Drilling and thread forming fastener

Claim 1

A drilling and thread forming screw comprising an elongated shank and means at one end of said shank co-operable with a tool for turning the screw, a. said shank having at the other end thereof an entering end portion having a tip centered about the longitudinal axis of the screw and comprising a terminal chisel edge intersecting and substantially perpendicular to the longitudinal axis of the shank with a symmetrically disposed substantially flat side surface on each side of said terminal chisel edge, each diverging angularly outwardly at an included angle of about 90* to 150* and each intersecting the shank periphery intermediate the terminal chisel edge and said means for turning the screw, b. said shank including a trailing portion having a plurality of integral thread convolutions, the convolutions adjacent the entering end portion having a progressively increasing diameter, the greatest diameter of the thread convolutioNs being greater than the diameter of the entering end portion, c. said entering end portion having a pair of oppositely disposed substantially longitudinally extending flutes therein with intermediate strengthening shank portions between the flutes to provide sufficient bearing surface for efficient drilling and thread forming action, d. each of said flutes being defined by first and second opposed curved flute sides having at least portions with a common axis of curvature and a connecting substantially flat flute side, each of said first curved flute sides extending from the shank periphery adjacent said trailing portion into and beyond said terminal chisel edge, thereby intersecting the adjacent of said symmetrically disposed flat side surfaces and a portion of the other of said symmetrically flat side surfaces at opposite sides of said terminal chisel edge, e. the intersection of each said first curved flute side with the respective adjacent of said symmetrically disposed flat side surfaces providing a cutting edge disposed at an angle of about 10* to 80* to said terminal chisel edge, the web between the respective cutting edges having a thickness of about 0.01 to 0.04 inch, f. each of said first curved flute sides being convexly curved at least adjacent said cutting edge, a plane tangent to the convexly curved surface at the cutting edge being inclined at an angle with respect to a plane containing the longitudinal axis of the shank so as to traverse the plane containing the longitudinal axis at an angle of about 5* to 25* intermediate said chisel edge and said means for turning the screw, whereby to provide a positive rake angle at said cutting edge, g. said terminal chisel edge being disposed to initially engage and extrude material from the workpiece, thereby progressively exposing the workpiece to said cutting edges and the thread convolutions.