GD&T questions

Are You Using GD&T Correctly?

Posted by on May 27, 2008 in GD&T questions

`So often in using GD&T, people are worried about using it correctly. And this can certainly be a valid concern — geometric tolerancing consists of symbols and rules for how to use them. But we also need to remember that GD&T is a language for communicating design requirements. And like any other language, there may be several ways to say the same thing. When asked about the proper use of GD&T on a sample drawing, I usually classify individual callouts in one of three ways: 1. A-OK;  2. Illegal;  3. Legal, but doesn’t add any value. Example of #1: While the use of datum D as a pattern may seem confusing, this datum usage is perfectly fine. Example of #2: This is illegal because flatness cannot reference a datum. (Seems obvious, but I’ve seen this several times on actual drawings!) Example of #3: This one is a little harder to assess. The feature control frame itself is legal, but it’s actually redundant with the 0.2 provided by the height tolerance. There is no way that the top surface could exceed 0.2 anyway, due to the plus/minus on the height dimension. So keep in mind that GD&T is more than just learning the symbols; it also involves many rules and the interplay...

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Does GD&T Require a Diameter Symbol?

Posted by on Apr 11, 2008 in GD&T questions

Here’s another common question that comes up in a GD&T class:  Suppose we are applying a position tolerance to a hole. It would typically have a diameter symbol in front of the number, as in the first example below.  But what if we omit the diameter symbol in front of the number, as in the second example? Would the shape of the tolerance zone still be assumed as cylindrical?   Answer: No, it’s not cylindrical. Without a diameter symbol, a tolerance zone defaults to two parallel planes (unless the BOUNDARY concept is invoked). So the second drawing above is ambiguous; the zone will be two parallel planes, but we don’t know the direction of those two planes. Two solutions:  First, if the intent is to control the position in all directions, you must add a diameter symbol as in the first example above. Second, if we really intended two parallel planes, we must graphically indicate the direction of those planes:  Now, it is clear that we are controlling position in the left/right direction. Of course, that means that there is no position control in the up/down direction. So let’s take it one more step: This example creates two sets of parallel planes — one in the vertical direction and...

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Dimensions with Datum Targets

Posted by on Mar 6, 2008 in GD&T questions

Another question came in recently, having to do with basic dimensions and their use with datum targets. If you are up to speed on GD&T, you should know that a basic dimension is any number enclosed in a box: The purpose of this is to show a theoretical dimension, without any tolerance. (Even a general title block tolerance does not apply!) Instead, the feature that is being dimensioned will have some GD&T that provides the actual tolerance for manufacturing. Now here’s the question: datum targets are usually located using basic dimensions, but there is no GD&T to provide a tolerance: Is this legal? What governs the accuracy of where those datum targets are? Yes, it is legal, and here is the key: Geometric tolerances are applied to features of a part. A datum target is an imaginary point, line, or area that is simply used for fixturing or gage setup. It’s not the responsibility of a product drawing to worry about the tolerancing of a gage or fixture! The ASME Y14.5 standard says it this way in paragraph 4.6.2: “The location and size, where applicable, of datum targets are defined with either basic or toleranced dimensions. If defined with basic dimensions, established tooling or gaging tolerances apply.” Thus, most GD&T...

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What Is Resultant Condition?

Posted by on Feb 13, 2008 in GD&T questions

If you are familiar with GD&T, you’ve probably heard of “virtual condition.” It is a number that represents a worst-case combination of a feature’s size along with its geometric tolerance. For instance, suppose we have the following example of a hole, with a size tolerance of 6 ± 0.2 mm:                          The worst case for assembly purposes would be the smallest hole that is also out of position: 5.8 – 0.3 = 5.5.  This virtual condition represents the “usable” area that the mating pin must fit within:                       Now, virtual condition is usually easy to identify with — ”think of assembling pins and holes together. But sometimes I may be concerned about the outside boundary, created by the largest hole that is also off center:                         This outer boundary, called resultant condition, is calculated as: LMC + stated geo tol + any bonus, or: 6.2 + 0.3 + 0.4 = 6.9 mm. This represents the area where any portion of the hole’s edge may possibly fall.This resultant condition is not of concern when dealing with assembly of holes and pins! But suppose we are simply punching...

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It’s Just Semantics…

Posted by on Feb 5, 2008 in GD&T questions

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...

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What’s the Deal with Concentricity?

Posted by on Dec 17, 2007 in GD&T questions

Here’s a question I received at the beginning of a recent GD&T training session.  Often I’ll ask participants if there is anything in particular that they are looking to get out of the training. One gentleman didn’t hesitate:   “I’m here to learn why we shouldn’t use the concentricity symbol. No one has been able to tell me why I’m not supposed to use that symbol! Our drawings show a runout tolerance instead, but I’ve never heard a good answer why.” First, what this person was told by others is indeed true: the concentricity symbol is often discouraged, and another choice such as position or circular runout is usually better.   To answer the question, we must analyze how the terms are defined. A dictionary definition describes concentric as “having a common axis or center.” That is what we often require for parts such as a camshaft or guide pin holes, right? But the problem is in how we find that axis or center. There are different ways to derive an axis. The definition of concentricity in the GD&T standard (ASME Y14.5M-1994) is given in paragraph 5.12. “Concentricity is that condition where the median points of all diametrically opposed elements of a figure of revolution are congruent with the axis (or center point) of...

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