Radius (R) — Partial Contours, From the Center

R placed before a value states a radius: the distance from a feature's center to its surface, controlling the partial circular contours — fillets, rounds, arcs — that a diameter cannot describe.

Definition

A radius is a straight line from a center to the feature's surface; the symbol R before the value (R10) states the surface lies that distance from the center. On the drawing, the leader is drawn from the actual center with a single arrowhead touching the arc — the leader itself points out the center, so no separate center mark is required.

What the tolerance produces

A radius size tolerance R10 ±0.2 defines a zone between two concentric arcs — R9.8 and R10.2 — and the entire produced contour must lie between them about the same center. It is the arc-shaped cousin of the circularity zone, except these arcs are fixed by size rather than centered by a datum axis. Where wiggle inside that zone would matter, the drawing upgrades to CR.

R vs. ⌀, and the full-radius case

• A contour that closes around its center takes ⌀; a partial arc takes R
• A radius that must be an uninterrupted sweep without flats is noted FULL R (or ALL R when every fillet on the part is included)
• Where a leader would be unclear on a small radius, a dot replaces the arrowhead on the surface
• Spherical radii take SR — a radius measured to a sphere, not a cylinder

When plain R is not enough

Plain R lets the produced surface wander anywhere inside the two limiting arcs — including back-and-forth reversals that still measure in tolerance. On a seal track or bearing seat those reversals are the failure mode, which is why the standard offers controlled radius: CR adds one requirement — the surface must be a fair curve, continuous, with no reversals.