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Effect of Magnetic Field on MR-Fluid in Ball End Magnetorheological Finishing

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Effect of Magnetic Field on MR-Fluid in Ball End Magnetorheological Finishing Effect of Magnetic Field on MR-Fluid in Ball End Magnetorheological Finishing Harry Garg1,Vipender Singh Negi1, Ashish Singh Kharola1, Rohit Sharma1 1Optical Devices & Systems CSIR-CSIO, Chandigarh, 160030, India *Corresponding author: Optical Devices & Systems, CSIR-CSIO, Chandigarh, 160030 India [email protected] Abstract: MRF is one of the precision to a class of smart controllable materials whose finishing processes in which magnetic field is rheological behavior can be controlled externally used to drive abrading forces for Finishing 2D by using some energy field. In the absence of and complex 3D surfaces. In this paper ball end magnetic field, an ideal MR-fluid exhibits magnetorheological finishing (BEMRF) has been Newtonian behavior and on the application of analyzed for fluid behaviors under the influence the external magnetic field, it exhibits of strong magnetic field. Polishing action in MR- magnetorheological effect. The iron particles in fluid depends on magnetization, magnetic non-magnetic fluid acquire dipole moments intensity, fluid composition and relative motion proportional to the magnetic field strength and between fluid and surface to be finished. Here when the dipolar interaction between the fluid stiffness is key functionality which is particles exceeds their thermal energy, the determined in terms of magnetorheology effect, particles aggregate into chains of dipoles aligned In most precision optics this behavior plays a in the field direction. Because energy is required predominant role in obtaining very high to deform and rupture these chains, this precision of the order of 70nm or even less. In microstructural transition is responsible for the this context viscosity of fluid has be analyzed in onset of a large controllable finite yield stress. the presence of magnetic field produced by the when the external field is removed, the particles electromagnet. As fluid viscosity is a function of return to their random state and fluid exhibits its magnetization, volume fraction and magnetic original Newtonian behavior. of MR-fluid in the intensity, they have been described to generate presence of magnetic field. the magnetorheological effect in fluid. The Field-responsive behavior of MR fluids is often represented as a Bingham plastic having Keywords: Flux density, Surface finish, a variable yield strength [3] for stresses t above Magnetorheology. the field dependent yield stress t 0 ()H the flow is governed by Bingham's equation. 1 Introduction . t= t()H + h g for t≥ t ()H Magnetorheological fluids are mainly 0 0 -3 dispersion of particles made of a soft magnetic Below the yield stress (strains of order 10 ), the material in a carrier oil. Most often MR-fluid is material behaves visco elastically. t = Gg , t< t 0 ()H made up of particles of carbonyl iron in silicone 3 oil. The characteristics of MR-fluid is 1 2 determined in terms of yield stress which t 0 (H )= 6fm 0 MHs 2 2 directly depends on the magnetic properties of t0 ()HHµ fm0 the fluid and availability of the magnetic field in . space. This yield stress represents the maximum where t is the applied shear stress, g is the of the stress versus strain. since the gel-like shear rate, G is the complex material modulus, h structure will break when the stress would have is dynamic viscosity determined by the base reached this maximum value. fluid composition, and the field-induced shear MR-fluid was invented by Rabinow [1] in stress t 0 ()H depends on the magnetic field late 1940s. Interestingly, this work was almost strength H. The strength of the fluid increases as concurrent with Willis Winslow's [2] work on the applied magnetic field increases but this electrorheological (ER) fluids. MR-fluid belong increase is non-linear since the particles are Excerpt from the Proceedings of the 2014 COMSOL Conference in Bangalore ferromagnetic in nature, and magnetization in magnetorheological finishing slurry. Unlike different parts of the particles occurs non- conventional rigid lap polishing, the MR-fluid's uniformly [4]. MR-fluids typically exhibit shape and stiffness can be magnetically dynamic yield strength of 50–100kPa for applied manipulated and controlled in real time. The magnetic field of 150–250kA/m [5]. The optic's final surface form and finishing results ultimate strength of MR-fluid is limited by are predicted through the use of computer magnetic saturation. The ability to electrically algorithms. Center for optics manufacturing manipulate the rheological properties of MR- (COM) in Rochester, N.Y. has developed fluid attracts attention from a wide range of automate MRF Process for finishing lens industries, and numerous applications are (Kordonski and Golini 1999).Since then, more explored [6][7]. These applications include number of polishing techniques are evolved shock absorbers, damping devices, clutches, using magnetorheological fluid. Few of them are brakes, actuators, and artificial joints[8]-[18]. Magnetorhelogical finishing (MRF), Magnetorheological jet finishing (MRJF), 2 Magnetorehological Finishing(MRF) Magnetorheological abrasive flow finishing (MRAFF), Magnetorheological abrasive honing Precise Finishing of internal surfaces and (MRAH), and ball end magnetorheological complex geometries has always been target for finishing (ball end MRF). achieving advance technology in various field of science. abrasive with small multiple cutting 3 BEMRF(Ball-End edges are generally employed to get desired surface finish characteristics and geometrical Magnetorehological Finishing) accuracy by removing unwanted material from BEMRF comprises of a central rotating core, the work piece. the traditional finishing stationary electromagnet coil, and copper cooling processes such as grinding, lapping and honing coils wrapped over the outer surface of the works on this mechanism of finishing. but due to stationary electromagnet coil for continuous development of new materials which are difficult cooling. The cooling medium is supplied by a to machine due to complex geometrical shapes, low temperature bath. A magnetically generated available traditional finishing processes are alone ball end finishing spot of MR polishing (MRP) not capable of producing required surface finish fluid at the tip surface of the rotating core is used and other characteristics of the product. as a finishing segment to finish the work piece Advancement in last few decades in non- surfaces. The flow of MRP fluid at the tip of the conventional machining processes has relaxed rotating core is not continuous. It means, the limitation of hardness requirement of the whenever MRP fluid is required to be cutting tool. some of these machining processes conditioned after a certain period of finishing are EDM, ECM, USM, AJM etc. Cutting of operation only then was it made to flow to the tip material using predefined motion between surface of the rotating core through a peristaltic cutting edges with respect to cutting surface pump. Otherwise, an already formed ball end imposes limit in finishing complex surfaces. to finishing spot of MRP fluid is used continuously overcome this problem, the multiple cutting for the finishing operation. edges in some loosely bonded form are directed The BEMRF tool has comparatively less to follow the intricate geometries to be finished. limitations on finishing of different work piece This process lacks controlled flow of the surfaces, as compared with regular MRF. The abrasive over the desired surface. This possess finishing spot of MRP fluid formed at the tip the limitation for finishing complex geometry surface of the central rotating core can be easily and sometimes impart surface and subsurface made reachable for the different 3D surface damages. profiles. The vertical tapered tool tip, with One such process which address the control finishing spot of MRP fluid, can be moved and and direction of abrasive bound slurry is performed finishing with the help of a computer Magnetorheological finishing (MRF). controlled program over different kinds of Magnetorheological finishing is a precision surfaces in a work piece, such as projections at surface finishing technology. Optical surfaces different angles or in-depth pocket profiles; are polished in a computer controlled whereas finishing of these surfaces in the work Excerpt from the Proceedings of the 2014 COMSOL Conference in Bangalore piece are likely to be inaccessible by a regular 4 Modeling BEMRF MRF process owing to rotating wheel size or Considering the schematics described in mechanical interferences. MR jet finishing was figure 1. The model is formed and analyzed developed to finish the internal surface of a steep using Comsol Multiphysics. Due to interaction concave and spherical dome, where the jet was of fluid and magnetic field i.e. laminar flow and impinged on the work piece surface from the Magnetic field are considered for further bottom and the work piece surface was rotated analysis. following are the governing equations relative to the MR jet. In this process, the which are used create mathematical model of relative movement of different 3D complex work BEMRF. piece surfaces, with respect to the MR jet, may have challenged the task. The newly developed 4.1.1 Laminar flow finishing process can be found in more industrial applications in the area of MRF processes. Momentum Equation é T 2 ù r(.).(()uÑ u = Ñê -p I +m Ñ u + Ñu -m(.) Ñ u Iú ë 3 û Rotation of Rotary Valve tool +F Electromagnet Ñ.(ru )= 0 Outer Core Reference Temperature 293.15K Density 2000kg/m3 Wall u=0, No slip boundary condition. Velocity field u=0, Pressure P=0 Inner Inlet P0=4bar Outlet P0=1bar Volume Force Fx=d(UB)/dx, Fy=d(UB)/dy To-and- fro 4.1.2 Magnetic Field motion of work piece Amperes law Ball end MR-fluid --1 1 Work Piece Ñ ´ ()m0 mr B- s v ´ B = J e BA= Ñ ´ DE= e 0e r BH= m0 mr Figure 1 BEMRF Tool. Magnetic Insulation In mechanism of BEMRF pressurized MRP n´ A = 0 fluid enters from the top end of the central Initial Value rotating core.
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