z-axis problem

Hi there

I recently build a prusa mendel I2 and after I wired it up and calibrated it I have the following problem:
When I start a print, the z-axis homes and goes up 2 cm, when that happens, one z-axis stepper always jams and the x-axis becomes uneven.
I made a video of it: [drive.google.com]
this only happens when starting a print! when I move it around using pronterface, everything's fine.
How can I fix it? should I just lower the z-axis max speed?

Edited 1 time(s). Last edit at 12/13/2014 04:58AM by omega-xis.

So, if I could try to add some direction. It seems that this issue comes into play whenever the machine "homes" itself, correct? In that case, I would adjust the home rate settings. I would lower the Z-axis Vmax but also lower the Acceleration for homing.

Like yoUmake-3d noted, try adjusting the homing settings in your firmware. Could be that the one z-axis stepper is skipping steps because its trying to step too quickly.

I use Marlin, and this is what I have:

//// MOVEMENT SETTINGS
#define NUM_AXIS 4 // The axis order in all axis related arrays is X, Y, Z, E
#define HOMING_FEEDRATE {50*60, 50*60, 120, 0} // set the homing speeds (mm/min)

Note that the HOME_FEEDRATE is in mm/min, whereas most machine speeds are in mm/sec. I would try something like 80mm/min first off and see if that helps. If it does, try increasing it until you see your issue again. Then you know your bounds.

If that doesn't work, try adjusting the current on your motor controllers - could be you're choking one stepper out and its missing steps.

well. I lowered the Z homing speed by 10% and it's solved for homing and movement (I don't mind the Z being super-slow, it doesn't need to be fast!) but it mainly occurs when I start a print, the machine homes itself and then it goes up 20 units to pre-heat the extruder, then it goes down again to start printing.
it's those 2 movements! those are not part of the Z homing speed, those are "non-print moves". I use marlin to but I can't find an overall max speed setting, only in slicer but that's for ALL axis, I only want it lowered for the Z.

#define DEFAULT_MAX_FEEDRATE          {500, 500, 5, 25}    // (mm/sec)
#define DEFAULT_MAX_ACCELERATION      {9000,9000,100,10000}    // X, Y, Z, E maximum start speed for accelerated moves. E default values are good for Skeinforge 40+, for older versions raise them a lot.
#define DEFAULT_ACCELERATION          3000    // X, Y, Z and E max acceleration in mm/s^2 for printing moves
#define DEFAULT_RETRACT_ACCELERATION  3000   // X, Y, Z and E max acceleration in mm/s^2 for retracts

These are the defaults from github. Try to lower the max feedrate for Z and the max acceleration for Z

that fixed it somehow, I still have another x-axis problem but that's for another topic.