Arc Length — Dimensioning Along the Curve

A small arc drawn over a dimension value states the value is the distance measured along the curve — the arc's true length — not the straight chord between its endpoints.

Definition

The arc length symbol — a small arc — is placed above the dimension value, stating the value is an arc length measured on a curved outline: the distance along the curve itself, not the straight chord an ordinary linear dimension would report. The symbol applies to the surface nearest the dimension, which matters on multi-surface parts where two candidates sit within sight of one leader.

Chord vs. arc: why the symbol exists

A linear dimension across a curved outline reads as a chord — the straight-line distance between endpoints. The chord is always shorter than the arc, and the gap widens as curvature increases; on a 90° arc of radius 50 the chord is 70.7 but the arc is 78.5 — nearly 8 mm of unexplained difference that a shop discovering it mid-process will not thank the drawing for. The arc over the value removes the ambiguity with one stroke: the length along the curve is the specification.

Where it earns its keep

• Curved slot paths and wear surfaces whose developed length drives the mating part
• Trim and flange edges on formed parts, where the bent length differs from the flat-pattern length
• Roll-formed or wrapped features whose material consumption is computed from developed length
• Anywhere the curved outline is dimensioned and the chord value would be ambiguous with it

Measurement

Layout verification — the arc length is checked along the curve: a flexible rule, a radius-and-angle computation from a probed center, or a CMM curve fit. It is never verified with calipers across the endpoints — that measures the chord.

Drawing hygiene — if the value's meaning is chord, dimension it as an ordinary linear dimension and let the geometry show it; the arc symbol is only for lengths measured along the curve.