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!

2 Comments

  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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