1. Field of the Invention.
The present invention relates to thread forming dies and particularly to rolling dies having a unique thread profile geometry which will produce a seam-free space type thread. More specifically, this invention is directed to the production of seam-free threaded fasteners and particularly to the generation of rolled form threads characterized by a root which is wider than the thread crest and by an absence of seams, laps and craters. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art.
As pointed out in the above-referenced co-pending application, which is hereby incorporated herein by reference, the formation of a threaded fastener by subjecting a generally cylindrically-shaped portion of a preformed metal blank to a thread-rolling process is well known. The co-pending application discloses and claims a novel technique, and rolling dies for use in the practice of such technique, for forming machine-type screws wherein the thread is free of fissures at the crest of the thread. A machine screw is generally characterized by a helical thread having a crest width which is substantially equal to the root width and in the technique of the co-pending application the profile of the thread forming die is generated about the pitch line of the thread.
The desirability, and thus potential marketability, of a machine screw characterized by the absence of crest seams has precipitated a desire to extend the technology of the referenced co-pending application to the manufacture of threaded fasteners characterized by what is known in the art as "space type threads". A "space type thread" is characterized by a coarse pitch and a root which is wider than the thread crest. Fasteners having a "space type thread" include type B, type AB, type A, wood screws and lag screws along with the customized variations of such fasteners.