Surface Roughness of Optical Glass Under Magnetorheological Finishing

Magnetorheological finishing (MRF) is a novel optical machining technology. The mechanism of MRF is introduced, the polishing experiment of optical glass of MRF is conducted through changing the polishing parameters such as magnetic field intensity, content of polishing abrasive and rotation speed of polishing wheel. Through proper controlling of the finishing parameters, the polished surface roughness of optical glass is Ra 1.132 nm measured by an AFM. Introduction Magnetorheological finishing is an advanced ultra-precision machining technology. It was developed by W. I. Kordonski etc. in 1990s. Owing to its controllable and flexible polishing process in MRF, subsurface damage couldn’t be produced on the workpiece, and the polishing process can be easily controlled by a computer. The problem of poor efficiency of traditional aspheric optical polishing can be effectively solved. High quality surface of optical glass can be achieved by controlling the parameters in MRF. MRF can achieve the manufacturing request of optics in aviation, space and defense area and has a broad prospect of application. As a kind of smart material, the viscosity of magnetorheological fluid (MR fluid) can be continuously changed with the change of magnetic field intensity. The MR fluid in the magnetic field stiffens quickly, then returns to its original fluid state when left the field, accordingly the shear stress of it can change from hundreds to over one thousand times. So MR fluid is a kind of controllable medium and a large number of applications of this smart material have been widely used. According to the properties of the controllable character of MR fluid with the magnetic field intensity, the MR fluid dispersed with appropriate amount of fine abrasives is used as an actuating medium to polish hard and brittle materials such as optical glass. High precision and ultra smooth surface can be achieved with MRF [1]. The material removal rate in MRF can be controlled by altering the magnetic field intensity. In addition, the polishing properties and machining path can be controlled in process. This makes it possible to finish aspheric apparatus of optical glass with high efficiency and precision. In the process of MRF, MR fluid dispersed with certain concentration of fine abrasive is injected to the region between the workpiece surface and the rotating wheel, at the same time a controllable high gradient magnetic field is imposed to this region. The viscosity of the injected MR fluid alters in a very quick time and changes to a bulging ribbon with high shear stress. The gradient magnetic field makes the Fe particle settle near the underside of the MR fluid, the abrasive particle float on the upside on the other hand. The fine abrasives of the MR fluid are uniformly dispersed on the surface of the bulging ribbon, and form a continuous rotating polishing wheel which acts as a polishing tool in free abrasive polishing. The bulging ribbon can achieve precision minor removal of the workpiece [2-3]. The principle of MRF machining process is shown in Fig.1. The workpiece is positioned above a moving surface that supports and carries a shaped ribbon of MR fluid into and through the polishing zone. The DC field from an electromagnet, which located just below the carrier surface and centered under the workpiece, stiffens the ribbon before it contacts the part. Two machine configurations can be set up: one is a rotating aluminum plate with a shallow trough along its rim. The other is the vertical wheel configuration used in this experimental MRF machine. Fig.2 is the schematic diagram of the model machine. MR fluid is pumped from the stirring Key Engineering Materials Online: 2004-03-15 ISSN: 1662-9795, Vols. 259-260, pp 662-666 doi:10.4028/www.scientific.net/KEM.259-260.662 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-13/03/20,19:03:49) Journal Title and Volume Number (to be inserted by the publisher) 663 device up to the nozzle, where it is emitted onto the polishing wheel. The rotation of the wheel takes the fluid under the workpiece, where it is acted upon by the magnetic field. The magnetic field stiffens the ribbon and removal is occurred in this region. Then MR fluid is collected back to the stirring device through the recovery device [4-5]. Fig.1 Schematic of MRF Fig.2 MRF model machine set-up Experimental Conditions The surface quality of optical glass has been studied by experiment through changing polishing parameters such as magnetic field intensity, content of polishing abrasive and rotation speed of the polishing wheel. The influence pattern is discussed as well. The polished surface of optical glass is measured and analyzed using an AFM. In the experiment, the workpiece is K9 optical glass, the experiment is finished on the own developed MRF experimental machine. Results and Discussion The Effect of Magnetic Field Intensity. In the effect of magnetic field intensity on MRF surface roughness experiment, the speed of polishing wheel is 85 rpm, the speed of workpiece is 1000 rpm, the gap between workpiece and polishing wheel is 1.5 mm and the MR fluid has 6% cerium oxide as polishing abrasive. Fig.3 is the effects on the surface roughness Ra of electro-magnetic field intensity in MRF. It can be seen from the figure that surface roughness Ra increases with the increasing of magnetic field intensity. When the magnetic field intensity is increased, the shear yield stress of the U Y X Key Engineering Materials Vols. 259-260 663