Importance

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Abstract

A twist drill has a pair of concave-shaped main cutting edges continuous outwardly to respective center cutting edges, and a pair of corner cutting edges receding at an angle from the outer ends of the respective main cutting edges with respect to a drill rotation direction. Respective ends of the concave-shaped main cutting edges are situated on a straight line almost parallel to a plane passing through a drill axis. These cutting edges have a function of minimizing wear and chipping of corner portions and damage to leading edges in a drilling operation using no cutting oil, or a drilling operation wherein a very small amount of cutting oil is sprayed to the drill, or in a drilling operation on relatively hard steel. They also have a function of facilitating breaking of cut chips into pieces during a drilling operation using cutting oil and during a drilling operation on steel of relatively low hardness such as general structural rolled steels and low carbon steels.

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First Claim