Patent ID: 8545141

Claim:
A twist drill having a cutting edge forming a concave curve in drill axis end view, a spiral flute having a flute spread angle set to the range of 80° to 110° in drill axis-perpendicular sectional view, and a thinning flute reducing the width of a chisel edge, wherein a region of said spiral flute taking part in chip control is formed along a first circle along said cutting edge and a second circle inscribed in the first circle in drill axis-perpendicular sectional view, said first circle and the second circle are both tangent at the same point on a third circle expressing the web thickness, the radius R of said first circle is in the range of 0.40 D to 0.70 D and the radius R 1 of said second circle is in the range of 0.15 D to 0.30 D respectively assuming that D represents the drill diameter, and a drill rotational direction front portion of said spiral flute is formed on said second circle to be closer to the drill center than a fourth circle concentric with the drill center and having a diameter of 0.65 D.