debris and/ or any present lubricant. Were you feeling a noticeable friction change if you spin the motors by hand, at a specific point or points?

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i just did what sean did, it helped a lot. but my right trapezoidal is to tight as well. cant get it off (or loosegnig) by turning though. :-

dont wont do disassemble everything

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After a few days, it all seems fine for me now - I haven't disassembled to check but I would guess the nut is still too tight, but the stepper motor can push through. Probably isn't good for its lifespan though, but hopefully it continues to loosen up over time.

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Hi Sean,

good to hear, you've solved the issue. In case the problem reappears, please contact our support.

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Hey @jakoob I just finished assembling my mk3 kit and I encourter the same problem as Sean word for words

" during the process of assembling it, I noticed that one of the two "trapezoidal motor nuts" fits very tightly on the screw shaft. One spins quite freely, I can flick it with my finger and it will move a few rotations up or down - but the other does not move well at all, and requires me to use my other hand to hold the screw shaft still to be able to move the nut at all. If I do not hold the screw shaft, the motor is forced to turn when I try to turn the nut. It seems that during the calibration, the motors for the Z axis try to turn, but get stuck."

I have made a video of the wizard calibration test and my printer keeping on failing on the z-axis. The left "trapezoidal motor nuts" seem to be way to tight and thus stuck the extruder to move upward (it is litterally stuck!!)

What should I do?

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Hi Pablo (@pabtra), try retightening the nut it might help, if not, please contact our support using live chat at shop.prusa3d.com to get new ones.

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I had the same problem, with the spiral rod (leadscrew) coming out the z axis left motor.

There was a defective spot in the lead-screw that made the trapezoid nut stick during calibration.

I checked both trapezoidal nuts and the Z-screw covers on the same defective leadscrew & they all got hung up on the same spot though it was not a visible defect.

example motor: https://shop.prusa3d.com/en/mk3-printer-...

I was able to oil the trapezoidal nut and keep wearing it out forcing it over and over to get it to spin by. I was able to calibrate the machine and so far I dont have any issues however I have not printed anything that tall enough to reach the defect spot yet. Im just going to go with it for now.

I am very disappointed because Ive encountered a problem with every single kit thus far.

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Sean will be eternally grateful.
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