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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).
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 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.
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.
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 finish-machined by this process.
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).
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.
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.
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.
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.
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.
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.
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.
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.
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.
