Profile Of A Line

Profile of a Line is a GD&T tolerance that controls the allowable variation of an individual 2D line element of a feature, typically a cross-section of a curved or contoured surface, relative to the true profile. The tolerance zone consists of two parallel curves or lines that the actual cross-section must remain within. Without datums it controls form; with datums it also controls orientation and location.

On the shop floor, profile of a line is applied when a part's local shape matters more than the entire surface, such as a radius, edge contour, or CNC-machined transition that must precisely match CAD in a specific cross-section. In practice, a CNC toolpath must produce each targeted section within the tolerance band, accounting for cutter radius compensation, spring passes, tool wear, and material deflection. This callout is common in architectural millwork for reveal edges, formed extrusions, turbine contours, and cam profiles. Inspection is typically performed with a CMM or optical comparator for complex geometries. The programmer chooses this tolerance to ensure functional fit, sealing, airflow, or aesthetic accuracy that general size tolerances cannot achieve. Without datums it controls form only; with datums it also fixes orientation and location, making it a versatile constraint for critical features.