Geometric tolerances: when dimensions alone are not enough
Shape, orientation, placement and throw — the GPS system and datums explained
A shaft can have a completely correct diameter at all measuring points and still be bent like a banana. Ordinary dimensional tolerances say something about size, but nothing about shape, direction or placement. For that, you use geometric tolerances, which on Danish and international drawings fall under GPS — Geometric Product Specifications — and the standard DS/EN ISO 1101. It is a symbolic language you must be able to read to understand what a drawing actually requires.
§Four families of tolerances
ISO 1101 divides geometric tolerances into four main groups. Form tolerances describe the shape of a single surface or line. Orientation tolerances describe how a surface faces another. Location tolerances describe where a feature is positioned. And runout tolerances describe how much a surface runs out when the piece is rotated about an axis. Each group has its own symbols, which appear in a tolerance box on the drawing along with the tolerance value.
| Group | Examples |
|---|---|
| Shape | Straightness, flatness, roundness, cylindricity |
| Orientering | Parallelism, perpendicularity, angular accuracy |
| Placement | Position, concentricity, symmetry |
| Runout | Single cast, total cast |
§Datum: the reference everything is measured from
Most geometric tolerances only make sense in relation to something else — and that something else is a datum. A datum is a theoretically perfect reference point, reference line or reference plane designated on the part with a letter (A, B, C). When a drawing requires a surface to be perpendicular to datum A, it means the surface must stand perpendicular to precisely the surface marked A. The sequence of datums (A before B before C) tells how the part should be clamped and oriented when measured.
§Position tolerance and tolerance area
One of the most widely used placement tolerances is position — typically for holes. Instead of specifying the hole with plus/minus tolerance in two directions, the position tolerance defines a circular (or cylindrical) zone within which the hole's center must lie. This gives a larger and more accurate tolerance zone than a square plus/minus window, because function rarely cares in which direction the hole is offset — only how far. This makes position tolerances both fairer to production and easier to verify.
- 01Roundness — how close a cross-section is to a perfect circle
- 02Planeness — how even a surface is, independent of other surfaces
- 03Perpendicularity — how close a surface stands at 90° to its datum
- 04Runout — how much a surface deflects when the workpiece rotates on its axis
§From drawing to measurement
Geometric tolerances are closely related to how the part is inspected. Roundness and cylindricity are often measured on a roundness tester or coordinate measuring machine (CMM) while runout can be easily checked with a dial indicator while the part is rotated between centers. Because the tolerances refer to datums the measurement setup must mirror the datum sequence the drawing specifies. Understand the symbols and you know both what the part should be able to do and how to prove it can.
“The dimensions tell how large the part is. Geometry tells if it fits into the world at all.”