GD&T questions

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...

Occasionally users of GD&T suggest that everything be simplified by just boiling all 14 symbols down to just two or three.  (What, you didn’t know there were 14 symbols? Click here for a handy chart!) There is some logic to what these people are saying — namely, that many GD&T symbols overlap others, and position and profile can be used in such a way as to cover the others.  But as you might guess, there are pros and cons to this. First, realize that position always controls two qualities: location and orientation. Location is obvious, but don’t forget orientation — because position extends all the way through the depth of a feature, it will control any tilt or angling of that feature. Profile of a surface, if used with datum references and basic dimensions tying it back to those datums, can control all four required qualities: location, orientation, size, and form (shape). Since it covers ALL of these, it can be argued that the other GD&T symbols could be ignored and simply use this one symbol (well, two if you count profile of a line). But there are two problems with this minimalist philosophy: For one thing, it may sometimes be necessary to really only control a particular aspect, such as...

This blog post may seem like splitting hairs to some of you, but it’s a question that came up in my class.  And you know the saying: if someone has a question, chances are that others are thinking the same question. When inspecting parts for runout (and a few other characteristics), you may know that the classical method involves holding a dial indicator to the surface and then watching for the highest and lowest reading.  This difference is then compared to the specification allowed by the drawing.  Here’s a visual of runout being checked on an end face: The absolute value of highest to lowest gage reading is often called “TIR,” or “total indicator reading.” In the past it was quite common to specify runout by adding a note to the drawing such as “.040 max. TIR.”  (Nowadays, it is more proper to use GD&T to control this, especially because the former method is ambiguous when it comes to identifying the datum.) Well, somewhere along the line another acronym crept into the vocabulary: “FIM,” which is “full indicator movement.”  It essentially means the same thing — the total variation from highest to lowest gage point.  But the Y14.5 standard uses FIM exclusively in its explanations for runout. Is there...