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Y-Axis Motor Sychronization

Discussion in 'OpenBuilds Forum Help' started by ricklach, May 19, 2022.

  1. ricklach

    ricklach Well-Known
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    I have a home built 5' x 10' CNC powered by a black box. When the carriage travels along the y axis I always get a few bumps, always in the same places, no matter what direction the carriage is travelling. I have taken great pains to check the rails to ensure that they are parallel and without any abnormalities along the length of each rail. Today, while watching the machine work I tried to find any discontinuities that could be causing the problem. The only thing that I could determine for sure was that the "bump" always occurred in the same places. To me it seemed like a synchronization problem between the left and right motors. I reasoned that if one side were travelling ever so slightly faster than the other side I would get the same result. After the "bump" the motors seemed to be back in sync until the next bump. All of this is just conjecture on my part but it seems to be a plausible theory. My question is: is this type of behaviour possible because the motors get out of sync and if so is there any thing I can do to fix it, or are there other plausible causes that I may have ignored and if so where should I start to look?
     
  2. Alex Chambers

    Alex Chambers Master
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    The motors will only get "out of sync" if a problem causes them to "miss steps". Although electronic issues can cause this, mechanical problems are more likely. They don't then get "back into sync" and the effects of missing steps is almost always cumulative - it gets worse each time there is a bump.
    Could you post a video showing the issue?
    Alex.
     
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  3. ricklach

    ricklach Well-Known
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    I have attached a video as requested. Over the motor noise you will hear several bumps or click noises - that is where the problem occurs. You will also note that the gantry stops by itself at about the 1M mark - I have yet to figure out why that happens. After homing the machine I did a check to see if the two gantry support pieces on the y-axis were aligned. By my measurement, the right gantry was 2.5mm out of alignment with the left gantry and that may be the cause of the problem. If that is the case, how do I go about aligning the two. I did read that two limit switches on the Y-axis might fix this problem and if that is the case how do I set them up?
     

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  4. Alex Chambers

    Alex Chambers Master
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    If the two sides of the Y axis are out of sync to start with, power off, disconnect motors from controller (important - turning motors by hand while connected to your controller can damage your stepper drivers) and turn the leadscrew by hand until they are the same distance from the end frames. Also check that your frames are square - measure the diagonals is the most accurate check.
    Alex.
     
  5. Peter Van Der Walt

    Peter Van Der Walt OpenBuilds Team
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    If you don't have soft limits, homing and max travel configured, CONTROL doesn't know how far it can send the machine.
     
  6. ricklach

    ricklach Well-Known
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    My y-axis is belt driven, but I catch your drift. I will tinker with it to see if I can fix the problem.
     
  7. Peter Van Der Walt

    Peter Van Der Walt OpenBuilds Team
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    Unable to view your video till I am back in the office on Monday (unlisted youtube links are more viewable while on the road than uploads are) but with belts:
    - check pulley grubscrews - replace if hex head is stripped. Slipping pulleys can cause the issues you describe
    - check belt tension - pull to about 12kg
     
  8. ricklach

    ricklach Well-Known
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    No slipping pulleys, all have been tightened and set screws have loctite. Tension in the belts is good. The gantry is square. There is no adjustment I can make with a belt and pulley system. Soft Limits, max travel, and limits are all set. Max travel is set for 2.3M. However I can only jog about a metre at a time. I am out of ideas.
     
  9. Alex Chambers

    Alex Chambers Master
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    Will have a look at the video when I get onto my PC - probably tomorrow (UK time)
    Alex.
     
  10. Christian James

    Christian James Journeyman
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    What motors do you have on that machine? It sure sounds like its under a lot of stress.
     
  11. Peter Van Der Walt

    Peter Van Der Walt OpenBuilds Team
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    See my earlier reply, then complete the proper set up (So CONTROL can move within the confines of a properly defined (via Max Travel) working envelope that is properly Established (via homing) and safe (via Soft Limits):

    Otherwise it sends a max move of 1000mm
     
  12. ricklach

    ricklach Well-Known
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    Solved the setup problem! Still need help with the bump.
     
  13. Peter Van Der Walt

    Peter Van Der Walt OpenBuilds Team
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  14. ricklach

    ricklach Well-Known
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    Peter, any comments on the video I uploaded?
     
  15. Peter Van Der Walt

    Peter Van Der Walt OpenBuilds Team
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    Thanks for the reminder (But still, unlisted YouTube links easier to view anywhere anytime)

    Can't locate the source of the noise from the video - but you are right there at the machine. One's own senses are the absolute best at locating things like mechanical vibrations through the sense of touch on your fingers, and through stereoscoping hearing - locating directional vectors of noises.

    Take the motors off move things by hand - feel where it bumps/knocks - get hands on with it
    Listen and walk around the machine - zoom in on where the sound comes from - go on a proper hunt and find whatever is making the noise

    I see its not really an OpenBuilds machine - linear rails making funny bump noises can be missing balls in the recirculating carriage, or damage to the rail. I see ballscrews - again easy to knock out some balls during installation that could cause noises and accuracy issues. You really just got to get in there, strip down if needed and do a proper investigating. Something is damaged, something is loose, etc

    That sound is what it sounds like when Grbl's default 10mm/s2 acceleration setting is kept - it accelerates very slowly thus sounding like its stressing. Turning up the acceleration will make it sound more normal
     

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