Z-axis height way off

Discussion in 'Calibration, Help, and Troubleshooting' started by richgain, Apr 21, 2017.

  1. richgain

    richgain Active Member

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    I've just had a really frustrating and puzzling experience. The part I have just printed is one half of a puzzle cube and should be 100 x 100 x 100 mm (minus a 0.1 mm offset on each side).
    X and Y are spot on at 99.8 mm on each axis. The height however, is 114.7 mm - way off!
    Printed in PLA and Polymaker Polysupport on my E3D Big Box Dual Titan Hybrid

    IMG_4153.JPG IMG_4154.JPG

    I've checked the STL file and it's perfect. I've also checked the Simplify3D generated code file and the final layer has a Z value of 99.8, which should be right.
    My Z axis steps/mm (printed straight from the EPROM using M503) are 1600 - unchanged from default.

    Here's my only clue - the puzzle is meant to be 7 blocks high with each block being 14.08 mm tall. The bottom block layer is a bit short at 13.8 mm. The middle 5 blocks are all much too tall at 17.8 mm.
    And the top layer, the only one without support material, is pretty much dead on 14 mm tall.

    Any ideas what on earth is going on here?
     
  2. richgain

    richgain Active Member

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    Nobody?!

    OK, well I've had a nice hot bath and laid there thinking for a while (which often helps, but not quite as much as the morning shower ...) and I remembered that when I was 30, I had an IQ of 157 (based on a postal Mensa application form - I didn't join). Now I'm almost double that age, and my little grey cells are undoubtedly dying off at an ever-increasing rate, but I don't think I have lost all my skills as a diagnostician yet (albeit, they are probably better at silicon-based hardware than carbon-based wetware these days) so it shouldn't be beyond me to solve this little mystery by applying some logic.

    I was leaning towards thinking that the only plausible answer was skipping motor steps due to a too low driver current (I haven't actually tested any of this yet), but I couldn't work out how that would make the object taller. Surely, skipping steps on the Z axis would mean less movement and therefore a shorter model??
    Then I realised that there is one possible explanation for the increased height of the print. The 'Oozeless Tool Change' script that I used for this dual-extrusion print includes the following line every time the print-head is changed, just before moving to the dock position:
    Code:
    G91 ; relative positioning
    G1 Z2 F360 ; move Z axis by 2 mm
    G90 ; absolute positioning
    This drops the bed by 2 mm at 6 mm/s (360 mm/min) (gravity assisted - no problem!)
    (Incidentally, Alex's original script included a 5 mm Z move, but I had already decreased this to 2 mm, simply to save time).

    Then, at the end of the purge/prime/wipe cycle, the print head moves to the correct position to recommence printing and then bed must then be raised by 2 mm to restore the correct Z height, e.g.:
    Code:
    G1 X170.140 Y51.320 F12000
    G1 Z85.600 F360
    The printer is trying to raise the bed by 2 mm at the same speed, but against gravity!
    If the current was fractionally too low, the motors would be most likely to skip at this point, and failing to restore the bed back up to the correct position would definitely result in the layer height being fractionally taller, and the whole print significantly taller as a result.
    This would also explain why the top set of blocks printed at the correct height - no support means no tool changes, so no 2 mm drop on every layer.

    I think the next step is to measure my voltage on the Z axis driver and increase it a bit before running some test prints.
    Any suggestions for a good setting?
     
  3. Ephemeris

    Ephemeris Well-Known Member

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    Stock steppers and Rumba with GeeTech stepper drivers? The stock Wantai 42BYGHW609 steppers are rated at 1.7 amps/phase. You could try running them at 1.5 amps as a test of your theory.
     
    richgain likes this.

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