Controlled Radius (CR) — Fair Curves, No Reversals

The CR variant of a radius: the surface must both stay inside the radius tolerance zone and form a fair curve — continuous, without reversals — so the contour cannot wiggle its way through a zone it technically fits.

Definition

CR is placed before the value exactly where R would be (CR10). It does everything R10 does — same two-arc tolerance zone, same size control — and adds one sentence: the produced surface must be a fair curve without reversals. Points along the surface may not oscillate within the zone; the contour reads as one continuous arc, not a legal-looking wiggle.

The failure mode it prevents

A plain radius in tolerance can still be a mess: an S-shaped contour whose extremes both sit inside the R9.8–R10.2 zone passes inspection and still chatters a seal, rocks a bearing seat, or hunts a cam follower. CR closes that gap — the geometry that matters at contact is continuity, and continuity is now a requirement, not a hope.

Where it earns its keep

• Bearing and bushing seats, where reversals concentrate load at the peaks
• Dynamic seal tracks, where a wavy contour scours the seal lip
• Cam and follower contours, where the follower must track without hunting
• Machining note: CR surfaces are produced by slowing feed to prevent chatter reversals — a real process constraint, not just notation

Standards note: ASME only

ISO 1101 has no CR symbol. The same functional intent is specified in ISO by controlling the contour with profile of a surface, whose zone both bounds and smooths the result. Drawings that must work in both systems either state the requirement in a note or convert the callout to a profile control.