bcn sigma y axis belt tension/backlash



  • I just received a Sigma from matter hackers. The y axis was having some problems. First is that it crashes the end of travel at the left front of the machine. It turned out the gantry was 5mm off square. I fixed that by rotating the left pulleys.

    How tight should the belts be? I can squeeze the y axis belts so they touch midspan with almost no force. Maybe an ounce.

    Next is that every printed object is a little short on the y axis. Circles are out of round about 0.015". Circles have a flat spot front and back. I was trying to check the backlash by issuing g codes and monitoring a dial indicator. I installed pronterface. X axis G commands work as expected but Y axis commands will not move the stepper motor unless they are about 5mm or more. A G0Y1 from Y=0 does nothing. 10mm works though. This is not the case with the x axis where it responds as expected.


  • Moderator

    1. The belts don't need to be too tight (i.e. you don't need to be able to play music with them). They just need to be tight enough so the belt is not skipping teeth on the pulley.
    2. My guess would be that the Y axis pulley is loose on the motor shaft. There are some set screws that lock it in place. Maybe those need to be tightened.


  • the drive pulley is tight.

    Am I expecting too much? Would a round object 1 inch in diameter be expected to be out of round 0.015" to 0.025"?

    I did a threaded part but at that tolerance the nut would have to be 0.030" larger to fit and would be uselessly loose just to prevent binding in the tight dimension.


  • Moderator

    No, I don't think you are expecting too much. You are talking about a 0.4 to 0.5 mm difference between the X and Y axis (we normally work in metric). That is pretty high. The diameter of the entire circle might vary by up to 0.2 or 0.3 mm, but the X and Y axis should still be consistent with each other to within at least 0.05 mm (i.e. the circle should remain circular). Flat spots on the circle would indicate to me that there is some kind of slop in the Y axis.



  • i've had a lot more time to experiment and nothing seems to help. I've tried tightening y axis belts a little and tightening them a lot. Tightening them a lot, 2 turns, on the screws actually made the circle off by .7 mm instead of the normal .35 to .5mm.

    Are the 6 mm belts are too narrow for the dual extruder gantry?

    The pullies are gt2 20 tooth 6mm wide.


  • Moderator

    I don't see how the thickness of the belts would make any difference.

    Here's a test you can do for backlash. Move the X and Y axis around a bit using pronterface, so that the holding current is turned on. Then try to move it by hand. If you can easily wiggle the Y axis back and forth a little bit, then there is something wrong mechanically. Ideally, the printer should hold it firmly in place and you should not be able to move it until you apply a fairly significant force.


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