Right z-Axis Motor skips steps  

Re: Right z-Axis Motor skips steps

Are you sure it only skips coming down, but not going up? That seems odd because going up requires more torque whereas coming down requires almost no torque. Is the carriage coming down much faster than it goes up? The speed going up and down should be the same. Maybe it's the motor on the left that is "skipping" ahead and the one on the right is doing exactly what it should be doing?

At the top of the screw, does it fit freely in the holder? It should be able to "orbit" a little without interference.

Re: Right z-Axis Motor skips steps

It is clearly the right side. I ran a self test, which passed, selected calibrate/auto home, which was normal, then calibrate Z. The X-axis assembly was driven all the way up with no issues or skips, then on the way down about 8 cm down, the right side stopped moving and the left continued a few mm, then stopped with the message that calibration failed, check the axes, then re-run calibration. I re-ran the self test then Z calibration. I hit the reset button when the X-axis assembly didn't go all the way up ... I assume it sensed enough resistance that it thought it was all the way up. I did a factory reset ... because I'd hoped the Z calibration would have run all the way up to level things and I could watch and listen again to see if the right side was not stepping because it wasn't trying to step (missed step electrically) or was binding in some way (mechanical miss). I said yes to the wizard after the reset. Self test successful. XYZ calibration drove X-axis assembly up to the stops, did the check on steel sheet, clean nozzle, and paper sheet, then did the 4 point calibration. It seemed successful although there was one instance of a very slight sound from the right side during descent. (at that point I wondered if I might have tightened a screw too much on a bearing?) After 4 point calibration, the x-axis raised, i put the steel sheet on the bed, and then as the x-axis lowered, the right side again seemed to bind and the x-axis was no longer level. I hit reset. I manually drove the x-axis assembly up to the stops using settings/move axis/z axis, then repeated XYZ calibration. It failed in the same manner ... some binding during descent. It did a minor reset and manually drove the assembly up to the stops, then up and down to see if/when binding occurred. Never on upward motion, as far as I could tell. minor bind/missed step or three on descent, similar behavior on repetitions. I drove it all the way up then tried Z calibration. Sufficient binding on the way down that it stopped at the third point to indicate bed leveling failed. I rechecked the ability of the top of the screw rods to move freely within the Z-axis tops, which they can do.

I'm getting ready to disassemble so I can check the Z-bearings and the trapezoidal nuts to see if those are in any way off nominal.

Re: Right z-Axis Motor skips steps

Disassembly check and reassembly:

1. Removed long screw holding electronics box door closed (step 8.36). Then removed two screws holding electronics box to frame (step 8.14) . Not clear these steps are necessary. I replaced all three screws after next step.
2. Removed 2 screws holding each Z-axis top (step 4.9) to release the top of the z-axes.
a. The first time I did this, I also removed the screws holding the trapezoidal nuts (step 8.4)…which allowed the nuts that the screws closest to the frame to fall out. It Is very hard to get these nuts back in place. WAIT!
b. It would be great if one could simply hand screw the z-axis long screws to remove the x-axis assembly, but the extruder interferes with the frame and this can’t yet be done.
3. Removed the screws holding the z-axis motors to the holders (step 8.4), allowing the motors to lower to the table top. Note that unscrewing the z-axes (step 4.7 pictures 1 and 2) still results in interference between the frame and extruder.
4. Removed the smooth rods from the holder (step 4.7 picture 3), which allows their removal and freeing the x-axis assembly from the rods and frame.
5. Removed the screws holding the trapezoidal nuts to the x ends (step 4.6), taking care not to dislodge the nuts these screws go into.
6. Checked free movement of smooth rods in bearings.
7. Rethreaded trapezoidal nuts on screw rods and ensured they travel freely.
8. Checked z-screw covers were screwed against the motors, but not too tight. (step 4.3)
9. Replaced x-axis assembly over screw rods, holding so that extruder tip did not get damaged.
10. Replaced smooth rods, sliding them through bearings on x-axis assembly, then inserted them in holes in z-motor supports.
11. Hand twisted z-axis motor screw shafts to coarsely level x-axis (step 4.7 pictures 1 and 2). I felt some resistance on the right side…but hope this is due to not having them level or twisting the two sides at different rates.
12. Lifted z motors into place and replaced screws mounting them to supports (step 4.6). I checked again for resistance at this point (step 4.7 picture 2), trying carefully to move both sides the same amount. I felt resistance on the right side, and the resistance was coming from the z-motor, not the bearing. I am concerned there is a problem internal to the motor.
13. I dislodged the smooth rods from the lower supports and confirmed them moved up and down freely. (step 4.7 picture 3)
14. I replaced the Z-axis-top-left part (step 4.9 pictures 1 and 2) then the Z-axis-top-right part (step 4.9 picture 3).

Checkout after disassembly and reassembly:

1. Plugged printer in and turned it on. Got message that “Printer has not been calibrated yet. Please follow the manual, chapter First steps, section Calibration flow.
2. I did a factory reset All Data to ensure all self check and calibrations were done.
3. Selected yes to run the wizard. After checking X and Y axes, Z axis check passed, the z-axis went up (was only a cm or so from where I had left it after reassembly, not having adjusted the panda probe) but as it moved down to perform automatic homing, at least 7 steps were missed, a pair, a pair, a single, and a pair. Self check passed.
4. XYZ calibration – Calibration failed! Check the axes and run again. Please check our handbook and fix the problem. Then resume the wizard by rebooting the printer.
5. I used the menu/settings/move axis/z axis to raise the x-axis assembly as far as it would go. It would not go to the stops. The right side was visibly higher than the left.
6. I reset the printer, then raised the z axis to the stops to level it. I selected z axis calibration and the carriage stopped about 3 cm below the top with “Calibration failed! Check the axes and run again.”
7. I turned the printer off and manually leveled the z axis with the bottom of the top portion of the metal frame.
8. I turned the printer on again, got the “Printer has not been calibrated…” message and ran XYZ calibration. As usual, the printer drove up against the z stops, then descended. I think 2 or 3 steps were missed on the right side, but the misalignment did not seem as severe. 4 point calibration started. The four calibration points went fine. The carriage went up. I placed the steel plate on the heatbed as instructed. During the descent 2 to 4 four steps were missed. Bed leveling failed at the third point (front right of the print surface) “Sensor didn’t trigger. Debris on nozzle? Waiting for reset.
9. I pushed reset. “Orinter has not been calibrated yet…”
10. I went to support in the LCD menu:
a. Firmware: 3.4.1-1356
b. EINSy_10a
c. E3Dv6full
d. Date: Oct 4 2018
e. XYZ cal details: Y distance from min Left 10.54 mm Right 10.50 mm
f. Neasured skew 0.04 deg
g. Slight skew 0.12
h. Severe skew 0.25 deg
i. 0;0 point offset X -0.86 mm Y 0.04 mm
j. Belt status X 265 Y 291
11. I shut off the printer and manually raised the carriage by hand turning both screws to level them again using the metal frame as a reference.
12. I turned the printer on again, “printer has not been calibrated.” I used the menu to select Z calibration. It drove up to the stops then downwards. A few steps appeared to be skipped with audible sounds. It failed at the bottom without moving through the first point.

Re: Right z-Axis Motor skips steps

You may have a defective motor. That's very unusual, but possible.

At this point, I would disconnect the motors from the Einsy board, then carefully lay the printer on its back (supporting the frame on a cardboard box) and completely remove both Z axis motors. It should be possible to do that simply by removing the four screws that attach each motor to the mounting bracket, leaving the smooth rods in place. Rotate the threaded rods to back them out of the X carriage.

That way, it will be possible to carefully check each motor in isolation for unusual mechanical resistance to turning. If one motor is bad, I think that Prusa will send you a replacement at no charge.

Re: Right z-Axis Motor skips steps

Check that both your X axis smooth rods are fully pushed in, the sides of plastic parts should be perfectly parallel to the sides of the gantry. If the bottom one is not fully in when you are lowering the axis it may be creating more friction on the way down again.

Re: Right z-Axis Motor skips steps

Prusa shipped me a replacement right side Z-Axis motor and the printer now works perfectly! Thanks to Prusa for the great service and to all for their helpful suggestions.

RE: Right z-Axis Motor skips steps

Just wanted to add that I have also had the same issue. After a  couple of hours with Prusa support, they agreed to send a new motor. They thought it might be a loose rod (the left side wasn't all the way down), then the trapezoidal nut (which was damaged) but it turned out that those two symptoms were caused by the faulty motor. Luckily I had a spare trapezoidal nut so when I installed it, the same issue occured. 

RE: Right z-Axis Motor skips steps

Also, if you are having issues with a stepper motor losing steps and you call Prusa support, be aware of the following.

Support will get you to check multiple parts to eliminate them from the diagnosis (be ready with your tools!). However, their diagnosis process stops if they find an issue with the trapezoidal nut. If the nut is damaged, they will assume the nut was the cause of the issue and the process will stop. Make sure that you keep the call open and test whether the nut is truly the issue. Use your spare if you have one or swap right to left or left to right if you don't have a spare. 

Luckily, I had a spare nut and tried it while making them stay on the support call. This showed that the motor was still lagging behind and that it was the motor that caused the damaged nut, not the nut making the motor lose steps. This saved a lot of time, otherwise I would have received a replacement nut, installed it, had the problem reoccur, go through support again and then wait for a replacement motor to arrive.

The support call was good but took longer than I expected so be prepared to spend an hour or two troubleshooting.

RE: Right z-Axis Motor skips steps

often this Is caused by the dust shield nut over  the motor bearing being screwed down too tightly