DRILL.

摘要

The present invention provides a drill which has high cuttability and high strength, can prevent the occurence of burrs at the outlet of a bored hole and the increase of an expanded margin of the bored hole, can be produced economically and can be ground again easily. In order to improve cuttability of a single chisel edge (3) formed between the center of the drill tip and a first main cutting edge (1), a rake face (30) is formed and in order to improve drill strength, a chisel edge non-formation portion (B1OB2) is disposed between the center of the drill tip and a second main cutting edge (2). A margin portion (5) disposed at the peripheral edge portion on the first main cutting edge side at the drill tip and having a cutting edge non-formation portion (11) is freely slidable to the inner peripheral surface of the bored hole, exhibits a guide shoe function and improves stability of the outer peripheral cutting edge (5) formed at the peripheral portion of the drill tip on the second main cutting edge side at the time of drilling work. The outer peripheral cutting edge extends while forming an acute angle with the drill outer periphery when viewed from the direction of the drill axis, directs the flow of a material to be cut to the inside of the bored hole and prevents the occurrence of burns. The rake face associated with the chisel edge consists of a twisted curved plane having the same lead as that of the rake face (10) of each part of the drill so that production and re-grinding of the drill become easy.