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Thread: How to calculate a 'bonus' for a profile which is 2U0.3 AB(M)C(M)

  1. #1
    Associate Engineer
    Join Date
    Mar 2014
    Sao Paulo, Brazil

    How to calculate a 'bonus' for a profile which is 2U0.3 AB(M)C(M)

    Hello Srs.

    I'm trying to calculate a 'bonus' for a profile which is 2U0.3 AB(M)C(M)
    M as maximum material
    A n B n C are datums

    But I'm not finding any material which would support me on doing 'bonus' for unilateral profile.
    Do you guys know how to do this math?

    Everything I learned was only to calculate 'bonus' on bilateral tolerance.


  2. #2
    Project Engineer
    Join Date
    May 2011
    First, a minor terminology thing... the (M) that comes after a datum letter doesn't create a bonus tolerance. It does allow for extra tolerance in a way, but the proper name for it is shift tolerance. Bonus tolerance is what happens when the (M) follows the geometric tolerance number.

    But as to your specific question, it is difficult to calculate this shift tolerance because it involves two datums. (And it's not because of the unequal distribution of the profile tolerance.) Essentially, the shift tolerance is the allowable looseness that may result around those datums when the actual datum features are different -- such as a datum hole that is made at the larger diameter, rather than its (M). If you can picture this: you might have some looseness around your datum feature B, and also looseness around datum feature C. But we can't add those loosenesses together to get a grand total, because each looseness may act in a different direction (or different degree of freedom).

    So it comes down to this: If you can get away with a physical gauge to verify that profile tolerance, then it's easier because you can literally drop the part onto the gauge and wiggle it around those datums until the profile outline conforms to the profile tolerance boundaries. But if you need reportable data on the measurement then a CMM may be the only option; a CMM would do some math to "best-fit" the datums in order to report what the profile error is.
    Last edited by Belanger; 01-22-2016 at 07:44 AM.

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