Effective Training Inc. Message Board

Need to know the correct callout for Datum C related to Datums A & B per the attached drawing.

The question is when inspecting the part, it is set on Datum A (primary), located off the hole centerline of Datum B (secondary) and Datum C (tertiary) is the centerline of another hole, should the postional callout for Datum C be diametric or linear? Considering by locating the centerline of Datum B and squaring the part with the centerline of Datum C, Datum C can only be out of position relative to A and B linear?

Which of the callouts shown is correct, or is the dimensional scheme incorrect altogether?

Any help here would be greatly appreciated.

Thanks in advance.

DDB

Dear DDB,
Unless there is a benifit to using parallel planes, such as a bi-directional tolerance of position, axis are always bound by the geometrical boundary of a cylinder. Using the cylindrical tolerance zone permits the position tolerance to apply to the exact value of 50.5 as well as the exact value of 20.

Since you're sharing I would like to share a few points to consider:

The drawing appears to be ISO standards based, but there are drawing practices used which are inconsistent with ISO standards. Namely the placement of the dimension line to the size dimension rather than the tolerance frame (feature control frame). If the drawing is not ISO based, but rather ASME based I would suggest consulting the Y14.5 standard. If you don't know what I'm talking about here, then identifing a standard is more important than resolving your datum specification delema.

The specification of (2x) is confusing. Where is the second hole or is that optional? If 2x is meant to apply the tolerance and datum identification to both of the holes not labeld as datum feature B then remove the parentheses and the position A|B|C on the middle hole.

Consider your application: do the three holes really serve a different purpose in your application? It is possible, although I don't know your application, that all three holes should be identified as datum feature B and you won't need a datum feature C. It is also possible, based upon your part's function, that both the middle and lowest hole should be datum feature C. Only understanding your part's function can definitavely answer the question.

Good Luck

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Every Line has a meaning, every curve has a purpose