What is GD&T? GD&T, namely “Geometric Dimensioning & Tolerancing,” is a symbolic language with which to 1) research, 2) refine and 3) ultimately “encode” the functions of each feature of a machine part by specifying permissible limits of imperfection which guarantee assembly and operation prior to drawing release.
CAD (Computer Aided Design), and GAD (GD&T Aided Design) are the two equally important halves of the science of machine part geometry specification. CAD provides the means to generate, manipulate and communicate the nominal geometryof machine parts. GD&T, on the other hand, serves to research, refine, “encode” and guarantee the functions of machine part features. CAD without GAD is very “BAD,” because it represents just half the story and fails to guide downstream processes, whereas CAD with BAD GAD, the most common kind, is even worse because it misleads. Without GD&T, and of course syntactically correct and function based GD&T, meaningful tolerance stack-up analysis is impossible, manufacturing depends almost entirely on tribal understandings, and all coordinate metrology is pure invention on the part of the inspector. Thus the lack of any GD&T, and even worse of reliable GD&T, is demonstrably responsible for bumpy new product ramp up cycles, unnecessarily costly manufacturing processes, and questionable inspection results. In short, GD&T is a highly sophisticated “encodable” and “decodable” symbolic language for managing the risks involved in machine part design, manufacturing, inspection and assembly, and getting it right the first time.