Discontinuous Chip Formation

Abstract

Abstract The extent of fracture in the chips in a metal-cutting operation varies from cracks that are just discernible under the microscope to complete discontinuity where the metal removed is in the form of discrete particles. The fracture that occurs in all of these cases is either of the ductile shear type (which predominates in metal cutting) or the brittle tensile type. The characteristics and causes of these two types of fracture are first discussed and then applied to the problem of chip fracture. The very limited fracture that occurs at the point of a tool in “continuous cutting” is held to be of the tensile type, and the role of tool sharpness in this connection is discussed. It is shown by means of motion pictures and dynamometer data that completely discontinuous chip formation is entirely different from that in continuous cutting, being better described as a periodic extrusion process rather than one of simple shear. In discontinuous cutting the friction between chip and tool is static rather than dynamic in nature. The normal stress on the shear plane, as influenced by rake angle or tool friction, is found to be an important variable in determining whether a chip will be continuous or discontinuous. Other variables of importance include the cutting speed, depth of cut, number, shape, size, and hardness of inclusions in the metal cut, and tool rigidity. The influence of these several variables with regard to chip discontinuity is discussed and a number of examples are presented to illustrate the principal points that are made.