Limitation of the flexibility and motion of the tool configuration make lapping and polishing not practical for the treatment of complex surfaces, such as sharp corners, deep recesses, sharp projections, free-form surfaces, and interiors of complicated components.
Components used in critical applications in the aerospace, biomedical, and semiconductor industries require highly finished surfaces to achieve their desired surface functions, and many of these are currently accomplished manually despite increasing production costs. In fact, some of these are virtually unreachable by conventional techniques, and the lack of finishing technology acts as an obstacle to the technology innovation.

Abrasive Flow Machining (AFM)

AFM Process

Abrasive flow machining (AFM) was developed by Extrude Hone Corporation, USA, in 1960. Three types of AFM machines that have been reported in the literature are one-way AFM, two-way AFM, and orbital AFM. The most common used AFM is two-way AFM in which two vertically opposed cylinders extrude medium back and forth through passages formed by the workpiece and tooling (Fig. 4).
AFM is used to deburr, radius, and polish difficult to reach surfaces by the extrusion of an abrasive-laden polymer medium with very special rheological properties through the workpiece repeatedly. It is widely used in finishing processes to finish intricate shapes and profiles (Loveless et al. 1994; Raju et al. 2005). The polymer abrasive medium used in this process possesses easy flowability, good selfdeformability, and fine abrading capability. Layer thickness of the material removed is of the order of about 1–10 μm. The best surface finish that has been achieved is 50 nm and tolerances are ± 0.5 μm. In this process, tooling plays a tremendous role in the finishing of material; however, hardly any literature is available on this kind of process. In AFM, deburring, radiusing, and polishing are performed simultaneously in a single operation on various areas including conventionally inaccessible areas, and it can produce true round radii even on complex edges.
AFM reduces surface roughness by 75–90 % on cast and machined surfaces. It can process dozens of holes or multiple passage parts simultaneously and achieve uniform results. For example, air cooling holes on a turbine disk and hundreds of holes in a combustion liner can be deburred and radiused in a single operation.
AFM maintains flexibility while capable to perform jobs which require hours of highly skilled hand polishing in a few minutes; AFM produces uniform, repeatable, and predictable results on an impressive range of finishing operations. An important feature which differentiates AFM from other finishing processes is that it is possible to control and define the intensity and location of abrasion through fixture design, medium selection, and process parameters. It has wide applications in many areas such as aerospace, dies and moulds, and automotive industries.

Magneto-rheological Abrasive Flow Finishing (MRAFF)

Magneto-rheological abrasive flow finishing (MRAFF) was developed by Jha et al. in 2004 as a new precision finishing process using MR fluid for complicated geometries (Jha et al. 2007). This process employs determinism and in-process controllability of the rheological behavior of an abrasive-laden medium to finish intricate shapes. The desired properties of the base MR fluid used in MRAFF are as follows:

1. The fluid should be thermally stable and should have a high boiling point
2. It should be noncorrosive and nonreactive with the employed magnetic and abrasive particles

The representative MR fluid slurry consists of carbonyl iron particles (6 μm in mean diameter: 20 vol.%), silicon carbide abrasive (mesh size #800, #1,000, #1,200, or #1,500: 20 vol.%), and organic medium (60 vol.%).
In the MRAFF process, magnetically stiffened slug of MR polishing fluid is being extruded back and forth with a piston through or across the passage formed by the workpiece and fixture. Selective abrasion occurs only where the magnetic field is applied across the workpiece surface while keeping the other areas unaffected.
The schematic diagram of the process is shown in Fig. 5. Looking into the rheological behavior of the polishing fluid, it changes from nearly Newtonian to Bingham plastic and back when entering, traversing, and exiting the finishing zone, respectively. The abrasive cutting edges, which are held by carbonyl iron chains, rub the workpiece and shear the peaks away from its surface. The bonding strength of the field-induced structure of the MR polishing fluid and the extrusion pressure applied through the piston determine the amount of material sheared from the workpiece surface peaks by the abrasive grains.

Electromagnetic Field-Assisted Machining

Magnetic Field-Assisted Finishing (MFAF)

The combination of a magnetic field with the mechanical action of a magnetic tool against a workpiece gives rise to the magnetic field-assisted finishing (MAF) process (Ko et al. 2003; Yin and Shinmura 2004; Yamaguchi et al. 2007). The magnetic tools can be introduced into areas that are hard to reach by conventional technologies and by means of magnetic manipulation; they exhibit relative motion against the workpiece surface needed for finishing. This shows potential for overcoming problems associated with more conventional finishing processes.
Figure 6 shows the schematic diagram of MAF. The magnetic abrasive (magnetic tools) consists of iron particles and Al2O3 abrasive grains. The composite ingot is produced from the thermite process using aluminum powder and iron oxide powder. Subsequently, the ingot is then mechanically crushed and sieved to form the finished magnetic abrasive. The Al2O3 grains are contained both inside and outside of the resulting magnetic abrasive.

In a magnetic field, ferrous particles (including magnetic abrasive) suspended by magnetic force are linked together along the lines of magnetic flux. When the magnetic flux flows unimpeded through the nonferrous workpiece material, it might influence the motion of a ferrous particle – even if the particle is not in direct contact with a magnetic pole – and the magnetic field can be controlled from outside. The ferrous particle chains connected by magnetic force allows a flexible configuration, and given this unique behavior of the ferrous particles, it enables the application of the finishing operation to easily accessible surfaces and also to areas that are hard to reach by means of conventional mechanical techniques.

Float Polishing Using Magnetic Fluid

Magnetic fluids (MFs) were developed by Papell in 1965 to magnetically control fuel flow for the Apollo project in the zero gravity conditions of space (Papell 1965). They are found in applications such as seal components and are routinely used in voice coils, dampers, and rotary brakes.
MFs are stable colloidal suspensions of permanently magnetized particles, such as magnetite. This stability comes from the Brownian motion which keeps the particles, which are about 10 nm in diameter, from settling under gravity, and at the same time, a surfactant covers each particle to create short-range steric repulsion between particles, which prevents particle agglomeration in the presence of nonuniform magnetic fields.
Application of a MF to polishing and finishing processes was undertaken by Kurobe and Inamaka in 1983 (Kurobe et al. 1983). The resultant process, called magnetic field-assisted fine finishing, was developed as a new lapping technique for the controllable finishing of materials, such as semiconductors and ceramics in the electronics and precision machinery fields. Figure 7 shows the schematic diagram of magnetic float polishing. The MF engulfs the groove cut in the brass disk, and the polisher (a 1 mm thick rubber sheet) covers the magnetic fluid-filled groove. After which, the water-based polishing compound was supplied over the polisher. When DC voltage is applied, electromagnets placed above and below the disk create a magnetic field. As shown in Fig. 7, the polishing pressure is generated by the ferrous particles, which are attracted by the magnetic field, and pushes the polishing compound (via the polisher) against the work surface. The polishing action results when the upper pole, which is connected to the workpiece, and the disk rotate in opposite directions. This method has been experimentally proven that flexible surface finishing is possible, and the surface roughness and stock removal rate can be controlled by changing the current to the electromagnet.

Magneto-rheological Finishing (MRF)

The initial discovery and development of magneto-rheological fluids (MR fluids) and devices is credited to Jacob Rabinow, who studied them at the US National Bureau of Standards (now named the National Institute of Standards and Technology (NIST)) in the 1940s (Rabinow 1948). MR fluids typically consist of micronsized, magnetically polarizable particles dispersed in a carrier medium; some common media are mineral oil or silicone oil. Particle chains form when a magnetic field is applied. As a result, the viscosity of the MR fluids apparently increases (Kurobe et al. 1983).
The MRF process was firstly introduced commercially in 1998, and the manufacture of precision optics has since changed dramatically (Tricard et al. 2003). The developed process makes use of polishing slurry based on MR fluid, which can be mixed, pumped, and conditioned in their liquid state; however, in the presence of an applied magnetic field, it causes a viscosity change to a semisolid state creating a stable and conformable polishing tool. A typical composition of an MR fluid is 36 % carbonyl iron, 6 % abrasive (cerium oxide), 3 % stabilizer, and 55 % water.
Figure 8 shows the schematic diagram of the MRF processing principle. A workpiece is fixed at some distance from a moving surface, in order to ensure that the workpiece surface and the moving surface form a converging gap. In the area around the gap, a nonuniform magnetic field is generated when an electromagnet is placed below the moving surface. The MR fluid is delivered to the moving surface just above the electromagnetic poles and then pressed against the surface by the magnetic field gradient making the fluid a Bingham plastic before it enters the gap. Thereafter, the shear flow of plastic MR fluid flows through the gap, resulting in the development of high stresses in the interface zone and thus, as a result, material removal occurs over a portion of the workpiece surface.

The process can make significant improvements to the surface roughness, and flatness has since been adopted by major manufacturers of precision optics. In the aspect of both form accuracy and micro-roughness, the MRF process has demonstrated the ability to produce optical surfaces to tight tolerances. The surfaces, including aspheres, can be made with materials ranging from glass/glass ceramics (including fused silica, ULE, and Zerodur) to single-crystalline materials (including silicone and calcium fluoride) or polycrystalline materials (including SiC). 

Internal Surface Finishing Using Magneto-rheological Fluid-Based Slurry

A new type of slurry – referred to as MRF-based slurry – was developed in 2006 by Yamaguchi and Sato (Sato et al. 2007) and is especially appropriate for the internal finishing process of piping systems in micro- and nanotechnological industrial devices. The major feature of the MRF-based slurry is that the abrasives are smaller than the iron particles so that the iron particles can trap as many abrasive particles as possible.
The processing principle is illustrated in Fig. 9. The MRF-based slurry is introduced inside the work and is attracted to the finishing area by the field generated by magnetic poles (e.g., electromagnetic coils or permanent magnets). When the work is rotated and oscillated along its axis, there is relative motion between the abrasives and the work surface, and the entire inner workpiece surface can be finished. Under typical finishing conditions, the MRF-based slurry experiences a magnetic force, a centrifugal force, a friction force against the work surface, and the force of gravity; these combine and result in the dynamic behavior shown in Fig. 9b. This behavior continuously displaces the cutting edges of the abrasives in the MRF-based slurry, which improves the finishing efficiency.

Magnetic Compound Fluid Finishing (MCF)

Magnetic compound fluid (MCF), which basically consists of MF and MR fluid, was developed by Shimada et al. in 2001 as an intelligent fluid (Shimada et al. 2008). In the same conditions, the apparent viscosity of MF in a magnetic field is lower than that of MR fluid. However, the stability of the particle distribution in MF is better than in MR fluid. Therefore, by changing the mixing ratio of MF and MR fluid, the apparent viscosity and the particle distribution stability in MCF can be altered. This characteristic must be the greatest advantage of MCF. The structure of MCF is made up of chain-shaped magnetic clusters consisting of magnetic particles of different sizes. Magnetic particles of MF (a few nm in diameter) surround the magnetic particles of MR fluid (a few μm in diameter), and long clusters of magnetic particles are formed in the process.
Research into the application of MCF to float polishing has been conducted from 2002 (Shimada et al. 2008; Sato et al. 2010). Figure 10 illustrates schematic diagram of MCF float polishing and chain-shaped clusters consisting of MCF mixed with abrasive. As the MCF is subjected to a magnetic field, chain-shaped magnetic clusters composed of nanometer-sized magnetic particles and micrometer-sized magnetic particles are thus created along the lines of magnetic flux. The dimensions of the clusters depend on the composition of the MCF, the strength of the magnetic field, and the method by which the field is applied. The nonmagnetic abrasive particles are trapped by the clusters or distributed between clusters, and to increase the viscosity, alpha cellulose fiber has been used and interspersed with the clusters. When the magnet is moved, the clusters and abrasive exhibit relative motion against the work surface, and material removal by the micro-cutting action of the abrasive particles can be observed. This process can be diversely applied to free-form metal surfaces, ceramics, glasses, resin surfaces, inner surfaces of tubes, etc.