I am designing a “T” section piece of waveguide, comprised of two (2) pieces of waveguide placed at 90 degrees. The vertical piece is right in the middle of the horizontal section. Each piece is a small rectangular channel with .1 thick flanges on each end. The channel outside dimensions are 1.000 x .500 and .050 thick walls.

Our customer asked me to make the left inside surface of the horizontal channel Datum A and Datum B and the right inside surface of the SAME horizontal channel Datum D and Datum E. I pointed out that the inside rectangular hole is all the way thru the horizontal channel, and if it is labled Datum A and Datum B on the left, it is also the same on the right.

His reasoning was for inspection. Am I wrong? How can I use ASME Y14.5M1994 to explain to him that the customer is sometimes wrong?

Billy Garfield


Hi Billy,

Based on your description and the drawing, there is not enough information to give you an accurate answer. I would need to see the actual drawing and I need to know how this part fits and functions in the final assembly. Whether this is a final product drawing or a detailed assembly with separate parts at each end will influence how the drawing is dimensioned and toleranced, also.

For example, if this in a single part drawing that has one continuous hole through the full length of the part, then it would be appropriate to specify this hole as a single datum letter and reference it as a primary datum in the feature control frames.

However, if this is an assembly with two separate flange details at each end, then it is appropriate to specify the holes at each end as separate datum letters since they are separate features. However, if the part relies on both of these holes at each end to provide the primary location and orientation of the part, then you would reference them as co-primary A-B datum features in the feature control frames. For example, A-B.

Thanks for writing. It’s a good question. If you’d like to send more detailed information privately, I can give you a more specific answer.