The dynamic geometry of a twist drill point

Abstract

A twist drill is characterized by complex geometry which is a direct function of the sharpening method used. This geometry has been studied by several investigators attempting to understand the cutting mechanism of the drill point. Each cutting lip has been considered analogous to a single-edge cutting tool in which the normal rake angle varies from positive at the outerside to negative at the innerside. Optimum performance is achievable using straight cutting lips occurring only in certain combinations of flute shape and point angle. The chisel edge in a conventionally ground drill has no true point. This is mainly responsible for geometric errors in drilling. In a double plane sharpened drill point, a well defined chisel edge with four faces is produced. This point possesses a self centering action which facilitates drilling operations. The contribution of feed velocity to cutting velocity cannot be neglected especially at the center where they are equal. This paper studies the working geometry of a double plane sharpened drill point, taking the effect of feed velocity into consideration. The effect of feed can be neglected at the cutting lips without a remarkable error, but cannot be neglected at the central zone.