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Profile Tolerance on a Datum

Loon4ever
We are having a disagreement here where I work. The older inspector is of the opinion that a Datum CANNOT have a profile tolerance. The rest of us have never heard of this and even have numerous examples of customer prints where they have Datums with profile tolerances. The custoer isn't always right however.

I understand his reasoning, Datums are "Zero" and such can't/shouldn't deviate, but I also understand that this isn't a perfect world where Datums can't deviate. So personally I believe that there is nothing wrong with giving a datum a profile.

Can anyone point me to a specific document, Preferably the ASME Y14.5 spec that says that Datums can't be given a profile tolerance?

Thanks in advance.
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  • Datum surfaces most certainly can have profile tolerance. A datum target POINT could not, on its own, but as a set of datum target points (4 or more), profile (F) could be applied, or with 3 clamped and 1 not clamped. The datum surface has a special constraint though, it technically can never be outside of material (+ condition).

    For example: if your primary datum feature is 3 target points on a plane, those 3 target points are 0, and the rest of the surface except those 3 points can be +/- tolerance. But, if the entire surface is your primary datum feature, then it should "contact" the 3 highest points (the datum simulator), and the rest of the surface can only be minus material.

    As mentioned, flatness is a profile control that is only considering form, by having no datum reference frame.

    There might be specific examples where it isn't applicable for some reason, or fringe cases where it is questionable, but profile on datum features is definitely not prohibited by ASME Y14.5.
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  • Datum surfaces most certainly can have profile tolerance. A datum target POINT could not, on its own, but as a set of datum target points (4 or more), profile (F) could be applied, or with 3 clamped and 1 not clamped. The datum surface has a special constraint though, it technically can never be outside of material (+ condition).

    For example: if your primary datum feature is 3 target points on a plane, those 3 target points are 0, and the rest of the surface except those 3 points can be +/- tolerance. But, if the entire surface is your primary datum feature, then it should "contact" the 3 highest points (the datum simulator), and the rest of the surface can only be minus material.

    As mentioned, flatness is a profile control that is only considering form, by having no datum reference frame.

    There might be specific examples where it isn't applicable for some reason, or fringe cases where it is questionable, but profile on datum features is definitely not prohibited by ASME Y14.5.
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