Breakable composite drill screw

Abstract

A breakable composite drill screw comprises a first shank made of a metal unsusceptible of hardening, a driving head extending from one end of the first shank and engageable with a screw driving tool, a second shank made of another metal susceptible of hardening, the second shank being fixedly adjoined to the other end of the first shank to thereby provide a break zone at said other end, a drill bit formed by cold forging at an end of the second shank, a continuous thread formed by thread-rolling to extend around both outer cylindrical peripheries of the first and second shank wherein a portion extending from the break zone to the drill bit is hardened or quenched, and wherein said shanks are adjoined at their engaging surfaces to each other by projection-welding technique making use of a welding lug protruding from either engaging surface, said lug forming the break zone.

What is claimed is: 
     
       1. A breakable composite screw comprising: a first shank made of an austenitic stainless steel; a driving head extending from one end of the first shank and engageable with a screw driving tool; a flat surface formed at the other end of the first shank in a direction perpendicular to an axis of the first shank; a second shank made of a .[.low-carbon steel.]. .Iadd.metal susceptible of hardening .Iaddend.and being of the same diameter D as the first shank; a welding lug protruding centrally from an end of the second shank; the second shank being fixedly adjoined to the flat surface by projection-welding by means of the welding lug to thereby provide a break zone at the other end of the first shank; a drill bit formed by cold forging at an end of the second shank; a continuous thread which is formed by thread-rolling so as to extend around both outer cylindrical peripheries of the first shank and second shank wherein a portion extending from the break zone to the drill bit is hardened or quenched. 
     
     
       2. A breakable composite drill screw according to claim 1, in which the welding lug is of a diameter within the range of 0.5D to 0.8D. .Iadd.3. A breakable composite drill screw according to claim 1, in which the second shank is made of a low-carbon steel. .Iaddend.