Skipped Steps, Belt Tension and Gantry Squareness

Discussion in 'Motion System' started by garethky, Jan 15, 2021.

  1. garethky

    garethky Well-Known Member

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    So I want to post about an experience that I have had with my tool changer motion system.

    I have been playing around with alternative motors, specifically 0.9 degree motors, to try and lessen the noise and vibrations at certain frequencies. So I did a motor swap and the printer starts skipping steps. These were larger motors with more torque and the carriage was unloaded. So initially I thought for sure this must be electrical and I went and eliminated every obvious electrical fault. I swapped back to the steppers I had been using with success. I inspected every individual crimp for a loos connection. I changed which ports on the Duet I was using to eliminate a bad stepper driver. Nothing changed and it kept skipping steps.

    So I have long believed that this is due to uneven belt tension. I've searched the internet looking for info on the relationship between corexy, belts, tension and squareness and come up with almost nothing. So I started investigating. I tore down the carriage, unhooked the belts and saw that my gantry was very much not square. Following this procedure magically resolved my skipped steps issue: :D
    1. Reset the belt tension adjusters to 0
    2. Tear down the carriage and loosen the belt grips enough so the belts can come off the pulleys and put no tension on the system.
    3. Move the gantry to the front of the printer and check that it can touching both endstop setscrews without any pressure applied to the gantry (don't force it!)
    4. Even if it does touch, don't assume anything. Use calipers to check that the distance from the linear rail on the gantry to the front aluminum extrusions is the same when the gantry is in contact with the end stops. If it needs adjustment:
      1. Loosen the 8 cap screws that attach the gantry to the linear bearings. If that doesn't permit the gantry to rest on both endstops, you may have to remove some of the screws, try removing 1 from each side.
      2. If the endstop distances were not equal, adjust them until they measure equal to the gantry rail.
      3. Then tighten all of the bolts that are still holding the gantry to the bearing blocks and, if necessary, install any removed bolts and tighten.
      4. Re-check that the gantry touches both endstops with no pressure applied. If it is now askew, loosen the bolts and try again.
    5. Your gantry is now mechanically square
    6. Reinstall the belts and pull tight with hand pressure. If the gantry no longer sits square the belt tension is uneven:
      1. Pull on each belt and see which one needs to be tighter to bring the gantry back to square
      2. Tighten that one by 1 belt tooth and re-check for square
    7. Reassemble the carriage
    If you adjust the belt tension and it brings the gantry out of square, odds are very good the belt tension is uneven. I now think checking for square is a better tension check than you could do with a pluck test. And in any event, only a square gantry makes square prints.

    Anyone else had a similar experience?
     
  2. Joe Pomo

    Joe Pomo Well-Known Member

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    Before making it square, how much do you think it was off by? I just measured from the gantry to the back of the top plate, and one side of my gantry is closer by about 0.3mm.
     
  3. garethky

    garethky Well-Known Member

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    I didn't measure but mine was off by at least 5mm! Also my end stops were not even, out by about 0.5mm.
     
  4. Joe Pomo

    Joe Pomo Well-Known Member

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    Whoa, 5mm! Not that it would have helped in your case, but in my research, I found m-code M556 which can allegedly compensate for skew in X, Y and Z axes:

    "Though with care and adjustment a RepRap can be set up with its axes at right-angles to each other within the accuracy of the machine, who wants to bother with care and adjustment when the problem can be solved by software? This tells software the tangents of the angles between the axes of the machine obtained by printing then measuring a test part. The S parameter (100 here) is the length of a triangle along each axis in mm. The X, Y and Z figures are the number of millimeters of the short side of the triangle that represents how out of true a pair of axes is. The X figure is the error between X and Y, the Y figure is the error between Y and Z, and the Z figure is the error between X and Z. Positive values indicate that the angle between the axis pair is obtuse, negative acute."

    https://reprap.org/wiki/G-code#M556:_Axis_compensation
     
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  5. Spoon Unit

    Spoon Unit Well-Known Member

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    @garethky, could you explain how to adjust the belt tension please? I can't find any reference to checking this anywhere. e3d dozuki for the TC has nothing on this. Would also be interested to know how you found out. Thanks.
     
  6. garethky

    garethky Well-Known Member

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    I agree there is no info on setting the tension. I set mine to "Feels right to me" tension. There is this printable tension meter:
    Tension Meter for the GT2 belts of i3 MK3S+ or Prusa MINI+
    Since its all GT2 belts it should be about the same. You could use that to set one belt and then use the method above to bring the other one into agreement.
     
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  7. Spoon Unit

    Spoon Unit Well-Known Member

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    Thanks @garethky for the pointer to the model. I was looking more for the nuts and bolts of it though. Where are the tensioners and how should they be adjusted to achieve loosening / tightening.
     
  8. garethky

    garethky Well-Known Member

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    Oh! The idlers at the front of the printer are the tension points. They are on little arms and there is a set screw that pushes on the arm to add tension.

    tensioners.jpeg
     
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  9. Nicholasav

    Nicholasav Member

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    What motors, specifically did you swap with?
     

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