Thread-forming screw

Abstract

A thread-forming screw includes a tapered end portion formed by inserting a cylindrical blank between paired dies each having a straight portion and a tapered portion. The threads of each die are provided with undulations along their lengths corresponding to the circumference of the screw blank so as to form a trilobular thread on the screw which spirals around its circumference from one end of the screw to the other in a direction opposite to that of the threads. The cylindrical blank provides increased material at the tapered thread-forming location for forming fully crested threads, while increasing the length of the tapered portion of the die and screw provides greater stress relief for reducing the force required for thread formation and allows the screw to be driven into a workpiece more easily while maintaining a high strip force. In a preferred embodiment, the tapered end of the screw has a constant minor diameter and two pitches in which the major diameter diminishes toward the end followed by two end pitches in which the major diameter remains constant.

Dependencies are inferred from the claim text.