nano-finishing

Journal of Materials Processing Technology 209 (2009) 6022–6038

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Magnetic field assisted abrasive based micro-/nano-finishing V.K. Jain ∗ Indian Institute of Technology, Mechanical Engineering, Kanpur 208016, Uttar Pradesh, India

a r t i c l e

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Keywords: Micro-/nano-machining Magnetic Abrasive Finishing Abrasive Flow Machining Magnetorheological Abrasive Flow Finishing Magnetorheological Finishing

a b s t r a c t Micro-/nano-machining (abbreviated as MNM) processes are classified mainly in two classes: traditional and advanced. Majority of the traditional MNM processes are embedded abrasive or fixed geometry cutting tool type processes. Conversely, majority of the advanced MNM processes are loose flowing abrasive based processes in which abrasive orientation and its geometry at the time of interaction with the workpiece is not fixed. There are some MNM processes which do not come under the abrasive based MNM category, for example, laser beam machining, electron beam machining, ion beam machining, and proton beam machining. This paper gives a comprehensive overview of various flowing abrasive based MNM processes only. It also proposes a generalized mechanism of material removal for these processes. The MNM processes discussed in this paper include: Abrasive Flow Finishing (AFF), Magnetic Abrasive Finishing (MAF), Magnetorheological Finishing, Magnetorheological Abrasive Flow Finishing, Elastic Emission Machining (EEM) and Magnetic Float Polishing. EEM results in surface finish of the order of sub-nanometer level by using the nanometer size abrasive particles with the precisely controlled forces. Except two (AFF and EEM), all other processes mentioned above use a medium whose properties can be controlled externally with the help of magnetic field. This permits to control the forces acting on an abrasive particle hence the amount of material removed is also controlled. This class of processes is capable to produce surface roughness value of 8 nm or lower. Using better force control and still finer abrasive particles, some of these processes may result in the sub-nanometer surface roughness value on the finished part. Understanding the mechanism of material removal and rotation of the abrasives in these processes will help in rationalization of some of the experimental observations which otherwise seem to be contradicting with the established theories. It also explains why a magnet used in MAF should have a slot in it even though the area under the slot has “non-machining” zone. It elaborates based on the experimental observations why to use pulse D.C. power supply in MAF in place of smooth D.C. power supply. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Fabrication of products deals with the building of machines, structures or process equipment by cutting, shaping, welding and assembling of components made of the same or different materials. Fabrication can be classified into two main categories: macro-fabrication and micro-fabrication. The first one considers the process of fabrication of structures/parts/products/features that are measurable and observable by naked eye (≥1 mm in size) while the second category deals/considers the miniature structures/parts/products/features which are not easily visible with naked eye, and have dimensions smaller than 1 mm (1 ␮m ≤ dimension ≤ 999 ␮m). There are various methods/ways by which micro-fabrication of products can be achieved (Fig. 1). However, two of them are the most commonly used: (1) material

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deposition, and (2) material removal. This article deals with the material removal processes only. Some of the traditional material removal processes can be used for micro-fabrication. However, they have some constraints hence advanced material removal processes are more commonly used for this purpose. Fig. 2(a) shows a classification of advanced micro-/nano-machining (MNM) and micro-/nano-finishing (MNF) processes. Majority of the advanced material removal processes can be employed for both, macro-machining as well as for micromachining. While scaling down the applications of a process from macro- to micro (␮)-machining, the ␮-machining process parameters have to be appropriately changed. Further, advanced micro-machining processes (abbreviated as AMMPs) are used for two main purposes: (i) shaping and sizing a part, (ii) finishing a part. For differentiating between these two classes, the first one is called as AMMPs and the second one as advanced micro-/nano-finishing processes (AMNFPs). The AMMPs can be further sub-categories as (i) those processes which use abrasive particles as tools for removing material from the work piece in the form of micro-/nano-chips,

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due to which the normal force exerted by an abrasive particle on the work piece changes. This change in normal force changes finishing rate and critical surface finish that can be achieved by the process under the given finishing conditions. 2.1.1. Nano-finishing processes without external control of forces In this category following types of abrasive based nano-finishing processes are briefly discussed: Abrasive Flow Finishing (AFF), Chemo-Mechanical Polishing (CMP) and Elastic Emission Machining (EEM).

Fig. 1. Methods of micro-fabrication.

and (ii) those processes which use direct energy for removal of material by melting and/or vaporization, or electrochemical or chemical reaction (Fig. 2(a)). The AMMPs deal with fabrication of microstructures, generation of micro-features (say, micro-grooves, micro-cavity, micro-channels, etc. [Jain, 2009]). This paper deals only with those processes which fall in the category of abrasive based micro-/nano-finishing processes. Some of the abrasive based advanced (micro-/nano-finishing) and traditional finishing processes are given in Fig. 2(b). 2. Abrasive based advanced micro-/nano-finishing processes In today’s advanced engineering industries, the designers’ requirements on the components are stringent, for example, extraordinary properties of materials, complex shaped 3D components (Fig. 3(a, i–iv)), miniature features, nano-level surface finish on complex geometries which are not feasible to achieve by any traditional methods (say, thousands of turbulated cooling holes in a turbine blade, Fig. 3(a, iv), making and finishing of micro-fluidic channels in the electrically non-conducting materials (say, glass), etc. Such objectives can be achieved only through the advanced manufacturing processes in general and advanced machining processes in particular. In this section, the working principles of abrasive based advanced micro-/nano-finishing processes (Fig. 4) are discussed. As shown in Fig. 2(a), AMPs (Jain, 2002, 2009) are capable of performing micro-machining operations. To limit the length of the article, only some of these advanced abrasive based (micro-/nano)finishing processes are critically reviewed in the following sections. 2.1. Advanced Abrasive Finishing Processes The Advanced Abrasive Finishing Processes can be divided into two groups to understand their working principles. First one includes Abrasive Flow Finishing (AFF), Elastic Emission Machining (EMM) and Chemo-Mechanical Polishing (CMP) where forces on the work piece acting during the finishing process are not possible to control externally. The second one includes Magnetic Abrasive Finishing (MAF), Magnetorheological Finishing (MRF), Magnetorheological Abrasive Flow Finishing (MRAFF), and Magnetic Float Polishing (MFP). In these processes, it is possible to externally control the forces acting on the workpiece by varying electric current flowing in the electromagnet coil or by changing the working gap while using a permanent magnet. A change in the electric current changes magnetic flux density in the working zone

2.1.1.1. Abrasive Flow Finishing. AFF process was originally identified for deburring and finishing critical hydraulic and fuel system components of aircraft in aerospace industries. It can polish anywhere that air, liquid, or fuel flows. Rough, power robbing cast, machined, or EDM’d surfaces are improved substantially regardless of their surface complexities Fig. 3(a) shows that the AFF medium acts as a ‘self deformable stone’ adapting itself according to the shape and size of the work piece (concave, convex, hexagonal or turbulated holes). It has been used for finishing micro-fluidic channels made on glass and ceramics. It uses two vertically opposed cylinders (Fig. 4(a)), and extrudes abrasive medium back and forth through a passage formed by the workpiece and tooling. To formulate the AFM medium, the abrasive particles are blended into the special viscoelastic polymer, which shows a change in viscosity when forced to flow through a restrictive passage Fig. 4(a) shows radial force (Fn ) responsible for indentation of an abrasive particle in to the work piece, and axial force (Ft ) responsible for removal of material in the form of a micro-/nano-chip. Abrasive action is accelerated by a change in rheological properties of the medium when it enters and passes through the restrictive passage (Rhoades, 1988, 1991). The viscosity of polymeric medium plays an important role in finishing operation (Jha, 2006). AFF can be applied to a wide range of finishing operations that require uniform and repeatable results (Kohut, 1989). Forces (radial and axial) acting during AFF have been evaluated (Gorana et al., 2004) and correlated to a change in surface roughness value achieved after AFF. Under certain finishing conditions, it has been found that the material removal during AFF can take place by ploughing mode (material piled up in the sides) also (Fig. 3(b)) (Gorana et al., 2004), other than chipping mode (Jain and Jain, 1999). Fig. 3(b) shows a ploughing mode of material removal during AFF process. Active grain density (Gorana et al., 2004; Jain and Jain, 2004) has been found to influence finishing rate and depends on the finishing parameters such as abrasive concentration, extrusion pressure, abrasive mesh size and medium viscosity. A stochastic methodology has been proposed (Jain and Jain, 2004) to evaluate the interaction between spherical (assumed shape) abrasive grains and workpiece surface. This simulation enables the prediction of active grain density at different concentrations and mesh size of abrasive particles which finally control the quality and rate of change in surface finish improvement. A good correlation is obtained between the predicted and microscopically observed active abrasive grain density. Empirical models have been proposed for material removal and change in surface roughness (Ra ). Authors (Jain et al., 2001a) have investigated the effects of various process parameters on the viscosity of the medium. It is found that abrasive concentration, medium temperature and abrasive mesh size have significant effect on medium viscosity. It is also found that an increase in viscosity of the medium results in an increase in material removal rate, but decrease in surface roughness (Ra ) value until the critical surface roughness is achieved. Further, an increase in finishing rate becomes zero at critical surface roughness value. Jain et al. (1999a) have analyzed material removal and surface roughness produced during AFF using finite element method. In this

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Fig. 2. (a) Classification of micro-machining processes (USM: ultrasonic machining; AJM: abrasive jet machining; AWJM: abrasive water jet machining; WJM: water jet machining; EBM: electron beam machining; LBM: laser beam machining; EDM: electro discharge machining; IBM: ion beam machining; PBM: photon beam machining; PCMM: photo chemical micro-machining; ECMM: electro chemical micro-machining; ECSMM: electro chemical spark micro-machining; EDG: electro discharge grinding; ELID: electrolytic in-process dressing). (b) Micro-/nano-finishing processes.

attempt, a classical abrasion theory has been applied. The model is based on accumulated plastic flow, by repeated indentation of moving abrasive particles. The dependence of surface roughness value on various process parameters has been analyzed and the theoretical results are found to be in good agreement with the experimental results obtained from AFF process. The AFF results have shown that the axial force, radial force, active grain density and grain depth of indentation, have a significant influence on the scale of material removal. The minimum depth of indentation and minimum load required for chip formation, are found to correlate well with the mode of material deformation. The theoretical and experimental results show that the rubbing mode of material deformation dominates in the present study; however, some evidences of ploughing during AFF are also reported (Gorana et al., 2004, 2006). To improve the performance efficiency of AFF process, drill bit guided (DBG) AFF process has been proposed (Sankar et al., 2009a). The major difference between AFF and DBG-AFF machines is in its

tooling. In AFF machine, circular fixture plate allows the medium to flow as a cylindrical slug. The abrasive intermixing (or reshuffling) purely depends on medium self-deformability and for most of the time the same active abrasive grains keep taking part in finishing. The abrasive particles follow the shortest contact length (straight line) in AFF. In DBG-AFF process, the cylindrical slug gets divided in two halves while entering in the finishing zone; at the exit side these two halves recombine resulting in better intermixing of the medium. The abrasive intermixing depends not only on the medium self-deformability but also on the pressure from the drill bit being exerted on the medium (reciprocating axial flow, flow along the flute, and scooping flow—all the three flows take place at the same time). Due to the combination of different modes of flow, the workpiece (AISI 4340)–abrasive contact length is no longer a straight line, rather it becomes inclined (Fig. 5(a)). Hence, the number of peaks that can be sheared off in a single cycle increases, leading to higher material removal rate hence finishing rate also

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Fig. 3. (a) Abrasive Flow Finishing medium adapts the shape of the workpiece (i) octagonal, (ii) concave, (iii) convex, (iv) turbulated hole, and (b) a schematic diagram of material removal by ploughing.

improves as compared to AFF process. The medium in contact with the drill bit (inner region of abrasive mixed medium slug) tries to follow the path of the flute at an angle . However, it is found that the abrasion is taking place at an angle  1 which is different from the flute angle () and the difference has been expressed as  2 (Fig. 5(b)). The finishing zone becomes more restricted because of the presence of the drill bit; hence, the abrasive particles in DBGAFF exert higher pressure on the workpiece surface as compared to the case of AFF. Nano-finishing of heterogeneous materials such as metal matrix composites (MMC) is a challenge for manufacturing engineers. Here, material removal also depends upon the position where an abrasive particle strikes the workpiece (i.e., matrix material, reinforcement, or interface of these two). Let an abrasive (particle in the medium) hit the MMC workpiece. If it hits matrix material (Al alloy), it removes the material in the form of a micro-chip. If abrasive (SiC) encounters reinforcement (SiC) then the abrasive tries to detach (or pull out) the reinforcement from the base material. The reinforcements pullout takes place only if Fa > Freq where Fa is applied axial force and Freq is the force required to pullout the reinforcement (i.e., resistance offered by the MMC for pull out). Fig. 6(a) shows a photograph of reinforcement pullout during AFF of MMC and Fig. 6(b) shows a model proposed (Sankar et al., 2009a,b) to represent reinforcement pullout. If Fa < Freq then the abrasive may rotate or cross over the workpiece surface peak without any effective material removal, or remain embedded in the medium (Fig. 6(c)) (because it cannot machine the material due to the condition Fa < Freq (may be due to higher depth of penetration, or unable to pull out the abrasive in MMC)). The average surface roughness achieved in finishing Al alloy/SiC MMC is 0.2 ␮m and this can be further improved by optimizing the process parameters. For achieving the best out of any manufacturing process, parametric optimization is essential which has been attempted in case of AFF (Jain and Jain, 2000; Jain et al., 1999a,b) using artificial neural networks (ANN). The optimization has been performed using backpropagation neural networks. The optimization results of ANN have been compared with the optimization results obtained by

employing Genetic Algorithm (GA) to establish the validity of neural networks approach. The important advantage in case of neural networks is that the process optimization can be performed in the absence of mathematical model of the process, and its accuracy depends on the accuracy of the experimental observations used for training of ANN. This feature of ANN is important because mathematical modeling of the process is a pre-requisite for all the classical optimization methods, and in some cases, it is very difficult to develop a mathematical model. 2.1.1.2. Chemical Mechanical Polishing. Chemical Mechanical Polishing (CMP) is mainly used in the semiconductor manufacturing industries and it is a planarization process which involves a combination of chemical and mechanical actions. In this process, the chemical reaction takes place between the silica slurry and the work material. The reaction products so formed are removed by mechanical action (abrasion) (Nanz and Camilletti, 1995; Hayashi et al., 2001). A schematic of CMP planarization process is shown in Fig. 4(b). The wafer is pressed downward by carrier and rotated against the polishing pad covered with a layer of silica slurry. A similar variant is Chemo-Mechanical Polishing in which driving force for material removal is chemical action between abrasive particles and work material followed by mechanical action for the removal of reaction products (Vora et al., 1982; Komanduri et al., 1997). This process is expected to overcome many problems of surface damage associated with hard abrasives, including pitting due to brittle fracture, dislodgement of grains, scratching due to abrasion etc., resulting in smooth, damage free surfaces (Komanduri et al., 1997). To explain material removal in CMP, abrasion mechanism in solid–solid contact mode has been proposed (Luo and Dornfeld, 2001). Liu et al. (2003) have presented polishing kinetics and the mechanism of material removal from the silicon substrate. They found that the smaller size (15–20 nm) silica sols (abrasives) perform better than the larger size (50–70 nm). It gives higher average polishing rate (200 nm/min), and it keeps changing with polishing time. It gives lower damage and lower value of surface roughness aftr CMP. Ahn et al. (2004) compared CMP using colloidal silica

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Fig. 4. Nano-finishing processes (a) Abrasive Flow Finishing (AFF), (b) Chemo-Mechanical Polishing (CMP), (c) Elastic Emission Machining (EEM), (d) Magnetic Abrasive Finishing (MAF), (e) Magnetorheological Finishing (MRF), (f) Magnetorheological Abrasive Flow Finishing (MRAFF), and (g) Magnetic Float Polishing (MFP).

based slurry and alumina (Al2 O3 ) based slurry. The colloidal-based silica slurry produced a desirable fine finish on aluminum surface with a few micro-scratches. Saka et al. (2008) found that during fabrication of advanced semiconductor devices, undesirable nanoscale scratches are produced. Polishing both sides concurrently gives defect free surfaces having better parallelism compared to single side polishing and less adherence of particles (Jhansson et al., 1989; Wenski et al., 2002). Another use of CMP substrate is in thin film transistor (TFT) technology and polishing of IC wafers (Venkatesh et al., 1995; Chang et al., 1996). 2.1.1.3. Elastic Emission Machining. This process attracts the attention because of its ability to remove material at the atomic level

by mechanical means and to give completely mirrored, crystallographically and physically undisturbed finished surface (Tsuwa et al., 1979). The ultra fine abrasive particles strike the individual atoms/group of atoms and separate them out from the parent surface (Fig. 4(c)). It has been found that the material removal process is a surface energy phenomenon in which each abrasive particle removes a number of atoms after coming in contact with the workpiece surface (Mori and Yamauchi, 1987). It has been established theoretically and experimentally that atomic scale fracture can be induced elastically producing ultrafine surface finish without plastic deformation at atomic scale (Mori et al., 2002). In EEM, the material removal occurs at the atomic level, hence the surface finish obtained is close to the order of atomic dimensions (2–4 Å). The type of abrasive and size of abrasive grains used (in the

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Fig. 5. Abrasion direction due to active abrasive grains in DBG-AFF (a) macro-view, and (b) micro-view.

nano-range) have been found to be critical to the material removal efficiency. 2.1.2. Nano-finishing processes with external control of forces This class of processes includes MAF, MRF, MRAFF, and MFP. In these processes, the force acting on the work piece surface through the abrasive particles is controlled externally by changing the magnetic flux density as discussed in the following paragraphs. 2.1.2.1. Magnetic Abrasive Finishing. In case of a flat large size workpiece made of hard-to-machine material, the processes discussed so far do not qualify to give nano-level surface finish. Magnetic Abrasive Finishing (MAF) is the process which is capable of precision finishing of such workpieces (Shinmura, 1987). In this process, usually ferromagnetic particles are sintered with fine abrasive particles (Al2 O3 , SiC, CBN, or diamond), and such particles are called ferromagnetic abrasive particles (or magnetic abrasive particlesMAP). Fig. 4(d) shows a schematic diagram of a plane MAF process in which finishing action is controlled by the application of magnetic field across the machining gap between the workpiece top surface and bottom face of the rotating electromagnet pole. The magnetic field acts as a binder and retains ferromagnetic abrasive particles in the machining gap. Normal component (Fmn ) of the magnetic force due to magnetic field is responsible for abrasive penetration inside the workpiece surface while rotation of the ferromagnetic abrasive brush intact to north pole results in tangential force (Ft ) (not shown in the figure). The sum of the forces Ft and the tangential component of the magnetic force (Fmt ), (Fc = Ft + Fmt ) is responsible to remove material in the form of tiny chips (Singh et al., 2005a,b; Kremen, 1994). The MAP join each other magnetically due to dipole–dipole interaction between the magnetic poles along the lines of magnetic force, forming a flexible magnetic abrasive

brush (FMAB) (usually, 1–3 mm thick). In case of unbounded (unsintered) magnetic and abrasive particles (homogeneously mixed powder), the abrasive particles get entangled in between the chains and within the chains formed by ferromagnetic particles (Singh et al., 2005a). MAF uses this FMAB for surface and edge finishing. The FMAB has multiple cutting edges and it behaves like a multi point cutting tool to remove material from the workpiece in the form of tiny chips. Since the magnitude of machining force caused by the magnetic field is very low but controllable, a mirror like surface finish (Ra value in the range of nanometer) is obtained. MAF can also be used to perform operations such as polishing and removal of thin oxide film from high-speed rotating shafts. Researchers (Jain et al., 2001b; Kim, 1997; Komanduri, 1996) have applied MAF to finish external and internal surfaces of cylindrical workpieces. Apart from rotary motion of the cylindrical workpiece, axial vibratory motion is also introduced in the magnetic field by the oscillating motion of the magnetic poles or the workpiece to accomplish surface and edge finishing at a faster rate and with a better quality (Komanduri, 1996; Fox et al., 1994; Yamaguchi and Shinmura, 1999, 2004). The process is highly efficient, and the material removal rate and finishing rate depend on the workpiece circumferential speed, magnetic flux density, working gap, workpiece material properties, and size, type and volume fraction of abrasives. The process performance is also affected the presence or absence of a slot in the magnet (Jayswal et al., 2004). Finishing of stainless steel rollers using MAF process to obtain final Ra of 7.6 nm at an average finishing rate of 7.08 nm/s has been reported (Komanduri, 1996; Fox et al., 1994). MAF can produce mechanical and electronic components with high accuracy and very low surface roughness value having hardly any surface defects. This process has also been applied for micro-deburring (Fig. 7) using permanent magnet in place of electromagnet (Madarkar and Jain, 2007).

Fig. 6. Mechanisms of material removal during AFF of MMC. (a) Reinforcement pullouts in MMC in the agglomerated area, (b) corresponding model of indentation in the matrix due to reinforcement pullout in Al alloy/SiC MMC, and (c) abrasive embedded in the surface.

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Fig. 7. Drilled hole edge (a) before and (b) after deburring by MAF.

2.1.2.2. Magnetorheological Finishing. The high precision lenses are usually made of brittle material such as glass, which tends to crack during machining/finishing. To overcome the difficulties being faced in finishing the lenses, a technology has been developed to automate the lens finishing process known as Magnetorheological Finishing (MRF) (Fig. 4(e)) (Kordonski, 1996). This process relies on a unique “smart fluid”, known as Magnetorheological (MR) fluid which is a suspension of micron sized magnetizable particles such as carbonyl iron particles (CIPs), dispersed in a non-magnetic carrier medium like silicone oil, mineral oil, or water. In the absence of magnetic field, an ideal MR-fluid exhibits Newtonian behavior. Magnetorheological effect is observed on the application of external magnetic field to the MR-fluid. In the presence of external magnetic field it behaves as non-Newtonian fluid. Fig. 8(a) shows the random distribution of CIPs and abrasive particles in

the absence of magnetic field. Fig. 8(b) shows that the CIPs magnetize when the magnetic field is on, and move towards the rotating wheel where magnetic field strength is higher. Fig. 8(c) shows that the abrasive particles are in contact with the workpiece (Lens) surface but intact with the fluid (ribbon), and the CIPs are closer to the rotating wheel. The normal magnetic force (or penetrating force) is transferred to the work surface through the abrasive particles, and it results in abrasive penetration in the work surface. Due to the relative motion between abrasive particles and work surface, material removal takes place in the form of micro-/nano-chips resulting in nano-finishing. Because energy is required to deform and rupture the chains, this micro-structural transition is responsible for the onset of a large controllable finite yield stress (Furst and Gast, 2000). There is an increasing resistance to an applied shear strain,  due to this yield stress. When the field is removed, the parti-

Fig. 8. (a–c) Material removal mechanism in Magnetorheological Finishing, and (d) MRF experimental setup developed at IIT Kanpur (Mathur et al., 2003).

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Fig. 9. Experimental results of MRF (a) initial surface topography, and (b) after finishing for 50 min (Mathur et al., 2003).

cles return to their random state and the fluid again exhibits its original Newtonian behavior. Fig. 8(c) shows work piece and fluid interaction region. The fluid strength (static yield shear stress) increases nonlinearly as the applied magnetic field increases because the particles are ferromagnetic in nature and magnetization in different parts of the particles occurs non-uniformly (Ginder and Davis, 1994). The upper limit of the strength of MR-fluid is decided by magnetic saturation. The ability of electrically manipulating rheological properties of MR-fluid attracts attention of a wide range of industries, and numerous applications are explored. The MRF process is used for finishing optical glasses, glass ceramics, plastics and some non-magnetic metals (Lambropoulo et al., 1996; Carlson et al., 1996; Klingenberg, 2001). This finishing process is capable to produce surface finish of the order of 10–100 nm peak to valley height, and 0.8 nm RMS value in finishing optical lenses (Kordonski, 1996). A setup shown in Fig. 8(d) was used to nano-finish glass lenses for 50 min. Fig. 9(b) shows a change in surface texture and surface finish compared to original surface roughness and texture (Fig. 9(a)). The surface roughness value changed from 53 nm to 11 nm in 50 min (finishing rate = 0.84 nm/min) (Mathur et al., 2003). 2.1.2.3. Magnetorheological Abrasive Flow Finishing. The abrading forces in AFM process are least controllable by external means, hence lack of determinism. To preserve the versatility of AFF process and at the same time to have determinism and controllability of rheological properties of abrasive laden medium, a new hybrid

process termed as Magnetorheological Abrasive Flow Finishing (MRAFF) has been developed (Jha and Jain, 2004) by combining AFF and MRF (Fig. 4(f)). Abrasive mixed viscous base medium acts as a “self deformable stone” and overthrows shape limitation inherent in almost all traditional finishing processes. This process has the capability of finishing complex internal and external geometries up to nano-level surface roughness value. It imparts better control on the process performance as compared to AFF process due to in-process control over abrading medium’s rheological behavior through magnetic field (Rabinow, 1948). MRAFF process comprises MR-polishing fluid having fine abrasive particles dispersed in it. On the application of magnetic field, the CIPs form a chain like columnar structure with abrasives embedded in between and within the chains. Fig. 10(a) and (b) show the actual photographs taken by an optical microscope for the case when no magnetic field is applied and the structure formed in the presence of magnetic field, respectively (Jha, 2006). The abrasive particles held by CIPs chains abrade the workpiece surface and shear the peaks from it. The amount of material sheared from the peaks of the workpiece surface by an abrasive grain depends on the bonding strength provided by the magnetic field-induced structure of MRpolishing fluid and the extrusion pressure applied through piston. The best finish obtained by the present setup and specified machining conditions (Jha and Jain, 2008) is 30 nm on stainless steel work piece. However, experimentation with optimum process parameters would give still lower Ra value of the surface finished by MRAFF process.

Fig. 10. Formation of CIPs chain structure (a) in absence of magnetic field and (b) on the application of magnetic field (Jha, 2006).

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The finishing action in MRAFF process relies mainly on bonding strength around abrasive particles, in magnetorheological polishing (MRP) fluid due to cross-linked columnar structure of CIPs. The fluid flow behavior of MRP-fluid exhibits a transition from weak Bingham liquid like structure to a strong gel like structure on the application of magnetic field. The rheological properties of MRPfluid play an important role in MRAFF action which mainly depends on CIPs and silicon carbide particles size, their volume concentration, magnetic properties and magnetic field strength. Experiments conducted on silicon nitride using silicon carbide (SiC), boron carbide (B4 C) and diamond abrasives proved MRAFF capability in nano-finishing hard ceramics (Jha and Jain, 2006). This process has immense possibilities of applications especially in case of finishing of complex shaped 3D components. 2.1.2.4. Magnetic Float Polishing. The finishing processes discussed in the preceding sections have been developed for flat surfaces, cylindrical surfaces or their combinations leading to complex 3D surfaces. However, finishing of spherical surfaces is equally important for which the above discussed processes do not qualify. The Magnetic Float Polishing (MFP) process has been developed (Fig. 4(g)) to meet this requirement. The best surface finish obtained using this process on the ceramic balls is 4 nm Ra and 40 nm Rmax , and the best sphericity obtained on the Si3 N4 balls is 150–200 nm. This process is assisted by magnetic field to support abrasive slurry in finishing ceramic balls and bearing rollers without having scratches and pits (Umehara et al., 2005). This technique is based on the ferro-hydrodynamic behavior of magnetic fluid that levitates a non-magnetic float and abrasive particles suspended in it by the application of magnetic field. The levitation force applied by the abrasives is proportional to the magnetic field gradient which is extremely small and highly controllable. MFP can be a very cost-effective and viable method for super finishing of brittle materials with flat and spherical shapes. A bank of strong electromagnets is arranged (alternately north and south poles) below the finishing chamber. The ferro-fluid is attracted downward towards the area of higher magnetic field and an upward buoyant force is exerted on non-magnetic materials to push them to the area of lower magnetic field (Rosenweig, 1966) (Fig. 4(g)). The buoyant force acts on a non-magnetic body in magnetic fluid in the presence of magnetic field. The abrasive grains, ceramic balls, and acrylic float inside the chamber are of nonmagnetic materials, and all are levitated by the magnetic buoyant force. The drive shaft is fed downward to contact the balls and press them downward to reach the desired force level. The balls are polished by the relative motion between the balls and the abrasive particles under the influence of levitation force. Si3 N4 balls have been finished by MFP for high-speed hybrid bearing in ultra highspeed precision spindles of machine tools and jet turbines of aircraft (Tani and Kawata, 1984). 3. Analysis of selected abrasive based nano-finishing processes Modeling and theoretical analysis of the selected abrasive based nano-finishing processes have been carried out to explain some of the results obtained during experimentation. These results have not been satisfactorily explained in the existing literature. 3.1. Critical surface roughness During MAF, AFM and MRAFF processes, it has been observed that with the increase in finishing time, the surface roughness value (Ra value) keeps on decreasing. Beyond a certain finishing time, the relationship between the finishing time and surface roughness

Fig. 11. Relation between surface roughness and finishing time.

value (Ra value) becomes asymptotic (Fig. 11) except the minor fluctuations within a small band of Ra value. This behavior was observed while comparing experimental and computational results (Jayswal, 2005) obtained for MAF process. To understand this behavior exhibited in Fig. 11, let us analyze the models proposed to explain material removal in MRAFF process which is applicable to MAF as well as AFM with minor modifications. Fig. 12(i) shows an abrasive particle held by chains of iron particles (Jha, 2006). The downward or indenting force is a normal component of magnetic force in MAF and MRAFF processes, and radial force (Fr ) in AFM process. Because of the cutting force (axial in AFM and MRAFF, and tangential in MAF), this bunch of iron and abrasive particles moves in the forward direction and shears/removes a very small amount of material in the form of micro-chip (Fig. 12(ii)). The size of the chip removed depends on the magnitude of radial force, axial force, and the ratio of axial force and the force required to shear off the roughness peaks. When this bunch of iron and abrasive particles moves further, it separates the micro-/nano-chip (MNC) from the workpiece (Fig. 12(iii)). Similar mechanism of material removal with minor modifications works in case of MAF process. In the same way, material is removed in the form of micro-chip in case of AFM (Fig. 12(iv)). This phenomenon of removal of material in the form of MNC is repeated by each bunch of iron/abrasive particles. As a result, the height of the surface irregularities keeps decreasing as shown in Fig. 13(i)–(iii). It also suggests that the iron and abrasive particles size should be larger than the top width of a valley. At a stage between Fig. 13(i) and (iv), one can get the minimum attainable surface roughness value but that stage is not known in priori. At this point, the pre-finished marks/scratches are completely removed and the abrasives creat their own finishing marks (peaks and valleys). At the stage (iv) (Fig. 13), the abrasive particles further penetrate into the workpiece and their depth of penetration depends upon the radial force, and the ratio of axial force and the force required to shear off the surface peaks. This indentation depth decides the critical surface roughness value attainable in these processes. Unless finishing conditions are changed, the value of (critical) surface roughness remains unchanged with time. However, real life situation is slightly different because the size of iron and abrasive particles varies in a range hence the depth of penetration also as shown in Fig. 13(v). It shows different depth of penetration for different size of abrasive particles depending upon the penetrating force acting on a particular abrasive particle. As a result of this, the critical surface roughness value obtained after finishing is not an unique number rather it fluctuates over a small range or band. This fluctuation in critical surface roughness value

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Fig. 12. (i–iii) Three stages of material removal in case of MRAFF process, and (iv) micro-chip formation in AFM process.

Fig. 13. Stages ((i)–(iii)) material removal from the workpiece surface, (iv) Critical surface finish obtained for the given finishing condition, (v) Cross-section along AA.

can also be caused due to the non-homogeneous mixture of the iron and abrasive particles (MAF and MRAFF processes), or viscoelastic medium and abrasive particles (in AFM). This variation may also result due to the inclusions of MNC in the medium before it is replaced by the fresh medium. The experimental results of MAF clearly indicate that the surface roughness approaches to the ‘critical surface roughness’ value during the experiments (Fig. 11). The critical surface roughness value for the given finishing conditions is higher for the case of higher viscosity in case of AFM. However, this also has an upper limit depending upon the finishing conditions. It requires further theo-

retical and experimental investigations to set these upper limits of the ultimate surface finish achievable from AFM and other abrasive based nano-finishing processes. 3.2. Rotation of abrasive particles This phenomenon is common for MAF, MRAFF and AFM processes where abrasive particle is held either by ferromagnetic (iron) particles or viscoelastic polymer. During finishing by any of these processes, one of the following conditions may prevail in the finishing zone (Fig. 14).

Fig. 14. (a) Cutting just to start, (b) material removal in the form of micro-/nano-chip, (c) rotation of the particle, (d) forces and depth of penetration, (e) three conditions of cutting force (Fc ) and force required (Freq ) for removal of material from the workpiece. Fc = available cutting force, Freq = Force required for cutting to take place. R and R’ are resultant forces.

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In the equilibrium condition, machining/finishing is just at the point of starting (Freq = Fc , Fig. 14(a)) while in the material removal condition (Freq < Fc , Fig. 14(b)) the material removal keeps taking place (a desirable finishing condition). In case of MAF (also in MRF and MRAFF) process when Fmn (normal component of the magnetic force) is more than the desirable magnetic field strength in the finishing zone (or higher radial force in case of AFM process), the abrasive penetration depth in the workpiece also proportionately increases (say, hs in Figs. 14(d) and 15). With the increased depth of penetration of an abrasive particle inside the workpiece surface, the projected area (=Ap ) accordingly increases hence the required force (Freq = Ap ×  s ) also increases to the extent that the available cutting force (Fc ) to remove the material in the form of MNC becomes smaller than the required cutting force (i.e. no cutting condition). Now, one of the two events can take place. Either the abrasive particle remains stagnant in this condition (without doing any cutting) unless favorable conditions prevail, or it slightly moves upward (or rotates opposite to the penetration direction) such that the reduced penetration depth (hs in Figs. 14(c) and 15) leads to the condition of cutting (Freq = Ap × s < Fc ). With the reduced projected area (Ap ), the required cutting force decreases. Under certain conditions of finishing, the event that the particle remains embedded in the surface also has equal probability of happening Fig. 6(c). As a result of this, although the penetration depth is increasing due to higher extrusion pressure (or due to higher magnetic field intensity) but Ra value is not changing (Fig. 16) substantially because Fc becomes smaller than the Freq . A non-uniformity existing in the strength of a FMAB, or that of different bunches of iron particles holding abrasive particles (MRF and MRAFF), is also practicable leading to the situation of Fig. 14(c). It can also explain the nature of Fig. 16 as follows. A slight decrease in the strength of a part of the FMAB (MAF), or a bunch of the iron particles holding abrasive particle in MRAFF or the medium holding the abrasive particle in case of (AFM), (or stiffened in case of MRF (or

Fig. 15. Depth of penetration during rotation of an abrasive particle in the brush/medium.

Fig. 16. Effect of radial/indentation force in AFM, on Ra (all other conditions remain unchanged including cutting force, Fc ).

Fig. 17. (a) Schematic view of plane MAF with a slotted pole and (b) solution domain for plane MAF with slotted pole.

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Fig. 18. (a) Variation of normal force on a single abrasive particle with distance from the centre of the tool (with one and two slots), and (b) variation of Ra value with finishing time (working gap = 4 mm, current = 1.12 A, voltage = 15 V, diameter of MAP = 100 ␮m, rotation speed of N-pole = 200 RPM, power input = 2.2 kW, size of the workpiece = 85 mm × 85 mm × 8 mm, initial Ra = 1.5 ␮m) (Jayswal, 2005; Jayswal et al., 2004).

MRAFF)), force R will slightly move the particle upward resulting in the reduction in depth of penetration from hs to hs (Fig. 14(c)).  , then the Fig. 14(c) also shows that if the force Fmn decreases to Fmn depth of penetration decreases by an amount equal to hs − hs = h and cutting restarts. 3.3. Slot in the electromagnet used in MAF As shown in Fig. 17(a), electromagnet used in MAF process has a slot in it. An attempt has been made to analyze merits and demerits of the presence of the slot, in term of variation in forces. This analysis has been carried out using finite element method (FEM). Fig. 17(b) shows the solution domain for the plane MAF with a slot in the magnetic pole (Jayswal, 2005; Jayswal et al., 2005a,b). As shown in Fig. 18(a), the normal magnetic force (Fmn ) becomes negative under the slot while under the edge of the slot, the force becomes larger than that under the flat area of the magnet. When the force becomes negative, there will be ‘no penetration’ by the abrasive particle in the workpiece under the slot but the abrasive particles near the slot edge will have larger depth of penetration than the flat area because at the boundary of the magnet, the magnetic force (Fmn ) value is very large for all the three cases of the magnets (with no slot, with one slot, with two slots) (Fig. 18(a)). This increase in Fmn at discontinuities of the magnet leads to an increased finishing rate as well as lowered Ra value until the critical surface roughness is achieved. This is possible only when the workpiece is given feed in X and Y directions to finish the whole workpiece surface. This characteristic can be seen in the Fig. 18(b) where a comparison is made between the performance of a magnet with a slot and without a slot. The magnet with a slot gives better surface finish than the magnet without a slot.

The MAF experiments were conducted (Singh et al., 2005a,b) using initial ground surface (Fig. 20(a)), with DC power supply without feed to the work piece (Fig. 20(b)) and with feed (in X and Y directions) to the work piece (Fig. 20(c)). These atomic force micrographs clearly indicate that the ground surface has high peaks and valleys resulting in surface roughness value of 500 nm or so. When this surface is finished by S-FMAB without feed to the workpiece, the average surface roughness value is reduced to 100–200 nm as can be seen in Fig. 20(b). At some places it has high peaks while at other places it has low peaks or good surface finish. The only reason which seems feasible to explain this characteristic is that the S-FMAB may be having non-uniform strength because of inhomogeneous mixing of iron and abrasive particles. When the workpiece is given feed (X and Y) relative to the magnet, such high peaks get sheared off when they interact with the strong part of the S-FMAB. It is quite obvious while comparing Fig. 20(b) and (c) (Singh, 2005). The performance of S-FMAB (DC-MAF) was further investigated in terms of the texture of the finished surface. Fig. 21(a) shows the surface texture obtained by the use of S-FMAB. Some randomly distributed peaks are clearly visible which are not desirable. These high peaks show mainly high noise level and to a small extent non-uniformity in the finished surface also. To find the solution to this weakness of S-FMAB, the experiments were conducted using pulsed power supply to the electromagnet. Fig. 21(b) and (c) shows the texture obtained by the use of P-FMAB. The surface is more uniform and has lower Ra value. However, when using 0.4 duty

3.4. Methods of energizing FMAB in MAF The electromagnet in MAF is usually energized by the DC power supply (the finishing process is called as DC-MAF) during experimentation. However, keeping in view the limitations of the DC-MAF process, the attempt was made to investigate this process with pulsed power supply to the electromagnet (the process is called as PC-MAF). DC-MAF results in the static flexible magnetic abrasive brush (S-FMAB) while PC-MAF results in a pulsating flexible magnetic abrasive brush (P-FMAB) (Fig. 19).

Fig. 19. Methods of energizing FMAB.

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Fig. 20. Surface produced by surface grinding and S-FMAB. (a) initial surface topography obtained by grinding, (b) surface texture obtained by S-FMAB without feed to the work (current = 0.88 A, working gap = 1.25 mm, RPM = 90, lubricant = 2%, time = 30 min), (c) surface texture obtained by S-FMAB with feed to the workpiece (current = 0.75 A, working gap = 1.75 mm, grain mesh size = 800, number of cycles = 9).

Fig. 21. Comparison of surface produced by S-FMAB and P-FMAB (a) lays obtained by S-FMAB (duty cycle,  = 1.0, t = 45 min), (b) lays obtained by P-FMAB ( = 0.4, ton = 2.0 ms) (Some grinding marks are left on the finished surface), and (c) lays obtained by P-FMAB ( = 0.08, ton = 2.0 ms) (no grinding marks).

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Fig. 22. Problems at very low duty cycle in PC-MAF (gap = 1.5 mm, RPM = 200, finishing time = 15 min). (a)  = 0.08, ton = 2 ms, and (b)  = 0.16, ton = 1.5 ms.

cycle, some grinding marks are left on the finished surface which are removed when still lower duty cycle (=0.08) is used (Singh et al., 2005b). Atomic force microscopy of the work pieces finished with PFMAB are further examined. It is found that some pieces with 0.08 duty cycle have deep pits (Fig. 22(a)), and deep scratches as shown in (Fig. 22(b)) which are not found at higher duty cycles. Hence, an attempt is made to further investigate for its reason. Two components force dynamometer is designed and fabricated (Singh, 2005; Singh et al., 2006). The normal magnetic force and the tangential cutting force are measured in case of DC-MAF (SFMAB) and PC-MAF (P-FMAB). In case of S-FMAB, the variation in Fc is within approximately 4 N and in Fmn it is approximately within 10 N (Fig. 23(a)). In the case of P-FMAB, Fc does not change substantially, i.e. minimal fluctuation is seen (because it is originated from the motor power used for rotating the magnet whose power is not changed during the process as can be seen in Fig. 23(b)). On the other hand, Fmn fluctuates between 1000 N and 3000 N (Fig. 23(b)) in case of PC-MAF. The high magnitude of fluctuation in force Fmn leads to such kind of deep scratches and pits. Hence, very low duty cycle is also not recommended to get the defect free surfaces. However, the question arises why such a high fluctuation in the magnetic normal force (Fmn ) occurs? It is explained as follows. The magnetic force is directly dependent on the magnetic flux density which depends on the current supplied to the electromagnet. Hence, current versus time variation during PC-MAF is recorded, and it is found that during a voltage pulse current varies by a factor of approximately three (Fig. 24(a)). Secondly, even during the off period the current never attains zero value due to the induced voltage as can be seen in Fig. 24(a). During PC-MAF, at the beginning of the voltage pulse the FMAB is quite strong and a large magnetic flux density is produced (Fig. 24(b, i)). But, during the off period, the

FMAB becomes weak and due to lower strength it starts breaking (Fig. 24(b, ii)) till the on-time of the next voltage pulse restarts. This making and breaking of the P-FMAB results in intermixing of the used and unused abrasive particles leading to a more uniform finished surface and higher finishing rate as compared to S-FMAB. That is why in some cases finishing rate during P-FMAB is as high as three times that of S-FMAB. 3.5. Texture and Measurement of Finished Surface During the MNF process, one may get the same surface roughness value but different surface texture which is important from wear point of view when the finished part is put in assembly. Fig. 25(a) shows an Atomic Force Micrograph of a piece finished by MRAFF process using SiC abrasive particles which gave Ra = 0.10 ␮m. It gave quite distorted surface texture. The same piece was further finished using the medium having diamond particles as abrasives (Jha, 2006). Then the finished surface was analyzed under the microscope and the surface texture obtained is shown in Fig. 25(b). It had the same surface roughness value (Ra = 0.10) as in the previous case when it was finished by SiC. To some extent Rq and Ry values are changed. This texture in Fig. 25(b) would definitely increase the product life against wear and tear as compared to the surface texture shown in Fig. 25(a). Hence, surface texture should also be examined from wear and tear of the part point of view, rather than surface roughness values alone. Fig. 26(a) shows SEM photograph after 30 cycles of MRAFF. Fig. 26(b) schematically shows that the measured surface roughness Ra value also depends upon where it is being measured. If, say, it is measured with 4 mm measurement length, and the valley comes under this measurement length then it gives the Ra value as 0.07 ␮m (Fig. 26(c)). On the same workpiece, if it is measured away

Fig. 23. (a) Force variation with time in S-FMAB (current = 0.75 A, working gap = 1.25 mm, RPM = 180), and (b) force variation with time in P-FMAB (duty cycle = 0.08, working gap = 1.50 mm, on-time = 2.0 ms, RPM = 200).

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Fig. 24. (a) Variation of pulse current and pulse voltage with time during PC-MAF. (b) (i) Formation of FMAB during on-time, and (b) (ii)partial breaking/falling of FMAB during off-time of a voltage pulse (Singh, 2005).

Fig. 25. Initial surface finish (Ra : 0.28 ␮m). (a) Finished surface after MRAFF for 2000 cycles with SiC abrasive (Ra : 0.10 ␮m), and (b) finished surface after MRAFF with diamond abrasive (Ra : 0.10 ␮m).

Fig. 26. Preliminary experimentation. (a) SEM micrograph of a finished work piece surface, (b) effect of cut-off length during the measurement of roughness and location of valleys, (c) intermediate roughness profile, (d) final roughness profile. (Jha, 2006).

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from this point where the valley does not appear in this measurement it gives a better Ra value (=0.04). Hence, the measurement of Ra value at one or two points does not give a clear picture about the surface texture of the finished part (Fig. 26(d)). 4. Conclusions From the presented experimental results and analysis, following conclusions can be drawn: • Each of the nano-finishing process is characterized by its ultimate (or critical) surface finish (normally varies in a small band) which can be produced by that process, and it depends on the finishing conditions used during the process. • During PC-MAF, use of low duty cycle is recommended but the surface integrity should be carefully examined for the defects such as deep scratches, pits, etc. Increase in magnetic flux density alone does not give higher finishing rate unless cutting force is high enough to remove material. • A single slot in the magnet during MAF gives higher finishing rate as compared to the magnet without any slot. • Knowledge of only surface roughness is not enough. Surface roughness plots should be examined carefully. Surface texture is also important from the wear and tear of the product point of view. Acknowledgements Author acknowledges the invitation by Prof. Ian Hutching, Chairman of 1st International Conference on Abrasive Based Processes, to give an invited talk on “Abrasive Based Micro-/Nano-Finishing Techniques—An overview”. The author sincerely thanks Mr. Ajay Sidpara, Ph.D. Scholar of Mechanical Engineering Department, IIT Kanpur for his help in the preparation of this manuscript. Thanks are also due to Dr. D.K. Singh of M.M.M. Engg. College, Gorakhpur; Dr. Sunil Jha of I.I.T. Delhi for sharing their results for the present manuscript. This paper’s contents were presented on 22 September 2008 at Churchill College, Cambridge (UK). References Ahn, Y., Youn, J., Chang-Wook, B., Yong-Kweon, K., 2004. Chemical mechanical polishing by colloidal silica-based slurry for micro-scratch reduction. Wear 257, 785–789. Carlson, J.D., Catanzarite, D.M., Clair, K.A., 1996. Commercial magnetorheological fluid devices. International Journal of Modern Physics B 10 (23/24), 2857–2865. Chang, C.Y., Lin, H.Y., Lei, T.F., Cheng, J.Y., Chen, L.P., Dai, B.T., 1996. Fabrication of thin film transistors by chemical mechanical polished polycrystalline silicon films. IEEE Electron Device Letters 17, 100–102. Fox, M., Agrawal, K., Shinmura, T., Komanduri, R., 1994. Magnetic abrasive finishing of rollers. Annals of CIRP 43 (1), 181–184. Furst, E.M., Gast, A.P., 2000. Micromechanics of magnetorheological suspensions. Physical Review E 61 (6), 6732–6739. Ginder, J.M., Davis, L.C., 1994. Shear stresses in magnetorheological fluids: role of magnetic saturation. Applied Physics Letters 65 (26), 3410–3412. Gorana, V.K., Jain, V.K., Lal, G.K., 2004. Experimental investigation into cutting forces and active grain density during abrasive flow machining. International Journal of Machine Tools Manufacture 44, 201–211. Gorana, V.K., Jain, V.K., Lal, G.K., 2006. Forces prediction during material deformation in abrasive flow machining. Wear 260, 128–139. Hayashi, Y., Nakajima, T., Kunio, T., 2001. Ultrauniform Chemical Mechanical Polishing (CMP) using a Hydro Chuck, featured by wafer mounting on a quartz glass plate with fully flat water supported surface. Japanese Journal of Applied Physics 35, 1054–1059. Jain, V.K., 2002. Advanced Machining Processes. Allied Publishers, Delhi. Jain, V.K., 2009. Introduction to Micromachinnig. Narosa Publishers, Delhi. Jain, R.K., Jain, V.K., 1999. Simulation of surface generated in abrasive flow finishing process. Robotics and Computer Integrated Manufacturing 15, 403–412. Jain, R.K., Jain, V.K., Dixit, P.M., 1999a. Modeling of material removal and surface roughness in abrasive flow machining process. International Journal of Machine Tools and Manufacture 39, 1903–1923. Jain, R.K., Jain, V.K., Kalra, P.K., 1999b. Modeling of abrasive flow machining: a neural network approach. Wear 231, 242–248.

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