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.