Thread forming fastener

Abstract

A thread forming fastener for threading into a nut anchor of a pliable material wherein there is produced during the driving of the fastener a combination of forces that encourages the nut anchor material to flow smoothly and to develop a mating thread in the nut anchor that minimizes stress cracking and radial hoop stress as the mating thread is being formed. The fastener comprises a drive head and a shank extending from the head and having a longitudinal axis, the shank including a core and a thread having a selected axial pitch, a selected thread height and a profile which in axial cross-section includes a leading face, a trailing face and a tip. The leading face is composed of a radially inner concave portion having a first radius and which transitions smoothly to a radially outer convex portion having a second radius, the transition occurring at a point on the common tangent to those portions. The trailing face is composed of a radially inner linear portion which transitions at a selected transition location to a radially outer convex portion having a third radius. The profile also includes a convex blending portion having a fourth radius which blends together the radially outer leading and trailing face portions at the tip. The thread profile produces force vectors during assembly with the nut anchor that provide enhanced mating thread strength and improved resistance to nut anchor thread breakdown.