Patent GB-823116-A (GB)
Title: Improved method and apparatus for simultaneously forming a plurality of multifocal lenses
Inventor: DALTON ERNEST THOMAS
IPC: B24B13/005
Filed: 1957-07-30
Abstract: 823,116. Grinding lenses. AMERICAN OPTICAL CO. July 30, 1957, No. 24203/57. Class 60. [Also in Group XXXVII] A method of simultaneously forming a plurality of multi-fqcal lens blanks L, Fig. 1, having areas 10, 11, which constitute different focal fields, and a cliff-like dividing line 12 between said fields, consists in mounting the blanks, which are each provided with a continuous surface of substantially the radius of curvature desired on the area 10 corresponding to one focal field, on a wheel-type block B, Figs. 3 and 9, with the axes of the continuous surfaces intersecting the axis of rotation of the block and with the surfaces spaced from the axis of rotation in accordance with the radius of curvature to be formed on the other of the areas 11 of the blanks, and then forming the other areas to a given spherical curvature and simultaneously forming the cliff-like dividing line between the areas. The radius of curvature for the areas 10, corresponding to the distance view. ing field of the blanks, is initially produced over the whole surface of the blanks by any of the known standard methods of grinding and polishing after which the blanks are secured to the block B, which is of a size selected to suit the radius of curvature to be produced on the area 11 corresponding to the reading portion of the blanks, by the device shown in Fig. 3. A blank is accurately positioned in a mould 40 selected to suit the shape of the blank and located in a recess in a base 32 by pins 42 which are adjusted so that the blank is a predetermined distance from the axis of the block B which is rotatablyadjustable on a spindle 35 secured to a radial arm 34 on a vertically-movable shaft 33 restrained from rotation by a rod 38 guided by a vertical rod 39. The blank L is coated on its upper, concave surface with a bonding agent in the form of a pellet or wafer of pitch or suitable adhesive and the block B lowered into contact with the bonding agent by actuation of an air cylinder 45 or other suitable means until the arm 34 contacts a spacer 46 which is disposed on an extension 47 of the frame and selected according to the radius of curvature to be ground on the areas 11. A vacuum line 44 connected to the under surface of the blank is used to provide a partial vacuum to hold the blank firmly in position during bonding to the block. A glass filler may be used to fill the spaces between the blanks to ensure even operation during grinding. The block is then mounted on a vertical spindle 50, Fig. 9, which is driven by a belt 61 from a motor 58 and carried on a table reciprocable to bring the blanks into engagement with coarse and fine abrading tools 51, 52, both of which are preferably diamond charged. From its initial midway position between the tools, the table is first moved at relatively high speed towards the coarse abrading tool 51 by a chain-belt 73 which is driven by an electric motor 71 and rotates a lead screw 66, axially secured to the table, in a sleeve 68 mounted in the frame 53 of the apparatus. To prevent damage to the blanks, the speed of the table is reduced by electro-magnetic relays stopping the drive from the motor 71 and replacing it with a double worm-and-wheel drive 77, 78, 81 from a motor 75 which rotates the sleeve 68, the lead screw now being prevented from rotation by magnetic braking in the motor 71. When the blanks are about to engage the tool, the speed of the motor 75 is reduced to provide a still further slow speed. The radius of curvature of the areas 10 of the blanks is then rough ground by the cup-shaped abrading tool 51 which is belt-driven from a motor 133 and arranged to grind a short distance above the dividing line 12. The tool is mounted on a spindle 129 carried on a housing 124 which can be adjusted by a hand-controlled worm drive 139, 141 around a quadrant 134 to pivot the tool about its lowest point 51a. The radius of curvature of the areas 10 is thus determined by the radius R, Fig. 5, and diameter D of the tools, the angular setting of the housing 124, and the distance of the blanks from the axis of rotation of the spindle 50. The abrading edge portions of the tool are formed to a predetermined angular shape so that the plane of the ledge formed along the dividing line between the fields is in the same plane as the optical axis of the blanks to prevent prism displacement when the line of vision passes from one area to the other. When the blanks have been fed to the predetermined depth of rough grinding, the motor 75 is stopped and the connections to the motor 71 connected in the reverse direction to traverse the blanks towards the fine abrading tool 52 which is formed and mounted similarly to the tool 51 but with its lower grinding and pivot point 52a coincident with the optical centre of the blanks and the dividing line 12. The slow feeds from the motor 75 are imposed as for the tool 51, and the final depth of feed is such as to reduce the height between the areas 10,11 at the optical centre of the blanks to a minimum. At the completion of the slow feed into the tool 52, a time delay is operated to ensure a high finish on the blanks before the table is returned to its initial midway position between the tools. The relays controlling the feeding of the blanks into the tools are operated by adjustable switches 89, 101, 102, 104, Fig. 11, which are carried on rotatably-adjusted drums on the front of the machine and actuated by a radial arm 87 carried on a shaft 83 which is disposed in the frame 53 and rotated by a gear 85 in engagement with a rack 86 depending from the table 54. An adjustable toothed annulus 85b is provided to eliminate backlash between the gear and rack. To aid setting up the apparatus, the relays can be operated by push buttons to jog the table in the desired direction. At the completion of grinding, the block is removed from the spindle 50 and mounted on a shaft 165, Fig. 13, driven by a belt 172,from an electric motor 167 which also drives a polishing pad 168 of plastic or other material. The pad, which is preformed to match substantially the curvature of the area 11 to be polished, is mounted in an angularly-disposed head 170 and may be supplied with a polishing medium. Finally, the blanks are removed from the block and the concave surfaces ground to the desired curvature. In an alternative process for producing multi-focal blanks L<SP>1</SP>,Fig. 18, the whole surface of the blanks is initially rough ground to the curvature of the areas 175 corresponding to the distance viewing portions of the blanks, before the blanks are positioned on the block B and the areas 176, which are the reading portions of the blanks, ground and polished as in the previous process. The blanks L<SP>1</SP> are then removed from the block B and remounted in a double row on a block C ; the blanks may have different curvatures on the areas 176. To position the blanks L<SP>1</SP> on the block C, the unfinished areas 175 abut concave side walls 185, 186 on the two halves 180, 181 of a hinged mould 179 while the edges of the finished areas 176 seat against sloping annular surfaces 187. A vacuum line is connected through channels 189, 190 and lateral openings 191, 191a to hold the blanks in positions arranged so that the blanks in one half of the mould are staggered relatively to those in the other half when the mould is closed. The block C, of a size selected in accordance with the radius of curvature to be finished on the areas 175, is positioned on a stub-shaft 193 in the lower half 180 of the mould and a supply of pitch or other bonding agent introduced through one or more openings 203 to fill the space between the block and blanks and also, if desired, cover the finished areas 176. A collar 197, which is slidable on the shaft 193, can be axially moved by rotation of a handle 202 and screw 201 to remove the block from the shaft after the blanks have been bonded and the mould opened. The block is then mounted on a motor-driven shaft 204, Fig. 20, and the blanks L<SP>1</SP> engaged by a cup-shaped abrading tool 207 which is of a diameter suitable to grind simultaneously the areas 175 of both rows of blanks and axiallymovable by a manually or automatically operated rack-and-pinion device 209. The block C is finally positioned on a further rotatable shaft for the areas 175 of both rows of blanks to be polished by a plastic or like polishing tool which is driven by an electric motor and supplied with a polishing medium. After removal from the block C, the concave surface of the blanks is ground and polished to the curvature desired.