Abstract: Improvements in numerical control systems associated with turning machines and by which a special mode of machining is carried out from those blocks of a part program having a special mode code associated therewith. A lead segment is automatically formed in the workpiece by the special block data from which signals are produced to represent (i) the angular starting location of the lead segment on the workpiece, (ii) the desired lead, and (iii) the ending location of the segment. A lead segment of desired lead and extent may thus be located angularly on a workpiece. A plurality of lead segments, contiguous or separated by zero-lead portions of a composite groove, may be formed circumferentially around the workpiece within an angular span of less than 360 degrees and with its groove portions accurately located in an angular sense.