Need some help with Z trigger height



  • This is my first experience with RepRapFirmware and the Duet 2 Wifi. I am coming from a Marlin/Prusa firware based system. For reference, I am using a BLtouch 3.1 with firmware version 3.1.1. Everything is basically working, I just have a few things I am trying to understand a little better.

    The printer is a modified Prusa MK3 Cartesian printer (bear frame). The heatbed is the MK52 PCB heater bed. As is common with this bed type, it tends to be not perfectly flat and changes shape a bit when heated. I will be using G29 mesh compensation.

    So, to the questions...

    I have followed the procedure to set the trigger height to be used in G31 in config.g, This should set the physical distance between the nozzle tip and the probe tip. This is not something I expect should change.

    However, once set, I find when the bed is warmed up, I can either wind up too close or too far from the bed. Since this is somewhat dependent on the bed temperature, is there a better way to probe to compensate for this slight variation before generating the mesh for the bed compensation so I don't have to babystep at the start of every print? Would using a G30 S-3 temporarily change the trigger height? If so, does this offset persist until it changes again?

    Does homing Z with the G30 change the trigger height or otherwise create an offset?


  • Moderator

    @smithkt said in Need some help with Z trigger height:

    Does homing Z with the G30 change the trigger height or otherwise create an offset?

    No, it uses the trigger height, but does not change it.

    It's a good idea to measure the trigger height with the bed and nozzle at temp to allow for any warping and expansion to take place. If the bed shape isn't super flat it can also be a good idea to make sure to move the probe to the same spot the nozzle was at when measuring the trigger height, otherwise the probe offset can be enough to give a different height since you're probing a bowl basically.

    If you're noticing you have to use babystepping both up and down it sounds like maybe your BLtouch isn't giving you super accurate results. You can try modifying the M558 command in config.g to get a little more probing accuracy.

    Try F60 T6000 A10 R0.75 S0.003



  • @Phaedrux

    Thanks for the response. I adjusted my trigger height while heated, created a new mesh and got a fairly decent first level print. I tried using the sensitivity of .003 for the probe, but it failed instantly. I think .003 might be a little ambitious. The Antclabs sight claims .005.

    More tuning is in order, but I feel like I now have a decent base to start with. Thanks again.



  • @smithkt said in Need some help with Z trigger height:

    @Phaedrux

    I tried using the sensitivity of .003 for the probe, but it failed instantly.t with. Thanks again.

    Did you include the A10 parameter?

    Frederick



  • For testing purposes I used A3 not 10. It failed on the first probe point. I think it's just a matter of the tolerance of .003 was a bit too ambitious. For now, I just set it back to the default of .03 and it's working. I will try more precise tolerances, but I'm just trying to get a feel for how this all works right now.



  • @smithkt said in Need some help with Z trigger height:

    For testing purposes I used A3 not 10. It failed on the first probe point.

    Something is wrong then.

    Normally a failure would only occur if the firmware was unable to get two consecutive readings within the range specified by the S parameter during the number of attempts specified by the A parameter.

    I don't know what could cause it to fail on the first attempt.



  • @fcwilt

    It failed on the first probe point, not first attempt. With the tolerance set to .003 and the number of probe attempts set to three, it failed to get two consecutive readings. This was a misunderstanding on my part.

    I have since changed it to A10 and .005 and have observed the probing of the 100 point mesh. On some points, it needs as many as five probes to be satisfied and move on.


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