Profile of a Line — Profile Tolerance

Controls the form of a line element of a surface along a specified section.

Definition

Profile of a line controls how far a single cross-section of a surface — almost always a curved one — may depart from its true profile. It is the first control in this library that is architecturally different from the Form group: where straightness or flatness are always self-referencing, profile can be specified with no datum reference at all (controlling form only) or with datum references, in which case it simultaneously controls orientation and location of the section's profile as well. That flexibility is Profile's defining trait, and it is worth stating plainly: Profile does not live in one fixed category the way the Form controls do.

The zone: two offset curves

The tolerance zone is bounded by two curves offset on either side of the true profile — the theoretically exact curve established by basic dimensions (or by a basic angle and radius in the drawing view where the profile appears). By default the tolerance is split evenly: half the tolerance outside the true profile, half inside. The actual profile of the one cross-section under evaluation must fall entirely between the two offset curves. Like circularity, sections are evaluated one at a time — profile of a line places no constraint on how adjacent sections relate to each other along the surface; that is the job of profile of a surface.

Datum-optional: two ways to specify it

Specified without datum references, the profile zone is free to float — the control limits only the shape of the section's profile, much like a Form control. Specified with datum references, the zone is anchored to the datum reference frame: the section's profile must now sit correctly relative to the datums, so orientation and location are controlled at the same time as form. This dual nature is why profile tolerances appear both on unfinished surfaces (form control) and on fully defined features (form + orientation + location), and why the frame must be read carefully — the same symbol means different things depending on what follows the tolerance value. As a frame, ⌒ 0.2 A B C reads as: profile of a line, zone 0.2 wide, referenced to primary datum A, secondary datum B, and tertiary datum C — while ⌒ 0.2 alone controls form only.

Unequal disposition of the zone

By default the zone is bisected by the true profile. When stock removal, coating thickness, or a sharp edge matters, the zone can be shifted so most of it falls on one side — the drawing states the offset explicitly (for example with the unequal-profile symbol and a value). This is an aside rather than a separate control: the zone width stays t either way; only its disposition relative to the true profile changes. Most profile callouts never need it, but coating-heavy or machined-stock features often do. For the full Ⓤ notation and worked disposition arithmetic, see Unequally Disposed Profile in the concepts section.

Measurement

Optical comparator / contour tracer (2D sections) — the natural method: project or trace the cross-section and compare against the true profile magnified; the band between the two offset curves is checked directly. Standard for stampings, extrusion dies, and cam profiles.

CMM section scan — probe along the cross-section, best-fit against the basic profile, and report the deviation band; used when the profile lives on a part too large or too irregular for an optical setup.

When to use it

• Curved cross-sections with no simpler geometry to fall back on — cam lobes, airfoil sections, formed sheet-metal ribs
• Sections whose shape matters but whose location is controlled elsewhere (or not yet) — the no-datum form-only callout
• Sections that must sit correctly relative to datums — the with-datum callout, controlling form and location in one frame
• Complex 2D contours that a radius-plus-tolerance scheme cannot describe honestly