机电工程专业英语-第二版 相关资料

插图：Thus the cooling properties of cutting fluid depend to a large extent on thecooling method. The cooling efficiency increases with heat conductivity and specificheat of the cutting fluid, with its heat of fusion and of vaporization, and also with itsviscosity. Cooling effect increases, too, with the flow rate of the fluid, the area ofapplication and with decreasing temperature of the cutting fluid.The lubricating action of the cutting fluid reduces the external friction betweenrake face and chip and between flank and machined surface~. Effectiveness of thelubricating action depends, therefore, in the first place on the degree of penetration ofthe coolant between those surfaces. It follows from the magnitude of specific cuttingforces that very often, i.e. when machining steel, pressures between chip and toolpoint and between workpiece and tool point may reach 20000~30000 kg/cm2. Notwithstanding such high pressures, the use of suitable cutting fluids results in thereduction of friction and wear.The washing action of a cutting fluid consists in removing the products ofabrasion, chip fragments and tool wear particle. In practice, cutting fluids exhibitgood washing properties having high wettability and because particles immersed inthe liquid move downstream.This action has an advantageous influence on the smoothness of the machinedsurface and on the reduction of tool point wear.Surface loosening action of the cutting fluid, also called the cutting action of thisfluid, results from the so-called Rebinder effect. Adsorbed molecules of cutting fluid,particularly of liquids containing surface-active additives, disintegrate, diffuse andpenetrate into microgaps resulting from defects in the crystal lattice. Because of thewedge-like action of the adsorbed liquid, a very thin surface layer of the machinedmaterial becomes "loosened".According to Rebinder's research, the thickness of this layer may reach 0.1 mm.Since the machined surface in the next pass becomes the work surface, plasticdeformation appearing in the chip formation area and slip of the chip elements occurmore easily and more frequently. This phenomenon is called internal lubrication.The efficiency of activated liquids increases with a reduction in unreformedchip thickness, since in this case the percentage share of the thickness of loosed layer is greater in relation to the total thickness.Increasing the cutting speed decreases the efficiency of activated liquids. This is due to a more ductile chip, having a much smaller number of larger gaps of all sorts （lack of preceding gap and of built-up edge）. Consequently, much fewer local vacuum spots are formed and the liquid does not penetrate everywhere, and dry friction may result.

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