Pages: 34 Published: Jan 2009
DISCUSSION IN THIS CHAPTER PERTAINS TO fluids that facilitate metalworking and machining operations. The nature of the various working operations is examined, along with the properties of the lubricants needed for each. Discussion includes metalworking fluid classifications, composition, formulating, and testing. Representative examples of the various metalworking fluid formulations are also provided at the end of the chapter. Metalworking is the process of converting the bulk metal into a component, or a part, and primarily involves two types of operations: Those that produce metal debris and those that produce no debris. The former type is classified as the metal removal operations and the latter type is classified as the metal forming operations. Cutting and grinding are examples of the first type and drawing, stamping, and bending are examples of the second type. All metalworking operations involve bringing two solids, a tool and a work-piece, together to create a new part or a shape. The process involves high friction, high pressures, high temperatures, and tool wear, and it is the job of the lubricant, or the metalworking fluid, to control them. Metalworking fluids accomplish this by providing cooling, lubrication, and protection against corrosion. They, therefore, improve the efficiency of the operation, and hence increase productivity. Irrespective of the type of the metal cutting operation, whether it is turning, milling, drilling, planing, shaping, broaching, or sawing, the mechanism of action of all cutting tools is the same. That is, the cutting is performed by the tool either as it moves across the metal surface being machined, or the tool is stationary and the metal surface moves against it. In either case, the process is accompanied by plastic deformation of the metal surface at the front of the cutting edge of the tool and the rubbing of the formed chip with the tool surface, as shown in Fig. 11.1. The temperature estimates in the cutting zone are 900°C on the tool's cutting edge, 500°C on the chip, and 200°C on the work-piece. As we move away from this zone, the temperature of the tool drops to 400°C on the tool's outer edge and to 200°C on the chip. About 75% of the heat generated is due to the deformation of the metal and the other 25% results from the friction due to sliding of the chip on the tool face. Metal deformation occurs due to shear or plastic flow along the shear plane that extends from the edge of the tool to the surface of the work-piece metal, see Fig. 11.1. Below the shear plane the metal is undisturbed; above it the metal is deformed and ultimately results in the formation of a chip. It is critical that the high temperatures in the cutting zone are decreased, otherwise extensive tool wear and rough finish of the work-piece will occur. In the case of the brittle materials, high temperatures in addition may cause the fracture of the metal along the shear plane, which will form a discontinuous or segmentai-type chip. This is undesired since it will interrupt the continuous cutting action. In the case of the ductile metals, high temperatures will also cause a built-up-edge on the nose of the tool, which will result in a severe plowing action and again poor finish of the work-piece surface will occur. The built-up edge is the stagnant mass of the metal that sheared away from the body of the chip by the high tool-face friction.
Paper ID: MNL11471M