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IndustryArena Forum > MetalWorking Machines > LittleMachineShop CNC Machines > LMS 3990 / HeavyMetalCNC conversion - backlash problem.
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  1. #1
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    LMS 3990 / HeavyMetalCNC conversion - backlash problem.

    LMS 3990 mill with the HeavyMetalCNC covnersion kit - but this isn't necessarily kit/mill specific


    (I have also asked HeavyMetalCNC for advice, but while I wait for a response, I though I would ask for some opinions here.)


    I have finally installed everything, dialed in all my axes, and have no binding and smooth moves.

    BUT – I have a big backlash problem on my Y-axis.

    X and Z – both have about a half a thou of backlash. Y – has .0331 !!!

    I tore everything down, set the Y axis gib perfectly, reinstalled the Y ballscrew and nut, and reattached everything to the frame. There is zero detectable play anywhere in the mechanical system. But when driving the axis – there is .0331 measurable backlash.

    If I tape a flag onto the stepper shaft, .0331 equates to about 1/8th of a turn before the ballnut engages. At 1/16th microstepping – that is about 400 steps…

    Test procedures -
    I measured by manually taking up the backlash, setting a test dial indicator to zero – then commanding a move away from zero, and return to zero – adjust the values until the dial does not move at all when taking up the backlash.

    i.e.
    -G0 Y -0.0331
    -G04 1000 (pause for a second so I can see the dial)
    -G0 Y0

    .0331 – needle does not move <- taking up the backlash in both directions.
    .0332 – needle moves a thou, returns to zero <- takes up the backlash, moves 1 thou
    .0431 – needle moves to 10 thou, returns to zero <- takes up the backlash, moves 10 thou

    I didn’t take any pictures or video, but I can if it will help explain better. (I am a total noob.)

    So I believe it to be entirely a ballnut/ballscrew issue – don’t know which of the two or a combination of both.

    What do you suggest?
    -Send the ballnut ballscrew back to vendor, and see if they can pre-load it better?
    -Try to pre-load it myself? (I don’t know how, or what size balls, etc…)
    -Buy a new ballnut/ballscrew pair, and hope they perform better? (X and Z are fine – so it may just be a one-off part issue)
    -Something else that I don’t know about that I can adjust?

  2. #2
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    Re: LMS 3990 / HeavyMetalCNC conversion - backlash problem.

    I used to have the same machine, check the thrust ball bearing on the Y Axis bracket, make
    sure that isn't the cause of the backlash.
    Just a thought.
    Larry

  3. #3
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    Re: LMS 3990 / HeavyMetalCNC conversion - backlash problem.

    Thank you - In the HeavyMetalCNC kit, it comes with two 608 style ball bearings (axial, not thrust), and they don't have any detectable axial motion. The screw position is very rigid.

  4. #4
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    Re: LMS 3990 / HeavyMetalCNC conversion - backlash problem.

    Update: I heard back very quickly from Bruce at HeavyMetalCNC, and he offered to redo the preload on the ballnut for me, so I am disassembling one more time....

    On the bright side, I can tear down the mill in 20 minutes now, after all the practice I have gotten.

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