Bed crashes after bed leveling

Discussion in 'Calibration, Help, and Troubleshooting' started by Ric Klaren, Mar 7, 2016.

  1. Ric Klaren

    Ric Klaren Member

    Sep 4, 2015
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    A bit stumped on the following issue. After running bed leveling from the LCD the head will crash into the bed. This happens even while the bed is nearly good enough to print on after autohoming (which I'm doing currently).

    (Aside note: at some point the bed leveling also worked reliably).

    A bit more history and things I tried:

    I received the Z fix parts and mounted them and verified the Z is smooth. Calibrated the Z offset so after autohoming a piece of paper fits snuggly between bed and head. All seems well.... Run bed leveling from the LCD. Start print and see that the head is pushed into the bed, cut power to abort things ASAP. Did various things like tweaking Z offset untill I noticed the problems only occurred after bed leveling and that the Z offset was actually not the issue.

    My test procedure is currently:
    1. auto home
    2. level bed
    3. move Z-axis in 0.1mm increments up
    4. repeat in other positions while using the LCD to move the head in X and Y
    I assume that if things are well I should not be able to crash the head into the bed. I can move the bed +/- 0.2mm 'into' the head.

    Things I tried:
    1. I took the rods/bed out once more and rebuild it so it's as tram as is possible. The Z moves really buttery smooth also when moving it slowly at the top of the rods, no snags. Verified this without the lead screws, and ran the bed up and down many times while watching the bearings if they were snagging (obviously with leadscrews ;) ). No change in the behaviour.
    2. I had been using pritt stick glue on the glass so far. Double checked that the calibration points were similarly 'glued' so reflection should be the same. No improvement.
    3. Clean the glass completely, rinse repeat, no improvement.
    4. Put paper beneath the glass, rinse repeat, no improvement.
    While writing this I realized I did not test with paper on top of the glass. In this case the head stays wide away from the bed. So it seems the IR sensor 'kindoff' sees the glass bed.

    Anyone have any thoughts on this?


    PS: I did not try fiddling around with the instructions on the page of that describes this particular probe (
    PPS this is on a single extruder Bigbox.
  2. nemith

    nemith Member

    Mar 9, 2016
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    I've been having the same problem. Same deal where it seemed to work before and now fails as well.
  3. 69charger

    69charger Active Member

    Sep 5, 2015
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    Had the same problem. Just kept moving the IR sensor up and down until it homed the Z axis perfectly. Disable steppers and move the X axis from side to side making sure the bed is level all the way across. The glass plate is crazy flat so I have yet to have a problem. I would much prefer a mechanical Z height sensor. The IR sensor does not work 100% of the time.
  4. UlrichKliegis

    UlrichKliegis Well-Known Member

    Sep 5, 2015
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    That motivates me to think about porting my FSR based ABL sensor to the BB. On my OrdBot, it failed exactly once in more than a year - when I had accidentally ripped out the sensor's connector cable. All the other times it runs with the precision of a swiss cheese.
  5. Rob Heinzonly

    Rob Heinzonly Well-Known Member

    Sep 20, 2015
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    Have had the same issue. At one time it was caused by a crash of the IR sensor holder with the oozing bucket. This pushed the IR sensor up, resulting in a bed-crash. The other crash occurred because the IR sensor was reading the edge of the bed. The print-head moved to far to the left. I had to adjust the x-axis endstop. I didn't notice that after the auto-home the print-head moved a couple of mm to the right and back, and thus positioned the zero-point at the wrong place on the bed.

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