Diagonal rod length on delta printer issues.



  • So my square is within .1mm as confirmed with and indicator on a CNC milling machine.

    I'm back to what error can be covered up so perfectly by using a wrong number when doing the math.

    Below is a quote by dc42 in the other thread I linked too. Can anyone elaborate?

    • Having the arms go vertical or even beyond when printing close to a tower isn't a problem. What you need to avoid is arms going below about 20 degrees to the horizontal

    Thanks
    Cory



  • i got similarly whacked results when my frame wasn't as straight as humanly possible (i.e. calibration aids suggested absurd rod lengths). After rebuilding the whole thing I mounted a dial indicator instead of nozzle and manually adjusted the delta radius until it was flat.

    if i recall correctly if the center of the bed is low you increase the delta radius (or smooth rod offset or whatever its called in the various firmware). I think it will vary with bedsize but a factor of 4 gets you close quite fast. I.e. your center is 1mm low, add 4 to the radius; split the difference when you overshoot. (or if i'm mistaken, its the other way around, but you'll see)

    after spending days with auto calibration, re-doing the whole thing by hand took an evening, rebuild and all. The wiki highlights the important aspects of the physical build, get that right and the calibration is pretty straight forward by hand.
    https://duet3d.dozuki.com/Wiki/Calibrating_a_delta_printer



  • Another source of error for me has been not having the belts parallel to the frame, caused by the toothed gears having slipped on the motor spindles. Measure belt-to-frame at each carriage and at the bottom just above the motor spindle, get them the same.



  • @bearer said in Diagonal rod length on delta printer issues.:

    i got similarly whacked results when my frame wasn't as straight as humanly possible (i.e. calibration aids suggested absurd rod lengths). After rebuilding the whole thing I mounted a dial indicator instead of nozzle and manually adjusted the delta radius until it was flat.

    if i recall correctly if the center of the bed is low you increase the delta radius (or smooth rod offset or whatever its called in the various firmware). I think it will vary with bedsize but a factor of 4 gets you close quite fast. I.e. your center is 1mm low, add 4 to the radius; split the difference when you overshoot. (or if i'm mistaken, its the other way around, but you'll see)

    after spending days with auto calibration, re-doing the whole thing by hand took an evening, rebuild and all. The wiki highlights the important aspects of the physical build, get that right and the calibration is pretty straight forward by hand.
    https://duet3d.dozuki.com/Wiki/Calibrating_a_delta_printer

    I loosened all the bolts and re squared the printer before the last tests. I also checked between the towers with the outside of calipers, really annoyed that I didn't think of that myself. They are within .25mm of being consistent. Didn't have any luck with the way the frame is made getting them any closer, I think one might have a bit of a warp in it.

    Also the subject of changing the radius to get to the head to track center to outwards... This works great, until I get to the area between the towers. Or opposite a tower, however you wish to describe it.

    @robm said in Diagonal rod length on delta printer issues.:

    Another source of error for me has been not having the belts parallel to the frame, caused by the toothed gears having slipped on the motor spindles. Measure belt-to-frame at each carriage and at the bottom just above the motor spindle, get them the same.

    Thanks, I just checked that, no luck there either.

    I'm pretty stumped by this and have to get away from it for a while. Unless anyone else has a better suggestion, I'm planning to decrease the print area size and see if I get better results with the correct arm length.

    Thanks
    Cory



  • Hi,

    My first three printers were mini-deltas kits. I spent hours and hours tweaking them.

    Then I got a Folger Tech FT-5 kit. It worked so much better with so little effort I got rid of the deltas.

    Frederick



  • @fcwilt said in Diagonal rod length on delta printer issues.:

    Hi,

    My first three printers were mini-deltas kits. I spent hours and hours tweaking them.

    Then I got a Folger Tech FT-5 kit. It worked so much better with so little effort I got rid of the deltas.

    Frederick

    Thanks, I guess.

    So after many hours I can not repeat the extremely flat results I had before with the incorrect rod length I posted a video of. I don't know if my attempt to re-square the printer made things worse, or if I was able to compensate better by feel to set the tilt. I get slightly better results with the incorrect rod length than I do with the correct number.

    I rigged up a probe and have ran many tests. Much easier to do, but harder to feel for what is happening than using an indicator.

    I am still of the mindset that if you do the math right with the wrong numbers it is impossible to get the results in the video of my first post. Would be nice to have access to the actual step data, or actual axis position data. Something doesn't add up.

    Cory



  • Correct rod length 291.06
    2_1559950825174_Screenshot_20190607_193736.png

    294.9 rod length
    1_1559950825173_Screenshot_20190607_165629.png

    Messing with the tilt some more.
    0_1559950825173_Screenshot_20190607_084354.png

    Unlike in the video I have a bad high spot by the Y tower no matter what I do. But the regardless more of the bed would be usable.

    Going to try and forget about this for a couple weeks while I'm on vacation
    Cory


  • administrators

    Do the rods bend at all as the effector moves? If so then that will change the effective length of the rods, affecting the geometry.

    I am suspicious of rods with springs between them, because the stretch of the springs, the force they apply to the rods and the direction of that force will depend on the XY position of the effector.



  • @dc42 said in Diagonal rod length on delta printer issues.:

    Do the rods bend at all as the effector moves? If so then that will change the effective length of the rods, affecting the geometry.

    I am suspicious of rods with springs between them, because the stretch of the springs, the force they apply to the rods and the direction of that force will depend on the XY position of the effector.

    I tried removing the springs early on and didn't see any real improvement. In the thread I linked to in the beginning I think they were having the same issue with magnetic rod links.

    It seems that longer rods help. But then you trade build height, torque, and resolution

    Thanks
    Cory



  • Going to reinstall the old control board to test when I get back from vacation. Seems that 3 motor and 3 home switch connections are about the quickest next test. Thanks
    Cory


 

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