MK4 loses Z-Calibration between prints (occasionally)  

RE:

I sometimes have the same issue. It also can also happen with the first print after power on.

Selecting Auto Home from the Control menu usually makes the printer find the correct Z height.

RE: MK4 loses Z-Calibration between prints (occasionally)

You mean that you loose the z live adjust value between prints or shutdown?

I noticed, that it is reseted to 0 again, but I had now effects on homing.

RE: MK4 loses Z-Calibration between prints (occasionally)

Hi,  I mean it tries to perform the nozzle cleaning when the nozzle is still a few cm above the print sheet.

When I stop the print and perform Auto Home from the menu it does lower the nozzle and when I print after that step all is fine. 

It just seems to forget the Z position.

RE: MK4 loses Z-Calibration between prints (occasionally)

I am having the same issue on my 5 day old MK4.  It attempts to find the bed several times and then aborts the print with a crash.  It's happened about 3 times so far, the first time from the Flashdrive before I installed Octoprint.  Anyone learn anything since this was first reported?

Running Firmware 5.1.2 with Octoprint

RE: MK4 loses Z-Calibration between prints (occasionally)

Hi,

After each switch off the printer loses not only z but x,y as well. It is natural and has been explained in the forum several times. When you switch it on it sets a default values. 

Usually each print starts with MBL routine starting whith the homing. If your failing print fails always after the switch of/on look at the .gcode file if it contains the MBL routin or homing routine.

Did you create the gcode fila via prusaslicer or downloading from somewhere?

I found this explanation. It describes how it works for x and y axes only, but for z it must be done as well.

Can you attach a zipped .gcode file?

RE: MK4 loses Z-Calibration between prints (occasionally)

Posted by: @paulz

Hi,  I mean it tries to perform the nozzle cleaning when the nozzle is still a few cm above the print sheet.

Could it be that your filament is 'difficult' to unspool? The Z-homing uses the extruder force sensor, and when you filament 'pulls' on the extruder while moving the head downwards, it can detect the bed too early.

RE: MK4 loses Z-Calibration between prints (occasionally)

Yes, I think it could be the tension in the filament.  I noticed it happens more frequently with a full spool  and when the previous print is taller so the extruder is parked higher at the start of the next print.   Im running inside a lack enclosure with the spool in a Sunluu dry box so there is more tension than the standard spool holder. I need to experiment some more to confirm.

RE: MK4 loses Z-Calibration between prints (occasionally)

Hi,

It's possible filament tension caused it. I cannot recall if it happened more on full spools than empty spools.  But as far as I can recall it has never happend after updating  to 5.1.2, so maybe the homing code has become more robust.

 Paul

RE: MK4 loses Z-Calibration between prints (occasionally)

So I printed a tiny "fish scale" to measure the tension in the filament.  My spool system requires 227-265 grams to unwind the filament (measured at the extruder).  I then used the scale on the extruder end of the filament to see if I could recreate the homing error.  The extruder pegged the fish scale at over 750 grams and homed like there was nothing  pulling on it.  Conclusion is the filament tension is not the cause.  I am running 5.12 firmware on the printer too so it's something else going on.  Maybe as I get more experience with the printer I can figure out the cause.  Appreciate the feedback.

RE: MK4 loses Z-Calibration between prints (occasionally)

This happens to me as well on MK4 5.1.3+13503. I'm mostly printing with Prusa Connect. In between prints the printer doesn't know it's Z height anymore, I would guess it happens about 75% of the time. As a result, the printer to move up and slowly move down to try to find it's home position, beeping several times in the process and often gives up or reports crash detection when there is no crash. At this point, I try to remember to reset the printer after printing every time.

Hopefully this will get fixed soon.

RE: MK4 loses Z-Calibration between prints (occasionally)

I was able to solve this by changing how I feed the filament into the extruder.  After doing some testing I discovered the touch-off sensor is very sensitive to side forces.  If you are feeding from a spool from above without a reverse Bowden tube the tugging by the x carriage on the filament spool will trigger the sensor.   I now use a reverse Bowden setup (same setup as the Prusa enclosure) fed from above-front-center. The reverse Bowden isolates the force sensor from the filament tension and prevents the tripping I was having.  Hope this helps.  

RE: MK4 loses Z-Calibration between prints (occasionally)

Thanks for your post, LarryD! I had this exact same problem with my top-loading enclosure solution that was feeding filament from a long top-mounted bar, and adding a tube immediately resolved this.