Profile of a Line — GD&T Reference

Controls

At any 2D cross-section taken through the part, all surface points must lie within a uniform band of width t centred on the true profile curve. Each cross-section is evaluated independently.

Without datums: controls form only — the profile floats to its best-fit position. With datums: controls the profile's orientation and location relative to the datum frame as well.

Example

A turbine blade must conform to its aerofoil profile within 0.08mm at each radial cross-section to maintain the correct pressure distribution and aerodynamic efficiency. Profile of a line is specified at each inspection slice rather than across the full 3D surface, allowing the blade to be checked section by section.

Inspection

CMM 2D scan at each defined cross-section; compare points to CAD nominal profile
Optical comparator — project shadow of cross-section onto scaled template
Profile projector with Mylar overlay at the specified magnification
Coordinate-to-nominal deviation reported as a colour map per slice

The drawing must specify where each cross-section is taken (e.g. at R=80, R=90, etc. for a blade).

Datum & Modifiers

Datum optional

Without datums: form control only (self-referential). With datums: location and orientation are also controlled. Most production applications use datums.

U modifier allowed

The U symbol specifies an unequal bilateral tolerance — e.g. the band is offset more to one side of the true profile than the other. Useful for controlled stock allowance.