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Auto-leveling works but nozzle hits the bed

BabidiBabidi
in Repetier-Firmware
I have a cartesian printer, with an inductive sensor and an aluminium bed for z probing. Auto leveling and storing it in eeprom with G32 S2 works in the sense that the Z axis corrects when it moves in X and Y. Probing is done for a 5x5 grid over the entire bed.

However, when I start a print the nozzle is too close to the bed. I have measured the correct value of z_probe_height, it's 0.8mm. But when I use this value the nozzle will touch the bed. Even when using a too low z_probe_height value of 0.1 mm the nozzle will still be too close. 

I have run some tests and after a G32 probing session I move the nozzle towards the bed at multiple points. When it hits the bed at any point the Z height is 0.4 instead of 0. Since the value is consistently 0.4 everywhere I think the leveling works. But off course it should be 0, so something must be wrong with the offset between the nozzle and the probe. 

I could try to use an even lower value of z_probe_height, a negative one but I feel this is not a proper solution. What is going wrong here?

Comments

  • RepetierRepetier
    Do you have z max homing? Or how do you home z? That is very importnant with this kind of problem.

    Bed coating also has a influence on this. As you measure disatnce to aluminium, you can add extra height for bed coating that way.
  • BabidiBabidi
    Yes I home to z max. No endstop on zmin.

    I don't use bed coating, just printing directly on the aluminium bed.
  • RepetierRepetier
    As you have always an error of 0.4 set bed coating to 0.4 and it should fit.

    Does not explain where the difference comes from, but from description I do not see the error. Normally changing probe height, homing and G32 S2 adjusts the error as expected.
  • BabidiBabidi
    Ok thanks for the solution. I already used a similar solution by further lowering the z_probe_height. 

    It still bugs me that something is wrong but for now I've spend enough time on trying to solve it and the solution works.

    If I find some new relevant data I'll post it here.
  • RepetierRepetier
    If lowing z probe height helps, it is the correct solution. I understood you that it didn't change a thing.
  • BabidiBabidi
    edited November 2016
    Well, at 0.8mm it just hit the bed. At 0.1mm it was still way too close and almost no plastic could be extruded. So I set it to -0.3mm and that gives a good and level first layer. So it works but the correct z_probe_height is definitely 0.8mm.
  • stripsstrips
    Are you probing with the same temp on bed as when you print? Inductive sensors will change distance at different ambient temperatures. So z probing should be at same temp always.
  • RepetierRepetier
    On the other hand inductive sensors get influenced by bed magnetic field when turned on. So you should heat up, turn heater off and measure directly if that makes a bigger difference for your printer. In fact some beds change shape when hot.
  • BabidiBabidi
    I always performed the leveling procedure with the bed heated. So the temperature difference cannot be the cause.

    But it might be the bed magnetic field. I will check if it makes any difference with the bed power turned off before leveling.
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