True Position Calculator

A position tolerance with a diameter symbol makes a cylindrical tolerance zonecentered on true position, the location the basic dimensions give from the datums. The feature's axis must lie inside it. Because the zone is a diameter, the deviation is twice the distance from true position to the measured axis:

r = √(Δx² + Δy²)  ·  position deviation = 2r

Bonus tolerance.With MMC in the frame, the zone grows by however far the feature's actual size has moved from MMC: for a hole, its size minus the smallest allowed; for a pin, the largest allowed minus its size. With LMC it is the reverse, measured from the least material size. At RFS there is no bonus. A feature outside its size limits is rejected on size and gets no bonus.

Virtual conditionis the constant boundary the modifier implies: at MMC a hole's smallest size minus the tolerance, a pin's largest size plus it; at LMC the reverse. At MMC it is the boundary a mating part is designed to, so the parts always assemble; at LMC it protects a minimum wall thickness or edge distance instead.

Worked example. A hole 10.0 to 10.2 mm, ⌀0.2 at MMC, measured 10.08 mm across and 0.08 mm off in X and Y. The deviation is 2 × √(0.08² + 0.08²) = 0.226, which fails ⌀0.2 alone. The bonus is 10.08 − 10.0 = 0.08, the zone becomes 0.28, and the hole passes using 81% of it. Load it with the button under the inputs.

Not covered:the axis's tilt along a long feature (check both ends), projected tolerance zones, composite frames and patterns, and three-dimensional deviation. The measured size you enter stands for the size the modifier is judged by: the actual mating size at MMC, the actual minimum material size at LMC.

Reference: ASME Y14.5-2018, Section 10, Tolerances of Position; Fig. 10-10 (bonus where a hole is not at MMC) and Fig. 10-13 (at LMC); Section 5 for virtual condition.

Background on the Drafter blog: Position, Feature control frames.