The torque and thrust arising from contact at the lips in spade drilling of ductile metals

Abstract

Measurements of the chip flow angle and of the chip length ratios (which afford estimates of the normal shear plane angle) along with calculations of the change of rake angle along the lips of spade drills reveal that removal by the lips in spade drilling is quasi-orthogonal. This, coupled with the realisation that for spade drills (where the drill diameter is large compared to the chisel edge length), the assumption that the lips pass through the centre of the chisel edge introduces no serious error, enables simple expressions for the torque and thrust force originating at the lips to be obtained. For spade drills without chip breaker slots, these expressions are found to give an accurate description of the torque and thrust when feed, point angle and the inner and outer diameter of tubular workpieces are varied. These expressions are subsequently modified to allow for the influence of chip breaker slots.