| 摘要 |
PROBLEM TO BE SOLVED: To provide a drill for hole boring processing that achieves both improvement in quality of a processing hole and improvement in durability of the drill.SOLUTION: A drill is provided with a torsion angle &bgr; of 25°-40° and three cutting edges 2. In the drill, a margin 5 provided on a land part 4 is set as a single margin, and the cutting edges 2 are constituted of at least three edge parts whose point angles sequentially become smaller from a rotation center toward an outer periphery. Further, the drill is provided with a planar thinning surface 7 on a cutting face of a region which ranges from an inner edge in a radial direction of a first edge part 2positioned on an innermost radial side of the cutting edge to at least an outer edge of the first edge part and includes a portion of a second edge part 2. |
