[Precision Piezo] Compensation for bending rods while probing



  • Is there a way to compensate for the bending of the rods while probing at the center of the bed (Rod length is 480mm)? This is with very high sensitivity and a fast probing speed (to ensure it picks up the point on the piezo disk)

    EDIT: Am positive the build plate is flat and does not have a belly in it. Difference from outer edge to center in Z is 0.15mm after probing (from mesh).

    0_1546788797205_IMG_2447.jpg


  • administrators

    That's exactly the sort of thing that G29 is for. Although it's usually called bed compensation, it's frequently used to compensate for sag in the gantry too. The firmware doesn't know whether the bed is bulging or the gantry sagging, the effect and the compensation needed are the same. But if you are sure the bed is flat and the gantry is sagging, don't use bed compensation taper!



  • @dc42 said in [Precision Piezo] Compensation for bending rods while probing:

    That's exactly the sort of thing that G29 is for. Although it's usually called bed compensation, it's frequently used to compensate for sag in the gantry too. The firmware doesn't know whether the bed is bulging or the gantry sagging, the effect and the compensation needed are the same. But if you are sure the bed is flat and the gantry is sagging, don't use bed compensation taper!

    The difference here is one is touching the bed while probing causing the bowing of the rods. When the printer is printing, the nozzle isn't touching the bed so the rods are straight. Wont this be different from when the data was captured when the rod was bending?

    I believe this is different from sag when the rods bend under the weight of the carriage and not being bent while probing.



  • If you are sure your bed is flat and rods are stright, then manually level the bed and dont use any compensation at all.


  • administrators

    I think what you mean is that the force between the nozzle and the bed while probing causes the rods supporting the nozzle and/or the bed to bend, and that bending delays triggering. But surely that would cause the height map to have a dip in it, not a bulge?



  • the force at which the piezo is triggered is very small. surely not enough to bend your rods .4mm. And still it would cause a dip..have you tried with a lower probe speed?
    Also the bulge is larger in the center than it is on the sides. Is this corexy? Then the center is the accumulation of sag in x and y axis. Whilst on the edges in the middle it is only the sag of 1 axis



  • @dc42 said in [Precision Piezo] Compensation for bending rods while probing:

    I think what you mean is that the force between the nozzle and the bed while probing causes the rods supporting the nozzle and/or the bed to bend, and that bending delays triggering. But surely that would cause the height map to have a dip in it, not a bulge?

    That’s exactly what I was trying to say. Thanks.

    I guess you are right as it would show up as a dip wouldn’t it.. I’ll have to do a little more investigating



  • @alexlin said in [Precision Piezo] Compensation for bending rods while probing:

    the force at which the piezo is triggered is very small. surely not enough to bend your rods .4mm. And still it would cause a dip..have you tried with a lower probe speed?
    Also the bulge is larger in the center than it is on the sides. Is this corexy? Then the center is the accumulation of sag in x and y axis. Whilst on the edges in the middle it is only

    It is a CoreXY with >400mm 8mm rods so it may be sag?

    I did try a lower probing speed but that just required more force (and bending of the rods) to trigger the piezo.



  • @chanders i think it is sag..I have a corexy hevo and had the same problem..used X/Y axis mount where the x-rod is hold at 2 places on each side which makes it a bit stiffer...
    Do you have a direct extruder, i.e. more weight than a bowden?



  • @alexlin said in [Precision Piezo] Compensation for bending rods while probing:

    @chanders i think it is sag..I have a corexy hevo and had the same problem..used X/Y axis mount where the x-rod is hold at 2 places on each side which makes it a bit stiffer...
    Do you have a direct extruder, i.e. more weight than a bowden?

    My CoreXY is also a HEVO. Its a bowden setup so not so much weight on the rods, even though the cables may be adding some weight to the gantry.

    Do you have any links to this mount you referenced?




 

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