Concentricity

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  • Concentricity

    Good Morning,
    I need verification on the method that PC DMIS uses to calculate the concentricity of a circle. This will be printed out for the Upper Mgt in an attempt to acquire a tolerance deviation. While I gave an answer, I need the support of this posting for my answer.

    The question is:
    When calculating concentricity is the deviation (ie.. dev = .900mm) a unilateral or a bilateral value? As a linear distance or a range?

    Thanks in advance,
    A.Gore
    sigpicA.Gore

  • #2
    It is from zero (the target, also the low limit) to the limit specified in your FCF. I may not understand your dillema and exactly what you need to know though.
    <internet bumper sticker goes here>

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    • #3
      PC DMIS does not do a true Concentricity. PC DMIS basically reports TP. You input a unilateral tolerance just as in TP. PC DMIS will report a TP style Deviation (radial deviation x2).
      Bill Jarrells
      A lie can travel half way around the world while the truth is putting on its shoes. - Mark Twain

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      • #4
        What the situation is, I was in a conversation with our water walkers in Engineering about the concentricity of one of our parts. Although I stated that the results were a linear distance from the center of the datum a (cylinder), he wanted me to investigate this a little more thoroughly. The result is a hypotenuse (sp), or a tp value from the theoretical zero. so the supporting answer is:
        a unilateral dimension from the theoretical zero. Am I right?

        Thanks,
        A.Gore
        sigpicA.Gore

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        • #5
          Originally posted by ag162 View Post
          What the situation is, I was in a conversation with our water walkers in Engineering about the concentricity of one of our parts. Although I stated that the results were a linear distance from the center of the datum a (cylinder), he wanted me to investigate this a little more thoroughly. The result is a hypotenuse (sp), or a tp value from the theoretical zero. so the supporting answer is:
          a unilateral dimension from the theoretical zero. Am I right?

          Thanks,
          A.Gore
          It is diametric. The result is the linear (radial) distance times two if I am not mistaken. You have a Cylindrical Tolerance Zone.
          Last edited by Wingman; 08-15-2007, 10:57 AM.
          Bill Jarrells
          A lie can travel half way around the world while the truth is putting on its shoes. - Mark Twain

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          • #6
            I am pretty sure it is the average range of all the diametrically opposing points back to centerline

            Kinda Like

            d1-d2=v1 (d1 being distance from point to centerline & d2 being the opposite point to centerline)

            Then the range of all v's

            Atleast thats how i understand it......??

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            • #7
              It is supposed to be what Spaz describes but CMMs cant measure that way, so it ends up being the radial deviation of centerlines TIMES 2 as Wingman describes. TP and Conc will give the same results.

              (It is the diameter about the datum that is defined by the centerpoint of the measured feature)
              Links to my utilities for PCDMIS

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              • #8
                Well, according to the Spec all median points must fall within a cylindrical tolerance zone. Median points are diametrically opposed points. PC DMIS does not calculate Concentricity according to spec. PC DMIS treats Concentricity as a True Position. The Centroid or Axis of the feature muct fall within the cylindrical tolerance zone.

                Not exactly correct but the true value of concentricity is suspect anyway (if you ask me).

                BTW, the 2nd feature is the PRIMARY DATUM feature. If you specify only one feature then the Workplane is the Primary Datum Feature.
                Bill Jarrells
                A lie can travel half way around the world while the truth is putting on its shoes. - Mark Twain

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                • #9
                  Thanks for the input. I learned something new today, while I was incorrect about how this is derrived, it is always good to ask and learn the correct answer.
                  However, if I put a part on a V block, used an indicator, zeroed out on the dat a cyl, rotating it, then moved to the area to be measured. Lets say that i had .15mm dev on the indicator, and I had a .1mm tolerance to dat A. How would I calculate that? Just for my curiosity.

                  Thanks,
                  A.Gore
                  sigpicA.Gore

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                  • #10
                    The answer you get with your method is a good answer - no doubling as a CMM does. However, checking concentricity in a vee block presents other problems due to form...
                    Links to my utilities for PCDMIS

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                    • #11
                      Linear distance from center to center X2.

                      I put a 20mm gage pin in a V block and made it origin, replace it with a 25mm gage pin and probed a circle. The distance between the 2 centers was 3.548, the concentricty result was 7.097 (3.548 X 2).

                      Experiment finished,

                      TK
                      sigpicHave a homebrew

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                      • #12
                        Originally posted by tking View Post
                        Linear distance from center to center X2.

                        I put a 20mm gage pin in a V block and made it origin, replace it with a 25mm gage pin and probed a circle. The distance between the 2 centers was 3.548, the concentricty result was 7.097 (3.548 X 2).

                        Experiment finished,

                        TK
                        Not so fast... he said indicator not CMM... you know, the old-fashioned way.
                        Links to my utilities for PCDMIS

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                        • #13
                          Why not use variables to calculate concentricity?

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                          • #14
                            Originally posted by spazus_maximus View Post
                            Why not use variables to calculate concentricity?
                            Why not throw concentricity out the window as the bad idea it really is.

                            1) I doubt the Engineers that use it really know why they are using it.
                            2) What does it 'really' return? When is it 'really' ever needed.
                            3) True Inspection is BIG $$ and there is dubious return.

                            Check with the Eng but I can almost guarantee they just want the two features to be on the same centerline. TP/ Coaxiality - call it what you want.
                            Bill Jarrells
                            A lie can travel half way around the world while the truth is putting on its shoes. - Mark Twain

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                            • #15
                              Originally posted by cmmguy View Post
                              Not so fast... he said indicator not CMM... you know, the old-fashioned way.
                              So fast, what? If I understood what Mr. Gore asked it was what did the demon spit out when the concentricity function was used. That is all MY experiment provided. Did I miss something? As to the indicator I was not commenting, K?

                              TK
                              sigpicHave a homebrew

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