Ball bar test results - trying to interpret - Page 2
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  1. #21
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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.

    Daryl

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  3. #22
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    Quote Originally Posted by Other Brother View Post
    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.

    Daryl

    Thanks Daryl, that is all very helpful. The one machine is Heidenhein 425, the other 530. I'm not sure if the servos in the TNC 425 can be tuned, but the 530 can. Thanks for all the advice/information.

  4. #23
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    I would certainly start with adjusting the gibs. The lateral play is telling you the table is moving around during the travel. Loose gibs would be the first place I would look -- as mentioned by Other Brother.

    Next would be the ball screw or possibly the nut.

    A single ballbar test can help point you in the right direction but having the ability to do before/after runs is really helpful to see if you are going in the right direction as you make repairs and adjustments. Don't get caught up on the positional accuracy the ball bar quotes -- real parts are the true test. The ball bar is simply a diagnostic tool.

  5. #24
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    Thanks Bonestock. We are currently waiting for some downtime to begin correcting the issues. I will be sure to look at gibs and the ball screw and components. I see how having a ball bar on hand would be useful to confirm corrective actions. The parts we are pulling off the machine are passing quality so I'm not overly concerned with the result of the ball bar, just looking to better understand it; this is the first time I've seen a ball bar being used.

    -Murdered Fiat

  6. #25
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    i have customers go thru this regularly.
    i always recommend that before you start BB testing you start with the basic
    have your machine releveled and squareness checked with a granite sq
    perform gib adjustments as needed to correct skewing
    perform ball-screw inspection /testing check at end of screw, check at end of table with power on by prying/pushing table both way. also inspect each coupler from axis motor to ball-screw if keys are used in coupler remove them and inspect key and key way.
    once the machine is know to be mechanical correct then have the ball bar test done, remembering that the BB is a super precision device.
    if your parts are passing QC with no problems and you are not trying to hold tight tolerance work then you in all honesty probably do not NEED a BB test.


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