Concentricity Help!

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  • #31
    Kevin,

    I am going to make a friendly challenge to the ASME definition of Concentricity. I understand the definition and while I agree that it may be the theoretically correct way to measure concentricity, it has been customary to measure concentricity by using the center point of the bestfit feature in question as the point to evaluate and not the midpoint of each and every diametrically opposed point. I don't know of a single CMM software that does it this way and every single one of them has a Concentricity Button on them. Maybe a GIDEP report should be issued.

    To further the argument that it is acceptable to do it this way, review the definition of a DIAMETER. It is the distance between to diametrically opposed points. Now, we measure and report diameters day in and day out on our CMMs. Are you saying that we shouldn't do that because it violates the definition? I would say that diameters are of higher importance than concentricity in the scheme of things, so if it is good for the diameter it is good for the concentricity.


    Your opinion?
    Links to my utilities for PCDMIS

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    • #32
      I guess I would say, the difference in a DIA and concentricity is, with a diameter you are evaluating the center pt. With concentricity you are evaluating multiple 'mid-pts', and the entire pt cloud. They are BOTH tolerances of location (as is symmetry, but we'll save that one for another time )

      I understand where the concern is (believe me I do). Just how many out there check concentricity this way? It is long and drawn out, no? Why not have a position symbol, and be done with it? (Here is where quality and inspection would need to voice there opinion in the design phase.)

      A Gentleman who is on the committee for 14.5 has for years trained our eng. and Mgr's and quality personal. I tried (I say tried) to argue these pt's when I first met him.

      Then I started to really get into GD&T. And I underwent a serious study regimen, making numerous phone calls/visits to many GDTP's - and discussing many topics and how they relate to CMM's.

      My friend said to me:"As your knowledge grows you can do 1 of 2 things. You can choose to be on the leading (and sometimes bleeding) edge of the sword, or you can continue to do as you always have."

      I chose the bleeding one

      Many times you will see my post's are a little 'anal' retentive when it comes to things dealing with GD&T and CMM's. This is why. I try to have the standard back up what I say on here. So...don't kill the messenger

      I am just trying to increase the awareness of other CMM programmers, who may not know - or may not have been correctly schooled in dimensioning and tolerancing according to ASME.

      You can either take heed, or take it with a grain of salt.
      The choice is up to you...

      Kevin
      RFS Means Really Fussy Stuff

      When all you have is a hammer - everything looks like a nail....
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      • #33
        Very well stated Kevin
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        • #34
          Excellent answer Kevin. I too have been involved in some interesting GD&T discussions, though I can tell your knowledge is stronger than mine. The issue here is usually flatness. I see too many people place the part on the surface plate, indicate the surface that requires flatness, and report that . "EXCUSE ME, you just checked parallelism." Flatness is all the points in a surface in relationship to itself. Concentricity is all the points in a diameter in relationship to the same polarly located point on the "datum diameter".
          When in doubt, post code. A second set of eyes might see something you missed.
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          • #35
            i have studied gd&t under bill tandler and asme. as bill once told me:"gd&t is a work in progress". at times i choose to do what i've always have- just because it makes more sense to me and not what someone comes up with in a "think tank" only to be changed next year. see my new post
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            Southern Man don't need him around anyhow!

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            • #36
              Kevin,

              I think you somewhat missed my point. I was suggesting that we make compromises all of the time when we use CMMs to measure features, concentricity included. I was using the diameter as another example. We obsess over diameters: how many points to take, what is the correct formula to use(best fit, mrs, mic & mcc), what size styli to use, scan or not to scan, blah, blah, blah... BUT by definition the CMM can't measure a diameter correctly, so we compromise.

              Diameter Definition:
              The length of a straight line segment passing through the center of a figure, especially of a circle or sphere, and terminating at the periphery.

              So that would leave you with only a bore gage to measure a diameter.


              Back to measuring concentricity, I would say that the centerpoint method used by every CMM software is also a reasonably acceptable compromise, until a better one comes along. It effectively is true position at RFS. I would also say that the current definition of concentricity maybe thoeretically impossible to measure on a cmm or any other gage for that matter. Unless your datums are centers, one can not achieve a reliable datum from which to measure, due to all of the other compromises - back to that diameter thing.

              The intent here was not to start a one-upmanship on knowledge or to hijack this thread, but to discuss the weaknesses of the CMM and to understand those weaknesses and to also understand the measurement compromises made to overcome those weaknesses.

              Out of compromises, come new definitions. You are right to maintain your vigilance on the theoretically correct way to do things. It is that vigilance that keeps the compromise as close to the original intent as possible. (gee it sounds like someone talking about the constitution, but that is another subject )

              Thanks
              J
              Links to my utilities for PCDMIS

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              • #37
                In no way was I trying to start a 'one-upmanship', But its a cool word

                I like to debate this stuff. We are the future, as well as the present. I do agree that we are a compromising bunch. For those of us that know that we are compromising. There are those out there that have no idea, and simply hit the button - because that is what it says on the drawing. Not there fault, it is how they are/were trained.
                I am all about trying to increase everyone's knowledge, including my own.
                Between you and I, I do agree with you. I know certain other people that would also, but the standard bangers won't budge with the ISO community. They have been compromising on different things for years trying to get them 'closer' together. 1 thing of note that they won't budge on is the envelope rule (rule#1). It's insane. Hellow bleeding edge

                I appreciate your professionalism ( and no I'm not leg humpin )

                Kev
                RFS Means Really Fussy Stuff

                When all you have is a hammer - everything looks like a nail....
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                • #38
                  Originally posted by kbotta
                  ( and no I'm not leg humpin )

                  Kev
                  I about died laughing on that. Thanks!!
                  Links to my utilities for PCDMIS

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                  • #39
                    Someone please explain how exactly PCDMIS calculates concentricity?? I've heard from a few now that it does not do it right. So now I'm in doubt about my dimensions and am reluctant to submit a first article report lol.
                    I used to be high on life but I built up a tolerance.

                    Brown & Sharpe Global Advantage
                    PCDMIS CAD++ v2011mr2
                    PH10MQ/SP600M


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                    • #40
                      It is giving you a position (TP) to a chosen datum feature. It is giving you a coaxial result.
                      RFS Means Really Fussy Stuff

                      When all you have is a hammer - everything looks like a nail....
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                      • #41
                        Here is real simple test. I measured 2 cricles. 1st ouput like concentricity (pcdmis), 2nd I set origin to 1st cir and dim tp on 2nd cir. Notice any similarities?

                        DIM CONCEN1= CONCENTRICITY FROM CIRCLE CIR1 TO CIRCLE CIR2 UNITS=MM ,$
                        GRAPH=OFF TEXT=OFF MULT=10.00 OUTPUT=BOTH
                        AX NOMINAL MEAS +TOL -TOL DEV OUTTOL
                        M 0.000 0.019 0.010 0.000 0.019 0.009

                        A1 =ALIGNMENT/START,RECALL:STARTUP, LIST= YES
                        ALIGNMENT/TRANS,XAXIS,CIR1
                        ALIGNMENT/TRANS,YAXIS,CIR1
                        ALIGNMENT/END

                        DIM LOC1= TRUE POSITION OF CIRCLE CIR2 UNITS=MM ,$
                        GRAPH=OFF TEXT=OFF MULT=10.00 OUTPUT=BOTH DEV PERPEN CENTERLINE=OFF DISPLAY=DIAMETER
                        AX NOMINAL MEAS +TOL -TOL BONUS DEV OUTTOL
                        X 0.000 0.006 0.006
                        Y 0.000 0.008 0.008
                        TP RFS 0.010 0.000 0.019 0.009
                        END OF DIMENSION LOC1

                        Both results the same...Should not be..
                        kb
                        RFS Means Really Fussy Stuff

                        When all you have is a hammer - everything looks like a nail....
                        sigpic

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                        • #42
                          concentricity calculation

                          Originally posted by kbotta
                          Both results the same...Should not be..
                          kb
                          the reason is: in "our software" the only difference in the calculation of true position vs concentricity is that you can use mmc/lmc in the true-position calculation.
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                          Southern Man don't need him around anyhow!

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                          • #43
                            I would concur.
                            Kev
                            RFS Means Really Fussy Stuff

                            When all you have is a hammer - everything looks like a nail....
                            sigpic

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                            • #44
                              And you can specify the location in true position whereas concentricity is always the same location as the reference feature.
                              PC-DMIS 2016.0 SP8

                              Jeff

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                              • #45
                                new avatar

                                Originally posted by kbotta
                                I would concur.
                                Kev
                                hey kev ROFL at new avatar. that is REALLY funny. WHO IS THAT?
                                Last edited by george frick; 04-19-2006, 01:23 PM.
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                                Southern Man don't need him around anyhow!

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