BlTouch and Z "sliding"

  • Hi,

    First, sorry for my English 😛

    I recently installed a BlTouch sensor and followed the great tutorial to set it up.

    Everything works great but...

    Checking the sensor I've noticed that at each prob the z offset is sliding by about 0.003mm. Sometimes less but the readings tend to 0.

    I started at 2.850 and after many reads it was below 2.7 i stopped the tests.

    However setting a Z offset at 2.7 makes a good result ( the best first layer I ever get ).

    What can cause that ?
    Can it be the 256 microstepping ? Maybe it can use to many CPU and make some lag (and yes it's a little overskill for the z axis)
    Or maybe the BlTouch need some time to heatup ?


  • administrators

    What speed are you probing at (F parameter in M558)? Try slower.

  • @dc42 I was at 100. Tried at 80 but same result.

  • administrators

    How are you checking the trigger height: using G30 S-1, or some other way?

    What are your Z steps/mm?

    Do you get the same result if you use lower microstepping, e.g. x16 with interpolation enabled?

  • @dc42 said in BlTouch and Z "sliding":

    G30 S-1

    Yes I'm doing the trick with the paper and the G30 S-1 to get the Z offset.

    Actualy my Z Steps/mm is 40960.
    I have a M8 rod with 1.25mm pitch.
    ( 200 * 256 ) / 1.25 = 40960

  • The shrink sleeving over the aluminum rod may be slipping. If it slips up, the rod drops down slightly. The easiest way is to take it apart and grab the rod and see if it slips as you pull out from the shrink sleeving. Its a known problem with the BLtouch that causes problems. I think the newer design has a machined groove on the rod so the shrink sleeving can shrink and hold position. I don't suggest you take it apart unless you know what you are doing.

    The position of the rod, the orientation of the magnet and how far the shrink sleeving is trimmed is critical to getting the BLtouch to work correctly. The easiest way to fix is to apply a small amount of CA or E6000 glue between the rod and original shrink sleeving without replacing the sleeving. I had to use a different shrink sleeving since it was binding against the inside and won't let the rod drop in a different situation. If you go that far, you have to make sure to take accurate measurements before you remove the shrink sleeving. Also mark the orientation of the magnet before you take it apart. The orientation matters to the hall sensor inside.

    The shrink sleeving and rod design on the BLtouch is one of the weakest part of the design. If you look at the rod, you will see lots of scoring marks from the trimming to get the shrink sleeving in the right distance. These scoring marks sometimes affect the rod and not allow it to drop properly also.

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