Concentricity Help!

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  • kbotta
    replied
    Here is a little more info - pulled off the web...

    Concentricity is complex and rare as it controls opposed median points to a datum axis. Concentricity will control location and only has some effect on the form and orientation of the feature. Concentricity will not control the form of perfectly oval parts but may have an impact on irregular or "D" shaped features.

    : Concentricity may be verified with dial indicators, a CMM or by other methods. If dial indicators are used, two diametrically opposed, mastered indicators are placed on either side of the feature and positioned and rotated about the datum. Several readings are obtained at each selected cross section along the entire length of the feature.

    : To measure for concentricity: Imagine or draw a directional compass on a piece of paper with the north, south, east and west indicated on the compass. Now locate one dial indicator at the east position and one dial indicator at the west position. Lets say that the east dial indicator is reading east and moving east at .010" and the west indicator is reading west moving west at .010". You would subtract the indicator readings from each other to obtain the concentricity tolerance which is .000= .010-.010 at that particular cross section. The indicators have "cancelled" each other out since they moved in opposite directions during measurement. Now rotate the part 90 degrees so the opposed dial indicators are at the north and south positions. Lets say that now you have a reading at the north position and moving south .005" and at the south dial indicator you have a reading of .005 moving in the south direction. You now add the reading together to get .010 = .005+.005. Since both indicators changed in the same direction (south) you would add the indicator readings to obtain the concentricity tolerance. You would repeat this process at many cross section along the entire length of the feature in question.

    : So the question one needs to ask when deciding on TIR vs. Concentricity is what do you want to control? surface deviations from a datum axis (TIR) or opposed points (sum and diff) relative to a datum axis (Concentricity)?

    : Hope this helps

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  • Underspec
    replied
    Will is hilarious...I'm sure he smokes a few bowls here and there...

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  • kbotta
    replied
    Will ferrel (sp?) the cow bell skit. I laughed so hard when I seen that...I just had to put it on here...

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  • cmmguy
    replied
    isnt that from sat nite live skit?? I can't remember who they were mocking.

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  • george frick
    replied
    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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  • Schrocknroll
    replied
    And you can specify the location in true position whereas concentricity is always the same location as the reference feature.

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  • kbotta
    replied
    I would concur.
    Kev

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  • george frick
    replied
    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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  • kbotta
    replied
    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

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  • kbotta
    replied
    It is giving you a position (TP) to a chosen datum feature. It is giving you a coaxial result.

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  • Underspec
    replied
    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.

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  • cmmguy
    replied
    Originally posted by kbotta
    ( and no I'm not leg humpin )

    Kev
    I about died laughing on that. Thanks!!

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  • kbotta
    replied
    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

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  • cmmguy
    replied
    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

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  • george frick
    replied
    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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