Words Are Important

Ah, yes.  I remember as a child being told that words are important!  And that is certainly true in the world of geometric dimensioning and tolerancing.  So many people think that GD&T is just a matter of learning the symbols, and it’s true that that is a key part of understanding the language.  But behind the symbols are many rules, acronyms, and definitions that can make a great difference if they are not fully understood.

One of the most significant examples is the confusion about the term concentricity. To a casual beginner, the word “concentric” sounds like a simple idea: two or more circles that share a common center.  But in the world of GD&T, concentricity has a very specific meaning that is more specific than what you’ll find in Webster’s Dictionary!   FYI — the same confusion applies to the symmetry symbol. (For more about concentricity, see this blog entry from a couple of years ago.)

Here are a few other miscellaneous terms to be careful with:

Datum — Technically, a datum is a perfect plane, axis, or point (or combination of these).  So when talking about the actual surface of a part, we shouldn’t call it “datum A,” because that surface may be imperfect: slightly concave, convex, etc.  The proper term for the actual part surface is “datum feature A.”

MMC — The “maximum material condition” is literally the size of a feature when it has the maximum amount of material allowed.  This is a simple idea that is usually covered near the beginning of any GD&T training. It is invoked upon a geometric tolerance by the circled M modifier after the tolerance number. However, few people are aware that when the same modifier appears after a datum letter, it is not called MMC.  Instead, it is referred to as MMB, or “maximum material boundary.” The reason it’s different is that a datum feature may have more than just a size tolerance; it could also have a geometric tolerance of its own, thus making the worst-case boundary different from the true MMC.  This is an idea that was clarified in the 2009 ASME standard.

Basic dimension — this one is not a difficult term.  But what gets me is that many people confuse it with “reference dimension.”  I guess they are a little similar; they both have no tolerance.  But the reasons are different.  A reference dimension (a number shown in parentheses) has no tolerance because it is not to be checked.  It is just for reference, or “nice-to-know” information.  A basic dimension, however, is linked to GD&T.  A basic dimension (a number enclosed in a rectangle) also has no tolerance — not even from the title block — because it establishes a perfect size, location, or angle from which a geometric tolerance is established.

These are just a few of the dozens of terms that are so important to understanding GD&T.  And especially for anyone who is preparing for the official ASME certification test, thorough knowledge of all these terms and acronyms is essential!


  1. Do you care to comment on position symbol vs. “true position.”

  2. In the Y14.5 standard, true position is defined as the theoretically exact location of a feature of size, as given by basic dimensions. So that’s still a legitimate term when talking to someone.

    However, the symbol itself is just called “position.” Yes, many people call the symbol “true position,” but technically that’s not correct. I think it’s a carryover from past practices where the symbol may have carried that name. I know that since 1982 it’s been just “position” — I’d have to dig through the 1973 or 1966 editions to see if the term was different back then.

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