Are You Using GD&T Correctly?

`So often in using GD&T, people are worried about using it correctly. And this can certainly be a valid concern — geometric tolerancing consists of symbols and rules for how to use them. But we also need to remember that GD&T is a language for communicating design requirements. And like any other language, there may be several ways to say the same thing.

When asked about the proper use of GD&T on a sample drawing, I usually classify individual callouts in one of three ways: 1. A-OK;  2. Illegal;  3. Legal, but doesn’t add any value.

Example of #1:

While the use of datum D as a pattern may seem confusing, this datum usage is perfectly fine.

Example of #2:

This is illegal because flatness cannot reference a datum. (Seems obvious, but I’ve seen this several times on actual drawings!)

Example of #3:

This one is a little harder to assess. The feature control frame itself is legal, but it’s actually redundant with the 0.2 provided by the height tolerance. There is no way that the top surface could exceed 0.2 anyway, due to the plus/minus on the height dimension.

So keep in mind that GD&T is more than just learning the symbols; it also involves many rules and the interplay of those rules can sometimes be confusing!

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