Segment 2 turns about fixed axis A and the centre 0 of its circular working surface b does not coincide with axis A. Spring 1 holds segment 2 against the surface being ground on workpiece a and tends to turn the segment counterclockwise. If centre 0 and axis A are close together, then with a small reduction in diameter of workpiece a, segment 2 swings through a relatively large angle, closing contact 3 which controls the grinding wheel feed mechanism.
$3415$SmF,FD$