<title>Air Bearings And Drive Systems In Single-Point Diamond Tool Machine Tools</title>

Abstract

AbstractThe paper discusses two topics that the author has found to be of importance in the appl-ication cf air bearings to diamond tool machine tools. The first topic discussed concernsthe potentially poor performance, in relation to a machine tool, of an air bearing journal,if it is designed for the maximum possible static load carrying capacity. The problem isassociated with a sometimes self- excited instability, associated with the compressibility of air. The second subject is concerned with two different types of non -influencing drivesystem , both of which have been used to promote a vibration -free mode of operation. One type of drive provides a uniform linear motion to an air bearing supported table on a diam-ond tool flycutter. The other type of drive has been used to provide angular velocity tothe Headstocks of both a diamond tool lathe and an aspheric generator.IntroductionThe reliable production of non- ferrous components, the surfaces of which have both theaccuracy of shape, and reflectivity necessary to satisfy the demands of the precision opti-cal industry, have placed, in turn, stringent demands upon the diamond tool machine tool.In order to satisfy these demands the machine tool designer has, among many other things,needed to pay particular attention to the quality of the main spindle bearing that he empl-oys, and also to ensure that the drive systems used do not impose vibrations, periodicerrors, or extraneous forces upon the main bearings or slideway systems.Over the past two or three decades, the hydrodynamic oil bearing, and occasionally thehydrostatic oil bearing have been the bearings most favoured in diamond tool machine tools.However, the last decade has seen, increasingly, the application of pressurised air bearingsto provide the main spindle bearing. All of the above bearing types have in common thebeneficial properties of fluid film bearings, i.e. an accuracy of rotation better than theaccuracy of manufacture of the components of which the bearings are made; and an absenceof the broad hand vibrations associated with rolling element bearings. These propertieshave been, of course, the reasons for the selection of fluid film bearings by the designersof diamond tool machine tools. However, if the designer decides to use an externallypressurised air bearing, possibly for the first time, it is, in the author's opinion, impor-tant for him to realize that an air bearing designed for the maximum static load carryingcapacity or maximum stiffness, may have a poor dynamic performance as a machine tool element,as the result of a phenomenen known as self excited pneumatic instability. This is not thecase with hydrostatic oil bearings, due to the incompressible nature of the fluid. The useof air, which is a compressible fluid, demands a bearing design philosophy which recognisesthis effect.A suggested approach to the design of air journal bearings is discussed below, as are twodifferent types of non- influencing drive that have been successfully employed on diamondtool machine tools.Air Journal bearingsThe static load carrying capacity of an externally pressurised air bearing is primarilya function of the pressure of supplied air and the bearing dimensions. The load carryingcapacity is also, but less importantly, a function of:-Orifice sizeNumber of orifices per rowNumber of rows (either one or two)Pocket diameterAxial position of rowIt should perhaps be acknowledged here that there are other types of externally pressurisedair bearings which do not use orifices as restrictors, but which will not be discussed inthis paper. Further, and for the purposes of simplification, this discussion is restrictedto the orifice compensated, or pocketed, type of bearing as shown in Figures 1 and 2.The designer, given a fixed conventional workshop air supply of say 551.5kNm2 (80LB /in2gauge), will want to satisfy himself that his journal bearing has a load carrying capacity