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IndustryArena Forum > WoodWorking Machines > DIY CNC Router Table Machines > Reversing direction causes motors to jerk
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  1. #1
    Join Date
    Nov 2019
    Posts
    11

    Reversing direction causes motors to jerk

    I'm in the process of putting together a kit from Avid CNC (CNC Router Parts). Everything is running, but I'm stuck on a problem. The motors run smoothly until they reverse direction. When they reverse, they jerk and make a CHUNK sound.

    I don't think the problem is with rack and pinion setup because

    1. It happens to all 4 motors, even the Z-axis ball screw.
    2. The motors jerk like this even when I run them disconnected from the machine.
    3. All of the motors run alright until the direction changes.

    So here's everything I know:

    1. I've tried lowering the acceleration, but it didn't seem to have any effect.
    2. I've tried both using backlash compensation and not.
    3. The steps/inch configuration is correct as measured with an indicator.
    4. Whatever the problem is doesn't cause much backlash. I generally get an error of ~0.003in the first time I move a motor, but after than I can move it back and forth a lot, often with no drift.
    5. So far, changing the tension on the rack and pinion parts hasn't had any effect.
    6. It happens to all 4 motors, even the Z-axis ball screw.
    7. The motors jerk like this even when I run them disconnected from the machine.
    8. All of the motors run alright until the direction changes.
    9. I've tried different microstep configurations from 2 up to 16. None of them had an effect.
    10. I've attached my linux-cnc config.

    This is my setup:

    Linux CNC
    NEMA 34 stepper motors
    Gecko GR214V drivers
    Mesa 6I25/7I76 I/O cards

    This is how I have the drives wired to the break out board:

    Gecko P1 pin 2 (stp) -> 7I76s TB2 pin 3 (step0+)
    Gecko P1 pin 3 (dir) -> 7I76s TB2 pin 5 (dir0+)
    Gecko P1 pin 4 (+5v) -> 7I76s TB2 pin 6 (+5vp)

    I've tried several timings, none of them had any effect. Here are two settings I tried:

    DIRSETUP = 100
    DIRHOLD = 600
    STEPLEN = 2000
    STEPSPACE = 1000

    DIRSETUP = 1000
    DIRHOLD = 1000
    STEPLEN = 4000
    STEPSPACE = 3000

    Here are two videos of what's happening:




    Anyone have an idea as to what the problem might be or what I should test next?

  2. #2
    Join Date
    Feb 2005
    Posts
    829

    Re: Reversing direction causes motors to jerk

    That looks like not enough current to the steppers.

  3. #3
    Join Date
    Nov 2019
    Posts
    11

    Re: Reversing direction causes motors to jerk

    Quote Originally Posted by nlancaster View Post
    That looks like not enough current to the steppers.
    Hmm ok. I'll try to verify the current getting to the motors. Why would a lack of current cause this?

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