GD&T Basics for Manufacturing
Geometric Dimensioning and Tolerancing (GD&T) is a system for defining engineering tolerances using symbolic language. It communicates design intent more precisely than coordinate tolerancing and ensures parts function correctly while maximizing manufacturing tolerances.
Why GD&T Matters
- Functional tolerancing: Specifies exactly what matters for part function, not arbitrary dimensions
- Bonus tolerance: MMC/LMC modifiers can increase allowable tolerance where function allows
- Reduces scrap: Parts within GD&T tolerance but outside coordinate tolerance are functional and can be accepted
- Interchangeability: Clear datum structure ensures consistent gauging across suppliers
- International standard: ASME Y14.5 (North America) and ISO 1101 (global) widely adopted
Key GD&T Symbols
| Category | Symbols | Controls |
|---|---|---|
| Form | Straightness, Flatness, Roundness, Cylindricity | Single feature shape |
| Profile | Profile of a line, Profile of a surface | Any curved surface |
| Orientation | Angularity, Perpendicularity, Parallelism | Feature relationship at an angle |
| Location | Position, Concentricity, Symmetry | Feature location relative to datum |
| Runout | Circular runout, Total runout | Surface variation rotating around datum |
Datum Selection Principles
- Primary datum: 3-point contact (plane surface). Largest practical surface for stability.
- Secondary datum: 2-point contact (line). Establishes rotation.
- Tertiary datum: 1-point contact (point). Fixes remaining degree of freedom.
- Functional surfaces first: Use mating/assembly surfaces as datums, not arbitrary surfaces.
MMC LMC RFS
- MMC (Maximum Material Condition): Part weighs most. Hole at min diameter, pin at max diameter. Adds bonus tolerance for location.
- LMC (Least Material Condition): Part weighs least. Hole at max diameter, pin at min diameter.
- RFS (Regardless of Feature Size): No bonus tolerance. Tolerance applies regardless of actual size.
Common Mistakes
- Over-tolerancing: Applying tight GD&T to non-critical features adds cost without function
- Invalid datum references: Datum must be physical. A centerline is not a datum — use the cylindrical surface.
- No tolerance on basic dimensions: Basic dimensions are theoretically exact and require GD&T callout for actual tolerance.