AiVibePosition Tolerance (True Position)
A position tolerance defines a zone, often cylindrical, within which the axis or centre plane of a feature such as a hole must lie, centred on its true position as defined by basic dimensions from datums.
The true, or theoretically exact, position of a feature is defined by basic dimensions from a datum reference frame. The position tolerance in the feature control frame sets the size of the zone around that point; for a hole specified with a diameter symbol, the zone is a cylinder. Material condition modifiers can add bonus tolerance as a feature departs from maximum material condition, which suits clearance holes for fasteners.
Position tolerancing is used for hole patterns, bolted joints, dowel locations, bearing bores and other features whose location relative to datums controls assembly. Compared with a square plus-or-minus zone, a cylindrical zone that circumscribes the square has about 57 percent more area, accepting functional parts that coordinate tolerancing would reject.
On a coordinate measuring machine the position deviation for a cylindrical zone is calculated as twice the radial distance of the measured axis from true position, after the part is aligned to its datums in the specified order. Datum order, material condition modifiers and the governing standard, ASME Y14.5 or ISO GPS, all affect the result. Functional gauges can verify positions at maximum material condition in production.
Key points
- Defines a zone around the true position set by basic dimensions
- Hole positions usually use a cylindrical tolerance zone
- Bonus tolerance can apply when modifiers such as MMC are specified
- Measured deviation is twice the radial offset from true position
Where AiVibe comes in
AiVibe designs and manufactures the AiAmbA AI Factory: edge devices plus AI agents that let people talk to CNC, PLC and robot controllers in plain language. Agents only propose changes, and a trained operator confirms each one.