THREE-BLADE DRILL

PROBLEM TO BE SOLVED: To secure high rigidity and excellent chip dischargeability, by devising a cross-sectional shape of a helical groove, on a three-blade drill.SOLUTION: In the three-blade drill, a helical groove 3 has a first curved surface 3a of a radius r1 of curvature and a second curved surface 3b of a radius r2 of curvature recessed toward the rotational center side of the drill continuing with the inner end in the radial direction of the first curved surface, in a cross-sectional view perpendicular to the axis of the drill, conditions of r1>r2 and also 0.25D≤r1≤0.40D and 0.15D≤r2≤0.25D are satisfied, and a cutting blade part thickness (t) of a body 4 in a position separate by 0.25D from the rotational center, is 40% or more of the circumferential length of a virtual circle S concentric with the rotational center of passing through a point separate by 0.25D from the rotational center.