GD&T, or Geometric Dimensioning and Tolerancing, has become an industry standard as well as a different way of describing tolerance. Rather than the traditional reference frame, GD&T emulates the constraints of putting something together, based on the kinds of spatial relationships of the parts. It includes symbols, rules and even vocabulary for communication among engineers.

It’s been proposed and circulated by different organizations, but in the United States, it was ASME, the American Society of Mechanical Engineers, that
formalized the standards for GD&T. The code was revised in 1994 to clarify the standards even further. A similar international standard is put out by the
International Standards Organization, or ISO. These codes create the framework for how the industry implements them.

Bob Salerno, co-founder of software company New River Kinematics, explained that in GD&T, the relationships are the most important. “It’s like how kinematics is the science of spatial relationships, and GD&T is a way of describing them,” he said.

One of the most exciting changes that has recently occurred is the development of methods for directly coupling the designer’s intent, as described through GD&T, to the inspection process. This effectively allows designers and measurement technicians to communicate exactly what is critical about a given part. It also puts the designer in control of the process and relieves the person doing the inspection from having to understand how this part mates to all the other parts.

The implementation and continued adaptation of GD&T has brought engineering and production a long way in the past 36 years since the code was adopted, and that is expected to continue as it’s adopted more universally.