L 7.1 CHIP FORMATION 

Consider Fig. 7.1, which represents Orthogonal Cutting. It is a schematic representation of an A shaping operation, in which the workpiece remains stationary and the tool advances into the workpiece towards the left. Thus, the metal in front of the tool gets; compressed very severely, causing Shear stress. This stress is maximum along a plane, called Shear plane. If the material of the workpiece is ductile, the material flows plastically along the shear plane, forming the chip, which flows upwards along the face of the tool.


Fig. 7.1

Strictly speaking, the complete plastic deformation of the metal does not take place entirely along the shear plane only, but it actually occurs over a definite area, represented by PQRS in Fig. 7.2. The metal structure starts getting elongated along the line PQ below the share plane and continues up to the line RS above the shear plane, where its deformation is complete. The complete area represented by PQRS, within which the metal deformation occurs, is known as Shear Zone. For the sake of clarity in explanation, the lines PQ and RS are shown as exactly parallel in the diagram, but actually they may not be so. They will actually be inclined to each other such that the shear zone contained between them will be of a wedge shape, with its thicker portion near the tool and the thinner one opposite to it. This shape of the shear zone is one of the reasons due to which the Chip curls. The produced chip is very hot and its safe disposal is very necessary. The various devices used for its disposal.


Fig.7.2 The Shear Zone.

• Mechanism of chip formation in machining ductile materials

During continuous machining, the uncut layer of the work material just ahead of the cutting tool (edge) is subjected to almost all-sided compression as indicated in Fig. 7.3.


The force exerted by the tool on the chip arises out of the normal force, N and frictional force, F as indicated in Fig. 7.3. Due to such compression, shear stress develops, within that compressed region, in different magnitude, in different directions and rapidly increases in magnitude. Whenever and wherever the value of the shear stress reaches or exceeds the shear strength of that work material in the deformation region, yielding or slip takes place resulting shear deformation in that region and the plane of maximum shear stress. But 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 long before total separation takes place. In the meantime, the succeeding portion of the chip starts undergoing compression followed by yielding and shear. This phenomenon repeats rapidly resulting information and removal of chips in a thin layer by layer. This phenomenon has been explained in a simple way by Piispannen [1] using a card analogy as shown in Fig. 7.4.


In actual machining chips also, such serrations are visible at their upper surface as indicated in Fig. 7.4. The lower surface becomes smooth due to further plastic deformation due to intensive rubbing with the tool at high pressure and temperature. The pattern of shear deformation by lamellar sliding indicated in the model can also be seen in actual chips by proper mounting, etching and polishing the side surface of the machining chip and observing under microscope.


The pattern and the extent of total deformation of the chips due to the primary and the secondary shear deformations of the chips ahead and along the tool face, as indicated in Fig. 7.5, depend upon

• work material

• tool material and geometry

• the machining speed (VC) and feed (so)

• cutting fluid application

 

• Mechanism of chip formation in machining brittle materials

The basic two mechanisms involved in chip formation are

• Yielding – generally for ductile materials

• Brittle fracture – generally for brittle materials

During machining, first a small crack develops at the tooltip as shown in Fig. 7.6 due to wedging action of the cutting edge. At the sharp crack-tip stress concentration takes place. In the case of ductile materials immediately yielding takes place at the crack-tip and reduces the effect of stress concentration and prevents its propagation as crack. But in case of brittle materials the initiated crack quickly propagates, under stressing action, and total separation takes place from the parent workpiece through the minimum resistance path as indicated in Fig. 7.6. Machining of brittle material produces discontinuous chips and mostly of irregular size and shape. The process of forming such chips is schematically shown in Fig. 7.7.