Engineering · GD&T
Profile of a line.
A 2D tolerance that controls a single cross-section of a curved or contoured surface. The tolerance zone is bounded by two parallel curves matching the basic profile, separated by the tolerance value. Less common than profile-of-a-surface but useful when only at-section profile matters.
01 · Definition
Curved tolerance band, single cross-section.
Profile of a line creates a tolerance zone defined by basic dimensions (the nominal contour) bounded on both sides by parallel curves at half the tolerance value. The toleranced cross-section line element must lie within that zone.
Without datums, profile of a line is a pure form control — the line element must match the basic shape but the cross-section can be located anywhere. With datums, the basic shape is also located in space relative to the datum reference frame.
The tolerance distribution defaults to bilateral (half on each side of the basic profile). Unilateral or unequal distribution (e.g., 0.005 outside, 0.000 inside) is specified with the Ⓤ (unequally disposed) modifier; the value after Ⓤ is the portion of the zone in the direction that adds material, so that example reads 0.005 Ⓤ 0.005.
02 · FCF and example
Cam profile cross-section.
A cam-lobe cross-section called out as ⌒ 0.003 A B. The drawing defines the basic cam profile with basic dimensions, and the cross-section line element must lie within ±0.0015″ (bilateral) of the basic profile, located by datums A and B.
The tolerance applies at every cross-section independently — each section can be inspected on its own without comparing to adjacent sections. For control across the full axial extent of the cam, you’d use profile of a surface instead.
03 · When line vs surface
2D vs 3D control.
- Profile of a line: at-section control. Each cross-section evaluated independently. Cheaper to specify, easier to inspect with traveling-line probe scans.
- Profile of a surface: 3D volumetric control. The full surface evaluated against the basic shape. More restrictive — captures axial deviation that line-profile would miss.
- Default to surface unless line is sufficient: profile of a surface is the more common choice. Profile of a line is appropriate when only the cross-sectional shape matters and you don’t care about variation along the axial direction.
Keep exploring
Related tools & references
Full feature-control-frame symbol reference.
The 3D version that controls the whole surface, not just line elements.
Form-only line control, with no datum reference.
See how profile zones stack across features.
Choosing realistic profile callouts.
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