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Magnetic Abrasive Finishing (MAF) is an advanced finishing method, which improves the quality of surfaces and performance of the products. Surface is finished by removing the material in the form of microchips by abrasive particles in the presence of magnetic field. The material is removed in such a way that surface finishing and deburring are performed simultaneously with the applied magnetic field in the finishing zone. The mechanism of super finishing in any finishing process is widely focused by the knowledge of forces involved in the process.
This paper deals with the detailed parametric study in super finishing of stainless steel SUS 304 thick steel tube. Statistically designed experiments based on Taguchi methods show that weight of abrasive, Revolutions per minute (rpm), magnetic abrasive diameter (Mesh No.) and finishing time have significant effect on the surface roughness obtained. Analysis of experimental data showed that change in surface roughness (ΔRa) was highly influenced by mesh number followed by percentage weight of abrasives, rpm of permanent magnet, and finishing time.
Key Words: Magnetic abrasive finishing (MAF), Design of experiments, Surface roughness.
Fine surface finish is in high demand in a wide spectrum of industrial applications. An internal magnetic abrasive finishing process was proposed for producing highly finished inner surfaces of work pieces. It is difficult to finish advanced engineering materials with high accuracy, and minimal surface defects such as micro cracks, by conventional grinding and polishing techniques. To minimize the surface damage, gentle/flexible finishing conditions are required, namely, a low level of controlled force. Magnetic field assisted manufacturing processes are becoming effective in finishing, cleaning, deburring and burnishing of metal and advanced engineering material parts. Magnetic abrasive finishing (MAF) is one of the non conventional machining processes which came to the surface in 1938 in a patent by Harry P. Coats.
The countries which are involved in the study and development of this process are USA, France, England, Bulgaria, Japan and Germany. In modern time, fine surface finish is in high demand with the development of industry manufacturing technology, in a wide range of industrial applications. A relatively new finishing advanced machining process in which cutting force is primarily controlled by the magnetic field and it can achieve highly finished surfaces that conventional techniques never achieve. MAF is a fine finishing technique which can be employed to produce optical, mechanical, and electronic components with micrometer or sub micrometer form accuracy and surface roughness within nanometer range with hardly any surface defects.
Finishing of bearings, precision automotive components, shafts, and artificial hip joints made of oxide ceramic and cobalt alloy are some of the products for which this process can be applied. This process can be used to produce efficiently good surface quality on at surfaces as well as internal and external surfaces of tube type work pieces. The method can, not only machine ferromagnetic materials such as steel, but can also machine non ferromagnetic materials such as stainless steel, aluminium and brass. The Abrasives generally rely upon a difference in hardness between the abrasive and the material being worked upon, the abrasive being the harder of the two substances. Shinmura et al. prepared two types of magnetic abrasives by sintering. In first sample, diameter of iron particle was varied and in second, diameter of abrasive particle varied.
They reported that diameter of iron particle effects both stock removal and surface finish. Influence of the diameter of abrasive particle on stock removal was comparatively small while surface roughness was remarkably affected. Shinmura et al. studied the effects of different machining parameters like magnetic flux density, vibration frequency and amplitude, machining time and pole-work gap on finishing characteristics using sintered magnetic abrasives. They concluded that the two parameters vibration and magnetic flux density remarkably affects the finishing efficiency. Yamaguchi and Shinmura studied the application of MAF for finishing of the inner surfaces of alumina ceramic components using diamond based magnetic abrasives.
The experiments performed on alumina ceramic tubes examined the effects of volume of lubricant, ferrous particle size, and abrasive grain size on the finishing characteristics. Shinmura concluded that with the increase in rotational speed of magnetic pole, the metal removal rate increases. They almost keep a linear relationship under given experimental conditions. The metal removal of brass work piece was highest with the artificial abrasives (alloyed Titanium Carbide and Iron). The diamond abrasives were mixed with iron to form diamond magnetic abrasives by Shinmura and Aizawa. He concluded that finishing efficiency increased with diamond magnetic abrasives with increase of speed of tool. He used them for finishing of ceramics to get high surface finish.
Results showed that machining depth increases with increase of mixing weight percentage of iron particles. Kim has developed a new type of magnetic abrasive composed of WC/Co sintered particles for cleaning the tubes, and found the optimal finishing parameters. Khairy prepared magnetic abrasives by blending of Al2O3 (15%) and iron powders (85%), compacting them by a bench press, sintering the mixture in a furnace at 1400 °C in an inert environment, crushing the compacts into small particles and then sieving to different ranges of sizes. The finishing of silver steel bars was studied with these sintered magnetic abrasives for various combinations of finishing parameters. Yamaguchi et al. proposed the finishing of SUS304 stainless steel bent tubes using aluminium oxide composite magnetic abrasive with a mean diameter of 80 μm.
It contains Al2O3 with grain size less than 10 μm sintered with iron in an inert gas atmosphere with high pressure and temperature. A two phase finishing process controlling the size of the ferrous particles was proposed to achieve efficient fine surface finishing. In particular, the use of 150 μm iron particles after 330 μm iron particles was found to be effective. Jain et al. finished stainless steel workpiece material (non-ferromagnetic) and observed that working gap and circumferential speed are the influential parameters affecting the material removal and surface roughness value. Mori has studied the process mechanism by using sintered magnetic abrasive particles. Lin et al. prepared sintered magnetic abrasives by typically mixing iron powder and Al2O3 powder with composition of 60:40 of wt% and compressing mixture into the cylindrical shape.
These compacts were sintered into a vacuum furnace. After sintering process, these cylinders were crushed to produce magnetic abrasives. These abrasives were used for the finishing of non-ferromagnetic material, SUS304. Wang and Hu studied on the inner surface finishing of tubing by magnetic abrasive finishing. They used three kinds of work materials i.e. Ly12 aluminium alloy, 316L stainless steel and H62 brass. They concluded that material removal rate of brass is highest amongst all three materials. Khangura et al. highlighted major existing technologies that are used to manufacture magnetic abrasives. They stated that amongst all the available varieties of magnetic abrasives, the sintered magnetic abrasives give highest surface finish on most of the work materials.
Processing principle and experimental setup
The poles, which consist of small permanent magnets ( six in number), placed inside the SUS 304 tube generate the magnetic field needed for attracting the magnetic abrasive to the finishing area by magnetic force. When the poles rotate inside the tube, the magnetic abrasive, driven by magnetic force, rotates along the inner surface of the tube along with the poles, and removes material from the surface. Manipulating the rotating poles along the tube axis causes the magnetic abrasive to follow the poles’ motion, finishing the entire inner surface of the tube.
The permanent magnets are fixed in the rotating disc by making six holes in the rotating disc and fixing the rare earth type magnets in these holes with the help of Epoxy resin. A schematic of experimental set up is shown in figure 1, which describes the principles of internal finishing. The space between work piece and permanent magnet is kept constant. The magnetic field strength depends upon weight percentage of the magnetic particles, present in the magnetic abrasive powder. Both the working gap and size of the workpiece are taken into consideration, while designing. A Magnetic abrasive particle through magnetic pressure finishes the work piece. Diamond based mixed abrasives are used as magnetic abrasives in this work.
FIG 1 RADIAL DRILL MACHINE WITH MAF SET UP
Internal magnetic abrasive finishing
An experimental setup is installed on radial drilling machine of varying rpm, consisting of permanent magnets (6 in no.s), work piece, working table, x-y stage. The permanent magnets fixed in the spindle can rotate at high speed as that of the rpm (varying) of rotating spindle of radial drill machine, while the work piece is fixed on the working table of the machine. When mixed type magnetic abrasives are introduced into the gap between the magnetic pole and the work piece, they are magnetically attracted by the magnetic pole and thus rotate following the magnetic pole.
Before starting experiments, the work piece surface finish and its mass are measured . The work piece is fixed between the jaws of bench vice of radial drill machine. The magnets and work piece are shown in Fig.1. The working gap is kept constant during experimentation. The magnetic abrasive powder, which is prepared just before each test by adding the lubricant, is fed to the finishing zone. The rotational motion to the permanent magnets is given through spindle of the radial drill machine. The finishing operation is continued for 300 s, and it is monitored with a stopwatch (0.01 s accuracy) to replace a part of the used magnetic abrasive powder in the finishing zone by the homogeneously mixed fresh (unused) powder. The work piece is taken out from its holder once the finishing operation is over.
After cleaning the work piece, its surface finish and mass are measured. Surface finish is measured using a digital surface roughness instrument having a least count of 0.1 µm (cut off length = 0.8 mm). Mass of the work piece was measured using electronic balance having an accuracy (or the error associated with a data point) equal to 0.001 g. The MAPs are joined to each other magnetically between magnetic pole and work piece along the lines of magnetic force. Ferromagnetic and abrasive particles are held together by the magnetic field, in the form of flexible magnetic abrasive brush.
The abrasive particles of the flexible magnetic abrasive brush remove the peaks of the irregularities on the surface of the work piece being finished. Let us understand, how loosely bounded MAPs do finishing (or cutting). To hold together the mixture of magnetic and abrasive particles for a longer time period as compared to unbounded MAPs, lubricating oil (5 wt.%) is added to the mixture of ferromagnetic and abrasive particles. After addition of oil, mixture forms conglomerate. The conglomeration helps the MAPs in staying in weak bounded condition in the working gap in the initial stage. This conglomeration increases effective working time of MAPs before replacement. Only those abrasive particles which are in direct contact with the rotating work piece surface, remove material by shearing in the same way as done in AFM.
It is envisaged that some abrasive particles get blunt and if these abrasive particles (entrapped between the ferromagnetic particles) continue to rub (without cutting) against the work piece surface, improvement in the surface finish becomes slow. Since the abrasive particles in contact with the work piece surface wear out, the force required for cutting increases and it may exceed the holding force acting on the conglomerate (or the abrasive particles loosely held between the ferromagnetic particles). As a result, such abrasive particles are dragged towards the bottom side of the steel tube work piece where magnetic field is very weak.
It is also envisaged that due to heat (heat generated due to friction between the abrasive particles and work piece, and due to machining/finishing of work piece), the lubricating oil (weak bounding material) will evaporate and it will further weaken the bounding between the abrasive and ferromagnetic particles. When such weakly bounded (or unbounded) abrasive particles reach towards the bottom of the work piece, they fall down in the collecting tray due to the combined effect of gravitational force as well as centrifugal force. Fresh powder mixture is constantly added to maintain its finishing efficiency.