Drill and drilling method

摘要: A drill includes a body 20 and pilot shaft portion 30 that projects from distal end of the has smaller diameter than are concentric. The main 31, flat surface 32 is at tapered 23 between an outer peripheral 31 certain angle β. odd number twisted grooves 22 on thereof three cutting edges 25. 25 ranges to α. auxiliary 24 continuous respective 34. extend surfaces 33. 34 formed According method forming hole in overlapped plates, can be plates or thick material process steps until obtaining desired reduced tool life long processing accuracy high.