Decoding (Not Interpreting) GD&T

Though design, machining and quality personnel all spend considerable time interpreting Geometric Dimensioning and Tolerancing (GD&T) information, Bill Tandler says there shouldn’t be any “interpretation” at all. GD&T is a language for part designs that uses context and symbols to characterize the imperfections allowed in each part feature. In theory, a GD&T callout should carry the same meaning to everyone. In reality, though, the language is complex enough that “tribal understanding” has crept in. Two engineers who work together may not realize they are misreading or misapplying GD&T, because both of them make the same error. When personnel from different tribal understandings come together, however, the result can be a part that doesn’t match the design intent.

Mr. Tandler is the president of Multi Metrics, Inc. (Menlo Park, California). He says that a supply chain could become significantly more efficient if everyone involved had the same ability to decode GD&T information. New products could be introduced more quickly and cheaply, he says, because manufacturing problems resulting from miscommunication could be avoided instead of being discovered on the shop floor. Meanwhile, new suppliers could be brought online more easily, and mid-process design changes could be introduced smoothly throughout the supply chain. Achieving this universal decoding is the goal of his company’s new Web-based utility, “e-GAD.”

The name is short for Electronic GD&T-Aided Design. This suite of online applications aims to guide all of the personnel involved with a part, as needed, through the processes of accurately applying and reading GD&T information. The utility consists of a Best Practices Guide and an Online Advisor. The former is a look-up reference on GD&T, while the latter is a self-guided training resource. Modules ranging from 5 to 30 minutes each use animation and audio to provide instruction in specific GD&T concepts—in effect providing just-in-time training.

Not all tribal knowledge needs to disappear. Mr. Tandler says the utility is password-protected, and therefore can be made unique for each company or supply chain using it. ASME or ISO standards can be followed, for example, and GD&T encoding schemes for a company’s particular part families can be included. In fact, the company’s own part drawings can provide relevant examples, making it that much easier for the user to get to the one correct interpretation of a GD&T callout quickly.