Self-drilling and self-tapping fastener and method of manufacture thereof

Claim 1

A drilling and thread forming screw comprising an elongated cylindrical 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 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 milled 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 intersecting flute sides, each of said first 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 of said first flute sides 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, f. each of said first flute sides being uniformly concavely curved, a plane tangent to the uniformly concavely curved surface at the chisel edge being inclined at an angle with respect to the longitudinal axis of the shank so as to transverse the axis at an acute angle intermediate said chisel edge and said means for turning the screw, whereby to provide a positive rake angle at said cutting edge, g. the curvature of each of said first flute sides being sufficient whereby 1. planes tangent thereto at the chisel edge and at the opposite or shank-exiting extremity intersect at an angle of about 40* to 100*, 2. the points of tangency of the tangent planes are on the same side of a plane through the axis of the shank and the intersection of the tangent planes is on the opposite side, and 3. the curved flute side and a plane through the axis of the shank traverse at two spaced locations, so as to redirect the normal chip flow and thereby to fragment the chips by flexure, h. said terminal chisel edge being disposed to initially engage and swage material from the workpiece and progressively expose the workpiece to said cutting edges and the thread convolutions.