MAGNETO ABRASIVE FLOW MACHINING

Magneto Abrasive Flow Machining

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3. INTRODUCTION Magneto abrasive flow machining (MAFM) is a new technique in machining. The orbital flow machining process has been recently claimed to be another improvement over AFM, which performs three-dimensional machining of complex components. These processes can be classified as hybrid machining processes (HMP)—a recent concept in the advancement of non-conventional machining. The reasons for developing a hybrid machining process is to make use of combined or mutually enhanced advantages and to avoid or reduce some of the adverse effects the constituent processes produce when they are individually applied. In almost all non-conventional machining processes such as electric discharge machining, electrochemical machining, laser beam machining, etc., low material removal rate is considered a general problem and attempts are continuing to develop techniques to overcome it. The present paper reports the preliminary results of an on-going research project being conducted with the aim of exploring techniques for improving material removal (MR) in AFM. One such technique studied uses a magnetic field around the work piece. Magnetic fields have been successfully exploited in the past, such as machining force in magnetic abrasive finishing (MAF), used for micro machining and finishing of components, particularly circular tubes. The process under investigation is the combination of AFM and MAF, and is given the name Magneto Abrasive Flow Machining (MAFM). 3.1

Problem Definition

Magneto Abrasive flow machining (MAFM) is one of the latest non-conventional machining processes, which possesses excellent capabilities for finish-machining of inaccessible regions of a component. It has been successfully employed for deburring, radiusing, and removing recast layers of precision components. High levels of surface finish and sufficiently close tolerances have been achieved for a wide range of components . In MAFM, a semi-solid medium consisting of a polymer-based carrier and abrasives in a typical proportion is extruded under pressure through or across the surfaces to be machined. The medium acts as a deformable grinding tool whenever it is subjected

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to any restriction. A special fixture is generally required to create restrictive passage or to direct the medium to the desired locations in the work piece. 3.2. Background Extrude Hone Corporation, USA, originally developed the AFM process in 1966. Since then, a few empirical studies have been carried out and also research work regarding process mechanisms, modeling of surface generation and process monitoring of AFM was conducted by Williams and Rajurkar during the late 1980s. Their work was mainly related to online monitoring of AFM with acoustic emission and stochastic modeling of the process. Loveless et al. and Kozak et al investigated the effect of previous machining process on the quality of surface produced by AFM and the flow behavior of the medium used in the process. Fletcher and others reported studies on the rheological properties and the effect of temperature of the medium used in AFM. Przyklenk conducted parametric studies of AFM. Research work concerning mathematical modeling, simulation of material removal and surface generation with the help of finite element and neural networks was presented by different researchers. Steif and Haan suggested the presence of ‘dispersive stresses’, which enable wear of the surface during abrasive flow processing. The dispersive stresses are generated because of the difference between stresses acting on abrasive particles and those acting in the surrounding medium. Jones and Hull reported the modification of existing AFM by applying ultrasonic waves in the medium for machining blind cavities. The orbital flow machining process suggested by Gilmore has been recently claimed to be another improvement over AFM, which performs three-dimensional machining of complex components. These processes can be classified as hybrid machining processes (HMP)—a recent concept in the advancement of non-conventional machining. The reasons for developing a hybrid machining process is to make use of combined or mutually enhanced advantages and to avoid or reduce some of the adverse effects the constituent processes produce when they are individually applied. Rajurkar and Kozak have described around 15 various processes under this category.



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3.3. Aim and Specific Objectives This report discusses the possible improvement in surface roughness and material removal rate by applying a magnetic field around the work piece in AFM. A set-up has been developed for a composite process termed magneto abrasive flow machining (MAFM), and the effect of key parameters on the performance of the process has been studied. Relationships are developed between the material removal rate and the percentage improvement in surface roughness of brass components when finishmachined by this process. 3.4. Method In almost all non-conventional machining processes such as electric discharge machining, electrochemical machining, laser beam machining, etc., low material removal rate is considered a general problem and attempts are continuing to develop techniques to overcome it. This report presents the preliminary results of an ongoing research project being conducted with the aim of exploring techniques for improving material removal (MR) in AFM. One such technique studied uses a magnetic field around the work piece. Magnetic fields have been successfully exploited in the past, such as machining force in magnetic abrasive finishing (MAF), used for micro machining and finishing of components, particularly circular tubes. Shinmura and Yamaguchi and more recently Kim et al., Kremen et al. and Khairy have reported studies on this process. The process under investigation is the combination of AFM and MAF, and is given the name magneto abrasive flow machining (MAFM). 3.5. Results & Discussion Analysis of variance (ANOVA) has been applied to identify significant parameters and to test the adequacy of the models. A magnetic field has been applied around a component being processed by abrasive flow machining and an enhanced rate of material removal has been achieved. Experimental results indicate significantly improved performance of MAFM over AFM.



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4. OVERVIEW AFM was developed in 1960s as a method to deburr, machining.

This provides

improvement in surface roughness and material removal rate, polish intricate geometries. The process has found applications in a wide range of fields such as aerospace, defence, and surgical and tool manufacturing industries. Extrusion pressure, flow volume, grit size, number of cycles, media, and work piece configuration are the principal machining parameters that control the surface finish characteristics. Recently there has been a trend to create hybrid processes by merging the AFF process with other non-conventional processes. This has opened up new vistas for finishing difficult to machine materials with complicated shapes which would have been otherwise impossible. These processes are emerging as major technological infrastructure for precision, meso, micro, and nano scale engineering. This review provides an insight into the fundamental and applied research in the area and creates a better understanding of this finishing process, with the objective of helping in the selection of optimum machining parameters for the finishing of varied work pieces in practice .MAFM is a new non-conventional machining technique .It produces surface finishes ranging from rough to extremely fine. Here chips are formed by small cutting edges on abrasive particles.The use of magnetic field around the work piece. It deflects the path of abrasive flow. Here ‘Microchipping’ of the surface is done. The various limitations of Abrasive Flow Machining are overcome like: 1. Low finishing rate. 2. Low MRR. 3. Bad surface texture. 4. Uneconomical.



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5. NON-TRADITIONAL MACHINING In present world of competition, product quality is main requirement of the customer. It is impossible to get required degree of accuracy and quality with conventional methods of machining. So it is required to move towards the application of non-traditional methods. The newer machining processes, so developed, are often called modern machining process or unconventional machining process. These are unconventional in the sense that the conventional tools are not employed for material removal. The energy in its direct or indirect form is utilized. Some of the non-traditional processes are: 1.

Electro Chemical Machining (ECM)

2.

Electro Discharge Machining (EDM)

3.

Ion Beam Machining (IBM)

4.

Laser Beam Machining (LBM)

5.

Plasma Arc Machining (PAM)

6.

Ultrasonic Machining (USM)

7.

Magnetic Abrasive Flow Machining (MAFM), etc.

These non-traditional methods cannot replace the conventional machining processes and a particular method, found suitable under the given conditions, may not be equally efficient under other conditions. A careful selection of the process for a given machining conditions is therefore essential. Furthermore, the machining process has to safely remove the material from work piece without inducing new sub-surface damages, the machining of work piece by means of magneto abrasive flow machining (MAFM) could be such a process. Unlike traditional grinding, lapping or honing processes with fixed tools, MAFM applies no such rigid tool with important advantage of subjecting the work piece to substantially lower stresses.



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6. EXPERIMENTAL SET-UP 6.1 MAFM set - up. An experimental set-up is designed and fabricated, it is shown in fig:6.1. It consisted of two cylinders (1) containing the medium along with oval flanges (2). The flanges facilitate clamping of the fixture (3) that contains the work piece (4) and index the set-up through 180° when required. Two eye bolts (5) also support this purpose. The setup is integrated to a hydraulic press (6). The flow rate and pressure acting on piston of the press were made adjustable. The flow rate of the medium was varied by changing the speed of the press drive whereas the pressure acting on the medium is controlled by an auxiliary hydraulic cylinder (7), which provides additional resistance to the medium flowing through the work piece. The resistance provided by this cylinder is adjustable and can be set to any desired value with the help of a modular relief valve (8). The piston (9) of the hydraulic press then imparts pressure to the medium according to the passage size and resistance provided by opening of the valve. As the pressure provided by the piston of the press exceeds the resistance offered by the valve, the medium starts flowing at constant pressure through the passage in the work piece. The upward movement of the piston (i.e. stroke length) is controlled with the help of a limit switch. At the end of the stroke the lower cylinder completely transfers the medium through the work piece to the upper cylinder. The position of the two cylinders is interchanged by giving rotation to the assembly through 180° and the next stroke is started. Two strokes make up one cycle. A digital counter is used to count the number of cycles. Temperature indicators for medium and hydraulic oil are also attached. 6.2 The Fixture.



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The work fixture was made of nylon, a non-magnetic material. It was specially designed to accommodate electromagnet poles such that the maximum magnetic pull occurs near the inner surface of the work piece.

6.3 The Electromagnet. The electromagnet was designed and fabricated for its location around the cylindrical work piece. It consists of two poles that are surrounded by coils arranged in such a manner as to provide the maximum magnetic field near the entire internal surface of the work piece. 6.4 The Abrasive Medium. The medium used for this study consists of a silicon based polymer, hydrocarbon gel and the abrasive grains. The abrasive required for this experimentation has essentially to be magnetic in nature. In this study, an abrasive called Brown Super Emery (trade name), supplied by an Indian company, was used. It contains 40% ferromagnetic constituents, 45% Al2O3 and 15% Si2O3.

Figure 6.1: The Workpiece



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Figure 6.2: Schematic illustration of the magneto abrasive flow machining process (1.Cylinder containing medium, 2. Flange, 3.Nylon fixture, 4.Workpiece, 5.Eye bolt, 6.Hydraulic press, 7.Auxiliary cylinder, 8.Modular relief valve, 9.Piston of Hydraulic press, 10.Directional control valve, 11.Manifold blocks, 13.Electromagnet).



Figure 6.3: Typical Machining Centre.


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7. PROCESS PARAMETERS Following process parameters were hypothesised to influence the performance of MAFM: 1. Flow rate (volume) of the medium, 2. Magnetic flux density, 3. Number of cycles, 4. Extrusion pressure, 5. Viscosity of the medium, 6. Grain size and concentration of the abrasive, 7. Work piece material, 8. Flow volume of the medium, and 9. Reduction ratio.

Table 7.1: Levels of Independent Parameters.



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7.1 Design of experiments With the help of experimental design, the effect of process variables on the output of the process and their interaction effects have been determined within a specified range of parameters. It is possible to represent independent process parameters in quantitative form as: Y ∑ f(X1, X2, X3… Xn) e, where Y is the response (yield), f is the response function; e is the experimental error, and X1, X2, X3… Xn are independent parameters. The mathematical form of f can be approximated by a polynomial. The dependent variable is viewed as a surface to which the mathematical model is fitted. Twenty experiments were conducted at stipulated conditions based upon response surface methodology (RSM). A central component rotatable design for three parameters was employed. The magnetic flux density, medium flow rate and number of cycles were selected as independent variables. The reason for choosing these variables for the model was that they could be easily varied up to five levels. MR and percentage improvement in surface roughness value (∑Rs) were taken as the response parameters. Cylindrical workpieces made of brass were chosen as the experimental specimen. An electronic balance (Metler, LC 0.1 mg) and a perthometer (Mahr, M2) were employed for the measurements of MR and surface roughness, respectively. The roughness was measured in the direction of flow of the medium. The experimental specimens were chosen from a large set of specimens in such a way that selected specimens had inherent variation in their initial surface roughness values in a narrow range. It was not possible to remove this variability completely; therefore percentage improvement in surface roughness (∑Rs) has been taken as the response parameter. The roughness values were taken by averaging the readings at several points on the surface.



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8. PRINCIPLE The volume of abrasive particles is carried by the abrasive fluid through the work piece. Abrasives are impinged on the work piece with a specified pressure which is provided by the piston and cylinder arrangement or with the help of an intensifier pump. The pressure energy of the fluid is converted into kinetic energy of the fluid in order to get high velocity. When a strong magnetic field is applied around the work piece, the flowing abrasive particles (which must essentially be magnetic in nature) experience a sideways pull that causes a deflection in their path of movement to get them to impinge on to the work surface with a small angle, thereby resulting in microchipping of the surface. The magnetic field is also expected to affect the abrasive distribution pattern at the machining surface of the work piece. The particles that otherwise would have passed without striking the surface now change their path and take an active part in the abrasion process, thus causing an enhancement in material removal. It is to be mentioned here that although the mechanical pull generated by the magnetic field is small, it is sufficient to deflect the abrasive particles, which are already moving at considerable speed. Therefore it appears that, by virtue of the application of the magnetic field, more abrasive particles strike the surface. Simultaneously, some of them impinge on the surface at small angles, resulting in an increased amount of cutting wear and thereby giving rise to an overall enhancement of material removal rate.

(a)

(b)

Figure 8.1: (a) Off-state MR fluid particles (b) Aligning in an applied magnetic field. www.123seminarsonly.com


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Figure 8.2: Principle of Material Removal Mechanism



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9. ABRASIVE MEDIUM The mainly used abrasive media is a Silicon based polymer, hydrocarbon gel and the abrasive grains.The abrasive required is essentially magnetic in nature for the proper machining process to take place. An abrasive called Brown Super Emery (trade name), supplied by an Indian company is normally used. It contains 40% ferromagnetic constituents, 45% Al2O3 and 15% Si2O3. SiC with silicon gel is also used as an abrasive media.Also diamond coated magnetic abrasives can be used to finish ceramic bars.

Figure 9.1: Mechanism of Magneto Abrasive Flow Machining



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10. MAFM MACHINES MAFM Machines are classified into 3, namely:1. One-Way Machines 2. Two-Way Machines 3. Orbital Machines 10.1 One-way machines. One way MAFM process apparatus is provided with a hydraulically actuated reciprocating piston and an extrusion medium chamber adapted to receive and extrude medium unidirectionally across the internal surfaces of a work piece having internal passages formed therein. Fixture directs the flow of the medium from the extrusion medium chamber into the internal passages of the work piece, while a medium collector collects the medium as it extrudes out from the internal passages. The extrusion medium chamber is provided with an access port to periodically receive medium from the collector into extrusion chamber. The hydraulically actuated piston intermittently withdraws from its extruding position to open the extrusion medium chamber access port to collect the medium in the extrusion medium chamber. When the extrusion medium chamber is charged with the working medium, the operation is resumed.



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Figure 10.1: Unidirectional MAFM Process 10.2 Two-way machines. Two-way machine has two hydraulic cylinders and two medium cylinders. The medium is extruded, hydraulically or mechanically, from the filled chamber to the empty chamber via the restricted passageway through or past the work piece surface to be abraded. Typically, the medium is extruded back and forth between the chambers for the desired fixed number of cycles. Counter bores, recessed areas and even blind cavities can be finished by using restrictors or mandrels to direct the medium flow along the surfaces to be finished.



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Figure 10.2: Two–way MAFM Process 10.3 Orbital machines. In orbital MAFM, the work piece is precisely oscillated in two or three dimensions within a slow flowing ‘pad’ of compliant elastic/plastic MAFM medium. In orbital MAFM, surface and edge finishing are achieved by rapid, low-amplitude, oscillations of the work piece relative to a self-forming elastic plastic abrasive polishing tool. The tool is a pad or layer of abrasive-laden elastic plastic medium, but typically higher in viscosity and more in elastic. Orbital MAFM concept is to provide transitional motion to the work piece. When work piece with complex geometry translates, it compressively displaces and tangentially slides across the compressed elastic plastic self-formed pad which is positioned on the surface of a displacer which is roughly a mirror image of the work piece, plus or minus a gap accommodating the layer of medium and a clearance. A small orbital oscillation (0.5-5 mm) circular eccentric planar oscillation is applied to the work piece so that, at any point in its oscillation, a portion of its surface bumps into the medium pad, elastically compresses (5 to 20%) and slides across the medium as the work piece moves along its orbital oscillation path. As the circular eccentric oscillation continues, different portions of the work piece slide across the medium. Ultimately, the full circular oscillation engages each portion of the surface. To assure uniformity, the highly elastic abrasive medium must be somewhat plastic in order to be self-forming and to be continually presenting fresh medium to the polishing gap.

Figure 10.3: Orbital MAFM Process (a) Before start of finishing (b) While finishing. www.123seminarsonly.com


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11. ULTRA-HIGH PRESSURE PUMPS. High pressure pumps are an alternative to create pressure. The intensifier pump creates pressures high enough for machining. An engine or electric motor is used which drives a hydraulic pump. Pressures from 1,000 to 4,000 psi (6,900 to 27,600 kPa) are achieved which is given into the intensifier cylinder. Hydraulic fluid pushes a large piston to generate a high force.The plunger pressurizes fluid to a level proportional to the relative cross-sectional areas of the large piston and the small plunger.

Figure 11.1: An ultra-high pressure pump



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12. MECHANISM OF MATERIAL REMOVAL. Solid particle erosion proposed by Finnie is considered as the basic mechanism of material removal in MAFM with some modifications. In abrasive jet machining the energy of the striking abrasive particle is imparted by the high speed of the medium stream, but in MAFM the required energy to the abrasive particles is provided by high pressure acting on the viscoelastic carrier medium. The medium dilates and the abrasive particles come under a high level of strain due to the pressure acting in the restriction. The momentum that abrasive particles acquire due to these conditions can be considered to be responsible for microploughing and microchipping of the surface in contact with the abrasive. Microploughing causes plastic deformation on the surface of the metal. Initially no material removal takes place. However, the surface atoms become more vulnerable to removal by subsequent abrasive grains. More abrasive particles attack the surface repeatedly, which causes the detachment of material often referred to as ‘cutting wear’. When a strong magnetic field is applied around the work piece, the flowing abrasive particles (which must essentially be magnetic in nature) experience a sideways pull that causes a deflection in their path of movement to get them to impinge on to the work surface with a small angle, thereby resulting in microchipping of the surface. The magnetic field is also expected to affect the abrasive distribution pattern at the machining surface of the work piece. The particles that otherwise would have passed without striking the surface now change their path and take an active part in the abrasion process, thus causing an enhancement in material removal. It is to be mentioned here that although the mechanical pull generated by the magnetic field is small, it is sufficient to deflect the abrasive particles, which are already moving at considerable speed. Therefore it appears that, by virtue of the application of the magnetic field, more abrasive particles strike the surface. Simultaneously, some of them impinge on the surface at small angles, resulting in an increased amount of cutting wear and thereby giving rise to an overall enhancement of material removal rate.



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Graph 12.1: Effect of magnetic flux density and medium flow rate on MRR

Graph 12.2: Effect of number of cycles and magnetic flux density on MRR

Graph 12.3: Effect of medium flow rate and number of cycles on MRR www.123seminarsonly.com


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13. RECENT DEVELOPMENTS

Besides Singh and Shan who applied magnetic field around the work piece in A. F. Mand observed that magnetic field significantly affect the material removal and change in surface roughness. Ravi Sankar et.al. tried to improve the finishing rate, material removal and surface texture by placing drill bit in the medium flow path called Drill Bit Guided AFM. The inner part of medium slug flows along the helical flute which creates random motion among the abrasive in inner region of the medium. This causes reshuffling of abrasive particles at outer region. Hence, comparatively more number of new and fresh abrasive grains interacts with the work piece surface. Also abrasive traverse path is longer than the AFM abrasive traverse path in each cycle. It results in higher finishing rate in DBG-AFM as compared to AFM. Material removal rate is found to decrease with decrease in drill bit diameter. Biing-Hwa Yanet.al., placed spiral fluted screw in the medium flowing path to improve surface quality. He rotated different shaped tiny rods at the centre of the medium flow path and used a low viscosity medium to finish. He concluded that the better surface finish is achieved due to centrifugal action caused by the rod on the abrasives and this process is called Centrifugal Force Assisted Abrasive Flow Machining (CFAAFM).



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14. ADVANTAGES 1. A very high volume of internal deburring is possible. 2. MAFM deburrs precision gears. 3. MAFM polishes internal and external features of various components. 4. MAFM removes recast layer from components. 5. Effective on all metallic materials. 6. Controllability, repeatability and cost effectiveness. 7. Less Time Consumption.

15. LIMITATIONS

1. Abrasive materials tend to get embedded, www.123seminarsonly.com


if the work material is ductile. 2. Require closed environment. 3. Require start up hole. 4. Mostly Magnetic materials.

16. APPLICATIONS 16.1 Automotives.


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The demand for this process is increasing among car and two wheeler manufacturers as it is capable to make the surfaces smoother for improved air flow and better performance of high-speed automotive engines. MAFM process is capable to finish automotive and medical parts, and turbine engine components. Internal passages within a turbine engine diffuser are polished to increase air flow to the combustion chamber of the engine. The rough, power robbing cast surfaces are improved from 80-90% regardless of surface complexities. 16.2 Dies and Moulds. Since in the MAFM process, abrading medium conforms to the passage geometry, complex shapes can be finished with ease. Dies are ideal workpieces for the MAFM process as they provide the restriction for medium flow, typically eliminating fixturing requirements. The uniformity of stock removal by MAFM permits accurate ‘sizing’ of undersized precision die passages. The original 2 micron ∑Rs (EDM Finish) is improved to 0.2 micron with a stock removal of (EDM recast layer) 0.025 mm per surface. 16.3

Laser Shops with materials as titanium, and steel (Thicker metal or composites).

16.4

Prototype, R&D, Maintenance and Repair Shops.

16.5

Controls Just-in-Time inventory requirements.

16.6

Metal Fabricators: Offer "clean edge" plate work.

16.7

Aerospace engine and control system components.



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Figure 16.1: Surface finish improvement before and after on (a) internal passages within turbine engine diffuser (b) medical implants (c) complete automotive engine parts.

Figure 16.2: Photomicrograph showing complete removal of EDM recast layer.

Figure 16.3: Microchiped surface of a metal.

17. CONCLUSION A magnetic field has been applied around a component being processed by abrasive flow machining and an enhanced rate of material removal has been achieved. Empirical modelling with the help of response surface has led to the following conclusions about www.123seminarsonly.com


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the variation of response parameters in terms of independent parameters within the specified range. 1. Magnetic field significantly affects both MRR and surface roughness. The slope of the curve indicates that MRR increases with magnetic field more than does surface roughness. Therefore, more improvement in MRR is expected at still higher values of magnetic field. 2. For a given number of cycles, there is a discernible improvement in MRR and surface roughness. Fewer cycles are required for removing the same amount of material from the component, if processed in the magnetic field. 3. Magnetic field and medium flow rate interact with each other .The combination of low flow rates and high magnetic flux density yields more MRR and smaller surface roughness. 4. Medium flow rates do not have a significant effect on MRR and surface roughness in the presence of a magnetic field. 5. MRR and surface roughness both level off after a certain number of cycles. MAFM is a well-established advanced finishing process capable of meeting the diverse finishing requirements from various sectors of applications like aerospace, medical and automobile. It is commonly applied to finish complex shapes for better surface roughness values and tight tolerances. But the major disadvantage of this process is low finishing rate. The better performance is achieved if the process is monitored online. So, acoustic emission technique is tried to monitor the surface finish and material removal .Various modelling techniques are also used to model the process and to correlate with experimental results. But experts believe that there is still room for a lot of improvements in the present MAFM status.

18. REFERENCES 1.

Singh S, Shan H. S, “Development of magneto abrasive flow machining process”, International Journal of machine tools and manufacture, Issue number 42 (2002), 953-959.



2.

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L.J Rhoades, Kohut T.A, Nokovich N.P, Unidirectional abrasive flow machining, US patent number 5, 367, 833, Nov 29th,1994.

3.

Gorana V.K, Lal G.K, “Forces prediction during material deformation in magneto abrasive flow machining”, Journal of manufacturing systems, Issue number 260 (2006),128-139.

4.

V.K Jain, R.K Jain, “Modeling of material removal and surface roughness in magneto abrasive flow machining process”, International Journal of Machine tool & manufacture, Issue number 39 (1999), 1903-1923.

5.

R.E Williams, “Stochastic modeling and analysis of abrasive flow machining”, Journal of Engineering for Industry, Issue number 114 (1992), 74-81.

6.

Petri K.L, Bidanda B, “A neural network process model for magneto abrasive flow machining operations, Journal of manufacturing systems, Issue number 17 (1998), 52-64.

7.

Jha S, Jain V.K, “Design and development of the magneto rheological abrasive flow finishing process”, International Journal of machine tool & manufacture, Issue number 44 (2004), 1019-1029.

8.

http://www.tnmsc.cn


MAGNETO ABRASIVE FLOW MACHINING

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