Abstract: A drill comprising a pair of cutting edges extending from the center of rotation of the drill symmetrically with respect to the center and curved outwardly away from the direction of rotation of the drill with a greater curvature in the center portion than in the outer peripheral portion of the drill when seen in a bottom view. The cutting edge is formed, close to its starting end, with a rake face having a rake angle approximately zero to enable the starting end portion of the edge to cut a workpiece with greater ease.

Abstract: A drill comprising a pair of cutting edges extending from the center of rotation of the drill symmetrically with respect to the center and curved outwardly away from the direction of rotation of the drill with a greater curvature in the center portion than in the outer peripheral portion of the drill when seen in a bottom view. The cutting edge is formed, close to its starting end, with a rake face having a rake angle approximately to zero to enable the starting end portion of the edge to cut a workpiece with greater ease.

Abstract: A single-toothed ball end mill provided with a cutting edge including a starting end, positioned near the center of rotation of the end mill and having an upright portion. The cutting edge is defined by a line curved outwardly in the direction of rotation of the end mill when viewed from the bottom. The cutting edge has its greatest curvature at the center portion of the end mill so as to permit the cutting edge to initiate a cutting operation with its starting end and to thereafter perform the cutting operation progressively in a radially outwardly direction.