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William Tandler

Metrology

Is GD&T a Lot of Gol-Derned Trouble?

Yes, and worth it—Smart GD&T workshop No. 1

Published: Wednesday, October 10, 2007 - 21:00

What is GD&T, anyway? The Greatest Design Tool ever, or a whole bunch of Gol-Derned Trouble? Actually, geometric dimensioning and tolerancing is both, and worth every penny.

Only with the help of GD&T can we test and guarantee the functionality of parts and assemblies on the design floor instead of on the assembly floor, which is an all too common and awfully expensive alternative. But grasping GD&T and making it actually work is truly a lot of trouble. So, are we going to leave it lie, or make it work?

As Stanley Parker, the alleged inventor of GD&T, discovered early one frosty morning (the year was 1940) in northern Scotland, CD&T—classical dimensioning & tolerancing—is almost useless for documenting functional limits of imperfection, as well as for communicating them unambiguously to manufacturing and inspection. Having spent the previous day rejecting a run of critical parts for a shipment of torpedoes, and having set them aside for further analysis the next day—based on CD&T—he discovered that they had been stolen from the inspection department after hours. As it turned out, functional testing in the wee hours of the night proved the parts were perfectly functional, which led to their installation and, ultimately, to shipment of the torpedoes. Instead of recalling the shipment and firing the manufacturing crew for criminal actions, a most reasonable response, Parker congratulated them on their pluck and began to wonder how he could have rejected so many perfectly functional parts. He then proceeded to tear the science of CD&T apart in an effort to discover any weaknesses, and weaknesses were all that he found.

The dimensioning and tolerancing tools in the CD&T tool box turned out to consist of a single item—the toleranced nominal dimension. Could one tool do the job? Although reasonably capable of controlling size,the envelope rule hadn’t been made a requirement. Furthermore, he realized that toleranced nominal dimensions are incapable of differentiating between reference features and controlled features, and even worse, wholly incapable of defining coordinate systems, which we need for coordinate metrology. CD&T also uses square instead of cylindrical tolerance zones to constrain the location of holes, permitting a nonfunctional larger offset along the diagonal than that allowed from left to right. Finally, he realized that CD&T provides no way to increase a tolerance on location as a bore gets larger or a shaft gets smaller, and is therefore unable to take advantage of the increase in clearance.

Today we’re the proud inheritors of Parker’s symbolic language—now referred to as GD&T—which represents a set of tools with which to communicate design intent unambiguously to manufacturing and inspection, and, much more importantly, to ensure that what we communicate is worth communicating and represents a functional, assemblable part.

This is the first in a series of monthly workshops, in which we look forward to reducing the Gol-Derned Trouble to help our friends take real advantage of the Greatest Design Tool ever. We intend to make each workshop fun and, above all, useful.

In our next article we’ll cast light on the anatomy of feature control frames and demonstrate how to “decode” rather than “interpret” GD&T. In the third Smart GD&T workshop we’ll cast a very bright light on the confusing world of the material condition modifiers (S), (M), and (L), and discover the difference between “tolerance zone size” modifiers and “tolerance zone mobility” modifiers.

Please join us by suggesting topics you’d like us to address. Put us through the ringer and see if we can come through shining for you.

Discuss

About The Author

William Tandler’s picture

William Tandler

William Tandler’s professional experience includes three years at the Hewlett Packard R&D laboratory in Palo Alto, CA, where he helped develop a quadruple mass spectrometer; five years with the laser manufacturer Coherent Inc. as international marketing manager; followed in 1975 by the founding of Multi Metrics Inc.

Tandler is currently a member of the ASME Y14.5.1 Mathematization Committee; a subject matter expert for Working Group 4 of the ASME Y14.5 standard focused on datums; and has been a contributor to ISO TC213 in the realm of datum reference frame construction.