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Thread: Ball Bar Calibration

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    Question Ball Bar Calibration

    I'm trying for the first time to do a ball bar calibration. The CMM has just had it's yearly calibration & my boss wants us to start to do monthly ball bar calibration to find out when the CMM would need to be calibated next. So this would be the first run. The bad news I've never done one before The guy who calibrated themachine was nice enough to give me a sheet with what position I would place the ball bar in to do the calibration but that is about as far as I get. Can anyone explain how to go about this &/or if someone has a simple program I can take a look at? Thanks
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    Quote Originally Posted by ptedrum View Post
    I'm trying for the first time to do a ball bar calibration. The CMM has just had it's yearly calibration & my boss wants us to start to do monthly ball bar calibration to find out when the CMM would need to be calibated next. So this would be the first run. The bad news I've never done one before The guy who calibrated themachine was nice enough to give me a sheet with what position I would place the ball bar in to do the calibration but that is about as far as I get. Can anyone explain how to go about this &/or if someone has a simple program I can take a look at? Thanks
    PC DMIS 4.2
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    Instead of trying to duplicate the ball bar cal create a "golden part" program. Retain a certain part and identify it as yor "golden part". It really doesn't matter what it is as long as it resembles what you typically check. Write a program to check a variety of features then run your "golden part" monthly and compare the results to previous months.

    This will probably be less time consuming than the ball bar cal and closer reflect the daily use of your CMM.

    HTH.
    Perry
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    That sounds like a really good idea but after the money that was spent on the ball bar I would get my *** kicked for not using the ball bar, thou you method makes a lot of sence & sounds like a better idea than the ball bar, none the less I should try to use the ball bar.

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    If you got the Ballbar from Renishaw it should have come with a software pakage that will walk you thru most all you need.

  5. #5
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    Cool

    Unless your bar ball has been calibrated traceable back to the national board of standards, what you are planning to do and what B&S did the other day, is NOT a calibration, but a certification of repeatability. They use the bar ball to confirm your machine repeatedly measures the distance between the ball the same all over the table, but if the balls are 6.75" reported apart, there is nothing saying that they really are 6.7500" apart, they could really be 6.7552" or 6.7473" and you would not know it. That is not likely. The machines rarely need true calibration and that is why B&S only does a certification unless you ask and pay for more. But you should be aware of the difference between a calibration to standards and a certification that repeatability meets factory specs. HTH
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    Quote Originally Posted by Wes Cisco View Post
    Unless your bar ball has been calibrated traceable back to the national board of standards, what you are planning to do and what B&S did the other day, is NOT a calibration, but a certification of repeatability. They use the bar ball to confirm your machine repeatedly measures the distance between the ball the same all over the table, but if the balls are 6.75" reported apart, there is nothing saying that they really are 6.7500" apart, they could really be 6.7552" or 6.7473" and you would not know it. That is not likely. The machines rarely need true calibration and that is why B&S only does a certification unless you ask and pay for more. But you should be aware of the difference between a calibration to standards and a certification that repeatability meets factory specs. HTH
    Actually, what they do is a volumetric accuracy and repeatabilty test.

    The volumetric is done with the ball bar. The repeatabilty is done with your qual sphere. Eh? pretty good for a late bloomer.


    James Mannes

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    Cool

    Quote Originally Posted by JamesMannes View Post
    Actually, what they do is a volumetric accuracy and repeatabilty test.

    The volumetric is done with the ball bar. The repeatabilty is done with your qual sphere. Eh? pretty good for a late bloomer.


    Yep, I sit corrected.
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    The Ball bar we have we just recieved from ball tec is traceable back to the national board of standards (ANSI/ASME-B89.4.1-1997) also our cmm was done to the same standard when it calibrated. I guess my real question is how do I make a program using a ball bar (monthly) to determine weather or not the CMM is deviating and would need to be re-calibrated.

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    I just went thru this argument with b & s again.I just had my 2 machines done to the ball bar & koba bar.What b & s does does not meet b89 spec.It says per 5.5.1 in no case should the ball bar tests alone be regarded as providing a measurement of machine accuracy.In this standard,accuracy is assessed in the linear displacement accuracy section(para. 5.4).even the tech that wast here admitted that.That is why I have went another route from now on.I have had this argument with b & s for 6 years.

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    Volumetric Efficiency is the greatest amount of variation between any two sphere to sphere measurements. Set up your 1st bar orientation, manually create the spheres, do an alignment to them, switch to dcc and measure them, record the distance. Repeat this with all of the bar orientations. Find the VE.
    GDTPS - 0584

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