Accuracy question

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  • Accuracy question

    I have a cylindrical part with a 4" bolt circle on one end with the 3, 10-32 tapped holes located at 30, 150 and 270 degrees. The way this is dimensioned, the 30 and 150 degree holes are located at a specific dimension from the centerline of the cylinder with a tolerance of +/- .0005, and then the bolt circle has a +/- .0005 tolerance.

    I aligned with this end geometry in Y- with the 30 and 150 holes rotated to X+, and then measured the distance from the center to the two tapped holes relative to the Z axis and also reported polar radius to see if, should the bolt circle be out of spec, if I could get a better idea of location of the tapped holes. I found the distance from center to two tapped holes both off by .001+ and the bolt circle to be under by .0015. The person I was measuring this for said they didn't trust the CMM and proceded to manually measure getting slightly different results.

    Is the CMM capable of these small tolerances and is there a better way I could approach this.
    sigpic
    Just a scooter pilot

  • #2
    What is the stated accuracy of your machine from last certification?

    Yes there are a few CMMs that can hold that. I suspect you can come closer with your machine than the alternative method. Even if they got "slightly better" on the surface plate, they are still out, right?

    The only thing I can add is you are probably using the pitch function and checking the minor diameter. You could use flex plugs to check the pd of the threads. HTH

    P.S. If you are feeling onery you could inquire how the design engineer expected this to be inspected, much less manufactured. hahahahahahahaha
    sigpic"Hated by Many, Loved by Few" _ A.B. - Stone brewery

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    • #3
      Thanks for your response Wes.

      Our CMM was certified about 3 weeks ago and found well within the stated .0004 volumetric accuracy and I was using flex-type plugs.
      sigpic
      Just a scooter pilot

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      • #4
        I would think using the pitch function would be more accurate than the flex plugs. It might be worth a try.

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        • #5
          Jo-plugs would be the best way, if you use the thread take a bunch of hits 12 at least and use the guass filter. I would recheck it on a surface plate. With pins and a height stand. Set up on a plate will be key to reproducing valid numbers.
          I talk dirty to my cmm. Justn

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          • #6
            Just A Comment

            Just wanted to add my thoughts. The threaded holes are only going to be as accurate as the bored holes. I have in the past had them give me a part without tapping to check to see if the bored or drilled holes are to spec. If not, there is no way tapping will bring it in to spec. Some of our engineers will sometimes call out positions to the minor diameter knowing how difficult it is to measure close tolerance positions to P.D.
            Larry

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            • #7
              Originally posted by justncredible View Post
              Jo-plugs would be the best way, if you use the thread take a bunch of hits 12 at least and use the guass filter. I would recheck it on a surface plate. With pins and a height stand. Set up on a plate will be key to reproducing valid numbers.
              Excellent point. I still say too many people use the CMM for everything, we need to remember the old ways.
              When in doubt, post code. A second set of eyes might see something you missed.
              sigpic

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              • #8
                Originally posted by Larry View Post
                Just wanted to add my thoughts. The threaded holes are only going to be as accurate as the bored holes. I have in the past had them give me a part without tapping to check to see if the bored or drilled holes are to spec. If not, there is no way tapping will bring it in to spec. Some of our engineers will sometimes call out positions to the minor diameter knowing how difficult it is to measure close tolerance positions to P.D.
                Larry
                In general that is correct. but...
                Unless the threaded holes are milled then there is no real correllation. Tapping tends to follow the holes but not always, depends on the quality of the taps. The tolerance on the minor is many times that of the PD.
                Links to my utilities for PCDMIS

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                • #9
                  One of the biggest fiasco's I was ever involved in was due to a tap that did not run true to the through hole. I had measured the hole using a close fitting pin. It was a 1/2 x 13 2b thread, there was a tooling ball that was on a 2 inch long post that went into this hole. The ball was over .100" out of location. It turned out that the part was tapped by hand with a 2 foot long pipe that was L shaped so the tap started in at an angle. The part was about 3/4 of an inch thick and the threads at the bottom were visually off to one side. When you looked closely at the threads.

                  Another thing the needs to be understood is that no matter what method you use you will get different results if you do not use the exact points to set up on. A CMM uses three or more points to set up a plane these points may or may not be the exact high spot on the plane. A surface plate by nature can only hit on the three highest points on the plane. So you are going to get a different number. A CMM is going to measure at a certain height whether that is in the thread on the pitch dia if you used pitch correction, on a pin or on a threaded Jo plug. By hand you will never be able to exactly measure at that location. Results with a CMM should be Similar but will never be the same as when done by hand especially when you get into the 4th decimal point.
                  Tolerance challenged ... Living in the world of unseen lines.

                  This software isn't buggy its an infestation

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                  • #10
                    The input from all who responded is much appreciated.

                    The CMM was used for this measurement mainly because the directive from product engineering has been to use the CMM as "first priority" for measuring these tight-toleranced parts. The machine being used is a standard pfx MicroXcel 7105 w/TP2 and I am questioning whether this machine, very recently calibrated well within its .0004 volumetric accuracy, is capable of accurately peforming these measurements.

                    I felt that geometric dimensioning and and the use of jo-plugs in the part and a precision plate from our Moore jig borer could quickly perform a go/nogo test.
                    sigpic
                    Just a scooter pilot

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                    • #11
                      It is good enough to measure the part, use the filters and do a GR&R on it. If set up right you should be capible to about 3 tenths. I can get mine to <20% on a .0002 tol. for a GR&R. Diffrence is I take scans with 100 points that I can then apply the filters to. Take a "boatload" of hits on the plugs, filter it to std dev 2 and outlier to 5.

                      Does the bolt ø have a TP callout or just the ±.0005?
                      I talk dirty to my cmm. Justn

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