Z axis home position moves

I set up z axis home position exactly as it should be and after I run job, hot end starts scraping on heated bed glass meaning z axis has moved down or something else happened?
I've broken two hot end on this, please help!!

Things get a little larger when warm, so you need to do your Z axis homing when everything is at the temperature used during a print.

However, if you're breaking hot ends on the print bed, you may be losing Z steps occasionally. You should probably adjust your Z axis mechanics to limit the force that can be applied downward. For example, the Z axis nuts can separate from my X axis ends if something prevents the carriage from coming down.

njadric, I had this problem once, too. I'd set the z-axis just right, get a suitable print, then find the head crashing into the bed on the next print. Not breaking anything, but definitely deforming the kapton on the print bed, and not letting any extrudate out, to speak of.

My problem wasn't cold z axis homing, nor lost z steps. What was happening was heat from the hot end was deforming the PLA extruder block. The extruder block was sagging just a bit into the hole in the x-carriage. It hadn't sagged when I started the first print, so it went fine. But by the time the second happened it was sagging pretty good, I guess. After printing up a new extruder in ABS with one of my good prints, the problem is completely gone.

HTH.

Karmavore, I had PLA extruder body which warped during print and I replaced it some time ago with ABS, but problem persists.
I think I'll set up z-axis home position when both hotend and heated bed are at their operating temperatures and then I'll post my findings here.

on marlin on 7 you can have it retract say ie 5mm then re home when it homes

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njadric Wrote:
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> Karmavore, I had PLA extruder body which warped
> during print and I replaced it some time ago with
> ABS, but problem persists.
> I think I'll set up z-axis home position when both
> hotend and heated bed are at their operating
> temperatures and then I'll post my findings here.

Ok, I've done this and now it works perfectly! Seems like calibration needs to be done in working conditions.