Drill screw and cutters for making same

Claim 1

A milled self-drilling fastener having a drilling tip at one end of the fastener shank, said tip comprising a pair of flutes extending at equal opposite angles with respect to the axis of said fastener lying generally on opposite sides thereof, a pair of heel portions extending intermediate said flutes, said heel portion intersecting to define a narrow chisel edge, said heel portions having convex curvature in a direction perpendicular to said chisel edge, and substantially no curvature in a direction parallel to said chisel edge, said flute defining a cutting edge and a drag surface, said heel portions being configured in substantially immediately increasing cross-sections in planes parallel to the axis of the fastener as distance along said planes from said axis increases thereby buttressing said cutting edge and enhancing resistance of said tip to breaking during drilling, each said flute having a compound configuration formed by a flat surface section extending inwardly from the outer edge of the shank and intersecting at least a portion of the cutting edge and a curved section including at least said drag surface, said curved section having a substantially uniform radius of curvature in a plane perpendicular to the axis of the flute.