True Position — Position Tolerance

Locates a feature (hole, pin, slot) relative to datum features at a basic location.

Definition

Position tolerance controls where a feature of size — a hole, boss, slot, or similar — is allowed to be located, relative to specified datums. It defines a tolerance zone (a cylinder, for a round hole) centered on the feature's true position: the theoretically exact location established by basic dimensions from the referenced datums. The feature's actual axis or center plane must fall entirely within that zone. It's the most widely used GD&T control because it maps directly onto real function — bolt patterns, dowel holes, mating bosses — rather than an arbitrary drafting convention.

Why a circular zone instead of ± coordinates

Tolerancing a hole's location with plus/minus X and Y dimensions creates a square tolerance zone. A hole at the corner of that square is off-location by more than either the X or Y tolerance alone (the diagonal), which is looser than intended in some directions and tighter than necessary in others. Position's circular zone matches the way a round pin actually behaves in a round hole. It is also more generous than coordinate tolerancing at the same worst-case error: a Ø1.0 circular zone allows essentially the same maximum offset as a ±0.354 coordinate zone (the square's corner), yet covers about 57% more usable area — parts that would fail under ± dimensioning often pass legitimately under position.

• Square zone (± X/Y dimensioning): tighter than intended along the diagonal, looser than intended along the axes — inconsistent by direction.
• Circular zone (position): matches how a round pin actually sits in a round hole — equal protection in every direction, and about 57% more usable area than the square it replaces, at the same worst-case offset.

MMC and bonus tolerance

When a position tolerance is applied at Maximum Material Condition (Ⓜ), the stated value applies only when the hole is at its smallest allowed size. As the actual hole is produced larger than MMC, the position tolerance gets a bonus equal to that difference — a bigger hole is allowed to be located less precisely and still function, because it still clears the fastener. This is where position tolerancing earns its reputation as a cost-saver: it lets real, imperfect parts pass that a fixed tolerance would reject.

Reading a feature control frame

⌖ ⌀0.5 Ⓜ A B C reads as: a position tolerance of 0.5 mm diameter, evaluated at MMC, referenced to primary datum A, secondary datum B, tertiary datum C.

Measurement

Functional gauging — a pin sized at virtual condition (MMC minus the tolerance, for an internal feature). Fast and repeatable; standard for high-volume production.

CMM coordinate measurement — measures actual location against the basic dimensions. Used for lower-volume runs, complex patterns, or when a dedicated gauge isn't practical.

When to use it

• Bolt circles and mounting patterns, where the hole needs to clear a fastener despite real-world location error
• Dowel and pin locations, where two parts must align without over-constraining the fit
• Any hole pattern currently toleranced with ± X/Y coordinates — position almost always permits more real variation for the same functional requirement
• Features where MMC bonus tolerance can meaningfully cut scrap on hard-to-hold locations