Location tolerances control the position of features relative to datums. They ensure that features are placed correctly for proper fit and function in assemblies.
1. Position
Controls the exact location of a feature relative to one or more datums.
- Most commonly used GD&T tolerance
- Typically applied to holes, pins, and slots
- Tolerance zone: usually a cylindrical zone
👉 Ensures accurate placement for assembly and interchangeability.
2. Concentricity
Controls how closely the center (median points) of a feature aligns with a datum axis.
- Applies to cylindrical features
- Based on derived median points, not surface
👉 Used for high-precision rotating parts (rare in practice due to complexity).
3. Symmetry
Controls how equally a feature is distributed about a datum plane.
- Applies to features like slots or tabs
- Tolerance zone: two parallel planes equally spaced from datum
👉 Ensures balanced geometry about a center plane (also rarely used).
💡 Important practical note:
In modern engineering, Position is heavily preferred, while Concentricity and Symmetry are rarely used because they are difficult to measure. Engineers usually replace them with Position or Profile for better manufacturability and inspection.
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