GD&T basics for the CQE exam
Geometric Dimensioning and Tolerancing (GD&T) is a symbolic language on engineering drawings that defines the allowable variation in form, size, orientation, location, and runout of a feature. It replaces vague “make it close” notes with exact tolerance zones that any inspector can verify the same way.
The five tolerance categories
| Category | Symbols | Controls |
|---|---|---|
| Form | △ flatness ◯ circularity ⎯ cylindricity ― straightness | Shape of a single feature; no datum needed |
| Profile | ⋄ profile of a line △ profile of a surface | Shape of a curve or surface; may or may not reference a datum |
| Orientation | ∥ parallelism ⊥ perpendicularity ∠ angularity | Angle relationship to a datum; datum required |
| Location | ⊕ position ◕ concentricity ⸻ symmetry | Where a feature is relative to a datum; datum required |
| Runout | ↑ circular runout ⇑ total runout | Surface variation during one rotation about a datum axis |
Key concepts
Datums are theoretically perfect planes, axes, or points used as a reference framework. They are established from actual part surfaces (datum features) in a defined order (primary, secondary, tertiary) that constrains all six degrees of freedom.
MMC (Maximum Material Condition): the feature contains the most material—largest shaft, smallest hole. When the MMC modifier Ⓥ is applied to a tolerance, a feature gets bonus tolerance as it departs from MMC.
LMC (Least Material Condition): the feature contains the least material—smallest shaft, largest hole. The LMC modifier Ⓛ works in reverse.
Bonus tolerance example: a hole is specified ⌀20.0 ± 0.5 with a position tolerance of 0.3 at MMC (⌀19.5). If the hole is produced at ⌀20.3 (0.8 mm larger than MMC), the bonus is 0.8 and the total allowed position tolerance = 0.3 + 0.8 = 1.1 mm.
Common exam traps
- Form tolerances need no datum. Flatness, circularity, cylindricity, and straightness are self-contained. If a problem references a datum for flatness, that is an error.
- Circularity vs cylindricity. Circularity is a 2D cross-section check (perfect circle in one slice). Cylindricity is 3D (the entire cylindrical surface at once).
- MMC gives bonus; RFS does not. The default is RFS (Regardless of Feature Size)—no bonus. Bonus only applies when the MMC or LMC modifier is explicitly shown.
- Runout always requires a datum axis. Runout measures surface behavior during rotation, so a reference axis is always necessary.
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