Z probe offset for homing only calculates relative offset



  • Let's say my Z probe is set as such...

    G31 P500 X0 Y-48 Z10

    I do a 4 corner check multiple times and get this...

    Leadscrew adjustments made: 0.348 0.347 0.362 0.346, points used 4, deviation before 0.351 after 0.000
    Leadscrew adjustments made: 0.359 0.342 0.342 0.348, points used 4, deviation before 0.348 after 0.000
    Leadscrew adjustments made: 0.352 0.360 0.360 0.363, points used 4, deviation before 0.359 after 0.000

    and so on. It's always approximately .35 off in each corner. So, I thought that my offset was wrong. I tweaked the offset by +.3 and then by -.3 and neither one made a difference. I then set the probe to...

    G31 P500 X0 Y-48 Z15

    And my results are the same...
    Leadscrew adjustments made: 0.347 0.349 0.344 0.348, points used 4, deviation before 0.347 after 0.000

    Shouldn't the corners be somewhere around 5.35mm off?



  • Hi,

    So you have four Z axis steppers?

    The deviation you are seeing is not going to be greatly effected by the offset, something else is going on.

    Frederick



  • Yes, I do have four motors. But that wasn't my question, @fcwilt. Read through the entirety 😉



  • @gnydick said in Z probe offset for homing only calculates relative offset:

    Yes, I do have four motors. But that wasn't my question, @fcwilt. Read through the entirety 😉

    I did.

    Bed leveling is a relative thing. The actually Z probe trigger height is not of interest.

    The process is determining the adjustments needed to get the bed level. It doesn't care what the actual reading is, just the differences for the four points.

    At least that is what it appears to be doing on my printer.

    Frederick



  • @fcwilt what is it measuring against if not the known Z position of the probe? if the probe is offset at z=10 and the corners are .35 off, or the z=15 and the corners are still .35 off, what is being subtracted to get .35?



  • @gnydick said in Z probe offset for homing only calculates relative offset:

    @fcwilt what is it measuring against if not the known Z position of the probe? if the probe is offset at z=10 and the corners are .35 off, or the z=15 and the corners are still .35 off, what is being subtracted to get .35?

    Hi,

    Consider readings of 10.1, 10.2, 10.3 and 10.4. You subtract 10 and get 0.1, 0.2, 0.3 and 0.4

    Now consider readings of 15.1, 15.2, 15.3 and 15.4. You subtract 15 and get 0.1, 0.2, 0.3 and 0.4

    In both cases the bed is out of level by the same amounts and that is what you need to know to make the adjustments to level the bed.

    Imagine a setup where you have thumbscrews you adjust to manually level the bed. It matters not if you do that by adjusting so that the nozzle just touches the bed or the nozzle just touches a 10mm thickness gauge sitting on the bed. In either case you end up with a level bed.

    Now clearly for actual printing Z=0 has to be the position where the nozzle just barely touches the bed - but that is a different issue.

    Frederick



  • @gnydick said in Z probe offset for homing only calculates relative offset:

    Yes, I do have four motors. But that wasn't my question, @fcwilt. Read through the entirety 😉

    Way to go how to influence people and make friends.... do you really think that is the way to get people to help you....


  • administrators

    @gnydick, does your bed.g file home Z before probing, or not?



  • @CaLviNx if someone answers a question based on reading half the post, do you want me to not tell them to read through thoroughly?



  • @dc42 this is my homez.g. it does both homing and probing.

    
    M913 X40 Y40 ; reduce current
    
    M561 ; clear any bed transform
    M290 R0 S0 ; reset baby-steps
    
    ;; Get the bed up fast and primary home
    G91 ; relative mode
    G1 S2 Z4 F200 ; raise head 4mm to ensure it is above the Z probe trigger height
    G90 ; back to absolute mode
    G1 X139 Y177.5 F2000 ; put head over the centre of the bed, or wherever you want to probe
    G30 ; lower head, stop when probe triggered and set Z to trigger height
    
    ; Probe 4 corners
    G30 P0 X240 Y255 Z-99999 ; probe near leadscrew
    G30 P1 X240 Y5 Z-99999
    G30 P2 X10 Y5 Z-99999
    G30 P3 X10 Y255 Z-99999 S4
    
    ; home again
    G91 ; relative mode
    G1 S2 Z4 F200 ; raise head 4mm to ensure it is above the Z probe trigger height
    G90 ; back to absolute mode
    G1 X139 Y177.5 F2000 ; put head over the centre of the bed, or wherever you want to probe
    G30 ; lower head, stop when probe triggered and set Z to trigger height
    
    
    G1 Z50
    
    M913 X100 Y100 ; restore current
    

    I'm guessing it's getting the difference based on the first G30?



  • @fcwilt I hear ya, but there has to be a base reference value. I think I figured it out though with @dc42 's help.


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