Recessed screw

Claim 1

A threaded fastener having a shank and an enlarged head meeting at a first line of juncture, and enclosing a substantially circular area centered on the longitudinal axis of the fastener, said head having a tool-engaging socket extending into its upper end, said socket being centered on and symmetrical relative to the longitudinal axis of the fastener and presenting in plan eight symmetrically disposed corners, each corner being defined by a pair of identical downwardly extending sidewall portions which intersect at a right angle, any series of four alternate corners and their defining sidewall portions outlining a square, the remaining series of four alternate corners and their defining sidewall portions outlining a second square in 45* angular relation to said first-mentioned square, each sidewall portion defining a part of an included angle of 135* with a sidewall portion defining a part of the next adjacent corner along an intersection equally spaced from such corners to provide a total of 16 identical sidewall portions, each of said sidewall portions being inclined inwardly significantly with respect to said axis of said fastener, said intersections defined by said included angles being inclined to said axis of said fastener at an angle greater than the angle of inclination of said sidewall portions, and the lines of intersection at said corners being inclined to said axis of said fastener at an angle greater than the angle of inclination defined by said included angles, a bottom wall of said socket being inclined inwardly with respect to said longitudinal axis at a greater angle than said sidewalls, and said bottom wall intersecting said sidewalls at a second line of juncture located above said first line of juncture and enclosing an area smaller than and lying within an axial projection of said circular area, so as to provide a substantial volume of material around said socket to resist shear stress when said fastener is driven.