GD&T is the only tool we have with which to manage machine part geometry perfectly. In particular, it’s the only tool we have with which to impose truly functional limits of imperfection on machine part features and actually guarantee assembly and operation prior to drawing release. GD&T is also the only tool we have with which to clearly communicate manufacturing objectives to the machine shop and with which to fully and functionally control coordinate metrology processes for manufacturing process assessment and refinement. Finally, the symbolic language of GD&T is also totally universal, does not depend on local languages and can be understood by engineers around the world.
The perfect imaginary world of GD&T is quite simple—it consists of only two things, 1) coordinate systems and 2) forests of tolerance zones which are oriented and located relative to them by basic dimensions. However, the symbolic language for specifying these items is unfortunately quite complex. And, as if its intrinsic complexity were not enough of a limiting force, the descriptions of its concepts, tools, rules and processes found in the two most important GD&T standards are often confusing and incomplete, namely in ISO 1101 and its numerous associated Standards, and in the ASME Y14.5 2009 Standard, which make it even more difficult to manage effectively. As a result, most of the GD&T “code” we find on models and drawings is dangerously “decorative” i.e. not function based, and rife with syntax errors, which users therefore have no alternative but to “interpret.” But if GD&T is open to interpretation, it is useless and must be abandoned.