When the force is applied by using tip of the tool an layer of work piece, the material starts deforming plastically and sliding over the rake force of tool, inducing shear stresses in layer of work piece material.

on further application of force by tool on work piece, the shear stress induced in work piece increases continuously, at some point shear stress induced will become greater than or equal to ultimate shear stress of work piece ( $\tau$ induced > $\tau$ ultimate )

Therefore shearing takes place at the tip of the tool. This implies that the mechanism by which chip formation takes place is the hearing and tearing action.

During machining of soft and ductile work piece, because of higher toughness continuous chip forms. whereas machining of hard and brittle material, because of lower toughness discontinuous chip forms.