Geometric tolerances and ISO GPS

A classical plus/minus tolerance states something about how large a part may be — but says nothing about its shape or relative position. An axle can be within its diameter tolerance and still be skewed or oval. Two holes can each have the correct diameter and still be offset so an assembly does not fit. Geometric tolerances (in English GD&T) control precisely what the size tolerances cannot: form, orientation, location and runout.

Geometric tolerances fall under ISO GPS (Geometrical Product Specifications), a unified standard system that binds design, manufacturing and inspection with one clear language. The basic standard for geometric symbols is DS/EN ISO 1101 which describes tolerances for form, orientation, position and runout as well as the rules for how they are specified and interpreted. Datums and datum systems are described in DS/EN ISO 5459. The point of GPS is that a specification can be interpreted the same way by all parties — including the measuring equipment that will later inspect the part.

Each tolerance is specified in a tolerance frame: a symbol for the characteristic, a tolerance value (often a tolerance zone as a width or diameter), and — for orientation, location and runout — a reference to one or more datums.

A datum is a theoretically exact reference (a surface an axis or a plane) that a geometric tolerance is measured against. If you choose datums wrongly you measure from something that does not match how the part is actually used or clamped. So datums must reflect the part's function: clamp it thinking how it will sit in reality and let the surface that carries or centres it become your primary datum. A datum system can consist of primary secondary and tertiary datums that together lock the part uniquely in place.

A geometric tolerance defines a zone which the actual element must lie within. The zone can be the space between two parallel planes (for example for flatness) a cylindrical space (for example for a position tolerance on an axis) or a band. It is a more precise way of thinking than plus/minus because it describes a three-dimensional space rather than just a number range — and because it ties to the part's function.

• 01Function over numbers: the tolerance describes what the part should do, not just how big it is.
• 02Fewer conflicts: design, production and quality control read the same specification the same way.
• 03Looser over-tolerance: only tighten the property that matters and can loosen the rest.
• 04Traceable inspection: the person measuring knows exactly which zone and which datum should be checked.

The GPS system is built on some overarching principles: that a specification must be unambiguous, that form and dimension tolerances are by default independent of each other unless otherwise stated, and that you can apply material conditions to a tolerance when an assembly's function depends on whether a part is at its maximum or minimum material size. Only use advanced specifications you fully understand — an incorrectly applied material condition significantly changes the meaning.

“Dimensions tell how large a part is. Geometric tolerances tell whether it works.”

— Principle in ISO GPS