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...
Learn MoreHere’s an interesting tolerance question: Have you ever seen a plus/plus tolerance? (or minus/minus?) The ASME standard does not mention this, so a purist would say that it is illegal (see paragraphs 2.2 and 2.3 of the Y14.5 standard). But I’ve seen examples such as a hole dimensioned with: The real problem is that, with a casual glance, you might not realize that a part made at the “nominal” size of 1.25 is a bad part! (The size limits are 1.252 – 1.255.) So the obvious question is: Why did the engineer (or designer) choose this odd method of expressing a tolerance? The most likely answer is because there is some desired fit with a mating feature. Depending on the assembly, they may want a press fit, transition fit, or clearance fit. So if the example given above is a hole, then a pin in the mating part may be dimensioned with a minus/minus tolerance. In this way we can say that the pin and hole each have a nominal size of 1.250 inches, but the respective tolerances will ensure that there is always a little looseness (clearance fit). In the metric system, an entire method of coding different limits and fits has been developed. A nominal size would be...
Learn MoreWhen it comes to calculating tolerance stacks, there are many different methods. Some people simply take blank paper, then make a quick sketch and scribble out some numbers. While that may work for a very simple stack, it’s obviously not a very methodical approach! A better way is to use Excel or even special software for tolerance stacks to calculate an answer. Even when using Excel, there are two schools of thought for performing a stack: Some folks prefer to use a stack method that separates everything into an absolute maximum or minimum dimension, and then create two columns to stack the max and min results. Others insist on translating everything to a nice symmetric plus/minus tolerance, centered around a nominal. Both methods will work — it often comes down to how you were taught to perform stacks. Here’s my two cents’ worth: I prefer the max/min method, especially when GD&T is involved. Because geometric tolerancing is based on limits (think about MMC and bonus tolerance) it is usually easier to plug those into a spreadsheet. For more information check out the Tolerance Stacks course that we offer. Most of the class is devoted to this max/min method, but we also teach the plus/minus method as an alternative....
Learn MoreFor those of you who actively use GD&T on a daily basis, you should be aware of the certification process for GD&T Professionals. The American Society of Mechanical Engineers (ASME) has established a credential for GD&T proficiency, called GDTP, which stands for Geometric Dimensioning and Tolerancing Professional. It is a testing process that measures your ability to interpret and apply tolerances correctly. There are actually two certification levels: the Technologist Level, which measures your ability to read GD&T; and the Senior Level, which tests not only interpretation, but also the application of GD&T to a design. One does not necessarily need to become a Technologist first; some people go right for the Senior Level, although they are required to document at least five years of experience in GD&T (and they have a more difficult test!). If you decide to pursue either level, be aware that the questions on the test can be taken from any section of the Y14.5 standard. Each chapter has a “weight” that determines the number of questions from the chapter that appear on the test. This means that there is more than just GD&T; you need to be familiar with the nuances of traditional tolerancing as well as the many definitions contained in that standard....
Learn MoreHere’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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