A strange phenomenon affecting X-axis and Y-axis stepping motors
Finally managed to reproduce a strange phenomenon affecting my X-axis and Y-axis stepping motors which changes their behavior causing unexpected results including crashing the nozzle into the print bed. I experienced this issue a few times during previous years but could not connect it to anything specific. Now finally found a way to reproduce it. Here are the steps:
Step 1: Switch on the Prusa Mini (in my case boot loader: 2.0.2, firmware: 4.4.1).
Step 2: Choose “Calibration | Mesh Bed Leveling” and wait until it finishes.
Step 3: Choose “Settings | Move Axis | Move Z” to 1. This will lower the nozzle and the Super PINDA probe will be switched off (too close to the bed) when checked for the next command.
Step 4: Choose again “Calibration | Mesh Bed Leveling” and now …
just hear how the X-axis and Y-axis motors sound changes (they become faster/stronger) as if the current they now receive is too much to handle. During the Mesh Bed Leveling cycle this occasionally causes the X-axis to skip a check point (the nozzle just wobbles instead of moving on X-axis) and when the measuring process continues the next checkpoints move the Super Pinda over the left edge of the print bed and as you may guess the nozzle goes down and crashes into the bed. That’s the reason why I was holding my finger on the power switch.
Here is the video: https://storage.googleapis.com/prusa3d-content-prod-14e8-wordpress-forum-prod/2023/11/199cbca4-prusamini_xy-axis_720p60.mp4
Notes: During the video the printer was vibrating/ringing a lot because the surface it was put on was not stable and there were many loose metallic objects attached to the frame, but this does not have any connection with the problem described. I did not post process the sound so the motors should sound the same during the whole video if there was no issue with them.
Does any of you have explanation what is really happening? Could you reproduce this strange behavior or it’s just my Prusa Mini specific bug?
Best Answer by PrusaTester2020:
Found what is causing the problem and how to fix it 🙂
The issue was preceded by executing in Prusa Mini menu “Settings | Move Axis | Move X” or Y or Z – it does not matter at all which axes or direction is chosen. This action changes the Prusa Mini startup settings from “M204 P1250.00 R1250.00 T1250.00” to “M204 P1250.00 R1250.00 T0” (travel acceleration changes from T1250.00 to T0) which is easily seen by executing “M503” command. According to official Prusa support this is expected behaviour. It looks like setting “T0” removes the travel movements speed limit and after executing “Calibration | Mesh Bed Leveling” or “Calibration | Auto Home” menu command the stepping motors are pushed to their limits.
To check if X-Axis motor struggles with the movement I temporary rose its current from the default 350mA value to 400mA (“M906 X400”). After this no more skipping occurred.
The next step was to find what actually causes the movement resistance in my case – the stepping motor itself (high unlikely), the buddy board (also not very likely), belt tension (already calibrated it) and the X-axis idler. In my case the X-axis idler was not well lubricated, and I also have a direct drive Bondtech LGX Lite extruder mounted on top of the heat break which adds a decent amount of additional weight on the X-axis. If in the future the skipping X-axis motor issue reappeared, I will have to rise X-axis stepping motor current permanently.
RE: A strange phenomenon affecting X-axis and Y-axis stepping motors
Found what is causing the problem and how to fix it 🙂
The issue was preceded by executing in Prusa Mini menu “Settings | Move Axis | Move X” or Y or Z – it does not matter at all which axes or direction is chosen. This action changes the Prusa Mini startup settings from “M204 P1250.00 R1250.00 T1250.00” to “M204 P1250.00 R1250.00 T0” (travel acceleration changes from T1250.00 to T0) which is easily seen by executing “M503” command. According to official Prusa support this is expected behaviour. It looks like setting “T0” removes the travel movements speed limit and after executing “Calibration | Mesh Bed Leveling” or “Calibration | Auto Home” menu command the stepping motors are pushed to their limits.
To check if X-Axis motor struggles with the movement I temporary rose its current from the default 350mA value to 400mA (“M906 X400”). After this no more skipping occurred.
The next step was to find what actually causes the movement resistance in my case – the stepping motor itself (high unlikely), the buddy board (also not very likely), belt tension (already calibrated it) and the X-axis idler. In my case the X-axis idler was not well lubricated, and I also have a direct drive Bondtech LGX Lite extruder mounted on top of the heat break which adds a decent amount of additional weight on the X-axis. If in the future the skipping X-axis motor issue reappeared, I will have to rise X-axis stepping motor current permanently.