GD&T questions

Words Are Important

Posted by on May 26, 2012 in 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...

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How Literally Should We Take the GD&T Standard?

Posted by on Aug 30, 2011 in GD&T questions

I hope everyone has had a great summer.   Here’s a topic that will be helpful even to seasoned experts in GD&T, and it kind of follows the previous blog entry… Often, when discussing the finer points of GD&T with others, we end up going to the official standard (or standards) that pertain to dimensioning and tolerancing in order to seek guidance.  But if you’ve been in the real world, you know that a technical document can’t capture every possible scenario. So we naturally look for the example or description in the standard that is closest to our real-world situation.  However, we have to make a decision whether we can make the leap of logic to say that a proposed design is still within the “spirit” of the standard.  This can sometimes be a sticky point! An aside:  The two major standards when it comes to GD&T are ASME Y14.5 and also the ISO series of standards (ISO has an umbrella of several standards that embrace GD&T, not just one book).   The predominant standard in North America — and the one I’m most familiar with — is ASME.  In some ways the two standards have different philosophies about the depth of coverage:  in some areas ASME tries to...

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Position and Profile for Everything?

Posted by on Mar 2, 2010 in GD&T questions

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...

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Unilateral Profile Tolerancing

Posted by on Jan 28, 2009 in GD&T questions

Let’s go back to the question box — “Why is profile allowed to be designated as one-sided? Can other GD&T symbols also be one-sided?” First, let’s address a couple of points: there are two profile symbols: profile of a line and profile of a surface. Yes, each of them can have the tolerance amount that is unilateral (one-sided) or bilateral (both sides of the intended profile). Also realize that the profile symbols can be “unequal bilateral” where there is tolerance on both sides of the perfect shape, but more tolerance exists on one side.  Some examples: No other GD&T symbol can use these options; there is no such thing as unilateral flatness or unilateral parallelism. Why is that? Well, when we talk about flatness or parallelism, the feature in question has no curvature. So the surface “is where it is.” Profile tolerances, however, have a curve (usually) and also one or more basic dimensions that describe the radius or other values. The prescribed radius is important, and if the curve dips in or out, it is directly impacting the given radius. With this in mind, I should point out the tolerance zones for some other GD&T symbols can float to one side or the other. For instance, a parallelism...

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What’s the Difference Between TIR and FIM?

Posted by on Nov 17, 2008 in GD&T questions

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...

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Can Basic Dimensions Be Omitted?

Posted by on Oct 29, 2008 in GD&T questions

A question came up recently about how GD&T relates to CAD dimensions:  The rules of GD&T say that basic dimensions are required when using a position tolerance. But does that always mean that the print must show these “boxed” dimensions? Answer:  No, it doesn’t. While classical GD&T seems to say that the boxed dimensions are required, there are two alternative ways to meet the rule.  First, if most or all of the dimensions on a print are to be basic dimensions, then a general note can be placed on the drawing that states “all dimensions shown are assumed to be basic” or something to that effect.  This means that the dimension is still shown, but without the box. Caution: if this is the case, make sure that the title block does not show any default plus/minus tolerances. The other option is one that is becoming more common: Leave the dimensions off the print, and add a note saying something like “all undimensioned features are dimensioned in CAD, and are basic.”  To its extreme, this means that the entire print can leave off all dimensions, requiring the reader to have access to the CAD data if they really want to know the dimension.  While there are advantages to this (the...

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