Chained ± dimensions link every hole into an accumulating dimension chain and often create square tolerance zones. Position tolerance uses theoretically exact dimensions and datums to define a cylindrical zone that better matches the assembly function of bolts, pins and patterns. ISO 1101:2017 defines GPS geometric tolerancing; ISO 5459:2024 covers datums and datum systems.
Values in this article support engineering screening during design review, RFQ and prototyping. They do not replace formal calculations, material data sheets, qualified procedures or contractual standards.
SIMPLE COMPARISON
If X and Y coordinates each allow ±0.20 mm, center offset at a corner reaches √(0.2²+0.2²)=0.283 mm, equivalent to a 0.566 mm diametral envelope. A position tolerance of ⌀0.40 limits radial offset to 0.20 mm uniformly and is easier to verify with a functional gauge or CMM. They are not directly interchangeable; derive functional tolerance from material boundaries and assembly clearance.
BUILD DATUMS FROM FUNCTION
The primary datum commonly simulates seating on the mounting plane; secondary controls rotation/one direction, tertiary completes location. Choose assembly-contact surfaces or locating holes—not unstable, hard-to-measure or irrelevant edges. Clearance holes may use maximum-material requirement and bonus tolerance, but only when design and inspection understand the rule.
INSPECTION METHOD
Define datum order, hole size, basic dimensions, position tolerance and material requirement on the drawing. State CMM alignment, functional gauge or vision method in the plan. Report actual diameter and position error, not only PASS. Apply fundamental rules such as ISO 8015:2011 consistently in the contract system.