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Thread: GD&T Help

  1. #1
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    GD&T Help

    Good Morning,


    We have come to a debate here at work and would like to employ the counsel of other for a settlement.
    We have a drawing that calls out MMC on the secondary and tertiary datums of the feature control frame.
    The problem is the position of the holes on my bolt circle pattern are out of tolerance and I do not know how to apply these "bonus" tolerances.
    I am able to continue with this part or I should red tag it.
    Let me explain a little more in depth. I was able to CMM this part.
    My worst hole in the hole pattern is .041" position.


    DATUM A is the plane on which the bolt circle lies.
    DATUM B is 17.000 ( .005), Actual is @ 17.005
    DATUM C is .160 ( .005)located @ 10, Actual is @ .165
    All Bolt circle hole 's are at nominal.


    I have added a screenshot of such drawing.question.pdf

  2. #2
    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by VMORALES View Post
    and I do not know how to apply these "bonus" tolerances.
    You need to learn how to apply and calculate the datum features bonus tolerance.

    Quote Originally Posted by VMORALES View Post
    I am able to continue with this part or I should red tag it.
    Reject a potentially good part? no..

    You have not provided enough information to evaluate the acceptable position tolerance of the bolt circle related hole features.

    What are the actual Datum features B and C? are they internal feature of sizes or external?

    What is the as-built sizes of your hole features?

    See: http://www.engineersedge.com/geometr...ies-2-book.htm
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  3. #3
    Principle Engineer Cragyon's Avatar
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    Quote Originally Posted by VMORALES View Post
    I do not know how to apply these "bonus" tolerances.
    I am able to continue with this part or I should red tag it.
    You should red tag yourself...

  4. #4
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    yea the actual DATUM A is the plane on which the bolt circle lies.
    DATUM B is 17.000 ( .005), Actual is @ 17.005
    DATUM C is .160 ( .005)located @ 10, Actual is @ .165
    All Bolt circle hole 's are at nominal.
    Datum B is an Internal Diameter.
    Datum C is as well an internal Diameter on the same BC.




  5. #5
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    Quote Originally Posted by Cragyon View Post
    You should red tag yourself...
    thanks

  6. #6
    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by VMORALES View Post
    yea the actual DATUM A is the plane on which the bolt circle lies.
    DATUM B is 17.000 ( .005), Actual is @ 17.005
    DATUM C is .160 ( .005)located @ 10, Actual is @ .165
    All Bolt circle hole 's are at nominal.
    Datum B is an Internal Diameter.
    Datum C is as well an internal Diameter on the same BC.

    So, you get a .010" bonus tolerance from Datum B and another .010" bonus tolerance from datum C.

    Quote Originally Posted by VMORALES View Post
    All Bolt circle hole 's are at nominal.

    And.... you're still not giving enough information to evaluate total tolerance available..

    What is the size specification for the hole features?

    We offer best in industry practical GD&T training see: http://www.engineersedge.com/GDT_Training.htm
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  7. #7
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    thanks for the response, why would i get .010 bonus tolerance? would it not be half of the total tolerance since it is a diameter? the holes that lie on the BC are .235 +/- .005 and i am measuring them out at .235.

    thanks in advance.

  8. #8
    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by VMORALES View Post
    thanks for the response, why would i get .010 bonus tolerance? would it not be half of the total tolerance since it is a diameter? the holes that lie on the BC are .235 +/- .005 and i am measuring them out at .235.

    thanks in advance.
    You should and maybe be measuring diametric tolerances - that is an unknown..

    Therefore, you get another .005 bonus tolerance from the as-built size of the hole feature.

    Total tolerance given by your MMC modifiers is: .005 + .010 + .010 + .020 = .045" total allowed position tolerance.

    Your part would be most correctly verified with a functional check gauge.

    This is as far as I go... You need significant dimensioning and tolerancing training. MMC modifier's require more time and typing then is intended for an internet forum.

    I'm not going to spend hours training you here.

    Check out the links provided and consider stepping up your working knowledge of ASME Y14.5-2009.

    Good luck..
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  9. #9
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    The extra tolerance from the datums (not really "bonus," but "datum shift") can't be added into the tolerance you're trying to measure. This is because they act in different directions. So it's a bit more complicated, with multiple variables.
    Check out some GD&T training, or do a web search for: datum shift tolerance calculation.

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