Twist drill

Claim 1

In a twist drill, an improvement in which with respect to an outer portion in each chisel edge line (AO,BO) at the drill point there is an additional inner helical groove (30,40) permanently efficient in cutting and united to an original helical flute, the improvement being characterized in that a cutting effect at an inner portion (EO,FO) of each chisel edge line adjacent to a center (O) of the drill is produced by formation of a hard built-up cutting edge (9), wherein with respect to the said inner portions (EO,FO) a secondary additional inner helical groove wall surface (32,41) having a same lead of helix as an original associated flute is formed in a secondary rake surface for the inner chisel portions, said grooves being extended to a vicinity of the center on a clearance surface of an opposite cutting lip passing through an associated intermediate point (E,F) dividing outer and inner portions of the associated chisel edge line; and wherein a seating face (KPO) suitable for creating the built-up edge (9) is provided with a narrow triangular region consisting of a primary rake surface (KP) of negative rake angle which is bordered by the inner portion (EO) and an intersecting curve between the additional innermost helical groove and a rearward clearance surface (6) of an opposite main cutting lip (2).