For metal cutting to be effective, it is necessary that tool takes the form of a large angled wedge. This tool must be driven asymmetrically into the work material, to remove a thin layer from a thicker body. The layer removed must be thin so that imposed stress on tool and work is within limits. Further a clearance angle must be provided on the tool to ensure that the clearance face does not make contact with the newly formed work surface.
The included angle of tool edge can vary from 55° to 90° to enable chip to divert by an angle of at least 60° as it moves away from the work, across the rake face of the tool. The whole volume of the removed layer from the work (chip) gets plastically deformed and a large amount of energy is needed for its formation and to make it move across the tool face. The knowledge of the process of chip formation is thus essential.
The formation of chip involves shearing of the work material in the region OA (plane extending from the tool edge to the position where the upper surface of the chip leaves the work surface).
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A very large amount of strain takes place in the region OA in very short interval of time, which result in fracture of the metal.
The cross section of chip is not rectangular because the metal is free to move in all directions (except at rake face of tool where it is constrained) as it is formed into the chip. The chip tends to spread side-ways and as a result the maximum width t2 is greater than the original depth of cut t1. The chip spread is small with harder alloys, but with soft metals when cutting with a small rake angle tool, t2/t1 > 1.5 can be observed. Usually the chip thickness is greatest near the middle and it tapers off somewhat towards the sides. The upper surface of the chip is always rough, usually with minute corrugations or steps. Even with a strong, continuous chip, periodic cracks are often observed, breaking up the outer edge into a series of segments.
As any volume of metal like abcd (in Fig. 22.11) passes through the shear zone, it is plastically deformed to a new shape a’ b’ c’ d’. The amount of plastic deformation or shear staring is related to the shear plane angle and the rake angle as shown in Fig. 22.11. It may be noted that shear strain = b/a in Fig. 22.11.
From Fig. 22.11, it will be noted that for each rake angle, there is a minimum strain when the mean chip thickness is equal to the feed (t2 = t1). For zero rake angle it occurs at shear plane angle of 45°. Also at zero rake angle the minimum shear strain in 2 and becomes less as the rake angle is increased. Fig. 22.12 shows how the change of shape of a unit cube after passing through the shear plane occurs for different values of the shear plane angle.
