GD&T (Geometric Dimensioning and Tolerancing)

GD&T (ASME Y14.5 standard) is a language of geometric symbols and rules that engineers use on drawings to precisely define allowable variation in part geometry, including size, form, orientation, location, and runout, eliminating ambiguity in part requirements. Unlike traditional coordinate dimensioning, which allows box-shaped tolerance zones, GD&T uses symbols like perpendicularity, parallelism, concentricity, and runout to define exactly how features must relate to datums and each other.

Programmers and setup technicians interpret GD&T to understand precisely which tolerances matter most, allowing them to prioritize operations and setups accordingly (for example, if position tolerance references a datum hole, that hole must be machined first with precision). Quality assurance teams use GD&T to determine the correct inspection methods and pass/fail criteria, ensuring parts are assessed based on design intent.

In CNC machining, understanding GD&T is essential for communicating between designers, programmers, and manufacturers about what precision matters and why.