The removal of extra material from a metal surface by shearing or cutting action is known as machining or metal cutting. The cutting takes place along a plane, which is known as shear plane. There is a cutting zone; if it is examined carefully we find that the severe plastic deformation occurs in this zone due to compressive force applied by the sharp-edged cutting tool. The extra material due to this deformation flows over the tool surface, known as chip, and this shearing zone is known as primary shear zone.
During flow of chip on the rake surface of the cutting tool, the temperature of newly formed chip increases due to friction and it gets welded automatically on the rake surface. But, due to compressive force applied by newly formed chip (just after the welded chip) causes secondary shear of the welded chip, and this shear is known as secondary shear. In metal cutting the line generated by the cutting motion is called generatrix and the line formed by feed motion is called directrix.
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