Prerequisite Knowledge before Learning GD&T

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 we’re OK there. But this print does not provide any tolerance for the “coaxiality” of the two diameters!  The general tolerance cannot help us here, so technically this print is ambiguous.

Of course, we could go on to discuss adding a geometric tolerance such as concentricity or position for the diameters, but the point here is to get you to think about basic drafting rules before you proceed to the GD&T.

One last prerequisite needed before attending a GD&T class: a willingness to learn the material, and in some cases unlearn preconceived notions! I’ve had numerous people come to a seminar as a refresher, but at the end they realize that they had a poor or erroneous understanding of some of the symbols. That’s not a criticism of them; it’s a compliment to their honestly and ability to evaluate technical concepts in a new light. We all learn something new each day — even us GD&T experts!

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