Research Engineering & Manufacturing, Inc.

Herman G Muenchinger

A thread-forming screw has a point or lead thread portion having a relatively steep entry angle with respect to the screw axis. Adjacent to the point thread is a tapered thread portion of about two to three pitches in axial length. This tapered thread has a shallow entry angle defined by the taper of the maximum diameter of the thread toward the point. This shallow entry angle may be approximately 5.degree. to 8.degree. or equal to about three times the helix angle of the tapered thread. This configuration of the tapered thread results in a balanced action of thread formation and axial in-feed of the screw, resulting in a lower end pressure to initiate the thread-forming in a workpiece. A roll-threading die is contoured to provide ridges and valleys to effect the tapered thread portion.

Dies for making thread-forming screw with step taper

Thread-forming screw with step taper

A thread-forming screw has a work-entering section for forming a thread in a workpiece and a work-holding section adjacent thereto. Both sections are of an arcuate, polygonal lobular configuration in cross-section. The amount of lobulation on the holding section is considerably less than the lobulation on the work-entering section, which is of such magnitude as to insure effective thread-forming at low torque. The lobulation on the holding section is preferably just sufficient to effect a locking action with the thread of the workpiece and also to improve the stripping torque of the screw from the workpiece. The work-entering end of the fastener also has a pilot thread of about one turn and with a uniform width over 360.degree. and sized approximately the same as the pilot hole in the workpiece to prevent cocking of the fastener as it is initially inserted into the pilot hole. Thread-rolling dies for forming the fastener have a die section with grooves of equal depth to roll the pilot thread.

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South Dartmouth, MA, United States of America

Herman G Muenchinger