Patent GB-648776-A (GB)
Title: Improvements relating to the formation of curved surfaces of prescribed figure by grinding and/or polishing
IPC: B24B13/06
Filed: 1947-12-12
Abstract: 648,776. Grinding and polishing lenses, mirrors and other curved surfaces. BURCH, C. R., and METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. Dec. 12, 1947, No. 32875. [Class 60] Apparatus for forming curved surfaces of prescribed figure, such as aspheric lenses and mirrors, by grinding or polishing comprises means for mounting a specimen for rotation about a fixed axis, a bar having fixed at one end a grinding or polishing pad which bears yieldingly against the surface of the specimen, with an effective area of contact which is of small dimensions and a mere fractional part, namely of the order of 5 per cent or less, of the prescribed surface, and means for reciprocating the other end of the bar on a locus which is the negative curvilinear tractrix of the surface of prescribed figure, that is a curve 11, Fig. 1, defined by constant intercepts on the family of tangents 12 drawn from the surface 10 of prescribed figure. It is stated that by reciprocating a grinding or polishing pad on the specimen in this manner the tangential component of force applied by the work to the pad will pass close to the driving axis of the bar and will approach this axis the more exactly the closer the surface of the specimen is to the prescribed Figure, and will thus be without effect on the normal component of force applied by the pad to the work; in conseauence the action of the pad will not be influenced detrimentally by changes in the coefficient of friction. Apparatus comprises basically a deferent bar 13, Fig. 2, which is oscillated, with progressive creep, about a pivot 14; the driving end of the pad bar is pivoted at 17 to an epicycle bar 16 which is pivoted at 15 to the deferent bar, the epicycle bar having secured thereto a cam rod 18 which co-operates with a fixed cam 19 whereby the point 17 traces out the required path 21 which deviates from the circular arc 20. In the preferred embodiment, the work 35, Fig. 4, is rotated on a vertical spindle. The grinding or polishing pad 43 is supported by pad bars 42 secured to a shaft 41 pivotally mounted in bars 40, 37 secured to a rod 39 which also carries a bar 38. The frame consisting of the bars 37, 38 and rod 39 is pivotally mounted on a shaft 28 connected to the upper ends of deferent bars 24, 25 the lower ends of which are secured to a shaft 26 journalled in bearings in a channel-shaped member 27 which is vertically adjustable on a base plate 29 by means of a nut and screw adjuster 30; the channel member 27 is adjustable about a vertical axis by means of a screw 31. The base plate 29 is secured on a support by bolt 33 and levelling screws 34. The rod 39 has secured thereto a cam ratio bar 45 on which is slidably mounted a member 46, secured in adjusted position by a set screw 47, and having a lateral extension 48 engaging a cam bar 49 pivotally mounted on a pillar 50 extending laterally from the deferent bar 25, the cam bar 49 engaging a cam 51 supported by a horizontally adjustable slide 52 carried by a vertically adjustable member. The shaft 26 carries a counterpoise 57. Oscillation of the parts is effected by a steel tape secured at one end to a sector 59 mounted on the shaft 26 and connected at its other end with a driving device, such as that described in Specification 593,759, which reciprocates the tape at selected amplitude and also causes it to creep between limiting positions. The shape of the cam 19 may be obtained by calculation; preferably, however, if a specimen having the required Figure is availabe the cam 19 may be cut substantially to the required shape and corrected as required by observing the orientation of the part with the surface of the specimen. For this purpose the normal polishing or grinding pad would be replaced by a rigid pad provided with an aperture extending normal to the surface through which the surface of the specimen may be examined autostigmatically. Alternatively a lens may be mounted on the surface of the pad to give interference images (Newton rings) with the surface of the specimen.