As we know, GD&T is a symbolic language with which to specify permissible limits of imperfection in manufactured parts, namely limits that maximize the parts’ operability, assemblability, and affordability. It’s complex only to the extent that the real world of imperfect geometry is complex. If we’re dealing with a slightly tapered, slightly bent, slightly tilted, and slightly offset shaft, which is supposed to fit into a slightly, equally distorted mating bore, we need concepts, tools, and rules that can address those details. That’s GD&T.

The less clearly we deal with the concepts, tools, and rules of GD&T, the more complex and confusing the subject becomes, making it necessary to interpret poorly framed verbal statements and badly stuffed feature-control frames, instead of being able to simply decode clear statements and properly encoded feature control frames. Therefore, crystalline concepts, tools, and rules are essential to make GD&T work, and are the only way to turn the jungle of GD&T into a well-laid-out, formal garden.

The world of datums
The many concepts associated with the word “datum” represent a particularly important part of this jungle—oops, garden—and our purpose in this workshop is to define them so clearly that we can use them with confidence and communicate them clearly.

We’ll deal with these concepts in the following order:

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