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  1. #1
    I'm very aware that I'm a newbie here and I should probably enter the forum a bit more gently but I have a problem I could really do with some help with and it's a biggie.
    I'm having an absolute nightmare trying to debug my new machine, I've been at it for seven days straight and I'm losing the will to live.

    It's a pretty standard 8x4' setup.
    2.2Kw spindle with VFD.
    SM 86HT115 Steppers on Y axis with DM860e controllers on 48v.
    SM 60STH100 on X axis with DM542 on 36v.
    SM 57STH76 on Z axis with DM452 on 36v.
    Generic Mach 3 controller board.

    My right side Y axis is continuously losing steps no matter what I do, up to 2-4mm over the course of running for 20mins or so at between 45 and 70mm/sec.
    I'm running two separate inputs for the Y axis motors.
    So far I have checked all motor tuning is correct.
    Checked the test tool paths I'm using are correct.
    Replaced the controller board for another one.
    Swapped over the two motor controllers, fault stays on the right motor.
    Physically swapped over the two motors, fault stays on right.
    Checked the rack pitch on both sides was identical.
    Checked shielding on everything is properly earthed to the chassis.
    Twisted the motor controller power supply wires.
    Tried multiple PCs, fault stays on the right.
    Ran the machine with the spindle off, fault stays on the right.
    Ran the machine with the X axis powered down, fault stays on the right.

    The only thing I've just noticed is one of the cables on the two Y axis motors is a different make to the other so I'll sort that out on Monday.
    I've also been told that resonance in the body of the machine could be the problem but I've no idea what to do about that.
    Feels like I'm clutching at straws now.

  2. #2
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 9 Hours Ago Forum Superstar, has done so much to help others, they deserve a medal. Has been a member for 9-10 years. Has a total post count of 2,862. Received thanks 351 times, giving thanks to others 8 times.
    Do the motors run in the same, or opposite directions?

    I'm guessing that if you're using a racks, then the motors will physically run in opposite directions.
    In which case, how has that been achieved?
    By the wiring having been swapped for one of the motors coils, or by changing the polarity of the direction wiring to the driver?

    If it's been done by swapping the direction wiring polarity, then the step/dir timing will likely be out of sync, which will result in the driver moving two steps out of position when the direction is changed (but only when the direction is changed in one way, the opposite direction change will be correct).


    The simplest test for loss of position, is to setup a DTI against the axis in question. Then run a simple g-code program that simply moves the axis back and forth a short distance 50 or a 100 times, and see if the axis returns to it's original starting point against the DTI.
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

  3. #3
    Why use 86mm motors?.
    48v is low for that size. Could be dropping out.
    You have DM860 drivers so use them.
    72vdc, no less.

  4. #4
    M-C has probably given you the correct answer to your issue in that it's a step timing issue because even though you swapped motors and drives you will have kept the same Step settings in Mach3 and the drives will both use the same pulse edge setup meaning the issue will remain on that side.

    Without getting too technical what's happening is that the STEP pulse is given in front of the DIR pulse so any steps given before the direction change are lost.
    The cure is easy just change the Active state of the STEP pin in Mach3 just for that side,

    The test is like M_C describes with small moves back n forth (bounce test we call it, see vid), 10mm is more than enough but I would do more than 100 as it's easier to see then. Notice on the video the indicator doesn't get touched until it's finished and goes back to zero slowly for the last move so the indicator doesn't get knocked.

    This test is worth doing on all your axis because it doesn't always show, esp on a Z-axis that doesn't change direction very often ie Profile cutting and pocketing where it just goes to depth and stays there until finished. Often problems like this only come to light when someone tries to do 3D or engraving work with lots of Z-moves.

    Dazp1976 point is valid also because 48V is low for a Nema 34 motor so you could be dropping steps depending on how you have the machine setup, esp on a rack n pinion setup which is very inefficient compared to ball-screw machines and have a lot higher friction to deal with. At higher feeds and with a heavy gantry on an 8x4 machine, you could easily be losing steps.
    Try running slowly and see if you are still losing steps, this will tell you straight away if you are dropping any due to lack of power/friction.



    Last edited by JAZZCNC; 27-03-2022 at 02:33 AM.
    -use common sense, if you lack it, there is no software to help that.

    Email: [email protected]

    Web site: www.jazzcnc.co.uk

  5. #5
    Quote Originally Posted by dazp1976 View Post
    Why use 86mm motors?.
    48v is low for that size. Could be dropping out.
    You have DM860 drivers so use them.
    72vdc, no less.
    I don't know much about this.
    I got all the components as a bundle from a local CNC building company.
    The 48v power supply is what came in it, should I change it to 72v??

  6. #6
    This is brilliant! Thank you guys
    I have been checking the discrepancy by measuring the distance from the end of the track to the carriage when the toolpath has finished.
    The discrepancy increases the more complicated the toolpath is so that would back up your advice.
    Also, yesterday I ran the machine with the right hand motor running as a slave to the left and there was no run out.
    I've not noticed any difference when changing the speeds.
    I will change the Active state and report back.

  7. #7
    Quote Originally Posted by Hatchett Job View Post
    I don't know much about this.
    I got all the components as a bundle from a local CNC building company.
    The 48v power supply is what came in it, should I change it to 72v??
    Yep.
    I'd move the 48v to the smaller DM452/542, ditch the 36v (keep it for bench testing other things maybe), and install a new 72v onto the DM860.
    I usually buy these types of items on Aliexpress, pretty easily available.
    This should improve any velocity anxiety when it comes to losing steps.

    Dual motor axis are best ran with one as a slave.
    Coils on both the motors should be wired identically to avoid inductance/resistance differences. Esp when it comes to 8 wire units.

  8. #8
    So I tried running my test tool path with step low active selected on A axis (my right hand Y axis motor) and then on the Y axis.
    Both times A axis gained steps, am I missing something?




  9. #9
    Quote Originally Posted by Hatchett Job View Post
    This is brilliant! Thank you guys
    I have been checking the discrepancy by measuring the distance from the end of the track to the carriage when the toolpath has finished.
    The discrepancy increases the more complicated the toolpath is so that would back up your advice.
    You need to do this in a methodical order. First, you need to identify if you are losing steps because of pulse timing problems or tuning/power/mechanical problems.
    Forget your toolpath and just do the bounce test first by making a simple G-code program to move the axis back n forth, this will tell you straight away if it's a pulse timing issue and if it's not then you can move on and look else were.



    Quote Originally Posted by Hatchett Job View Post
    Also, yesterday I ran the machine with the right hand motor running as a slave to the left and there was no run out.
    Ok, you are giving confusing messages now.? So are you saying you were not using slaving before.? If yes then how were you doing it.?
    -use common sense, if you lack it, there is no software to help that.

    Email: [email protected]

    Web site: www.jazzcnc.co.uk

  10. #10
    Thanks so much for helping me with this.
    Here's where I'm at.
    Like you said I made a test toolpath that jogs the Y axis up and down 100 times.
    First I ran it with motor A polarity reversed and sharing motor Ys signal from the control board. it worked fine.
    Next I ran it with motor A slaved off motor Y with motor A step low activated and it lost steps.
    Then I ran as above with motor Y step low activated and it lost slightly more steps.

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