BLTouch - Probe trigger height ineffective?

  • Hi,

    I have a problem with the first layer of the prints. I have set the Z trigger value, which should be the main factor determining how high the nozzle will be when it starts printing (of course the first layer height is the main determinent, however it depends on trigger height). My nozzle starts too high when printing and changing the Z trigger value doesn't seem to affect it. Also, babystepping seems to be ineffective (it doesn't lower the nozzle). Am I doing something wrong? Is there another detemining factor for the nozzle's height?


  • administrators

    Do you have a G31 command in config-override.g that is overriding the one in config.g that you are changing?

  • I don't have a config-override.g file.

  • administrators

    How have you set the Z trigger value, and how are you homing the Z axis of your printer?

  • That's my G31 command on config.g:

    G31 X30 Y0 Z2.2 P25 ; Set Z probe trigger value, offset and trigger height

    And the following lines are from homez.g:

    ; Lift Z relatively to current position
    G1 Z5 F4800
    G1 X100 Y100 F3000 ; Go to safe loaction

    Probably some of the G90s or 91s are unnecessary?

    The Z homing portion of homeall.g is as follows:

    ; Move Z down until the switch triggers
    G1 X100 Y100 F3000


    ; Absolute positioning

    ; Uncomment the following line to lift the nozzle after probing
    G1 Z5 F80

  • administrators

    Looks OK to me. Is your G31 command later than the M558 command in config.g (it needs to be)? Are you sure that you don't have a second G31 command somewhere?

    You can check the current setting of the G31 trigger height by sending G31 Tn where n is the P parameter in the M558 command

  • Out of curiosity have you setup your bltouch EXACTLY as it states in the documents from antclabs? The distance from the nozzle tip to the end of the plastic housing is important. 8.3mm from memory on mine which gives me a trigger height of 0.87mm

  • I had to add a negative value to my minimum z height since the firmware would block any negative z offset.

    M208 X0 Y0 Z-2 S1 ; Set axis minima

    G31 P25 X35.25 Y0 Z1.25 ; Set Z probe trigger value, offset and trigger height. The higher the "Z" value, the closer the bed will get to the nozzle.

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