Instead of discussing some technical point this time, let’s take a brief look back at the history of GD&T. Some people may think that GD&T is “just the latest fad” (I actually heard someone refer to it that way) and therefore they are implying that it’s not worth learning, since it may soon go away. But the facts show that GD&T has been around for a long time (50+ years), it applies tolerances in a logical and standardized manner, and it saves money — all reasons why it’s not going to fade into the sunset.
Supposedly, the story goes that a guy named Stanley Parker came up with the first GD&T concept having to do with position (or “true position” if you prefer). The time was World War II, and the location was Great Britain. As you might imagine, during wartime deadlines are critical, and Mr. Parker ran into a situation where some torpedo parts inspected according to traditional tolerances were rejected. But it turns out that they were actually functional parts, and those parts were sent on their merry way even though they didn’t seem to be to print.
He traced the discrepancy to the fact that traditional X-Y tolerances result in a square tolerance zone, but that parts outside the square may actually be good, so long as they are within a circle that encompasses the square’s corners:
See the logic? If the four corners of the square zone were functional — as the X-Y method clearly allows — then in most cases a circular area would be just as functional. And think how many parts may have been needlessly rejected! (Of course, if your process is capable, you should not really have any parts out near the edge, but that’s a different discussion.)
From there, GD&T has grown dramatically. Over time, Mr. Parker’s idea of “position” grew to include other concepts such as flatness, parallelism, runout, profile, and many more. And though GD&T became standardized by the military in the 1950s, it gradually became more popular among commercial industries, and has been used by many companies for well over thirty years. So don’t think of geometric tolerancing as a fad; think of it as the way we should have always done things!