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UNIT 6 FINISHING PROCESSES Finishing Processes

Structure 6.1 Introduction Objectives 6.2 Traditional Finishing Processes 6.2.1 6.2.2 6.2.3 6.2.4 Polishing 6.2.5 Buffing 6.2.6 Burnishing 6.3 Advanced Finishing Processes 6.3.1 Magnetic Finishing 6.3.2 Magnetorheological Finishing 6.3.3 Magnetic Float Polishing 6.3.4 Elastic Emission 6.3.5 Ion Beam Machining 6.3.6 Finished Surface Characteristics 6.4 Metal Additive Methods 6.4.1 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 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 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 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 , 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 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 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 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 , 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. Lapping is applied for both flat as well as cylindrical surfaces. It has been applied on the curved surface as well by designing the curved lap (e.g., lenses or spherical surfaces). It can give surface finish as good as 25 nm (nanometer) and tolerances as ± 0.4 µm. 6.2.3 Superfinishing This process is used to improve surface finish and reduce the surface defects created by other machining and abrasive processes. The cutting conditions in superfinishing are mild hence friction and wear are low on mating components. It is used for surface refinement of those surfaces which are dimensionally correct having good surface finish. This process can give mirror like appearance. In superfinishing, the abrasive bonded sticks reciprocate at high frequency (2-3 kHz) and short strokes (4-5 mm) as shown in Figure 6.4. Workpiece rotational speed is low (about 10-30 m/min) and the pressure applied on it is also low (0.1 to 0.2 MPa).

Figure 6.4 : Schematic Diagram of Superfinishing of Cylindrical Workpiece This process can yield mirror finish to the workpiece. This process is applicable to both ferrous as well as non-ferrous metals. It can produce surface finish as good as 0.010 µm (10 nm). Some of the automobile parts, which are superfinished, are crank shaft, brake drum, pressure plate, cam shaft, main bearing, etc. This process has been applied for both flat surfaces as well as cylindrical and spherical surfaces. 36

6.2.4 Polishing Finishing Processes It is also a finishing operation, and removes scratches, tool marks, pits and other defects by cutting action. Polishing wheels are made of leather, wool, or canvas having abrasive grains set up with glue or thermosetting resin on the wheel. The workpiece is held against the wheel, and latter is rotated at about 50 m/s peripheral speed. Polishing may be done manually or by machine. The machines are classified as end-less belt machine or wheel. It is not used for dimension correction but sometimes can be employed to maintain the tolerances (± 0.025 mm). Very fine abrasive powder (Al2O3 or diamond) is used for coating the wheel or belt. The mechanism of polishing involves two steps: first it removes material by the fine scale abrasive removal, and then softening and smearing of surface layers by frictional heating during polishing. 6.2.5 Buffing Buffing is similar to polishing with the difference that in buffing very fine abrasives are used on soft disks made of cloth. The abrasive grains in a suitable carrying medium like grease are applied at suitable intervals to the buffing wheel. Polished parts may be buffed to obtain an even finer surface finish. In buffing, the amount of material removed is insignificant but it gives high luster on the buffed surface. It does not affect the dimensional accuracy of the buffed parts. For a mirror finish, it should be free from defects and deep scratches. The abrasive mixed in lubricating oil/grease is usually aluminum oxide. Buffing wheels are usually mounted on buffing or simply buffing machines which are run by belt derives from motors. Buffing speeds (60-70 m/s) are usually higher than polishing. 6.2.6 Burnishing The burnishing of metals is a method for producing smooth surface by plastically moving away the raised micro-irregularity or peaks (in Figure 6.5) on the surface and pressing them into micro-cavities on the surface. The produced surface has high degree of finish. The tool used for burnishing is or roller type tool, which moves under pressure on a rotating workpiece. The tool during its movement deforms the metal plastically and presses it in the valleys. A small amount of spring back in the deformed metal also takes place. Finally, a smooth finished surface is obtained. Burnishing also improves the surface hardness of the workpiece due to strain hardening induced by the surface deformation of the micro-irregularities.

(a) (b) Figure 6.5 : Burnishing : (a) Before; and (b) After

6.3 ADVANCED FINISHING PROCESSES

The developments in the material science has led to the evolution of difficult-to-machine, high strength temperature resistant materials with many extraordinary qualities. Nano-materials and smart materials are the demands of the day. To make different products in various shapes and sizes, many times the traditional finishing techniques do not work. One needs to use non-traditional or advanced finishing techniques [Jain, 2002]. Further, the need for high precision in manufacturing was felt by manufacturers world over, to improve interchangeability of components, and to improve quality control and longer wear/fatigue life [McKeown, 1986, Taniguchi, 1983]. Achieving controlled surface finish on such components is equally important. Traditionally, abrasives either in loose or bonded form whose geometry varies continuously in an unpredictable manner during the process are used for final finishing purposes. Nowadays, new advances in materials syntheses have enabled production of ultra fine abrasives in the nanometer (nm) range without the need for comminution (a 37

Abrasive Machining process by which brittle materials are reduced in size). With such abrasives, it has Processes become possible to achieve nanometer surface finish and dimensional tolerances. There is a process (ion beam machining), which can give ultra precision finish of the order of the size of an atom or molecule of the substance. In some cases, the surface finish obtained has been reported to be even smaller than the size of an atom. This section deals in brief, with some of the advanced fine finishing/machining processes like (a) Abrasive Flow Machining (AFM), (b) Magnetic Abrasive Finishing (MAF), (c) Magnetic Float Polishing (MFP), (d) Magnetorheological Abrasive Finishing (MRAF), (e) Elastic Emission Machining (EEM), and (f) Ion Beam Machining (IBM). Since AFM has been described in detail in Block 4, it is not described here. 6.3.1 Magnetic Abrasive Finishing (MAF) In MAF process, magnetic abrasive particles (MAPs) which comprises of ferromagnetic material (as particles) and abrasive grains (say, Al2O3, SiC, or diamond), are used as cutting tools, and the necessary finishing pressure is applied by electromagnet generated magnetic field. Figure 6.6 illustrates the working principle of MAF process through a schematic diagram. The magnetic abrasive particles are joined to each other magnetically between the two magnetic poles (S and N) along the magnetic lines of force a flexible magnetic abrasive brush (FMAB). When there is a relative motion between the FMAB and workpiece, the abrasion takes place and gives a polished finish. This finish can be as good as approaching to the hundredth of a micrometer depending upon the process parameters and workpiece material. The performance of the process depends on the parameters like MAPs (type, size, and mixing ratio), working clearance between the workpiece and magnetic pole, rotational speed of work or pole, vibration (frequency and amplitude), properties of work material and magnetic flux density.

Figure 6.6 : Plane Magnetic Abrasive Finishing Figure 6.7 shows the surface finish before and after MAF on a workpiece [Jain et. al., 2001]. This process can also be used to improve the surface properties by the process of diffusion of particles in the workpiece surface. Proper tooling can be developed even to finish 3-D intricate shapes because FMAB is self-deformable and adapts to the shape of the workpiece but it is not as flexible as AFM medium. Achieving uniform surface finish near the discontinuities or edges in the magnetic poles is difficult. At the irregularities, the magnetic flux density is more hence the rate of change in surface roughness is also more. MAF has been successfully used to finish internal surfaces of the tubular workpieces as shown in Figure 6.8 [Kim, 1997; Komanduri, 1996]. The typical improvement in surface finish of processed workpieces is illustrated by the following data : Before Machining 38 Ra = 1.10 µm, Rq = 1.37 µm, Ry = 9.60 µm

Finishing Processes

Figure After Machining

Ra = 0.53 µm, Rq = 0.67 µm, Ry = 3.90 µ

Figure 6.7 : Surface Roughness Plots Before and After Magnetic Abrasive Finishing of a Steel Pipe [Jain et. al., 2001] The path of the jet of the magnetic abrasive particles is slightly deviated by the magnetic field (Figure 6.8) so that the MAPs abrade the internal surface but still keep moving in the axial direction as shown in Figure 6.8. The velocity of the jet of MAPs is another important parameter of the process of magnetic abrasive jet finishing of internal surfaces. Results have been reported [Fox et. al., 1994; Umehara, 1994] in the literature regarding finishing of stainless steel rollers using MAF to obtain final surface roughness Ra value as low as 7.6 nm from an initial surface roughness of 0.22 µm.

Figure 6.8 : Magnetic Abrasive Jet Finishing 6.3.2 Magnetorheological Finishing (MRF) Precision lenses are usually made of brittle materials such as , and they tend to crack during conventional machining/finishing. Even a single microscopic crack can drastically hinder a lens’s performance and make unsuitable for performing its intended function. Manufacture of a lens usually involves two operations – grinding and finishing. Grinding operation makes a lens close to the desired size while finishing removes the cracks and surface imperfections either created by grinding or could not be removed during grinding. Manual grinding and polishing are non-deterministic and a high local pressure may lead to sub-surface damage. To take care of these difficulties, magnetorheological- finishing (MRF) process has been developed which is automatic in nature. MRF process uses magnetorheological (MR) fluid, also known as smart fluid because it changes its properties under the influence of magnetic field. Figure 6.9 shows a schematic diagram for MRF process. MR fluid consists of colloidal suspension of magnetic particles and finishing abrasive randomly distributed. This smart fluid reversibly stiffens under the influence of magnetic field. The stiffness is directly proportional to the strength of the magnetic field. This temporary finishing surface (stiffened fluid) can be controlled in real time by varying the field’s strength and direction. This process is used for finishing , glass , plastics, etc.

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Abrasive Machining Processes

Figure 6.9 : Magnetorheological Finishing Process In MRF, a MR fluid ribbon is extruded between the workpiece and the rotating wheel rim. As a result of extrusion, wear of the workpiece (convex, concave, or flat) takes place. The extent of wear (or finishing) of the workpiece is governed by MR fluid properties, magnetic field strength, workpiece material properties, rotational speed of the wheel and fluid pumping pressure. It has been reported that the surface accuracy is achieved of the order of 10-100 nm (peak to valley). MRF is capable of removing sub- surface damage, correct the shape and polish/finish the work surface. 6.3.3 Magnetic Float Polishing (MFP) This process is employed for gentle finishing of very hard materials like ceramics, which develop defects during grinding leading to fatigue failure. To achieve low level of controlled forces, magnetic field is used to support abrasive slurry in finishing products like ceramic balls and bearing rollers. This process is known as Magnetic Float Polishing (MFP). The MFP technique is based on the ferro-magnetic behaviour of the magnetic fluid that can levitate the non-magnetic fluid and abrasives suspended in it by the magnetic field Figure 6.10. The levitation force applied by the abrasives is proportional to the magnetic field gradient and is extremely small and highly controllable. It is a good method for superfinishing of brittle materials with flat and spherical shapes [Tani and Kawata, 1984; Mori et. al., 1976].

Figure 6.10 : Schematic Diagram of the Magnetic Float Polishing Apparatus

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Figure 6.11 : Buoyant Force Acting on a Non-magnetic Body in the Finishing Processes Magnetic Fluid with Magnetic Field The set up in this case employs the magnetic fluid comprising fine abrasive particles and extremely fine ferromagnetic particles in carrier fluid (water or kerosene) (Figure 6.10). On the application of the magnetic field, the ferrofluid is attracted downwards, the area of higher magnetic field. At the same time, the buoyant force is exerted on non-magnetic material to push them upward which is the area of lower magnetic field (Figure 6.10 and Figure 6.11). The abrasive grains, the ceramic balls, and the acrylic float inside the chamber, all being of non-magnetic material are levitated by the magnetic buoyant force. The drive shaft is fed down to contact the balls and to press them down to reach the desired force level. The balls are polished by the relative motion between the balls and the abrasives. The material is removed by abrasion. 6.3.4 Elastic Emission Machining (EEM) This process removes material from the workpiece surface to atomic level by mechanical methods, and gives completely mirrored, crystallographically and physically undisturbed finished surface. It can give surface of the order of atomic dimensions (~ 0.2 nm to 0.4 nm) [Mori et. al., 1976; Mori et. al., 1987]. Using ultrafine abrasive particles to collide with the workpiece surface, it is possible to finish the surface to atomic scale by elastic fracture without plastic deformation, and the process is known as Elastic Emission Machining (EEM). Figure 6.12 shows working principle of EEM through the schematic diagram. The polyurethane ball used during EEM is of about 56 mm diameter. This ball is mounted on a shaft whose axis of rotation is inclined at about 45° to the vertical axis, and it is driven by a variable speed motor. The workpiece is immersed in a slurry of ZrO2 or Al2O3 (size 20 nm to 20 µm) as abrasive and water as carrier. The slurry is circulated in the gap by a diaphragm pump, and is maintained at constant temperature with a heat exchanger. The material removal is due to the slurry and work interaction, which leads to the erosion of the surface atoms by the bombardment of abrasive particles without the introduction of dislocation.

Figure 6.12 : Rotating Sphere and Workpiece Interface in EEM [Taniguchi, 1983]

This allows the thickness of the material removed to be controlled within 20 nm and surface roughness value as low as 0.5 nm. Material removal process is a surface energy phenomenon in which abrasive particle removes a number of atoms after coming into contact with the surface. 6.3.5 Ion Beam Machining (IBM) Nano-technology aims towards achieving ultraprecision machining of workpieces having capabilities of producing surface finish of the order of 1nm or less which means approaching towards the ultimate surface finish. Ion beam machining is a molecular manufacturing (or atomic size stock removal) process based on the sputtering off phenomenon of work material by bombardment of energized ions of 1 to 10 keV and 41

Abrasive Machining current density of 1 mill amphere/cm2. This process can be applied for the manufacture of Processes ultra-fine precision parts of electronic and mechanical devices [Shima et. al., 1990 and Shima et. al., 1995]. The sputtering off is basically a knocking out phenomenon of surface atoms of workpiece by the kinetic momentum transfer from incident ions to the target atoms. Removal of atoms from the work surface occurs when the actual energy transferred to the affected area exceeds the usual binding energy of 5-10 eV (Figure 6.13). Ions of higher energy may transfer enough momentum such that more than one atom causes a cascading effect in the layer near the surface, removing several atoms. Ions of still higher energy may get implanted deep within the material (Figure 6.13(b)) after ejecting out several atoms or molecules. However, the bombardment of high energy ions is not desirable as it may damage the workpiece surface. There are various factors that affect the machined surface characteristics. Such factors are properties of work material, ion-fetching gas, angle of incidence of ions, ion energy and current energy.

Figure 6.13 : Ion Beam Machining IBM can be employed for the sharpening of a diamond stylus for profilometer having tip radius of about 10 nm, and finishing asymmetric and aspheric mirror for telescope. IBM is an ideal process for nano-finishing of high melting point, hard and brittle materials such as ceramics, semiconductors, , etc. However, surface roughness value increases with the increase in the size of grain structure of workpiece, ion energy, and current density. 6.3.6 Finished Surface Characteristics The performance of the fine abrasive finishing processes is evaluated by their achievable surface finish, form accuracy and resulting surface integrity. These finishing processes have shown to yield accuracies of form (figure) in the nano-meter range. The order of surface damage observed during these advanced finishing processes ranges from micrometers to nano-meters. It makes their measurement a formidable task. Table 6.1 summarizes the attainable surface finish in various finishing processes. Working principles of seven such advanced finishing processes have been explained in brief. It is to be noted that the precise control of forces on abrasive particles controls the achievable surface finish. Fine finishing of brittle materials like ceramics, glasses, and semiconductor wafers can be easily done in nanometer range using these advanced Table 6.1 : Surface Finish Obtainable from Different Processes

Finishing Process Workpiece Ra (nm) Grinding 25 – 6250 Honing 25 – 1500 Lapping 13 – 750 Abrasive Flow Machining (AFM) Hardened Steel 50 with SiC abrasives Magnetic Abrasive Finishing (MAF) Stainless Steel 7.6 with diamond abrasives Rods

42 Magnetic Float Polishing (MFP) with Si3N4 4.0

Finishing Processes CeO2 abrasives Magneto-rheological Finishing (MRF) Flat BK7 Glass 0.8 with CeO2 Elastic Emission Machining (EEM) Silicon < 0.5 with ZrO2 abrasives Ion Beam Machining (IBM) Cemented carbide 0.1 fine-finishing processes. The exact mechanics of abrasive interaction with the workpiece surface in most of these processes is still a subject of in-depth research, and need involvement of multidisciplinary sciences.

6.4 METAL ADDITIVE METHODS

The methods discussed so far improve the surface finish of the pre-machined components by removing small amount of material mainly by shear deformation giving rise to miniature chips. There are some methods which improve the surface characteristics by adding the material (or metal) to it by different ways. Some of these methods are discussed here in brief. These methods include electroplating, galvanizing and metal spraying. 6.4.1 Electroplating It is commonly used for applying a metallic coating on metallic surfaces. Commonly used plating materials are chromium, nickel, copper, zinc and tin. Before plating, it is desirable that the surface has been buffed, and cleaned to remove grease, dirt, and buffing compounds. The elements of an electroplating system are cathode, anode, electrolyte and DC system for electric current supply (Figure 6.14). Anode is made up of the material to be deposited on the cathode. Electrolyte is made up of salt solution of the metal to be applied (or deposited). As the current flows in the cell, metal ions migrate from the anode to cathode where they give up their charge to the cathode and get deposited as metallic coating. Parts to be electroplated should not have sharp edges/corners to avoid excess deposition of the metal. Electroplating is used for protecting the surfaces against corrosion, wear, or abrasion. Sometimes it is also used for depositing metallic layer on the worn out part to restore the part of its original dimensions.

Figure 6.14 : Schematic Diagram of Electroplating 6.4.2 Galvanizing The workpiece to be galvanised is dipped in the molten metal bath for a definite period of time so that the molten metal gets deposited on it. Commonly used metals for deposition are zinc, tin, or an alloy of lead and tin. Before dipping the parts in the bath, they should be thoroughly cleaned and fixed in a solution of the zinc chloride and HCl acid. To get the uniform thickness of the coating, small objects (nuts, bolts, pins, etc.) are taken out of the bath and centrifuged till the coating gets hardened. Such coating provides effective protection against corrosion, and gives better appearance. 6.4.3 Metal Spraying The objective of metal spraying on a part is to impart some physical properties to the surface, better appearance, and decorative look, and sometimes for the production of coloured surfaces. A metal spraying system is shown in Figure 6.15. In a metal spray gun, appropriate metal wire is fed to the oxy-gas flame, which melts it. The molten metal 43 is sprayed through the nozzle with the help of compressed air. The atomized metal gets

Abrasive Machining embedded/adhered on the cleaned surface. For good adhesion, the surface should be Processes rough, and free of dirt, oil and grease. The coating can be applied to both metallic as well as non-metallic products. The compressed air helps in uniform spraying of molten metal, as well as promotes cooling of the workpiece surface.

(1) Oxy-fuel Gas, (2) Compressed Air, (3) Flame, (4) Workpiece, (5) Atomized Spray, (6) Melting, (7) Wire Figure 6.15 : Schematic Diagram of Metal Spraying System [Hajra Choudhury et. al., 1982] SAQ 1 (a) In MAF, the magnetic field affects the (i) Forces acting on the workpiece (ii) Strength of FMAB (iii) MRR (iv) All of these (b) MAF can be applied to : (i) Ferromagnetic material (ii) Non-ferromagnetic material (iii) Any kind of material (c) In MAF, magnetic flux density at the irregularities compared to flat areas is (i) More (ii) Less (iii) Is unaffected by the presence of irregularities (d) MAF is applicable for (i) External surfaces (ii) Internal surfaces (iii) Both types of surfaces (e) Smart fluid is used in (i) MAF (ii) MRF (iii) AFM (iv) EEM (f) A suitable method for the finishing of glass lenses is : (i) AFM (ii) MAF (iii) IBM (iv) None of these (g) Products like ceramic balls or ceramic rollers can be efficiently finished by (i) MAF (ii) AFM (iii) MFP 44 (iv) IBM

(h) MFP is applicable for the finishing of : Finishing Processes (i) MS balls (ii) Aluminum rollers (iii) Any material products of simple shapes (i) A surface finish of the order of 0.2 to 0.4 nm can be achieved by : (i) Honing (ii) AFM (iii) Electroplating (iv) None of these (j) In buffing, the expected buffing speed is approximately (i) 67 m/s (ii) 100 m/s (iii) 50 m/s (iv) None of these (k) In burnishing, material removal mostly occurs due to : (i) Chip formation by shearing (ii) Ploughing (iii) Plastic deformation and filling of cavities (l) The mesh size of the abrasive grains used in superfinishing is usually of the order of : (i) 80-200 (ii) 180-350 (iii) 400-600 (iv) Any one of these (m) Time required for honing operation is a function of (i) Workpiece material (ii) Abrasive mesh size (iii) Honing pressure (iv) All of these (n) In honing, abrasives are used in (i) Loose form (ii) Embedded in a stick (iii) Suspended in ferro fluid (iv) None of these (o) Lapping can induce thermal damage. (i) True (ii) False

6.5 SUMMARY

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Abrasive Machining This unit describes basically two types of surface finishing processes, viz. material Processes removal processes and material addition processes. Material removal processes are classified as traditional and advanced finishing processes. In material addition processes, material is added externally. Traditional machining processes include honing, lapping, superfinishing, polishing and buffing. Some of these processes are capable to give surface finish of the order of hundredth of a micrometer if appropriate finishing conditions are selected. But all these processes have constraints on size and shape, which can be finished. Such constraints are relaxed when advanced fine finishing processes such as MAF, MRF, MFP, EEM and IBM are used. These processes are automated and many of them are computer controlled. The abrasives used are very fine (fraction of the micrometer) in nature, and forces applied are also small fraction of one Newton. Another category of surface finishing processes like electroplating, galvanizing and spray painting are not finishing processes in real sense. These processes are used for protecting external surfaces from wear and corrosion, and to give good appearance for the exposed surfaces.

6.6 ANSWERS TO SAQs

SAQ 1 (a) (iv) (b) (iii) (c) (i) (d) (iii) (e) (ii) (f) (iv) (g) (iii) (h) (ii) (i) (iv) (j) (i) (k) (iii) (l) (iii) (m) (iv) (n) (ii) (o) (ii)

6.7 EXERCISES

Exercise 1 Explain the working principles of honing, lapping and superfinishing. Exercise 2 Write brief note on polishing and buffing operations. Exercise 3 How advanced finishing operations are different from traditional finishing operations? Compare shape and size constraints and achievable surface finishing in table form. Exercise 4 Describe a method to achieve a surface finish as good as the size of an atom or 46 molecule.

Exercise 5 Finishing Processes How metal additive processes are different from material removal processes (working principle point of view)?

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Abrasive Machining Processes BIBLIOGRAPHY

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