I have several of your published books and appreciate very much the insights and explanationsyou have provided for the ASME Y14.5M-1994 standard. Your published works have contributed greatly to my success in the area of GDT.
In the drawing above, I have a coplanar datum A as primary, into which are two holes that are used as secondary datum and tertiary datum C. Datum B is defined by a TOP to datum A. My question: Is it wrong for the secondary datum B to be defined by a TOP to datum A? If it is not wrong, is it considered poor drawing practice?
Jon Houck GDTP S-0507
Woodward Governor Company
Thanks for your kind words. Your question illustrates a common problem on drawings.
Although not explicitly prohibited in Y14.5, I do believe that most committee members would agree with the following explanation. I do not recommend this practice. With only a single datum reference (the surface) only perpendicularity is being controlled, not location. Therefore, a perpendicularity tolerance would communicate the requirement much more clearly. In my opinion, this would certainly be considered a questionable tolerancing practice.
If datum B was two or more holes, then a position tolerance could be used with only a single datum reference as shown on your sketch. With a pattern of holes being toleranced, the position control would be controlling the spacing (location of the holes to each other) of the holes, which then makes sense to use position.
I hope this answers your questions. If I missed something or if my answer is not clear, let me know and I will try again.