Q. Explain the mechanism of chip formulation in metal cutting.

1) In figure the tool is considered stationery, and the work piece moves to the right.

2) The metal is severely compressed in the area in front of the cutting tool. This causes high temperature shear, and plastic flow if the metal is ductile.

3) When the stress in the work piece just ahead of the cutting tool reaches a value exceeding the ultimate strength of the metal, particles will shear to form a chip element which moves up along the face of the tool.

4) The process is repetitive and a continuous chip is formed a highly compressed and burnished underside, and minutely serrated top side caused by the shearing action.

5) The place along which the element shears is caused the shear plane. Thus the chip is formed by plastic deformation of the grain structure of the metal along the shear plane.

6) Actually, the deformation does not occur sharply ace the shear plane, but rather it occurs along a narrow band.

7) The structure begins elongating along the line AB below the shear plane and continue to do so until it is completely deformed along the line CD above the shear plane in figure.

8) The region between the lower surface AB, where elongation of the grain structure begins, and the upper surface CD, where it is completed and the chip is born, is called the shear zone or primary deformation zone.

9) In figure, the shear zone is included between two parallel lines AB and CD. Actually however these two lines may not be parallel but may produce a wedge shaped zone which is thicker near the tool face at the right than at the left. This is one of the causes of curling of chips in metal cutting.