Jawaharlal Nehru Technological University Hyderabad B.Tech III METROLOGY AND MACHINE TOOLS Previous Question Paper (August - 2022 - R16)

The mechanism of chip formation involves the plastic deformation of the work material as the cutting tool advances. As the tool edge engages the workpiece, the material ahead of the tool is subjected to high compressive stresses. When these stresses exceed the yield strength of the material, it undergoes shear deformation along a specific plane known as the shear plane.

• Shear Zone: The transition from the undeformed workpiece to the chip occurs within a narrow region called the shear zone.
• Deformation: The material is compressed and sheared, causing it to flow along the tool rake face.
• Friction: Significant heat is generated due to the friction between the chip and the tool rake face, and the plastic work done during shearing.

Where r is the chip thickness ratio and α is the rake angle. The formation process is essentially a continuous shearing process where the material is forced to deform and slide over the tool face.

Chips are classified based on the material properties and cutting conditions:

• Continuous Chips: Formed when machining ductile materials (e.g., mild steel, aluminum) at high cutting speeds with a positive rake angle. These chips flow smoothly over the tool face without fracture, resulting in a good surface finish.
• Continuous Chips with Built-Up Edge (BUE): Occur when machining ductile materials under conditions of high friction and pressure. Particles of the work material weld to the tool tip, forming a temporary, hard, and brittle built-up edge that periodically breaks off, causing poor surface finish and tool wear.
• Discontinuous Chips: Formed when machining brittle materials (e.g., cast iron) or ductile materials with low cutting speeds and large feed. The material fractures along the shear plane before significant plastic deformation occurs, resulting in separate, small segments.

The choice of cutting parameters, such as cutting speed, feed, and tool geometry, plays a critical role in determining the type of chip produced, which directly influences the tool life and the quality of the machined surface.