When I teach a GD&T class, I have to presume that eveyone is “green” about the topic. Even if some folks have been using GD&T regularly, I find it best to start from the beginning. This ensures that everyone is on the same page, and it sets the stage for presenting the various GD&T topics that will be examined in the class. However, in order to effectively learn the GD&T system, there is some prerequisite knowledge. Before signing up for a GD&T class, make sure you are comfortable with basic blueprint reading, such as how to interpret the various views on a print (top, front, side, sections, etc.). You should also be familiar with plus/minus tolerancing (including unilateral plus or minus) and common drafting practices. Here’s a simple example: Common drafting practice tells us to assume that the corners in the right-hand view are 90 degrees, and we also assume that the inside and outside diameters are to be made on the same center line. But that raises two questions: What is the tolerance on the 90 degree corners? And what is the tolerance on the possible offset between the diameters? According to the general tolerance given for the print, the corners can deviate anywhere from 89-91 degrees. So...
Learn MoreFor engineers who regularly perform tolerance stacks, handling regular dimensions is pretty straightforward. And even when GD&T is involved, there is usually not much difficulty, until one encounters the MMC modifier. How can the effect of this “M” symbol be accounted for? First, a primer on what the effect of the MMC modifier is. Suppose the example shown below is applied to a pin: The “M” symbol tells us that the given position tolerance of 0.5 applies if the pin is made at its maximum size of 11.8 mm. But if the pin is made at any size less than that, then the position tolerance gets a corresponding “bonus” tolerance. Thus, each part that is made gets its own customized geometric tolerance. (Example: a pin made at 11.5 gets a position tolerance of 0.8, and a pin made at the smallest size of 11.2 gets a position tolerance of 1.1 mm.) The advantage to this system is that some parts that are made will get more positional tolerance, while still ensuring that those pins will assemble with the mating parts. With two-column tolerance stack calculations, then, we must be careful. Without the “M” symbol, we would simply add a line item in our stack to account for the 0.5...
Learn MoreWhen it comes to GD&T training, I am often asked which units of measurement are preferred. The answer: It doesn’t really matter! The GD&T system works the same using inches or millimeters; the only thing that changes is the number.The technical standard ASME Y14.5M-1994 uses SI units (millimeters). Paragraph 1.1.2 phrases it this way: “The International System of Units (SI) is featured in this standard because SI units are expected to supersede United States (U.S.) customary units specified on engineering drawings. Customary units could equally well have been used without prejudice to the principles established.”This may be humorous to those companies that have always used inches and continue to do so. (Weren’t we all told back in the late 1970s that everything would soon be metric?) The millimeter is widely used by countries besides the United States, and within the U.S. many industries have made the complete changeover to metric (including the automotive industry). But other industries, such as the aircraft industry, continue to use inches, as do smaller suppliers and machine shops.Obviously, dimensions and tolerances given in one system can be easily converted to the other, but several things need to be addressed when doing this. First, keep in mind that rounding error may occur. Second, there are...
Learn MoreNobody has to tell you that training is a valuable tool to help improve your skills and that of an entire group. But so often, a desire to implement training is thwarted by roadblocks. These roadblocks come in different forms. Here are a few, and ways to avoid or work around them: “We don’t have time.” This is the most common roadblock. There are always hot projects that can’t wait, especially in the world of engineering and design. But if your company considers training valuable, they should help you make time for it. To minimize the time away from your usual job duties, ask the trainer if the schedule can be broken apart. For our GD&T classes, I am willing to teach a few half-days that are spaced apart. Simply ask for this option, or see if the trainer offers a condensed version of the training. “We don’t have anyone on staff that can teach that.”  Well, that’s when you get on the Web and look around for consultants! I specialize in GD&T; other trainers have other specialties. Don’t be afraid to look outside your company for help; if you are unsure about a consultant’s qualifications, ask questions. How long have they been doing this? Do they hold certifications...
Learn More`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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Watch John-Paul talk about the benefits of GD&T training courses.