How to find Z=0 ?



  • Hello Everyone,
    I am in the process of replacing a lergeX board with a Duet2 in a Two Trees Sapphire s coreXY.
    This is my first printer.
    So far it’s homing to all axes except Z because I have a dummy carriage installed.
    My next step is to install the carriage with the hot end and find Z=0.
    The printer has a mechanical and movable end stop at min height; the bed moves down.
    The bed has three adjustable screws to level it.

    My question is, what is the easiest approach to set Z height properly?

    Thank You.



  • Do you have any kind of probe or endstop at Z physical up? (Which will be Z=0).



  • Yes, a mechanical end stop at min height



  • Unfortunately, printers that move their bed lend themselves to confusing terminology. Does "Min Height" = "closest to the center of the earth" or does it = "smallest Z coordinate", which, on a moving bed CoreXY will actually be "max height, furthest from the center of the earth".

    Phrased another way: The bed moves up to make Z get smaller, eventually reaching Z0. Is the switch at the physical top of that movement?

    And to begin to answer your question: Assuming it is at the physical top, wherever the switch is is Z0. This is unlikely to be where the nozzle is.
    In fact, in a properly designed coreXY moving bed, the nozzle will be a couple of mm above this (i.e. negative Z) to avoid collisions.

    To get the nozzle to JUST touch the bed, specify a tool offset for that nozzle, with G10. To very initially establish this offset:

    1. Home to Z0. This will be below the nozzle, somewhere.

    2. Jog until a piece of paper just barely slides out of the nozzle/bed juncture. Note the Z reading in the display. This will be a negative number.

    3. Add G10 P0 Znn.nn to your config.g, with that same number as a positive number.

    4. Reset the printer to make config.g take full effect, and test carefully!

    If you have multiple tools, each will get an offset. My Jubilee tool changer has two tools at this moment, and it has:

    G10 P0 Z3.38 X0 Y0
    G10 P1 Z2.59 X-0.196 Y0.183

    P.S. if the printer won't let you jog to enough negative Z in step (2), change your M208 to allow it. My jubilee is:

    M208 X-16.5:317 Y-10:385 Z-10:315



  • Be sure to set the zero position when both the bed and extruder are hot!



  • Phrased another way: The bed moves up to make Z get smaller, eventually reaching Z0. Is the switch at the physical top of that movement?

    Yes, that's how it is. Sorry about the confusion.



  • Got it. Then those steps should work.


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