Patent ID: 7334976

Claim:
A countersunk screw for fastening a metal part having a preformed sinkhole or for fastening a part formed of a soft material, including wood, not having a preformed sinkhole, the screw comprising: a threaded shank and a countersunk head, the countersunk head having a conical surface tapering at a countersinking angle in the direction of the threaded shank, the countersunk head having a plurality of ribbed protrusions protruding from the conical surface, the ribbed protrusions include outer bearing surfaces, the ribbed protrusions having flanks defining limit lines, the limit lines generally converging in the direction of the threaded shank, wherein the outer bearing surfaces jointly define a conical enveloping surface with a cone angle, the conical enveloping surface and the conical surface jointly define a total cone surface area, the countersinking angle of the conical surface being less than the cone angle of the conical enveloping surface, wherein the surface area of the conical enveloping surface lies within the range of at least 40% to about 60% of the total cone surface area, wherein the ribbed protrusions have an inner periphery and an outer periphery, the inner periphery being closer to the threaded shank than the outer periphery, the inner periphery being more narrow than the outer periphery, wherein when the screw is used for fastening the metal part having a preformed sinkhole the ribbed protrusions do not exert sufficient surface pressure to damage a contact surface of the sinkhole, and wherein when the screw is used for fastening the part formed of the soft material not having a preformed sinkhole the ribbed protrusions have a milling effect on the soft material to form a sinkhole.