Strange bed mesh compensation G29

  • I'm using a Kossel XL from Think3DPrint3D, with metal corners and metal effector, 0.9 degrees steppers and differential ir probe.
    Im printing on glass with black paper betwe and a heated aluminium plate.

    My bed distance is not optimal, about 0.1mm between highest and lowest point.
    As auto calibration wasnt able to fix this i tried bed compensation (G29) but the resulting heatmap is rather strange:
    When measuring one row of points, the measured z value increases linear from about -0.08 to 0.08.
    So as the next row of points is processed in the opposite direction, a wave pattern is created.
    Obviously that is not how my build plate is shaped, i cant see these waves on my plattform.

    When rotating the glass 90 degrees, the pattern stays the same (it does not rotate), same even i used the underside of the glass on top.

    I marked very roughly where the bed is too high (red) or low (bue):

    As i had the suspicion the same would happen during auto calibration (G32), i made a manual calibration using pretty much the same points as auto calibration and the result is much better and i could further optimize it by hand:
    M665 R177.17 L350.00 H374.77 B125.0 X0.47 Y0.08 ; auto-calibrated
    M665 R176.92 L350.00 H374.72 B125.0 X0.48 Y-0.10; manual calibration
    M665 R176.92 L350.00 H374.72 B125.0 X0.48 Y-0.25; manual calibration further modified (by hand)

    M666 X0.03 Y-0.41 Z0.38 ; auto-calibrated
    M666 X0.18 Y-0.49 Z0.30 ; manual calibration
    M666 X0.22 Y-0.52 Z0.25 ; manual calibration further modified (by hand)

    Any clue what can cause this?

  • Still couldnt fix this, i would appreciate some tips 🙂

  • administrators

    I believe that pattern is caused by backlash in the movement system, probably the Z carriage and associated joints. The actual nozzle height you are getting near the edges of the bed depends on whether you approached it in the +X or -X direction.

  • I will check that. Is there a way to change direction of the probing process, so instead of west-east it goes north-south? Just to check if the same issue occurs.

  • I couldn't find any backlash, but found my carriage adaptors were very soft and it was easy to twist them. I printed some stiffer ones and the effect is all but gone.

    Will compare it to the smart pcb effector using magnetic joints, currently still waiting for my zesty nimble to arrive.

  • oh my… this is what I'm seeing with my Nimble extruder. I had to disable mesh because I thought my PEI was getting uneven reflection because of wear.

    I need to stiffen um my x-carriage

  • I uploaded my version of the carriage adaptor:

    I could bend the "hook" to all sides about 8-10mm with moderate force.
    With the new one its more like 1-2mm, but i suspect half of it is because of using PLA instead of PETG.

    Probably not of much use for your Bigbox though.

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