Mechanism of metal removal

MECHANISM OF METAL REMOVAL

The metal cutting is a process of extensive plastic deformation to form a chip that is removed later. The basic mechanism of chip formation is essentially the same for all machining operations. In this process, a wedge-shaped tool with a straight cutting edge is made to move relative to the workpiece and a layer of metal called chip is removed. The metal is removed by a continuous shearing action of workpiece. The actual separation of the metal starts from the cutting tool tip as yielding or fracture phenomena. Then the deformed metal flows over the tool (rake) face. The chip after sliding over the tool rake face would be lifted away from the tool. During metal removal operation, there is a friction between the flowing chip and face of the tool.

1. PRIMARY AND SECONDARY DEFORMATION SHEAR ZONES

The metal in front of the tool rake face gets immediately compressed first elastically and then plastically. This zone is called shear zone because of the fact that the material in the final form would be removed by shear from the workpiece.

During continuous machining, the uncut layer of the workpiece just ahead of the cutting tool edge is subjected to almost all sided compression, hence shear stress develops. When the shear stress reaches or exceeds the shear strength of that workpiece in the deformation region, yielding or slip takes place which results in shear deformation in that region and the plane of maximum shear stress occurs. The narrow region around the plane of maximum shear stress is called primary deformation shear zone.

The forces causing the shear stresses in the region of the chip quickly diminishes and finally disappears while that region moves along the tool rake surface towards and then goes beyond the point of chip-tool engagement. As a result, the slip or shear stops propagating before total separation takes place.

During the metal removal operation, if the friction between the tool (rake) face and the underside of the chip is considered then the chip gets further deformed. It is called as secondary deformation. The narrow region around the plane of secondary deformation is called secondary deformation shear zone.

This phenomenon repeats rapidly resulting in formation and removal of chips in thin layer by layer.