GD&T

Geometric Dimensioning and Tolerancing — a standardized symbolic language for defining and communicating the allowable variation in the form, orientation, location, and runout of part features on engineering drawings, beyond simple linear dimensions and tolerances. GD&T controls: (1) Form — flatness, straightness, circularity, cylindricity. (2) Orientation — perpendicularity, angularity, parallelism. (3) Location — true position, concentricity, symmetry. (4) Runout — circular runout, total runout. Each GD&T callout consists of a feature control frame specifying: geometric characteristic symbol, tolerance zone (typically in mm), material condition modifier (MMC, LMC, RFS), and datum references. Per ASME Y14.5-2018 (US standard) and ISO 1101 (international standard — not identical, some differences in interpretation). GD&T is essential for: precision machined parts (pulley bores, bearing housings, shaft journals), mold design (for molded rubber parts), and any component where form and position tolerances are critical to assembly and function. Example: a pulley bore might specify perpendicularity of the bore axis to the hub face within 0.02 mm, and true position of the keyway within 0.05 mm at MMC. GD&T training (ASME-certified courses) is a core competency for mechanical engineers and machinists.