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  1. #1
    Hi

    I try to do as much as possible before bothering people by reading/watching video etc but i am out of my depth here now.
    Running a 720-Z for 8 months now slowly learning about it and using vectric vcarve pro.
    The other week after running a small shield in 3d finish it got to 99% and suddenly did a large L and at a 6mm depth straight through the work.
    Yesterday it ruined another piece when after about 1.5 hrs running it suddenly overrun the x axis by 20mm and carried on as tho nowt happened.
    Today i started all a fresh and again 99% 40 mins in and it suddenly plunged lower and finished the last run too deep and offset by 5mm and ruined the Pine wood.
    Running it at 44mm/sec .25 ball nose in pine.
    The only common denominator seems to be 3dfinsh but its all simple work nothing out of the box.
    Where could i start to track this fault (but i cant read g code) as i cant waste any more wood as its all for charity and it costs me.
    I have only done small sorts of work until these shields and never had any problem, but i am only a hobbyist and do little work with it. I also read here and agree with a previous poster about anything wrong with leadscrews etc and it has to be shipped to Germany and man it costs, a bit of a downer, but cant see why it would be these as no backlash or owt, it just has a moment of its own and goes off on one.
    Its on a dedicated PC (win 7) no screensavers or power save options and no internet connection to distract it

    Click image for larger version. 

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    Many thanks for any pointers Bernie
    Last edited by BernieNUFC; 20-03-2016 at 03:47 PM. Reason: added info about pc

  2. #2
    Hi Bernie,

    Ok well you need to start by checking the machine from head to toe for any mechanical issues. Mostly likely cause would be binding or failing bearings etc.
    Any of these and the motors will struggle and start losing position or worse case stall completely.
    It's actually quite common for things like this to happen on long jobs that run at higher feeds where torque is starting to drop off. The combination of heat which causes binding and steppers working in speed range that can be affected by resonance can also be a factor.

    If you can't find any mechanical issues and the G-code is correct then it's strong possibilty you are being affected by Mid band resonance. This happens like the name suggests when the motors are working around there mid rpm level which at 2500mtr/min then you could be approaching(depending on screw pitch).
    If you are being affected by this then you have limited options. Changing the feedrate to lower or higher speed is an easy test to see if you can move the motors out the frequency range where resonance affects drives/motors. If it is resonance then you'll likely struggle to go higher before motors stall.

    Next option and the one which is most likely to work is changing the micro stepping settings on the drives(If they have any). This can shift the resonance to different frequency which the drives can handle better.

    Replacing the drives with better drives is the last option and one that will most likley cure the issue if resonance is your problem.

    Checking all the wiring both on the machine and in control for lose connections is second thing to try after checking mechanical.

    If you post the G-code by zipping it up or in txt file (don't post all lines on here) then I'll take look to see can bad code.

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  4. #3
    3D Finish.zip

    Hi Jazzcnc
    Thats a very comprehensive reply and very thorough thank you, i will start doing the checks you mention, in the mean time i have attached the NC file.
    The machine has probably done less than 50 hours since new so i dont think there should be any parts that are ready to fail just yet, well there better not be.....:-)
    I am using WinPC-NC and the settings defaulted for the 720 so i am not sure where to start checking micro stepping.
    No loose connections as i have just relocated it and made everything a much better and tighter setup.

    Thanks again
    Bernie
    Last edited by BernieNUFC; 20-03-2016 at 07:01 PM.

  5. #4
    Quote Originally Posted by BernieNUFC View Post
    No loose connections as i have just relocated it and made everything a much better and tighter setup.
    This makes me suspicous not re-assured.!! . . . . Did it do this before Relocating.?

    Also Just because machine isn't very old doesn't mean it can't hit trouble. Infact it's about the right amount of hours when things could come loose and need tightening after settling down.!

  6. #5
    No it did not do it before the move but equally i did not do these files only the very short work jobs.

    I can understand about the bedding in fact last week after the first incident i gave it a good checkover just in case but could not find what might have caused the initial fail

  7. #6
    quick look at the code shows no issues so not that.

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  9. #7
    Checked every Allen key bolt, every screw and removed covers to check wiring etc, went through every connector to controller and then to PC, everything is tight now, only one screw holding an earth wire was loose enough to take a nip up.

  10. #8
    Quote Originally Posted by BernieNUFC View Post
    Checked every Allen key bolt, every screw and removed covers to check wiring etc, went through every connector to controller and then to PC, everything is tight now, only one screw holding an earth wire was loose enough to take a nip up.
    Have you checked for any axes binding. This is mostly likely cause.

    After checking this I suggest you try running the same G-code at different feedrates to test if resonance is the cause. If it runs fine at lower feed rates and at higher feedrates then it's most likely Mid band resonance.

  11. #9
    I will try it at 80% to see if it improves then, thank you

  12. #10
    Ran at 80%, motors got to 32.5c (did not seem too hot to touch) and i thought when it finished it had cracked it, What i thought this time that looked like a failure was a cutout which turns out to be a knot problem, so it may have worked, will try different wood but now i hope to adjust the speed at source
    fingers crossed
    Last edited by BernieNUFC; 21-03-2016 at 08:45 PM. Reason: Change of heart

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