GD&T (Geometric Dimensioning and Tolerancing)

GD&T (Geometric Dimensioning and Tolerancing) is a standardized symbolic language used on engineering drawings and 3D models to precisely communicate design intent and allowable variation. Unlike traditional plus-minus tolerancing, GD&T controls the form, orientation, location, and runout of features relative to datums, enabling clearer communication between design, manufacturing, and inspection. GD&T follows standards such as ASME Y14.5 (United States) or ISO 1101 (international). Geometric tolerances are specified using feature control frames (FCF), which contain the geometric characteristic symbol, tolerance value, material condition modifiers (if applicable), and datum references. Geometric characteristics are grouped into five categories: (1) Form controls (straightness, flatness, circularity, cylindricity) control the shape of individual features without datums. (2) Profile controls (profile of a line, profile of a surface) control the contour of lines or surfaces. (3) Orientation controls (angularity, perpendicularity, parallelism) control angular relationships relative to datums. (4) Location controls (position, concentricity, symmetry) control feature location relative to datums. (5) Runout controls (circular runout, total runout) control surface variation during rotation about a datum axis. Datums are theoretically exact reference points, axes, or planes from which measurements are made. A datum reference frame (DRF) typically uses three datums (primary, secondary, tertiary) to constrain all six degrees of freedom. Basic dimensions (shown in boxes) are theoretically exact and have no direct tolerance; the tolerance is defined by the feature control frame. Material condition modifiers affect tolerance application: MMC (Ⓜ, Maximum Material Condition) applies when a feature contains maximum material (smallest hole, largest shaft); LMC (Ⓛ, Least Material Condition) applies at minimum material; RFS (default, no symbol) means Regardless of Feature Size. When MMC is specified, bonus tolerance is available: as the feature departs from MMC toward LMC, additional positional tolerance equal to the departure amount is gained. Virtual condition is the worst-case boundary: for external features, VC = MMC size + geometric tolerance; for internal features, VC = MMC size − geometric tolerance.