Problem with Z movement  

Problem with Z movement

I finally received my MK3 and just finished assembling it this morning. This is my second Prusa printer that I assembled. My first one was a MK2 which has been trouble free for almost a year.

I did my initial testings and xyz calibrations of my MK3 and everything is normal. I did the first layer calibration and it printed fine but I noticed the extruder has trouble rising up after the print. It sounded like gears were slipping. The same grinding noise when the extruder reaches the max Z height except this happens around 7mm above the bed.

I did another z calibration and tried to move the extruder up the Z axis but it refuses to move up after the initial 6mm. I can hear the gear slipping but the extruder doesn’t move up. However, the LCD display showed the extruder is moving up. While I can’t raise the extruder I could lower it. Only after I lowered it then the extruder can rise up past the “stuck” position. By then the Z height is all messed up.

I tried to do a print but again the extruder refused to move up past 6mm height despite the LCD kept displaying the changing Z height measurements.

Any suggestion on what is causing this and possible solution. I just contacted Prusa but I hope to get some quick help here.