Location Tolerances and Controls
Locating features
Location controls are position, concentricity, and symmetry (the latter two are largely deprecated in favor of position and profile in current practice, but you must know them). Position locates a feature of size relative to a datum reference frame using basic dimensions; the tolerance zone is typically a cylinder for holes and pins.
Concepts to command
Know that position requires basic dimensions to locate the feature and a datum reference frame. Understand the difference between position of a feature of size (cylindrical zone) and position of a surface (two parallel planes). Know how MMC and LMC modifiers on position create bonus tolerance. Know the composite position control (two single-segment or composite frames) that separates pattern location from feature-to-feature location.
Traps that appear in questions
- Position without basic dimensions is incomplete; a toleranced location dimension cannot be used.
- Confusing the pattern-locating segment (with full datum reference) with the feature-relating segment (often with fewer datums).
- Assuming bonus tolerance applies when RFS is specified (it does not).
- Confusing concentricity (axis-based, historical) with position (zone-based, current preferred).
Rule of thumb
Position answers "where is it?" using basic dimensions and a DRF. If the callout lacks basic dimensions or datums, it is not a valid position control.
Worked example
A hole Ø10.0 ±0.2, position Ø0.4 at MMC to A|B|C: at MMC (Ø9.8) the allowed position tolerance is Ø0.4; at LMC (Ø10.2) the allowed position tolerance grows to Ø0.8 (0.4 + 0.4 bonus).