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  1. #1
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    Z-axis overshooting sometimes

    I am having an intermittent issue with my Fineline machine. When I go to run a program it will start with the z all the way up to the safe Z height, and then drop to the piece. Randomly sometimes it will make a barking noise and just plunge all the way into the piece like it gained a bunch of steps and dropped through the leadnut. I have since learned to hold my hand on the Estop button and listen for that when starting a new program, but would like to fix it permanently. I tried lowering the safe Z height and lowering the velocity and acceleration of the axis, thinking it would be a decent bandaid, but it still happens starting 2.5" lower. It only seems to be an issue in the top half of the Z travel. Before I start tearing it apart to figure it out, could it be something else? Are the ACME leadnuts a wear item that needs regular replacement?

  2. #2
    Community Moderator ger21's Avatar
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    Re: Z-axis overshooting sometimes

    Sounds like it's binding. HAve you oiled the screw/nut?
    Gerry

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    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  3. #3
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    Re: Z-axis overshooting sometimes

    Hmm, I can definitely say that I have never oiled it. I figured its some sort of plastic piece so it would need it. Its from here: https://www.finelineautomation.com/p...ly?taxon_id=39

    Wouldn't binding make it lose steps, not gain them and plunge into the table? Or is the binding making it skip teeth and drop?

  4. #4
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    Re: Z-axis overshooting sometimes

    compare the rated voltage of the z motor with the actual supply voltage, perhaps.

  5. #5
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    Re: Z-axis overshooting sometimes

    Quote Originally Posted by Stewey View Post
    compare the rated voltage of the z motor with the actual supply voltage, perhaps.
    It's not an issue with the motor for once on my machine! I upgraded to Clearpath servos and am utilizing the closed loop portion so the machine will shut off if the servo isn't behaving correctly.

    Could the backlash nut be spinning in the housing? I guess that would actually make it appear to have missed steps, not gained them.

    Hmm... Maybe the fast acceleration at the higher location is overcoming the spring tension. Would increasing the spring tension help overcome this?

  6. #6
    Community Moderator ger21's Avatar
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    Re: Z-axis overshooting sometimes

    If you have Clearpaths, then you can't be losing steps. The coupling may be slipping? Yes, you should definitely be lubricating the nut.
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  7. #7
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    Re: Z-axis overshooting sometimes

    Quote Originally Posted by ger21 View Post
    If you have Clearpaths, then you can't be losing steps. The coupling may be slipping? Yes, you should definitely be lubricating the nut.
    Exactly my thoughts. Its almost as if I am gaining steps, not losing them though. Like the lead screw is falling through the nut at the top half of the Z travel. This happens only in the top half and in the down direction. When running actual code the Z is dead nuts accurate.

  8. #8
    Community Moderator ger21's Avatar
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    Re: Z-axis overshooting sometimes

    The nut still may be binding, causing the couplers to slip.
    The motors should fault if they lose position, so you aren't gaining or losing steps at all. Must be a mechanical issue.
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  9. #9
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    Re: Z-axis overshooting sometimes

    Quote Originally Posted by drxlcarfreak View Post
    I am having an intermittent issue with my Fineline machine. When I go to run a program it will start with the z all the way up to the safe Z height, and then drop to the piece. Randomly sometimes it will make a barking noise and just plunge all the way into the piece like it gained a bunch of steps and dropped through the leadnut. I have since learned to hold my hand on the Estop button and listen for that when starting a new program, but would like to fix it permanently. I tried lowering the safe Z height and lowering the velocity and acceleration of the axis, thinking it would be a decent bandaid, but it still happens starting 2.5" lower. It only seems to be an issue in the top half of the Z travel. Before I start tearing it apart to figure it out, could it be something else? Are the ACME leadnuts a wear item that needs regular replacement?
    It could easily be lost steps, caused by binding or EMI noise problem that could cause lost position also, if it is not going to the command position in the program, can you cut and paste the G-code where this is happening, there could be something in your program also

    Remove your Z axes motor and see if the Z axes move up / down freely, this will eliminate anything mechanical problem with the Z axes
    Mactec54

  10. #10
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    Re: Z-axis overshooting sometimes

    Quote Originally Posted by drxlcarfreak View Post
    I am having an intermittent issue with my Fineline machine. When I go to run a program it will start with the z all the way up to the safe Z height, and then drop to the piece. Randomly sometimes it will make a barking noise and just plunge all the way into the piece like it gained a bunch of steps and dropped through the leadnut. I have since learned to hold my hand on the Estop button and listen for that when starting a new program, but would like to fix it permanently. I tried lowering the safe Z height and lowering the velocity and acceleration of the axis, thinking it would be a decent bandaid, but it still happens starting 2.5" lower. It only seems to be an issue in the top half of the Z travel. Before I start tearing it apart to figure it out, could it be something else? Are the ACME leadnuts a wear item that needs regular replacement?


    I used to have a Fineline machine with the ACME screw and had exactly the same problem you are describing. The machine worked fine for quite some time. Then, suddenly the problem manifested. Had me scratching my head for awhile, but I finally tracked down the problem. On mine, the coupling part that clamps to the ACME screw was slipping. When I assembled the parts, I used a product called Vibra-tite as a thread locker. Big mistake. It is not designed for such small clamping screws. I should have used blue Locktite. Anyway, the clamping screws loosened just enough to allow the screw to slip. Messed up a few pieces and made some bad cuts into the spoilboard before tracking down the problem. Dollars to donuts, you have loose clamping screws. I suggest looking there first. While you are at it, you should also check the clamping screws on the upper coupler.

    Note: Even if you used blue Locktite, it may be that you didn't get the screws tightened down enough. They are so darned small, that there is a real risk of stripping out the hex. In fact, when I retightened the screws, I managed to strip the hex on one of the screws. I was never able to get it back loose. I no longer have the Fineline (I built a better machine), but when I took the Fineline apart, I wasn't able to get the coupler off the screw. I could have cut the screw with a dremel, but didn't bother. I wasn't going to reuse it anyway. I mention it as a caution. My advice: GET NEW SCREWS.

    Gary

  11. #11
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    Re: Z-axis overshooting sometimes

    I actually sprayed the acme screw with silicone last night and it seems to have fixed the issue, now when I power the machine off there is so little friction the z axis noticeably sinks into the table, so I may wipe some off.

    Hmm, I will take a look at those bolts as well to see if I can crank them back down. Hopefully I don't strip them out!

    Thanks guys!

  12. #12
    Community Moderator ger21's Avatar
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    Re: Z-axis overshooting sometimes

    I still think the coupler is loose, and the lack of lubricant caused it to be noticeable. Adding lubricant lowered the torque requirement, so it's no longer slipping. But, it's still loose.
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

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