THE DYNAMIC GEOMETRY OF A TWIST DRILL POINT

A twist drill is characterized by complex geometry which is a direct f unction of the sharpening method used. This geometry has been studied by several investigators attempting to understand the cutting mechanis m of the drill point. Each cutting lip has been considered analogous t o a single edge cutting tool in which the normal rake angle varies fro m positive at the outerside to negative at the innerside. Optimum perf ormance is achievable using straight cutting lips occurring only in ce rtain combinations of flute shape and point angle. The chisel edge in a conventionally ground drill has no true point. This is mainly respon sible for geometric errors in drilling. In a double plane sharpened dr ill point, a well defined chisel edge with four faces is produced. Thi s point possesses a self centering action which facilitates drilling o perations. The contribution of feed velocity to cutting velocity canno t be neglected especially at the center where they are equal. This pap er studies the working geometry of a double plane sharpened drill poin t, taking the effect of feed velocity into consideration. The effect o f feed can be neglected at the cutting lips without a remarkable error , but cannot be neglected at the central zone.