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IndustryArena Forum > WoodWorking Machines > Commercial CNC Wood Routers > Chinese Machines > Problem with precision in 6040 cnc machine
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  1. #1

    Angry Problem with precision in 6040 cnc machine

    Hey all,
    This is my first post ????

    I have a problem with my cnc machine
    It's a 6040 with servo motors and a cooling option with a bath.

    Trying to mill a part but all of them have a problem with the dimensions of I mill a square 50*50mm it will be 50.2*50.6
    And if I do a circle of D12mm or will be 12.2 on x axis and 12.6 on y axis

    Did calibrate the steps using an indicator and have 0.02mm error

    Did cut a 90deg side in wood and it looks ok

    What can be the problem?
    And where I can find more info about the machine and how to calibrate

    Thank you very much!

  2. #2
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    Re: Problem with precision in 6040 cnc machine

    Hi,
    most probable explanation is that the machine is flexing under cutting forces.

    To establish this, or disprove it, do a 50mm square but VERY slowly with the lightest possible cut and measure the result.
    Now run the same toolpath but at regular cutting speed and much more tool engagement: measure the result.

    If the two runs differ significantly then machine flex is the cause.

    Craig

  3. #3

    Re: Problem with precision in 6040 cnc machine

    Forgot to add this info
    Using 3 mm flat tool
    1 mm engagement
    1 mm DOC
    And 200mm/min
    Is it fast for wood?

    And this is a new machine so maybe I'm missing something in the settings?
    Using usbcnc program

  4. #4
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    Re: Problem with precision in 6040 cnc machine

    Hi,

    And this is a new machine so maybe I'm missing something in the settings?
    Yes, that is possible but it's also possible that the machine is flexing. Are you going to try to determine if that is the case?

    Craig

  5. #5

    Re: Problem with precision in 6040 cnc machine

    Hey
    I did a 10*10 square and on the x axis it's 10.05 and on y axis it's 10.3
    Did it at 50mm/min speed extremely stow??

  6. #6
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    Re: Problem with precision in 6040 cnc machine

    Hi,
    do it at two speeds, or rather more importantly, two loads; lightly loaded and then heavily loaded and compare
    the measurements.

    Craig

  7. #7
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    Re: Problem with precision in 6040 cnc machine

    Normally machnists use indicators. Can be precission 10 microns or 2 microns or 1 micron. You should have tho one as well. With this indicator you can check accuracy, repeatability of the movement easy. You can try to push the spindle nose by hand and see on the indicator how rigid your machine is.

  8. #8

    Re: Problem with precision in 6040 cnc machine

    Yes I did this test and it's not great its 150-200um of movement
    But as far as I understand if I goo slow it should not affect the mill no?

  9. #9
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    Re: Problem with precision in 6040 cnc machine

    check repeatability as well ... backlash ... and so ...

  10. #10
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    Re: Problem with precision in 6040 cnc machine

    Hi,
    Yes I did this test and it's not great its 150-200um of movement
    But as far as I understand if I goo slow it should not affect the mill no?
    Ok, that gives a rough idea of how much your machine flexes when cutting forces are encountered. Its a fact of life. Even huge cast iron machines
    of 30 tons flex under load....but much less...say 1um per 750N of force....but they still flex.

    What you have to decide is how accurate you need any given part needs to be and then machine in such a manner that the flex does not exceed
    that tolerance. Regrettably small Chinese machines like this disappoint when that limit is established.

    Craig

  11. #11

    Re: Problem with precision in 6040 cnc machine

    Thank you for the tips
    After re calibrating the steps
    I did try to make a part with low speed end passes with small DOC and the x and y dimensions are the same
    But they are smaller then needed

    Do you recommend doing something to the machine or better compensate it in cam?

  12. #12
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    Re: Problem with precision in 6040 cnc machine

    Hi,
    have you positively established that flexure is causing the inaccuracy....or is it the machine settings, or perhaps a combination of both?
    You absolutely must dial the machine in to be accurate firstly without any cutting forces, ie spindle off and the tool above the material, then establish
    what inaccuracy occurs as you start cutting, and again when you are doing heavy cutting.

    I suspect a considerable fraction of the inaccuracy is due to flexure....and there is really nothing you can do in CAM to solve it, and often a complete rebuild with better materials, design
    and components required to improve the machine. More often than not its just not practical to improve these cheap machines. You'd be better off working to get the best possible
    accuracy by understanding the flexure and work within those limits.

    If that's not good enough for your work....get a better machine.....and this time pay particular attention to rigidity. Its the one aspect of of machine that cannot be corrected, its either built
    in or its not.

    Craig

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