A radius is any straight line extending from the center to the periphery of a circle or sphere.
A radius symbol, R, creates a zone defined by two arcs (the minimum and maximum radii). The part surface must lie within this zone. See Fig. 2-22.
Controlled Radius Tolerance
A controlled radius symbol, CR, creates a tolerance zone defined by two arcs (the minimum and maximum radii) that are tangent to the adjacent surfaces. Where a controlled radius is specified, the part contour within the crescent-shaped tolerance zone must be a fair curve without reversals. It is recommended that the CR be further defined with an engineering control specification. Additionally, radii taken at all points on the part contour shall neither be smaller than the specified minimum limit nor larger than the maximum limit. See Fig. 2-23. Where it is necessary to apply further restrictions to the part radius, they shall be specified on the drawing or in a document referenced on the drawing..