Principle of Cooling and Lubrication Cooling Ability of Cutting Fluids

CHAPTER 2 LITERATURE REVIEW

2.1 Cutting Fluid

Cutting fluids play a critical role on metallic surfaces in cutting and forming operations. During the machining processes of a metal, a considerable quantity of heat is generated. It happens mainly due to the high plastic deformation in the primary shear zone and also to the friction of the chip on the surface. The condition of friction and temperature will cause tool wear. It also results in a poor surface finish and incorrect dimension. Cutting fluids are used to reduce the detrimental effects of heat and friction on both tool and workpiece. The cutting fluid produces three positive effects in the process whish are heat elimination, lubrication on the chip and tool interface, and chip removal López de Lacalle et al., 2006. A cutting fluids effectiveness depends on factors such as the method used to apply the cutting fluid, temperatures encountered, cutting speed, and type of machining process Kalpakjian, 1991. A fluids cooling and lubrication properties are critical in decreasing tool wear and extending tool life. Cooling and lubrication are also important in achieving the desired size, finish and shape of the workpiece.

2.1.1 Principle of Cooling and Lubrication

Cooling give influences to machining in various ways. At the contact between the chip and tool, cooling can reduce the chip temperature. It also affects directly the friction force between the chip and tool. However, contact pressures are so high that the cutting fluid has no path by which it can completely penetrate the contact area. 5 Cooling is mainly indirect via modified conduction through the chip. For the cutting edge, the coolant always plays a major role in maintaining the machined material at ambient temperature. As coolers, cutting fluids decrease cutting temperature through the heat dissipation cooling. When water based fluids are used, cooling is more important than lubrication. It was experimentally proved Shaw, et al., 1951 that the cutting fluid efficiency in reducing temperature decreases with the increase of cutting speed and depth of cut. As lubricant, the cutting fluid works to reduce the contact area between chip and tool and its efficiency depends on the ability of penetrating in the chip-tool interface and to create a thin layer in the short available time. This layer is created by either chemical reaction or physical adsorption and must have a shearing resistance lower than the resistance of the material in the interface. In this way it will also act indirectly as a coolant because it reduces heat generation and therefore cutting temperature Sales et al., 2001.

2.1.2 Cooling Ability of Cutting Fluids

Aiming to classify the main cutting fluids based on their cooling ability, Sales 1999 developed a methodology which consisted in heating a standard workpiece and monitoring the cooling curve of it. This workpiece was fixed to the clutch of jigs and rotated at 150 rpm and its temperature was measured using an infrared sensor. The data acquisition started when the workpiece temperature reached 300°C and the measurement continued up to room temperature. Emulsions and synthetic fluids were applied using a concentration of 5. Synthetic fluids are containing water and additives. The synthetic oil 1 is different from synthetic oil 2 due to small variations in their formulas Sales et al., 2001. Figure 2.1 shows the results of this experiment. 6 Figure 2.1: Cooling curves of all fluids experiment Sales, 1999. The cooling ability in crescent order is dry cutting, neat, oil emulsion, synthetic-2, water and synthetic-1. The fact that synthetic oil 1 presented a cooling ability greater than water, which theoretically has greater convection ability, was a surprise. A deeper analysis of the curves behavior in high temperature showed that water presented lower cooling ability even than synthetic oil 2 and neat oil Sales et al., 2001. The explanation of these results may be found on the phenomenon occurring when a fluid like water, with low ebullition point 100°C, starts contacting a body in high temperatures. At this moment the quick heat transfer causes the liquid evaporation. This process reduces a little the hot body temperature, but the vapor forms a barrier preventing fresh volume of liquid, from reaching its surface and, therefore, decreases the heat transfer efficiency. Another important factor is the fluid wet ability, which is regularly higher for cutting fluids than for water. The higher wet ability of the cutting fluid implies in less splashing action and therefore a greater chance for heat exchange Sales et al., 2001. 7

2.1.3 Types of Cutting Fluids