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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.
  • DATUMS aren't perfect , DATUM SIMULATORS are. Start on Page 53 Y14.5-2009. All actual datums could have a " FORM " tolerance. So he is saying a DATUM -A- ( PLANE ) could have a flatness callout but not a profile ? Profile on a plane that is FORM ONLY ( not related to DATUMS ) is a Flatness callout..... Have him argue that one.
  • in reply to Schlag
    You tryna make this man annoy his company's FOG? Its not even Friday yet!
  • in reply to Schlag
    I have a zero tolerance policy on pretty much everything.....
  • 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.
  • in reply to Schlag
    I'm right there with ya, gotta toe the line Was just messin with ya =P
  • The "older inspector" confuses the real surface and the associated datum on it.
    If he is older enough, he confuses the granite surface where you put the part to simulate the datum !!!
    The granite was considered as a perfect plane, the datum was tangent to the surface, external side of the material, and minimizing more or less the max deviation...
    However, the surface of the part on the granit had defects... non measurable at this step Slight smile !
  • Well he still hates it but after 2 weeks of arguing with him he finally tried to put his money where his mouth was and was utterly dumbfounded that he couldn't find anything in the AMSE Y14.5 saying it wasn't allowed. Sometimes it's like arguing with a Wall. heh Thanks for all the responses.
  • Hopefully this conversation was had in such a way that the machinist learned about GD&T while gaining respect for your knowledge!
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