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# Question on Datum Reference Plane

## Question on Datum Reference Plane

(OP)
http://www.dimcax.com/gdt_web/february-02.htm

In the link above (assuming the drawing is correct), why is datum axis C not dimensioned off of datum axis B? Datum axis C is located off the edge, not hole to hole, why is that? Why bother having datum B in the FCF when defining datum C?
I thought that there is a datum coordinate system that is created through the intersection of Datum A,B, and C.

In the bottom it says that

"This framework made up of three mutually perpendicular planes could have been drawn at the angle shown so that one of the planes passes through the axis of datum feature C without changing the meaning."

Can there be multiple interpretations when determining the coordinate system? I thought it had to pass though datum C, making Figure A the answer to their question of
"Will the drawing below produce a part that most resembles Figure A or Figure B?".

How are we defining that top hole. It is referencing Datum A, but I don't see how it is using Datum B or C, it is using the edge of the part again.

Thank you for any clarification.

### RE: Question on Datum Reference Plane

Baratheons:

One thing that may be confusing you is that the example does not discuss the Simultaneous Requirements concept in Paragraph 4.19, page 79, in Y14.5-2009.

Traditionally, many are "trained" to think holes in a part must be located from the edges. However in this example, because of the general profile note, the outside edges are to A-B-C just like the 5mm hol. Given Simultaneous Requirements, the outside edges are located from the same reference frame as the 5mm hole. Or... where Datums (holes) B and C go the 5mm hole AND the outside edges "follow" together simultaneously.

Hope this helps

Certified Sr. GD&T Professional

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