There’s more to learning GD&T than just knowing the symbols. There is also a unique vocabulary — sometimes GD&T terms mesh with everyday usage, but sometimes a word can have a very specific meaning in geometric tolerancing.
Here are some examples:
- The symbol circularity is sometimes called roundness. The two terms are synonymous, but the ASME standard uses circularity.
- The most common GD&T symbol is position. People sometimes call it “true position,” but the ASME standard simply calls the symbol “position.”
The tricky ones, however, are concentricity and symmetry:
- While the words concentric and symmetric are sometimes used in a generic sense, the GD&T symbols for these are quite specific, and are often discouraged. (See below for an earlier blog entry on concentricity.) Also adding to the confusion is the fact that the ISO standard, which most other countries follow, defines these two GD&T symbols in the more generic sense.
- Runout and profile are sometimes used as general terms. But there is no GD&T symbol for “runout” or “profile.” Instead, those are categories which are each divided into more specific symbols; a print will show circular runout or total runout, along with profile of a line or profile of a surface.
- Also, there may be slight differences in pronunciation. For instance, datums can be pronounced with a long or a short “a” sound. I catch myself using both!
Finally, if you write about GD&T in a word processing program, you’ll find that the spell-checkers don’t recognize many of these terms! As I write this, the computer has flagged “tolerancing,” “runout,” and “datums” (I guess it thinks the plural of “datum” is always “data.”)
Admittedly, this is minor stuff, but it’s good to be aware of these things!