Z axis is inconsistent and the motors also move with Y axis

Hello all,

This is my first post to the forums. I bought a Prusa i3 a few months ago and I'm still having trouble with it. The biggest (among others) issue is that the Z motor moves inconsistently. My Prusa i3 has the Zennmaster auto bed leveling mechanisim, which is fully operable and works when expected.

I did some more investigating into this issue and I found that when I adjusted the z axis's steps per mm (via the M92 command), it did not improve the z axis movement. I performed some measurements and I found no pattern in the inconsistencies. What that means is that, the steps per mm did not fix this problem. The nozzle would continue to either crash into the bed or hover above the bed when I commanded the z axis to move to 0mm above the bed.

I did discover that, when I move the y axis independently, the z motors also moved as well. This is not intentional. I believe there is crosstalk in the wiring. Do you think that this also affected the z movement? What is the recommended fix to EMI issues in the wiring?

You shouldn't need to change z-steps per mm if the z axis is screw driven - calculate the required steps, set it and leave it.
If you have auto bed levelling, then very small movements in z as you move in X or y is normal behaviour, it's just the auto levelling attempting to keep the head at the same distance from the bed. If you move forward and back in Y do the z motors return to the same place?

Just to try and dig down to the root cause, with everything cold:
1. Move the z axis until the hot end just touches the bed
2. Move z up and down (using the move up and down in Z buttons, do not press the home Z button) do not move in X, y or heat anything up or extrude.
3. Do you still observe inconsistent z height? Try moving up and down different amounts at different speeds.

If you still see inconsistent height in this scenario, then it must be something up with your z axis:
Too low z motor current causing skipped steps (in one or both motors)
Stiction in the z axis (if the axis has a slight slant because the screws aren't at the same level, this can cause high friction)
Something is loose
Backlash in the z-nuts. I have never needed anti Backlash nuts in z - but other people swear by them.
You are trying to move the Z axis too fast or Z axis acceleration is too high - try lowering Z speed to 2mm/s and try again. Adjust Z acceleration and jerk to something low.
Possibly EMI - but I really really doubt this is the issue.

If you do not see inconsistent height in this scenario then possibly:
Z-height needs to be calibrated with the printer at working temperature, after it has been left at temperature for 5-10 minutes. Z height will change if temperature changes.
The hotend is slightly loose - often a problem with a pla carriage/extruder and no cooling if the printer is left stationary and hot for a long time.
The auto levelling has been set up incorrectly.
Your printer has not been rigidly assembled.

Edited 1 time(s). Last edit at 05/05/2014 04:24AM by konwiddak.

@areoalex:
Was this a crosstalk issue?
If yes, how did you fix it?

@konwiddak:
Why is it "normal behaviour" during auto bed level? When Y gets 100mm Gcode command and X gets no command then X should just not move, or am i wrong?
After auto level the Z is autolevelled so yes then it moves but during autolevel Z should not be moving either.