UNIT 6 FINISHING PROCESSES Finishing Processes Structure 6.1 Introduction Objectives 6.2 Traditional Finishing Processes 6.2.1 Honing 6.2.2 Lapping 6.2.3 Superfinishing 6.2.4 Polishing 6.2.5 Buffing 6.2.6 Burnishing 6.3 Advanced Finishing Processes 6.3.1 Magnetic Abrasive Finishing 6.3.2 Magnetorheological Finishing 6.3.3 Magnetic Float Polishing 6.3.4 Elastic Emission Machining 6.3.5 Ion Beam Machining 6.3.6 Finished Surface Characteristics 6.4 Metal Additive Methods 6.4.1 Electroplating 6.4.2 Galvanizing 6.4.3 Metal Spraying 6.5 Summary 6.6 Answers to SAQs 6.7 Exercises 6.1 INTRODUCTION The important attributes of any product are its shape, size, dimensional tolerances and surface integrity including surface finish. Surface finish is achieved from the last operation performed on the part, and this operation is known as finishing operation. In most of the cases, it also decides other surface characteristics of the machined part, for example, surface defects like micro cracks. If finishing operation raises workpiece temperature more than the permissible one, it may lead to surface defects like micro cracks, thermal residual stresses and warping. Hence, the selection of a right kind of finishing process is very important. For this purpose, awareness with wide varieties of available finishing operations is also essential. With this point in view, this unit describes material removal finishing processes (traditional finishing processes like honing, lapping, superfinishing, burnishing, polishing and buffing and advanced finishing processes like magnetic abrasive finishing, magnetic float polishing, elastic emission machining, and ion beam machining), and material addition processes like electroplating, galvanizing and metal spraying. At the end, a table has been provided which compares the capabilities of various material removal finishing processes. Objectives After studying this unit, you should be able to • know the need of surface improvement which can be achieved by either material removal processes or material addition processes, • understand the working principles of both types of surface finishing processes, 33 Abrasive Machining • choose appropriate process to achieve the desired surface characteristic, and Processes • compare capabilities of the available finishing processes on the shop floor. 6.2 TRADITIONAL FINISHING PROCESSES 6.2.1 Honing Honing is used to obtain fine surface finish on internal and external cylindrical surfaces, and flat surfaces of the workpieces, which may be metallic or non-metallic in nature. Honing operation is treated as finishing (or final) operation, which may correct the errors like out-of-roundness, taper, or axial distortion, which might have developed in the preceding machining operation. A honing tool (or stick) consists of either Al2O3 or SiC abrasives bonded by a vitreous or resin bond material. During honing, a large number of abrasive grains operate simultaneously in contact with work surface. As a result, force acting on the workpiece is comparatively large and hence material removal is also large. Figure 6.1 shows a honing, tool, which rotates and reciprocates in the hole during honing. In internal honing tool may hold a number of bonded abrasive sticks, which expand radially against the work surface. (a) (b) Figure 6.1 : (a) Hone for an Internal Cylindrical Surface; and (b) Typical Cross-hatched Scratch Pattern in Honing [Armergo and Brown, 1967] The resultant motion of a grain, therefore, is cross hatch lay pattern. Two universal joints (Figure 6.1) permit the honing tool to float so that it follows the axis of the hole. Application of cutting fluid does lubrication, cooling and removal of swarf. Finishing of external cylindrical surfaces is also done on the same principle except that the abrasive sticks are pressed on the outside of the component. Further, a large area of workpiece surface is covered during honing, hence rise in temperature is also low. Hence, surface damage in this process is comparatively low. Material removal takes place by shear deformation resulting in miniature chip formation. Honing of automobile cylinders is a common application. Some other applications of honing include gun barrels, hydraulic cylinders, and bearings. This operation can be performed after conventional machining operations but it cannot correct alignment errors in cylindrical components. It is a finishing process so it should not be recommended for bulk material removal. Further, selection of the abrasive grain type and size is made according to the workpiece material and the surface finish desired. The grain mesh size varies from 150 to 600. Reciprocating speed (m/min) during honing varies from 10 to 25 m/min while rotary speed (m/min) varies from 15 to 30 m/min depending upon the workpiece material properties and surface finish required. Higher cutting speed yields better surface finish but also higher temperature and faster wear of abrasive grains. The number of the abrasive sticks in a honing tool usually varies from 2 to 18 depending upon the diameter of the hole to be honed. The length of the abrasive sticks is about 34 0.5 times the length of the hole. Length of the abrasive sticks should be larger than the stroke length (protrude outside) at both the ends of the hole by the length approximately Finishing Processes equal to 0.25 times to their own length. The mechanics of material removal is somewhat similar to that of grinding, still differences exist. To facilitate penetration of abrasive grains into the workpiece surface, radial pressure is applied onto the grains. Large number of grains are simultaneously finishing the workpiece, and these grains are in contact of work surface for a longer period of time. Hence, the length of the chip is larger than that obtained in grinding. It is also believed that both cutting as well as ploughing mechanisms are responsible for material removal during honing. Cutting parameters in this case should be selected carefully to avoid glazing of honing stones. Light cutting force (or low penetration depth) may lead to glazing of the honing stone. 6.2.2 Lapping Lapping is employed when the surface finish to be achieved must be better than that achievable in grinding and honing. It is employed to achieve high dimensional accuracy, correcting minor imperfections in shape, and to achieve a close fit between mating surfaces. But it is more expensive operation than grinding and honing. During lapping, loose suspended abrasives in a vehicle are sandwitched between a lap (usually made of soft material) and the workpiece (Figure 6.2). Light pressure is applied between them and the two surfaces move relative to each other in a random fashion. As a result, workpiece abrades and tends to attain approximately the shape of the lap. The surface produced on the workpiece in this case is dull because removal of the material occurs in the random directions. There are two viewpoints about how the material removal takes place during lapping. According to one viewpoint it is suggested that the abrasives get embedded in the lap, and the lap acts like a grinding wheel and the material removal takes place in the same way as in the grinding. (a) (b) Figure 6.2 : Schematic Diagram of Lapping Operation : (a) Flat Workpiece; and (b) Cylindrical Workpiece [Armergo and Brown, 1967] According to the second viewpoint, the abrasive grains roll and slide between the work and the lap, taking tiny cuts on both the surfaces. Lapping is done either manually or on a specially designed machine. In both the cases, lap wear should be minimum while that of the workpiece should be high and uniform to achieve the desired surface characteristic, quickly and accurately. During lapping, to achieve uniform wear on the work surface, relative velocity, distance traveled, applied lapping pressure, etc. should be same in different regions of the workpiece. The variables affecting the performance of the lap include the material of the lap and workpiece, the vehicle used, the abrasives (type, size and amount), pressure applied, and relative motion between lap and workpiece. It is found that harder the lap material lower is its wear rate. However, there is no significant change in MRR by changing the lap material. The laps with embedded grains (or soft lap material) give better surface finish. With softer workpiece material, the MRR is higher 35 Abrasive Machining but surface finish is worse. In the same way, higher pressure leads to higher MRR but Processes poorer surface finish. The vehicle used in lapping serves multifarious functions like lubricant, coolant, and carrier. As a lubricant, it improves the cutting action while as a coolant, it lowers down the temperature of lap and workpiece both. Commonly used vehicles are lard oil, kerosene, benzene and low velocity fluids (including petroleum products). Higher viscosity fluids are good for coarser grains and higher pressure. The vehicle used should be non-corrosive, and hold abrasive particles under suspension. It should not easily evaporate. Commonly used abrasives are Al2O3 and SiC, but diamond, emery and boron carbide are also used. The grit size used in lapping is smaller (or higher mesh number) than those used in grinding and honing. It is normally 300-600 mesh size that is used in lapping but the range varies from 200 to 900. The material removal rate attains maximum with increase in the amount of abrasives. Finer grains give a better surface finish. Figure 6.3 : Effect of Pressure on Material Removal Rate (MRR) during Lapping It is found that MRR initially increases with increases in pressure, but at higher pressure it does not change significantly. Lapping pressure varies from 7-140 kPa. Increase in pressure beyond the limit may deteriorate surface finish (or increase Ra value). Increase in lap speed increases MRR while surface finish is affected only slightly.