Magnetic field-assisted finishing

MAF was initially developed as a machining process in the US in the 1930s, with the first patent in the 1940s. University research in the Soviet Union, Bulgaria, Germany, Poland, and US began in the 1960s with practical usage appearing by the 1980s and 1990s. The growth of the semiconductor, aerospace, and optics industries have resulted in the continued development of better methods for attaining high form accuracy and surface integrity.[2]

Magnetic Assisted Finishing or "MAF" is essentially the manipulation of a homogeneous mixture of magnetic particles and abrasive particles with a magnetic field to impart a machining force on a workpiece. Relative motion between the particle mixture and the workpiece surface result in material removal. Since MAF does not require direct contact with the tool, the particles can be introduced into areas which are hard to reach by conventional techniques. Additionally careful selection of magnetic particles and abrasive particles give rise to surface texture and roughness control that was previously impossible especially for hard to access areas.[2]

The magnetic field source in MAF is typically an electromagnet or a rare earth permanent magnet. A permanent magnet offers high energy density, lack of overheating resulting in a constant flux density, low cost, ease of integration into existing CNC equipment, and simplicity. Some applications require adjustment of the flux density during finishing, or require a switching magnetic field, which is only attainable with an electromagnet since the magnetic field in a permanent magnet cannot simply be switched off.

Relative motion between the magnetic/abrasive particle mixture and the workpiece is essential for material removal. There are several options for achieving the necessary motion. A common setup is the rotation of the magnetic pole tip. This is done by either rotating the entire permanent magnet setup or by rotating only the steel pole. Another method which is commonly utilized in internal finishing is the rotation of the workpiece, this is unfortunately limited to axial symmetric workpieces. In addition to rotational motion there is oscillatory and vibrational configurations that are applicable.

Start with the common expression for force on a magnetic dipole moment in a magnetic field,

${\displaystyle {\vec {F}}=\nabla ({\vec {m}}\cdot {\vec {B}})}$

From here, make the assumption that the moment of the magnetic particle is co-linear with the applied field. This is a reasonable assumption given the small size and high susceptibility of the magnetic particles. So the equation becomes,

${\displaystyle {\vec {F}}=m\nabla B}$

Using the following identities to obtain a more usable equation to describe the force experienced by a single magnetic particle,

${\displaystyle m=MV}$

${\displaystyle M=H_{k}\chi }$

${\displaystyle B=\mu _{0}H_{a}}$

Substituting the above definitions into the magnetic force equation yields,

${\displaystyle {\vec {F}}=\mu _{0}\chi VH_{k}\nabla H_{a}}$ [2]

where,

• ${\displaystyle H_{k}}$ is the maximum applied field for saturation of the magnetic particle
• ${\displaystyle H_{a}}$ is the applied magnetic field intensity
• ${\displaystyle B}$ is the magnetic flux density
• ${\displaystyle M}$ is the magnetization of the particle, assumed to be saturated
• ${\displaystyle m}$ is the magnetic dipole moment
• ${\displaystyle \nabla H}$ is the magnetic field gradient
• ${\displaystyle V}$ is the volume of the particle (assuming sphere shaped)
• ${\displaystyle \chi }$ is the material magnetic susceptibility
• ${\displaystyle \mu _{0}}$ is the permeability of free space

The combination of tangential force and normal forces exerted by the brush onto the workpiece is theorized to remove material from the top peaks of the surface asperities. This process is repeated as the contact between the brush and the surface continues during the finishing operation. Over time the surface roughness of the workpiece surface reaches a minimum value, this is due to the physical limitations of the current finishing setup. Specifically the selection of iron particles and abrasive particles dictates the minimum surface roughness that can be achieved. As the surface roughness decreases smaller abrasive particles are necessary to continue material removal.

MAF is capable of achieving roughness values ranging from 200 μm Ra down to 1 nm Ra with ease, demonstrating the degree of customization available to a MAF setup. The particle sizes for the magnetic particles in the brush dictate the finishing force which is governed by the magnetic force on a particle equation. however increasing particle size has adverse effects such as the inability to hold small abrasives and the presence of air gaps as a result of a larger packing factor. In order to alleviate these problems it is common practice to mix the magnetic particles with both large and smaller particles to "fill" the "holes" of the brush, the small particles effectively coat the larger particles within the particle chain. Close control of the surface texture and roughness can be manipulated through the selection of the right abrasive size and oscillation speed and spindle rpm. Generally speaking the faster the motion of the brush the more dense the finishing marks on the surface and the higher the surface roughness.[2]

MAF can be divided into three main categories, each defined by the type of magnetic particles utilized in the finishing operation. Each type has its specific niche that it may fulfill better than its counterparts therefore knowing the application of the process is key to selecting the proper finishing operation. The different MAF processes are listed in increasing surface roughness resolution while decreasing in applied force. This is primarily due to the reduction in iron particle size from one type of finishing to the next. These processes are just general terms and examples for some MAF setups, it is import to note that each of these process' have different variations to increase to applicability to other workpieces.

• Able to attain wide range of surface characteristics by careful selection of magnetic particles and abrasive particles
  • Roughness values from 200 um - 1 nm
  • Texturing
    • Enhance surface characteristics such as wettability or reducing friction
• Capable to accessing hard to reach areas
• Capable of modifying roughness without altering form
• Setup is independent of workpiece material
  • Can efficiently finish ceramics, stainless steels, carbides, coated carbides, and silicon
  • Flexible application of force and even pressure distribution reduce cost of assembly
    • Vibrations in the machining center and machining tool are not transmitted onto the workpiece surface

• Can be difficult to scale up to mass production operation
• Not as applicable to some "ordinary" finishing tasks where conventional finishing techniques can be used

• Kalpakjian S, Schmid S, "Manufacturing Processes" 5th Edition Pearson 2008
• Yamaguchi H, Sato T, "Polishing and Magnetic Field-Assisted Finishing" Intelligent Energy Field Manufacturing Interdisciplinary Process Innovations 2012
• Mori T, Hirota K, Kawashima Y, "clarification of magnetic abrasive finishing mechanism" Journal of Materials Processing Technology 2003
• Graziano A, Ganguly V, Yamaguchi H, "Characteristics of cobalt chromium alloy surfaces finished using magnetic abrasive finishing" ASME 2012
• Yamaguchi H, Riveros R, Mitsuishi I, Ezoe Y, "Magnetic field-assisted finishing of micropore X-ray focusing mirrors fabricated by DRIE" CIRP manufacturing technology 2010
• Yamaguchi H, Shimura T, "Study of the surface modification resulting from an internal magnetic abrasive finishing process" Wear 1999
• Rusetski A, Mokeev A, Korobko E, "Formation of a layer of magnetorheological fluid on the surface of the moving object in the gradient magnetic field" Journal of physics 2013
• Umehara, N., MAGIC polishing, Journal of Magnetism and Magnetic Materials, 252, 341-343, 2002.