GD&T True Position Explained
True position is the most commonly used GD&T control. It defines where a feature should be relative to datums, using basic (theoretically exact) dimensions. The tolerance zone is a cylinder centered on the true position. If the feature axis falls within this cylinder, the part passes.
The True Position Formula
True position is calculated as a diametral value: TP = 2 × √(ΔX² + ΔY²). The factor of 2 converts the radial deviation into a diameter so it can be compared directly to the position tolerance specified in the feature control frame.
Bonus Tolerance (MMC / LMC)
When a material condition modifier is specified, the part gets extra (bonus) tolerance as the feature departs from its MMC or LMC boundary. For a hole at MMC: bonus = actual hole size − MMC hole size. A 0.510″ hole with MMC of 0.500″ gets 0.010″ of bonus tolerance. This is added to the stated position tolerance.
Pro tip: Specifying MMC on hole patterns is common because it ensures assembly fit while allowing manufacturing flexibility. Holes that come out slightly larger than minimum get more position tolerance.
RFS (Regardless of Feature Size)
When no modifier is called out, RFS is the default per ASME Y14.5-2009. The position tolerance stays the same no matter what the actual feature size is. There is no bonus tolerance.
GD&T Symbol Quick Reference
| Symbol | Characteristic | Category |
|---|---|---|
| ⊕ | Position | Location |
| ◯ | Circularity (Roundness) | Form |
| ∥ | Parallelism | Orientation |
| ⊥ | Perpendicularity | Orientation |
| ◯ | Concentricity | Location |
| ∠ | Angularity | Orientation |