Apparatus for screw-threading

In combination with a lathe having attachment means for holding and slowly revolving, about its longitudinal axis, a work-piece to be screw-threaded with a tapering portion and a cylindrical portion, an apparatus for screw-threading said work-piece comprising:

• a tool holder ring eccentrically mounted around said work-piece;
• drive means for rapidly rotating said ring around said work-piece, said tool holder ring rotating in the same direction as said work-piece but at a relatively higher speed;
• a plurality of cutting elements radially secured in said tool holder ring, said cutting elements being disposed in a series of adjacent planes, the number of planes being equal to the length of said tapering portion divided by the pitch of said thread plus one, and said cutting elements having cutting edges and the distance of said cutting edges to the longitudinal axis of said work-piece decreasing from a first cutting element adjacent the head of said work-piece;
• and
• means for advancing said tool holder ring longitudinally of said revolving work-piece so that said cutting edges intermittently and successively tangentially contact said work-piece during each revolution of a cutting element, and so that the depth of the thread is accomplished by the continuous chipping of subsequent cutting elements set on said adjacent planes of said tool holder ring as said tool holder ring advances along said work-piece toward the head of said work-piece.