- What Is GD&T?
- GD&T history
- When Do We Use GD&T?
What Is GD&T?
A symbolic language rather than words on engineering drawings that explicitly describes nominal geometry and its allowable variation. The GD&T system has a strong mathematical base.
- Purpose of GD&T
- Proper application of GD&T
The purpose of GD&T is defined as describing the geometric requirements for part and assembly geometry.
Geometric tolerancing encourages a dimensioning philosophy called functional dimensioning. It defines a part based on how it functions in the final product, to insure the proper assembly of mating parts, to improve quality, and to reduce cost.
Proper application of GD&T will ensure that the allowable part and assembly geometry defined on the drawing leads to parts that have the desired form and fit (within limits) and function as intended.
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. e GD&T system has a strong mathematical base.
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!
When Do We Use GD&T?
1. Pride in workmanship. Every industry has unwritten customary standards of product quality, and most workers strive to achieve them. But these standards are mainly minimal requirements, usually pertaining to cosmetic attributes. Further, workmanship customs of precision aerospace machinists are probably not shared by ironworkers. 2. Common sense. Experienced manufacturers develop a fairly reliable sense for what a part is supposed to do. Even without adequate specifications, a manufacturer will try to make a bore very straight and smooth, for example, if he suspects it’s for a hydraulic cylinder. 3. Probability. Sales literature for modern machining centers often specifies repeatability within 2 microns (.00008"). Thus, the running gag in precision manufacturing is that part dimensions should never vary more than that. While the performance of a process can usually be predicted statistically, there are always “special causes” that introduce surprise variations. Further, there’s no way to predict what processes might be used, how many, and in what sequence to manufacture a part. 4. Title block, workmanship, or contractual (“boiler plate”) standards. Sometimes these provide clarification, but often, they’re World War II vintage and inadequate for modern high-precision designs. An example is the common title block note, “All diameters to be concentric within .005.”
Should We use GD&T on every drawing?
Some very simple parts, such as a straight dowel, flat washer, or hex nut may not need GD&T. For such simple parts, Rule #1 (explained in section 126.96.36.199), which pertains to size limits, may provide adequate control by itself. However, some practitioners always use GD&T positional tolerancing for holes and width-type features (slots and tabs). It depends primarily on how much risk there is of a part being made, such as that shown in Fig. 5-5, which conforms to all the non-GD&T tolerances but is nevertheless unusable.
All or Partly tolerancing?
On any single drawing you can mix and match all the dimensioning and tolerancing methods in Y14.5. For example, one pattern of holes may be controlled with composite positional tolerance while other patterns may be shown using coordinate dimensions with plus and minus tolerances. Again, it depends on the level of control needed. But, if you choose GD&T for any individual feature or pattern of features, you must give that feature the full treatment. For example, you shouldn’t dimension a hole with positional tolerance in the X-axis, and plus and minus tolerance in the Y-axis. Be consistent. Also, it’s a good idea to control the form and orientational relationships of surfaces you’re using as datum features.