Marlin Printrboard Z Motors fight when homing

I have just completed assembling a reprap prusa (makrfarm linear bearing kit).

I have all axis' moving correctly and homing. However, when I home the z axis (using prontrface), the z motors seem to bind. They vibrate and shudder. When I manually help them to the end switch, they don't offer resistance, but they seem to be having some kind of feedback problem. Adjusting the current? pots next to the z Allegro driver has no effect on this issue.

When the axis reaches the switch, the motors behave perfectly. When I move them with prontrface, they perform flawlessly. Once I send a home command, I suffer my z motor problem.

Should I change the homing feed rate in the firmware? Any ideas to what? I have looked for documentation on this, but it is hard for the finding.

I look forward to being a part of this community, and contributing what I learn.

If the General board is too general for this question, please forgive my newness. My searches for this topic showed results in the general forum...

Edited 1 time(s). Last edit at 06/05/2012 01:34AM by mbcomputerservices.

Using pronterface to move Z via the buttons find a good speed and use it to set the homing feedrate in Marlin.

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I agree with Sublime, this is definitely a homing speed issue. Marlin's default Z homing speed is too high for your Z steppers and they are missing steps. Homing speeds are just below the Steps per Unit, I believe, but I don't have it in front of me.

Also, apply a little household machine oil to your Z rods and the Z nuts and cycle them up and down about 50mm to work it in a bit. A little lube really helps and reduces wear and tear on the Z rods especially the threaded ones.

If you want a faster Z axis, you can also change out the Z motors go give larger motors with more torque. Thankfully, the Z axis is the one where the speed of the motors has the least affect on the print time.

Check and make sure that the Z-Steps/mm are right on the Printrboard. You can either fix it in the firmware, or with a M92 command for your start code in your slicing software (put it BEFORE the G28 homing command).

Another successfully resolved issue!

I can see now that the motors are skipping steps.
I changed the z homing speed from 4*60 to 3*60, the z axis works great now. Thanks for the tips.

Beekeeper, I will check my steps/mm. Thanks for this important reminder.

Beekeeper Wrote:
> Check and make sure that the Z-Steps/mm are right
> on the Printrboard. You can either fix it in the
> firmware, or with a M92 command for your start
> code in your slicing software (put it BEFORE the
> G28 homing command).

M500 will save the current settings to EEPROM.

Andrew Smith: Thank you for the advice. I didn't think that the M500 worked.

;just a thought run a M119 command and see if all endstop sensors show Closed. I did not see that I had a cable caught between my hotend and the Z endstop sensor.