Every GD&T symbol in ASME Y14.5 and ISO 1101 — form, orientation, location, runout and profile — with the control each one places on a feature and how it is inspected.
Complete reference tables and engineering data for gd&t symbols. All values verified against ISO, ANSI, and manufacturer standards.
GD&T is the engineering language for specifying part geometry and tolerances unambiguously. This reference covers the five categories of geometric controls as defined by ASME Y14.5 and ISO 1101 standards.
Plus/minus tolerancing creates rectangular tolerance zones. GD&T position tolerance creates cylindrical zones -- 57% more area for the same tolerance value. This means fewer rejected good parts. GD&T also clearly defines datum references, which eliminates ambiguity about how to fixture and inspect the part.
ASME Y14.5 (US standard): uses feature control frames with the "Rule of 1" -- form is controlled by the limits of size unless separately specified. ISO 1101 (international): uses the "Independency Principle" -- form and size are independent unless Envelope Requirement (E in a circle) is specified. ASME uses circled M for MMC; ISO uses M in a circle.
MMC means the tolerance applies when the feature contains the maximum amount of material -- smallest hole or largest shaft. If the actual feature departs from MMC, bonus tolerance is added to the geometric tolerance. This allows functional gauging: a fixed-size gauge that checks the worst-case (MMC) condition accepts all good parts automatically.
Use this reference to interpret GD&T symbols on engineering drawings. Form controls ensure the shape of individual features. Position (true position) is the most common location control for hole patterns. Profile of a surface is the most versatile control for complex 3D surfaces. When reading a drawing, first identify the datums, then the controls applied to each feature.
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