Magnetic field–assisted polishing is a nonconventional method in which the machining forces are controlled by a magnetic field. Consequently, finish polishing is achieved without the necessitate for expensive, rigid, ultra precision, vibration- and error-free machine tools by incorporating the magnetic polishing elements essential into the existing machine tools.
There are two types of magnetic field–assisted polishing: magnetic abrasive finishing (MAF), which uses a brush of magnetic abrasives for finish machining, and magnetic float polishing (magnetic fluid grinding), which uses magnetic fluid that is a colloidal dispersion of subordinate domain magnetic particles in a liquid carrier with abrasives. Although MAF originated in the United States during the forties, it was in the former U.S.S.R. and Bulgaria that a lot of the development took place in the late fifties and sixties. During the eighties, the Japanese followed the work and conducted research for a variety of polishing applications.
The machining system
Figure.1 shows a schematic diagram of MAF apparatus. A cylindrical work piece is clamped into the chuck of the spindle that provides the rotating motion. The work piece can be a magnetic (steel) or a nonmagnetic (ceramic) material; the magnetic field lines go through the work piece.
Fig.1 MAF schematic.
Axial vibratory motion is introduced in the magnetic field through the oscillating motion of the magnetic poles comparative to the work piece. A mixture of fine abrasives held in a ferromagnetic material (magnetic abrasive conglomerate, Fig. 2) is introduced between the work piece and the magnetic heads where the finishing procedure is exerted by the magnetic field. Typically, the sizes of the magnetic abrasive conglomerates are 50 to 100 microns and the abrasives are in the 1 to 10 micron range. With nonmagnetic work materials, the magnetic abrasives are linked to each other magnetically between the magnetic N and S poles the length of the lines of the magnetic forces, forming flexible magnetic abrasive brushes.
Fig.2 Typical magnetic abrasive conglomerates.
In order to attain uniform circulation of the abrasives, the magnetic abrasives are stirred periodically. Fox et al. (1994) adopted the following MAF conditions that caused mutually surface and edge finishing:
Roller speed Up to 1.3 m/s
Magnetic field density 0–0.53 Tesla (T)
Magnetic pressure 0–30 kPa
Abrasive type 80% Fe (40) + 20% SiC (1200)
Vibration frequency 12–25 Hz
Lubricant Dry or oil
Material removal process
MAF operates with magneto abrasive brushes where the abrasive grains arrange themselves by means of their carrying iron particles to flexibly comply with the outline of the work surface. The abrasive particles are held firmly against the work surface, while short stroke oscillatory motion is carried out in the axial work piece direction. MAF brushes contact and act upon the surface protruding elements that form the surface irregularities.
While surface defects for example scratches, hard spots, lay lines, and tool marks are eliminated, form errors like taper, looping, and chatter marks can be corrected with a limited depth of 20 microns. The material removal rate and surface finish based on the work piece circumferential speed, magnetic flux density, working clearance, work piece material, size of magnetic abrasive conglomerates including the type of abrasives used, and its grain size and quantity fraction in the conglomerate.
Fox et al. (1994) concluded that the average surface finish Ravg of a ground rod can be finished to about 10 nm. Increasing the magnetic flux density lift up the rate of finishing. High removal rates and the most excellent finish were obtained with an increase in the axial vibration amplitude and frequency. The axial vibration and revolving speed has to be taken into consideration for obtaining the best cross pattern that would give the greatest finish and high removal rate. Singh and his team (2004) recommended a high voltage level (11.5 V), low working gap (1.25 mm), high rotational speed (180 rpm), and large mesh number for improving the surface quality.
(i)Polishing of balls and rollers.
Conventional finishing of ceramic balls, for bearing applications, uses small polishing speeds and diamond abrasives as a polishing medium. The long processing time and the use of costly diamond abrasives result in high processing costs. Diamond abrasives at high loads could result in deep pits, scratches, and micro cracks. Consequently, the high processing cost and the lack of the machining system consistency form possible limitations. To minimize the surface damage, mild polishing conditions are required, namely, low levels of controlled force and abrasives not much harder than the work material.
A modern development in MAF involves the use of a magnetic field to support abrasive slurries in polishing ceramic balls and bearing rollers (Fig. 3). A magnetic field, containing abrasive grains and very fine ferromagnetic particles in a certain fluid such as water or kerosene, fills the chamber within a guide ring. The ceramic balls are between a drift shaft and a float.
The abrasive grains, ceramic balls, and the float (made from nonmagnetic material) are suspended well by the magnetic forces.
Fig.4 Magnetic finishing of nonmagnetic tubes.
The balls are preset against the rotating drive shaft and are polished by the mechanical abrasion action. Since the forces, which are applied by the abrasive grains, are extremely small and controllable, the polishing action is very fine. The process is economical, and the surfaces produced have little or no defects.
(ii)Finishing of inner tube surface.
Clean gas and liquid piping systems required to have highly finished inner surfaces that prevent contaminant from accumulating. When the pipe is slender, it is tough to produce smooth inner surfaces in a cost-effective way. Electrolytic finishing has many problems associated with the high cost of controlling the process conditions and disposing of electrolyte exclusive of environmental pollution. Figure.4 shows the two-dimensional schematic outlook of the internal finishing of a non-Ferro magnetic tube using MAF.
Fig.5 Magnetic finishing of magnetic tubes.
The magnetic abrasives, inside the tubes, are converged toward the finishing zone by the magnetic field, produce the magnetic force needed for finishing. By rotating the tube at a superior speed, the magnetic abrasives make the inner surface smoother. Figure.5 shows the case of ferromagnetic tube finishing where the magnetic fluxes frequently flow into the tube (instead of through the inside of the tube) due to their high magnetic permeability. Under such conditions, the abrasives hardly remain in the finishing zone when the tube is rotated. Geskin et al. (1995) achieved mirror finishing and eliminated burrs without lowering the accuracy of the shape.
Other MAF applications.
The process can be applied in several other fields, as described by Khayry (2000), Umehara et al. (1997), and Hitomi and Shinmura (1995):
1. Polishing of fine components such as printed circuit boards
2. The removal of oxide layers and protective coatings
3. Chamfering and deburring of gears and cams
4. Automatic polishing of complicated shapes
5. Polishing of flat surfaces.