Research on the Influence of the Diamond Wheel Wears on Machining Accuracy during Large Depth-to-Diameter Ratio Ultra-Precision Aspheric Grinding Process

During grinding large depth-to-diameter ratio aspheric surface part of high precision and high quality, the wearing of the diamond wheel would affect the machining accuracy of the aspheric surface part greatly. The influences of the wearing of the diamond wheel on machining accuracy of large depth-to-diameter ratio aspheric surface part are analyzed theoretically in this paper. The ultra-precision aspheric grinding system is researched out. In this system, the shape of the diamond wheel is ball-headed, and on-position dressing mechanism of the discharge principle is designed. After dressing, the profile accuracy of the ball-headed diamond wheel is very high, its diameter error is less than 0.1 μm, and the micro-blade group on the same height of the ball-headed diamond wheel is very well, it could be dressed directly without tearing down the workpiece when the diamond wheel is blunt. After dressing, this ball-headed diamond wheel could machine the aspheric workpiece directly. It resolved the problem that the assembling and wearing of the ball-headed diamond wheel affected the machining accuracy greatly. Finally, lots of grinding experiments are carried out on the grinding system. Grinding results show that profiles accuracy was 0.3 μm and the surface roughness is less than 0.01 μm. Introduction With the development of science and technology, the optical aspheric surface parts play more and more important roles in key instruments. Especially these parts are used in spaceflight and military equipments in evidence, such as imaging head of the reconnaissance satellite, the telescope of the heavenly body observation, reflector and medical microscope etc which need high accuracy aspheric parts [1-3]. However, it is still very difficult to machine aspheric parts with high accuracy, especially those concave parts with large depth-to-diameter ratio. Therefore many countries are paying attention to the study of optical aspheric parts machining. For aspheric surface part’s machining of large depth-to-diameter ratio, the wearing influence of the diamond wheel on the machining accuracy of aspheric surface parts is obvious. In order to improve the profile accuracy of the aspheric surface part and reduce its surface roughness, it is a hot study direction of many countries studies how to solve wearing and dressing question of the diamond wheels during grinding [4-6]. In this paper, grinding the large depth-to-diameter ratio aspheric surface part at the machine, the influence of the diamond wheel wearing on the profile accuracy of the aspheric surface part is firstly analyzed theoretically. Afterwards the on-position dressing mechanism of discharge principle are designed and manufactured. The dressing system solves the wearing question of the ball-headed diamond wheel bonded by cast-iron during the large depth-to-diameter ratio optical aspheric parts machining. Finally grinding experiments of the aspheric surfaces are carried out on the grinding system and experiments results show that the expected results are obtained. Ultra-precision Aspheric Surface Grinding System Key Engineering Materials Online: 2004-03-15 ISSN: 1662-9795, Vols. 259-260, pp 312-317 doi:10.4028/www.scientific.net/KEM.259-260.312 © 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-13/03/20,19:03:40) Journal Title and Volume Number (to be inserted by the publisher) 313 In order to succeed in machining such concave parts with large depth-to-diameter ratio, as shown in Fig.1, an ultra-precision aspheric surface grinding system is designed (see Fig.2). In this system, the grinder spindle of the diamond wheel is placed in an angle of 45°. The reason is mainly that the head of the aspheric surface can be expediently ground by the ball-headed diamond wheel, which is placed in the same direction, so that the ultra-precision machining of the large depth-to-diameter ratio aspheric surface part can be realized out [7]. Fig.1 Diagram of optic part and technical Fig.2 Photo of the aspheric surface grinding specification system Influence of Wheel Wearing on the Profile Accuracy of Aspheric Surface Parts Wearing Rate Influence of Different Contact Point of the Diamond Wheel. In the grinding system, according to the above-mentioned reason, the diamond wheel must be placed in some angle and its shape must be ball-headed if we want to grind the concave parts of large depth-to-diameter ratio. Because the diamond wheel spindle is placed with inclination mode, the contact points between the ball-headed wheel and the workpiece’s surface will vary continually. In Fig.3, when the wheel grinding from point A to point O, the contact point of the ball-headed wheel will vary from point B to point C, i.e. the BC arc section circle of the ball-headed wheel participates in the grinding process. According to the analyses, when the arc from point B to point C of the wheel participates in the grinding, with the difference of its contact section's length, the wearing rate of wheel will vary obviously, and when a grinding process goes on from point A to point O, the wearing rate of the wheel is the largest at point B and it is the smallest at point C. Therefore, there is different wearing rate in the wheel's different contact section. As a result, the profile accuracy of the aspheric surface part will be influenced greatly by that, and the error’s analyses have been showed in Fig.4. Fig.3 An analysis diagram of difference Fig.4 Influence of the wheel error on contact of the wheel surface accuracy of the workpiece X Workpiece O Z Perfect aspheric surface Grinding surface B point section circle C point section circle x 1