Profile of a line is a GD&T control that keeps each 2D cross-section of a surface within a band that follows the exact shape defined by basic dimensions. Without datums it controls shape only; with datums it also controls where each section sits and how it is tilted.
Take a slice through a surface, such as an airfoil, a cam or an extrusion, and you get a line. Profile of a line says that line must stay within a uniform band around its true profile, the theoretically exact curve drawn with basic dimensions. Each slice is judged on its own, usually in the view where the callout is shown.
It is the right tool when the shape of a section matters but the sections do not need to line up perfectly with each other, for example extrusions that may twist slightly along their length, or when it is used together with a looser profile of a surface that controls the whole face.
With datums referenced, the band is fixed in place relative to the datum reference frame, so the control covers size, location and orientation of each section, not just form.
At each section, the zone is two lines that follow the true profile at a constant distance apart, equal to the tolerance. By default the band is split equally either side of the true profile (bilateral). The unequally disposed modifier lets you shift it, for example all on the material side.
With no datums the band can float and rotate to fit each section. With datums it is locked to them.
Datums are optional. Without them, profile of a line is a form control. With them, it also locates and orients each section. Most drawings reference at least a primary datum.
| Modifier | With profile of a line |
|---|---|
| MMC or LMC on the tolerance | Not allowed. Profile applies to a surface, not a feature of size. |
| Datum modifiers (MMB/LMB) | Allowed on datum references that are features of size. |
| Unequally disposed (U) | Shifts the band so more (or all) of it is on one side of the true profile. |
| Between symbol | Limits the callout to the stretch of surface between two named points. |
Example: a cam lobe whose outline is defined by basic radii from datum axis A
Result: Every section stays in the 0.15 mm band, located from datum A, so the lobe passes.
Every GD&T symbol page: GD&T symbols chart
No. It can be used without datums to control shape only. Add datums when the section must also be in the right place and orientation.
Profile of a line checks 2D cross-sections one at a time. Profile of a surface puts one 3D band over the whole surface, so it also controls how the sections relate to each other.
The value is the total width of the band. By default it is split equally, so 0.2 means 0.1 either side of the true profile.
Not on the tolerance itself, because it controls a surface. Datum references to features of size can carry a modifier.
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