Manufacturing & engineering
GD&T
Geometric dimensioning and tolerancing
Definition
GD&T is a symbolic language for specifying the allowed variation in a part's form, orientation, location, and runout relative to datum features. In the US it is defined by ASME Y14.5, and it tells the manufacturer and inspector which geometry actually matters for fit and function.
Also known as: GDT, GD and T, geometric tolerancing, ASME Y14.5
Updated
What GD&T controls#
Plain plus-or-minus dimensions describe sizes. GD&T adds controls for geometry that sizes alone cannot describe well:
- Form: flatness, straightness, circularity, cylindricity
- Orientation: perpendicularity, parallelism, angularity
- Location: position, concentricity, symmetry
- Runout: circular and total runout
- Profile: profile of a line and profile of a surface
Each control sits in a feature control frame that states the characteristic, the tolerance zone, any material condition modifier, and the datums it is measured from.
Datums#
Datums define the reference frame for measurement. A position tolerance on a hole pattern, for example, means little until the drawing says which faces locate the part: usually a primary datum that seats the part, then secondary and tertiary datums that stop it sliding and rotating. Choosing datums that match how the part mounts in the assembly is most of the skill in applying GD&T.
Standards#
The current US standard is ASME Y14.5-2018, which ASME lists as reaffirmed in 2024. ISO publishes a separate family of geometrical product specification (GPS) standards. The two systems look similar on a drawing but differ in defaults and some interpretations, so a drawing should state which one applies.
GD&T in 3D models#
GD&T can be attached to the 3D model as semantic PMI instead of only appearing on a 2D drawing. STEP AP242 can carry that information between systems, and the CAx Interoperability Forum tests how well CAD programs exchange it.
GD&T and AI tools#
Text-to-CAD tools generate nominal geometry. They do not choose datums, assign tolerance zones, or reason about how a part is inspected. Any AI-generated part that has to fit something still needs a person to decide what matters and specify it.
Sources
Related terms
Tolerance
A tolerance is the allowed variation from a nominal dimension, such as 10.00 ± 0.05 mm. Tolerances drive fit, cost, and inspection, and they have to be specified deliberately because a CAD model on its own stores only nominal sizes.
PMI
PMI is the dimensions, tolerances, GD&T, materials, surface finishes, and notes attached to a 3D CAD model instead of, or in addition to, a 2D drawing. It is the core content of model-based definition.
MBD
MBD is the practice of making the annotated 3D model, with its PMI, the authoritative definition of a part instead of a separate 2D drawing. The model carries dimensions, tolerances, and notes directly to manufacturing and inspection.
Drawing
An engineering drawing is a 2D document generated from a 3D model, with views, dimensions, tolerances, and notes. Many shops still quote and inspect from drawings even when they also receive a STEP file.