Machining_theory

Machining_theory - Machining of Metals machining chip...

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1 Machining of Metals machining chip formation fundamentals; basic equations; strain rate; strain hardening, residual stresses; surface roughness; surface microstructure Machining by mechanical chip formation is widely used to remove material from a workpiece in a variety of configurations including: turning, milling, drilling, and tapping. It is a process capable of producing precise geometrical shapes with tight tolerances. Machining has a long history. In the 18 th century it was essential to the industrial revolution, as the only process capable of producing cylinder bores to tolerances sufficient for steam engines. Machining today is still widely used to remove large amounts of material, in the order of millimeters per pass, from ductile metals. Machining can also be used to form ductile chips in brittle materials. In production of aircraft parts under 75mm machining tolerances of +/- 5 μ m are reasonable. The basic process of chip formation is fundamental to all abrasive processes, e.g., grinding, honing, lapping, and abrasive wear. Machining remains economically important. The chip removal produces technically and scientifically interesting material responses. In single point cutting the process concentrates the mechanical power of a machine tool into small volumes of the workpiece by forcing a hard cutting tool with a small edge radius through an outer layer of the workpiece material. Deformations of the workpiece material during chip formation can exceed 300%, at strain rates that have been estimated to be as high as 10 6 s -1 (von Turkovich 1970). The tribological conditions at the tool-chip interface can result in macroscopic welding of the material to the cutting tool and dissolution of tool elements into the chip. The geometry of the deformation zones and the resulting microstructures in the workpiece depend on the geometrical and tribological interaction of the cutting tool with the workpiece material. Further understanding of fundamentals of chip formation is motivated by the desire for higher material removal rates, longer tool life, tighter tolerances and improved quality of machined surfaces. 1. Fundamental Geometrical Relations in Chip Formation The idealized, fundamental geometry of continuous chip formation for orthogonal cutting is shown in Fig. 1. This idealization is plane strain. It is a reasonable approximation for understanding the kinematics of the formation of a continuous chip and to be representative of the formation of the bulk of the chip in normal machining. Plane strain conditions exist sufficiently far away from the free surface at the edge of the chip, where plane stress conditions exist, and, sufficiently far from the nose of the tool where the flow is complicated by the curvature of the cutting edge.
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2 Figure 1. Idealization of orthogonal machining showing the basic geometry of chip formation on the shear plane. In this idealization (Fig. 1) the workpiece moves with a speed V against hard tool
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This note was uploaded on 04/29/2008 for the course ME 1800 taught by Professor Brown during the Winter '08 term at WPI.

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Machining_theory - Machining of Metals machining chip...

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