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GD&T Drawing Question - I have a drawing and it makes no sense to me whatsoever

I have a drawing and it makes no sense to me whatsoever. There is an A datum, "bottom" surface, as you would expect. There is a rectangular slot that is shown as Basic, while the OD of the part has a positional callout back to datum A (only). Then there is a four-bolt hole pattern, but each hole has a positional callout to both datums A and B. Oh, and the B datum is identified as this Feature Control Block. The diameter of the bolt hole pattern is a reference dimension.

Please, please either explain this to me, or tell me I'm not crazy and this doesn't make sense. Thanks in advance.



Involved In Discussions
I second that. The location callout associated with datum B calls out Datum B, creating a circular reference. For the slot, double check the notes and title block for a general callout. At a past job, the general callout was in a separate document that was called out in the print notes.

John Predmore

Involved In Discussions
I am thinking the datum B call-out refers to the pattern of 4 holes, whereas the featured control frame below 4 x 3.25 diam dimension refers to the location of the four individual holes, referencing datum B. That would resolve the circular reference contradiction which others point out. I don't recall whether Y14.5M 2009 allows this combination of symbols.

Jim Wynne

Super Moderator
I agree that there's a circular reference with regard to datum -B- and there appears to be no control over the diameter of the bolt circle. While we can guess about what was intended, the whole idea of standardized dimensioning is to eliminate the need for guessing,
Thank you all, for your feedback. I was afraid I was overthinking this. I will get with the customer about this. Again, thank you.
Now. I have a similar drawing, that almost makes sense to me. You can use the same basic structure as the attached drawing, but without the slot. This one has the bolt hole circle as a basic dimension, but now the bolt holes are the B datum. Again, does this make sense? Thanks again, in advance.


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