Z axis motors loose sync as printing commences with Marlin/Melzi 2.0

I have a show stopper problem on the Z axis of a Prusa i3 with NEMA-17 motors (wired in series, not parallel).
I can run the Z axis up and down all day manually through Pronterface and never loose sync between the two Z axis motors. However about 3 seconds into a test printing run (after the bed and extruder reaches working temp), the Z axis rises slightly from the home position, shakes rapidly making a burrrting noise, then goes to the middle of the bed to start printing. During this odd initial raised 'burrtt' near the home position the left and right motors loose rotational sync by around half to one and half turns. This destroys the X levelling...

Anybody know why the print run does some violent Z moves initially like this, when there seems to be no issues during manual movements ?

I've tried increasing the Z motor current limit pot to 0.8V, but the stepper driver gets hot (like near 100C) in a minute or two, so I backed the pot off again to 0.6V.

Martin

I am not an expert in firmware but is it possible that you set the Z speed or acceleration to something it can't handle? If you try to go to fast with steppers they can shutter like that. And maybe when you use pronterface manually it sets the speed?!? This is just a guess but seems logical.

Only other thing I can think of would be to heat everything up as if it were to print (hotend and heated bed) then manually try homing and moving Z axis to see if the problem returns. If it does it may be that with everything turned on you don't have enough current (big enough power supply).

Thanks for the ideas.
Following that I looked at Pronterface and Marlin settings for feed rate. One was in mm/sec and the other in mm/min. Pronterface was effectively set to a slower value than the firmware, hence it didn't jump on manual moves.
What helped a lot was finding this thread below
[forums.reprap.org]

The info in it was useful to test the Z feed at increasing rates, until it un-synced, which was at 280mm/min (4.7mm/sec) or higher. I found the Marlin firmware setting for Z feed rate was set to 5mm/sec, so I reduced it to 3mm/sec to be on the safe side of my test result, and it does not miss-sync now at all. Done two small test prints even, no change in the two motor positions (with respect to each other).

That thread also helped with another one of my forum questions.

Martin

Edited 1 time(s). Last edit at 01/13/2015 01:08AM by mr.sneezy.