Ball Bar Calibration

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  • 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
    PC DMIS 4.2
    Iota 2204
    Sharpe 32 controller

  • #2
    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
    Iota 2204
    Sharpe 32 controller
    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
    B&S Mistral
    3.207 Beta on XP

    Older'n dirt

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    • #3
      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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      • #4
        If you got the Ballbar from Renishaw it should have come with a software pakage that will walk you thru most all you need.

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        • #5
          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
          sigpic"Hated by Many, Loved by Few" _ A.B. - Stone brewery

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          • #6
            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.
            sigpic

            James Mannes

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            • #7
              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.
              sigpic"Hated by Many, Loved by Few" _ A.B. - Stone brewery

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              • #8
                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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                • #9
                  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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                  • #10
                    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.
                    sigpic GDTPS - 0584

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                    • #11
                      Originally posted by ptedrum View Post
                      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.
                      The certs you got from BalTec are really only for the Spheres and not for the length between the balls. (at least the last one I bought from them were that way.) James is right about them only checking volumetric accuracy. It is still a good intermediate check, but understand that you are checking the geometry of the machine and not much more. It may signal a machine gooing out. But the earlier recomendation of a GoldenPart is really the best way... Return the ball bar, take the restocking hit and move on... your boss should have asked you before hand.

                      Jim Harris: I agree with you fully... What B&S does with only a linear ball bar is not a calibration. They should be using a liner step gage... and everyone out there should call them on it and make them prove the machines are linear.
                      Links to my utilities for PCDMIS

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                      • #12
                        It sounds like ptedrum just wants to monitor his cmm and not actually try to adjust anything. He wants to do a verification.

                        The distance between the balls does not matter as long as it doesn't change! If his machine is truly calibrated so that 1" measures 1", 1' measures 1' etc. he can run the 'ball bar' and dimension the distance between the two. It doesn't matter what that distance is as long as it does not change. If it changes then there is a problem.

                        He could in effect use the ball bar as his "golden part". You guys have a golden part which is actually a part. Was it calibrated? No, you just make sure the distance between those two bores doesn't change.

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                        • #13
                          Originally posted by Goodluck View Post
                          It sounds like ptedrum just wants to monitor his cmm and not actually try to adjust anything. He wants to do a verification.

                          The distance between the balls does not matter as long as it doesn't change! If his machine is truly calibrated so that 1" measures 1", 1' measures 1' etc. he can run the 'ball bar' and dimension the distance between the two. It doesn't matter what that distance is as long as it does not change. If it changes then there is a problem.

                          He could in effect use the ball bar as his "golden part". You guys have a golden part which is actually a part. Was it calibrated? No, you just make sure the distance between those two bores doesn't change.
                          I agree with you, but he used the words calibration and even compared it to what he saw B&S do and mentioned that it was calibrated. Using a Ball bar by someone who does not do it every day, such as a CMM tech, is not easy. Moving it to the different posistions and get repeatable results so that you can say your machine is good will consume a lot of time.

                          What you said is possible. However, by his question, the OP seemed a bit on the beginner side. I dont want to discourage someone from trying something but using a golden part will accomplish the same thing with a lot less hassle, anyone can do it for him once a program is set up.
                          Links to my utilities for PCDMIS

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                          • #14
                            I just bought a machine checking gage from hk mfg.It comes with software that tells you if your machine is still square & liner.I will be using that from now on to keep track of my machines daily.It works pretty slick.

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                            • #15
                              Originally posted by JIM HARRIS View Post
                              I just bought a machine checking gage from hk mfg.It comes with software that tells you if your machine is still square & liner.I will be using that from now on to keep track of my machines daily.It works pretty slick.
                              Hey Jim, can you shoot me an email?

                              james.mannes@leggett.com

                              Thanks
                              sigpic

                              James Mannes

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