Ball bar test results - trying to interpret - Page 2
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  1. #21
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    Dec 2014
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    I agree with the other posters that think you wasted your money. A ball bar test is only as good as the guy setting it up and being able to interpret the results to you, which they did not do. I like seeing an indicator move while checking for lost motion and aligning machines.

    What I normally do with a box way machine is adjust all of the gibs, eliminate mechanical backlash in the screws/support bearings, then run a backlash program for all of the axis to set the backlash comp.

    Gib adjustment: Set-up a mag base and indicator, the base on the top half of the slide and the indicator on the way. Attach a large bar to the top half for leverage to push and pull. When a gib is adjusted properly you will see .0005" to .001" movement but the indicator will return to zero. If you push and pull and the reading stays at .001" adjust the gib slightly.

    Backlash on ballscrews: set-up a mag base and indicator with mag base attached to slide and indicator tip on the ballscrew where the balls ride. Jog the slide back and forth and you will see the lost motion when changing directions. On a good screw .0005" is the most I would want to see, if there is more than that I would get the screw rebuilt or replaced, but it also depends on your part tolerances.

    Support bearings on the ballscrews has already been covered by Matt, a mag base and indicator with indicator tip on the end of the screw. You don't want to see any lost motion here. If the screw is moving the bearings are either shot or they are not trapped in the housing properly.

    You never stated what control you have, but most controls have backlash compensation. Once the mechanical lost motion has been minimized I write a backlash program for every axis. The program moves the slide 1 inch, then I set an indicator to zero, move in the same direction previously moved .008", then move the other direction .008". The difference from zero on the indicator is the amount you put in the backlash compensation in the control. Note: using the pulse handle is not an accurate way to check backlash.

    Once you do these steps in the order I have written them your parts will check much better (the backlash might change after a gib adjustment). If your part tolerances are really tight and the control has pitch error compensation you can hire someone with a laser to set pitch error of the ballscrews. The servos can also be tuned so the commanded positions are more accurate, but that also depends on the control.


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