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Thread: Problem with oval holes

  1. #1
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    Default Problem with oval holes

    Hello
    I am having a bit of a nightmare with my CNC. I have recently changed to a Chinese knockoff of the Leadshine Easy servo series, more specifically these motors and drives https://goo.gl/2uFKXr. The reason for the upgrade was because the old Nema 34 steppers lost around 2mm worth of steps after 15 minutes of cutting…
    The new ones worked perfectly and the rapids was increased from 4000mm/min to 8000-10000mm/min (different settings for each axis).
    Or so I thought!
    I was making a little “pallet” with some 20mm holes to hold my ER20 collets. Somehow the hole was not round at all, off by 0.5mm, I also tried making some squares and they ended up pretty similar, see drawing below.



    What could cause this?

    I have checked for backlash and it is between 0.01-0.015mm (using ballscrews), therefore I almost can’t see how it could affect it that much?
    It is worth mentioning that I did not have this problem with the old steppers.

    So I am wondering if it is a tuning issue with the hybrid stepper/servos or maybe something about the timing of the outgoing pulses?

    Thanks in advance

    Best regards
    Andreas

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    Attached Thumbnails Attached Thumbnails Problem with oval holes-fil_001-1-jpg   Problem with oval holes-fil_001-2-jpg  


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    Default Re: Problem with oval holes

    Quote Originally Posted by androcles View Post
    Hello
    I am having a bit of a nightmare with my CNC. I have recently changed to a Chinese knockoff of the Leadshine Easy servo series, more specifically these motors and drives https://goo.gl/2uFKXr. The reason for the upgrade was because the old Nema 34 steppers lost around 2mm worth of steps after 15 minutes of cutting…
    The new ones worked perfectly and the rapids was increased from 4000mm/min to 8000-10000mm/min (different settings for each axis).
    Or so I thought!
    I was making a little “pallet” with some 20mm holes to hold my ER20 collets. Somehow the hole was not round at all, off by 0.5mm, I also tried making some squares and they ended up pretty similar, see drawing below.



    What could cause this?

    I have checked for backlash and it is between 0.01-0.015mm (using ballscrews), therefore I almost can’t see how it could affect it that much?
    It is worth mentioning that I did not have this problem with the old steppers.

    So I am wondering if it is a tuning issue with the hybrid stepper/servos or maybe something about the timing of the outgoing pulses?

    Thanks in advance

    Best regards
    Andreas
    Try running at the same speed as the older setup first? Leadshine drives are pretty inexpensive so the couple extra dollars you spend gives you better tech support than the knock-off guys.



  3. #3
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    Default Re: Problem with oval holes

    It could be a lot of things but I'd suggest on of the basics we get taught in tech school, do a "close visual inspection" to make sure everything I properly assembled. Follow that up with a physical inspection of all couplings to verify that set screws, clamps or whatever are tight. The fact that you changed steppers means that these components where touched in some way or another so there is a good possibility that a coupling is rocking someplace. Basically you want to review everything you did to the machine.

    If that doesn't turn anything up check the machine for proper physical movement for each axis vs what was requested. Keep an eye out for backlash. In other words verify your calibration for each axis.



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    Default Re: Problem with oval holes

    Regarding the speed - I tried cutting the circle at 3000mm/min and acceleration at 500mm/min which is where the measurements at the drawing are coming from. I tried several slower feedrates but no luck, only when I got down to about 2-300mm/min the hole was a bit better, but still oval in the same direction.

    I have measured each axis with a dial indicator and the backlash was never more that the 0.015mm and every coupling is checked..

    Yeah, I know that from know on... I saved 400 dollars which is a lot when studying. But the support is a F.....G joke. I had an E-mail correspondence with the company (JMC) as to why I couldn't connect the drive to the pc to tune it properly. First he/she said that I needed to make a special serial cable myself. I did that and it still didn't work, so I took of the plastic housing only to discover that the RJ-45 terminal wasn't even connected to anything on the PCB, it was only soldered to it... I sent a picture of it and they responded:
    "HiI am sorry for trouble you on this.
    But JMC factory only offer this driver which only can set parameters through the operate panel.


    you can check the user manual to set it easily."

    Best response ever



    According to a comment in this thread it seems to be some tuning issue?
    http://www.cnczone.com/forums/archiv...p/t-47838.html

    "Backlash affects you at MOTION XDIRECTION CHANGES - the monkey motion problems at 45 deg are happening while the motors are attempting to do the most/fastest relative changes in velocity - one is speeding up, the other axis doing the opposite SIMULTANEOUSLY... A simple look and see at the motors will show/prove this

    To make the motors "walk and chew gum" at the same time, they have to be timed EXACTLy the right speed relative to each other thought the motion cycle. We chased this all over hell and back until we FINALLY figured out how to properly tune for "follower error". Cutting ROUND circles is THE definitive test to assure you that you have servo follower error tuned properly.

    Trust, me, it isn't back lash and it ish't backlash comp. It is a problem in the instantaneious RATE of feed change that the axis are making that is causing the 45 deg out of round problems. If the controller is NOT changing the RATE at which the step pulses are instantaneously being fed to the steppers, they won't/can't keep the poper relative speed to each other and they thus CAN'T cut round holes."

    "
    It depends where it is out of round and by how much.

    Errors at the direction change points (12, 3, 6, 9 o'clock) are typically caused by backlash and take up errors in your drive'ball screws.

    Out of roundness at 45 degrees off the vertical or horzonal axis are due to servo tuning errors. At this point, both servor are affecting a high rate or change in opposite directions. Unless the tuning is perfrect, you;ll start getting follower error and out of roundness."











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    Default Re: Problem with oval holes

    Hello

    So I had the chance this weekend to tinker with the system.

    I was right about it not being backlash or anything loose in the system. I went through all the settings on the servo-drives and I found a setting for speed smoothness.. This was set at 8 on the X-axis and 2 on the Y-axis.
    The consequence of this difference between the two settings was the 0.5mm OOR on all circles. When adjusted to 8 there was a pretty aggressive speed smoothing going on.

    I adjusted both drives to 2 and behold, perfectly round holes and even sized diagonals in squares.

    It is so annoying what such a "small" thing can do to a system.

    - Andreas Landsfeldt



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Problem with oval holes

Problem with oval holes