The torque and thrust arising from contact at the chisel edge and margins in spade drilling of ductile materials

Abstract

Some simple experiments performed to elucidate the action at the chisel edge in spade drilling are described. These show that with drill point angles in the range 80–140°, chip formation occurs over the whole of the chisel edge width in the presence of stable dead metal zones (built-up edges) which form on the leading halves of the faces bounding to the chisel edge. Expressions for the torque and thrust force appropriate to removal at the drill lips are modified to reflect conditions at the chisel edge and the resulting expressions are shown to describe the dependence of the torque and the thrust on feed rate quite well. Experiments extended to include a 160° point angle drill revealed different behaviour in that the built-up edges were unstable and chip formation at the chisel edge was cyclic. Finally, the torque and thrust originating at the margins of the drill are quoted, but no explanation on the mechanism by which they are generated is offered.