Patent GB-656875-A (GB)
Title: Improvements in or relating to multifocal or bifocal lenses and to their manufacture
IPC: G02C7/06
Filed: 1947-08-07
Abstract: 656,875. Grinding lenses. AMERICAN OPTICAL CO. Aug. 7, 1947, No. 21621. Convention date, Oct. 11, 1939. [Class 60] [Also in Group XX] A method of forming at least two focal fields on a multi-focal lens comprises generating an optical surface on one of the fields to the desired radius of curvature to such a depth that a portion of the field extends within the area of the other field and is bounded by a line of revolution, generating a separate optical surface on the other field to the desired radius of curvature and simultaneously forming the optical surface of the other field to such a depth that the said line of revolution is substantially entirely removed with the result that the two fields substantially merge at or adjacent to the central portion of the line of division of the fields. The line of division extends across the full width of the lens as an approximately straight cliff-like edge 5, Fig. IV. For making bifocal lenses the blanks are moulded with two areas 1, 2 of different curvature, and the area 1 is ground by mounting a number of blanks on a holder 19, Fig. IX, which is rotated in contact with a rotary lap 20 by a crank 27 in such a fashion as to prevent the formation of scratch marks. At the end of this operation the area 1 has merged centrally with the area 2. This latter area is ground by blocking the blank on a holder 29, Fig. XII, which is carried on a shaft 30 secured against rotation but capable of angular adjustment about its own axis and vertical adjustment with respect to a supporting frame 45, a spring 43 being provided to bias the shaft upwardly. The frame 45 is oscillated about an axis 35a by a crank and connecting-rod which incorporates a Geneva mechanism to provide a " break-up " motion. The tool 32 has a bevelled edge 42 to facilitate grinding of the edge 5 and is mounted on a spindle journalled in a housing 33 and rotated by a shaft 34. The housing is angularly adjustable about an axis 35 and is loaded by a weight 52 adjustable along an arm 51 provided with a screw 49 to control the feed of the tool in a direction perpendicular to the direction of oscillation of the frame 45. To set the lens holder 29 to the correct height a gauge 71, Fig. XV, is used. This comprises a vertical bar 70 calibrated to facilitate adjustment of a cross bar 62 having centres 67 with a spring 63 interposed therebetween. The lens holder is replaced by a gauge block 61 and, with the gauge snapped into position on the machine as shown in Fig. XIV, the shaft 30 is adjusted until the lower end of the bar 70 may be swung across the surface of the gauge block with its tip barely in contact with the latter; the gauge block is then replaced by the lens on its holder. The bifocal lens produced is monaxial, i.e. the centres of curvature of the areas 1, 2 lie on a line passing through the line of division of the two areas. A trifocal lens may be produced by the same method, the area 74, Fig. XVII, being ground by the apparatus shown in Fig. IX, while the areas 75, 76 are ground by the apparatus shown in Fig. XII without reblocking the lens, each area being ground so that it merges approximately centrally of the line of division with the adjacent area. The surfaces 75, 76 will not, however, be monaxial. To produce a trifocal lens in which each area is monoxial with the adpoining one, a lens holder 83, Fig. XXI, is used. The lens is blocked on a part 84 adjustable on a part 85 by means of a pin and slot connection. To grind the areas 74, 75 the part 84 is adjusted so that the axis 79 passing through the line of division of these areas is in alignment with the axis 90 of the part 85 and the axis of the tool 32. To grind the area 76 the part 84 is adjusted until the axis 89 occupies the position previously occupied by the axis 90. Lenses with more than three fields can be produced by the use of lens holders designed to provide the required number of positions of the lens blank.