unit 1 metal cutting and chip formation - IGNOU

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Theory of Metal Cutting 8 1.3 CHIP FORMATION 1.3.1 Deformation in Metal Machining Figure 1.2 shows a schematic diagram of material deformation during cutting, and subsequently removal of the deformed material from the workpiece by a single point cutting tool. Because of the relative motion between the tool and the workpiece, material ahead of the tool face (rake face) is compressed (elastically and then plastically). Further, movement of the tool into the workpiece deforms the work material plastically and finally separates the deformed material from the workpiece. This separated material flows on the rake face of the tool called as chip. The chip near the end of the rake face is lifted away from the tool, and the resultant curvature of the chip is called chip curl. Figure 1.2 : Schematic Diagram of Chip Deformation The study of the mechanism of chip formation involves deformation process of the chip ahead of the cutting tool. Theoretical study of the material deformation in metal cutting is difficult and therefore experimental techniques have been resorted to for analyzing the process of deformation in chips. The methods commonly employed for this purpose are : (i) Use of movie camera for taking pictures of chip. (ii) Observing grid deformation during cutting. (iii) Examination of frozen chip samples obtained by the use of quick-stop device. Experimental study of chip deformation process has revealed that : (i) During machining of ductile materials, a plastic deformation zone is formed in front of the cutting edge (Figure 1.2). (ii) The distinctive zone of separation between the chip and workpiece where deformation gradually increases towards the cutting edge is called the primary deformation/shear zone. In shear zone extensive deformation occurs. The width of shear zone is very small. (iii) The plastic deformation involved in the formation of chips affects the hardness of material (strain hardening). Strain hardening increases when a layer undergoes deformation in the shear zone. 1.3.2 Chip Types The type of chip obtained from a machining process is characterized by a number of parameters e.g., the type of tool-work engagement, work material properties and the cutting conditions. Ernst has classified the chips obtained in machining processes into three categories : Type 1 : Discontinuous chip,
Type 2 : Continuous chip, and Type 3 : Continuous with built-up edge. When ductile materials at high cutting speed are cut by a single point cutting tool, ribbon like continuous chip (Figure 1.3(a) and 1.3 (b)) is obtained. The conditions that promote formation of continuous chips in metal cutting are sharp cutting edge, low feed rate (or small chip thickness), large rake angle, ductile work material, high cutting speed, and low friction at chip-tool interface. As shown in Figure 1.2, major deformation takes place in primary shear deformation zone (PSDZ) resulting in the formation of chip. Due to ductile nature of work material and reasonably high temperature in the PSDZ, the deforming material flows on the rake face of the tool as continuous mass rather than the one fractured/ruptured at small distances at the underneath of the chip as in discontinuous chip. Continuous chip results in good surface finish, high tool-life, and low power consumption. But disposal of large coiled chips is a serious problem, for many industries where tons of chips are produced every week. To get rid of this problem various types of chip breakers are used which are in the form of step or groove on the rake face of the tool (Figure 1.4). The chip strikes with this step/groove and gets broken in the form of small segments. Disposal of such small chips is not a problem. If the friction between tool and chip while machining ductile materials is high, some part of the chip gets welded to the rake face of the tool near its cutting edge. The welded material is extremely hard and its size keeps on increasing with time. Because of the hardness of the adhered materials onto the cutting edge, it participates in cutting to a certain extent. That is why it is named as built up edge (Figure 1.5). As the size of the BUE grows larger, it becomes unstable and it breaks. Some part from the broken BUE is carried away by the chip as well as on the machined surface (Figure 1.3). Figure 1.3 : Different Kinds of Chips : (a) Continuous; (b) Photograph of Continuous Chip; (c) Continuous Chip with Built Up Edge; and (d) Discontinuous Chip The chip with the adhered parts of the BUE is known as continuous chip with BUE. The adhered parts of the BUE on the machined surface make the machined surface rough, but the BUE protects the actual cutting edge of the tool from wear. Thus, cutting with BUE enhances the tool life (or tool cuts longer before regrind). Metal Cutting and Chip Formation 9
Page 1 and 2: UNIT 1 METAL CUTTING AND CHIP FORMA
Page 3: Figure 1.1(b) : Various Operations
Page 7 and 8: operation in which the cutting edge
Page 9 and 10: tu is called chip thickness ratio o
Page 11 and 12: F = Ft cosα + Fc sin α . . . (1.3
Page 13 and 14: V cosα V = c s . . . (1.19) cos(
Page 15 and 16: As the cutting progresses in the be
Page 17 and 18: (iii) external and internal frictio
Page 19 and 20: Example 1.3 Solution γ = 1.771 A c
Page 21 and 22: γ = tan (φ − α) + cot φ = tan
Page 23 and 24: SAQ 1 Vs = Vc × = 62. 83 sin ( 90
Page 25 and 26: (v) Determine the condition when φ
Page 27 and 28: THEORY OF METAL CUTTING This block

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