Homing Z and need for G29 S2?

  • Is it recommended to put a G29 S2 at the start of homeall.g and homez.g to clear any existing bed compensations to get an accurate Z home value consistently? I'd assumed the bed compensation numbers wouldn't come into play when homing Z, but I get as much as +.25 Z drift if I do a manual bed level and then say a print's gcode does a home and another bed level. It's as if the offset numbers sometimes compound and then every so often they correct and then start to drift again. When I noticed it I started homing Z and doing a bed level over and over to try to determine some pattern.

    The only repeatable scenario was total power off and then homing and doing the first bed level. That was consistent… but additional bed leveling while powered on, the Z would seem to drift as if it weren't being zeroed out by the homing.

    At first I thought it might have been that my BLTouch just had terrible repeat-ability but I noticed that I get consistent offset values for the 9 points I am probing, it's just the height of the grid off the bed varies based on Z.

    I am not sure when this started being an intermittent issue. I didn't notice it when I was running 1.19 (but I started to notice it around 120b10).

    All I know now is that if I do a G29 S2 to unload the compensation and THEN home Z I can get far more consistent values each time I home or do another bed level.

    I guess I expected homing Z would be more of an absolute and wouldn't take the bed compensation values into consideration. Is this expected behavior and if so, is a G29 S2 recommended before homing Z?


  • administrators

    Homing Z will reset the Z=0 position. If the head is not at the same XY position as one of the probe points when you home Z, this may give you a slightly different Z=0 level compared to running bed probing after homing.

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