Geometric dimensioning and tolerancing (GD&T) is a tolerancing technique that is used in the mechanical design process to control the fit and function of parts and assemblies. It complements traditional “plus and minus” dimensional tolerancing by letting you control 14 different geometric characteristics.
The geometric characteristics can be split into 5 different categories:
- Form Tolerances – used to control flatness, straightness, circularity and cylindricity.
- Orientation Tolerances – used to control parallelism, perpendicularity and angularity.
- Location Tolerances – used to control position, concentricity and symmetry.
- Profile Tolerances – used to control profile of a line or profile of a surface.
- Runout Tolerances – used to control cirular runout or total runout.
Unlike traditional dimensional tolerancing, that applies tolerances to dimensions, GD&T applies tolerances to features instead. Geometric tolerances are applied to features by adding feature control frames to the relevant feature on the engineering drawing.
The video below uses 3D animations to explain the different geometric characteristics in detail. It also covers other key concepts in GD&T, including:
- Datums, with an explanation of their importance, and how they are defined as used
- Material modifiers – how the Maximum Material Condition (MMC), Least Material Condition (LMC) and Regardless of Feature Size condition (RFS) are used to gain bonus tolerance depending on the actual size of a feature.
- The Envelope Principle, also called GD&T Rule #1, that is used by default in the ASME drawing standard, and the Independency Principle that is used by defaul in the ISO drawing standards. These principles establish the rules used to define whether the shape of a feature is controlled by its size.