I hope everyone has a safe and happy holiday season. This may help you with your home decorations…

In the book, Customer Centered Products, Ivy Hooks and Kristin Farry share some observations about the American culture’s urge to improvise…

We have worked with project managers and engineers who believed that solving the unanticipated problems under the gun was “the fun part of the project.”

In my own experience, I have also met many managers and engineers who would prefer to fix problems through testing, rather than through proper drawings and analysis.

Since we have a competitive edge in the U.S. with our comprehensive product development processes, let’s use them to our advantage. Let’s focus on identifying and solving problems in the design phase, before hardware is built.

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

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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.

“The economic success of manufacturing firms depends on their ability to identify the needs of customers and to quickly create products that meet these needs and can be produced at low cost.”

- Karl T. Ulrich and Stephen D. Eppinger,
Product Design and Development, 2004

A great book! Should be required reading for all product engineers.

My thought is that functional dimensioning supports Karl’s quote because it protects the customer needs and provides the largest possible tolerance for manufacturing.