Patent GB-664134-A (GB)
Title: Improvements in or relating to the manufacture of lenses
IPC: B24B13/005
Filed: 1949-12-08
Abstract: 664,134. Blocking and unblocking lens blanks. KODAK, Ltd. Dec. 8, 1949 [Dec. 8, 1948], No. 31532/49. Class 60 A method of unblocking a lens blank from a lens seat in a metal blocking tool in which it has been held by a thermoplastic blocking pitch or wax for a surfacing operation, comprises the steps of heating the blocking tool substantially to the melting temperature of the blocking pitch or wax while maintaining the lens blank below this temperature and then removing the lens blank from the lens seat while this condition of relative temperatures exists, whereby the pitch or wax will strip from the blocking tool with the lens blank, leaving an insignificant residue on the blocking tool. The lens blank is preferably blocked by precoating the surface of the lens blank adopted to engage the lens seat with a thin layer of blocking pitch or wax just sufficient in quantity to accomplish the adhesion of the blank to the lens seat, heating the blocking tool to substantially the melting-point of said blocking pitch or wax, pressing the coated surface of the lens blank into the lens seat until the blank is properly seated therein and cooling the blocking tool to set the pitch or wax. For processing the convex surface of a planoconvex lens 12, Fig. 3, the first step is to precoat the plane surface and edge of a plurality of blanks with just a sufficient layer of pitch 16 to accomplish the necessary adhesion of the blank to the seat in the blocking tool. The precoating may be accomplished by spraying a group of blanks, while held in a suitable tray or the like, with a spray gun, using a solution of pitch in a volatile solution, or using a spray gun which applies the pitch as a molten spray which solidifies upon striking the lens blank which may or may not be warmed, as desired. A rotatable table 20, Figs. 7 and 8, having means for indexing it 90 degrees at a time, has four shouldered recesses 21 for receiving the shanks 13 of blocking tools 10. An empty blocking tool is placed on the table at Station III. At Station IV the blocking tool is moved upwardly into an induction heating coil 22 by an air-operated piston 23; the admission of air to the cylinder 25 may be automatically turned on as the table is indexed to position a new blocking tool at Station IV and may be automatically cut off after the blocking tool has been held in the heating coil sufficiently long to heat the blocking tool to substantially the melting temperature of the blocking pitch. At Station I, the precoated lens blanks are pressed into the lens seats on the blocking tool 10 until the pitch is melted and the blanks are properly seated; in order to hold the lens blanks firmly in place, a weighted cap 26 may be placed on top of the lens blanks and left there until the pitch has cooled sufficiently to solidify completely. At Station II a jet 27 of water or cold air is directed into the bore 14 in the shank of the blocking tool:so as rapidly to cool the blocking tool to set the pitch without the cooling mechanism striking the blanks. The cooled blocking tool is removed at Station III to be placed on the abrading machine for surface processing. Ultimately, when there are blocking tools coming from the abrading machine with the surfaces of the blanks finished, or a blocking tool with unprocessed lens blanks is removed at Station III for processing, a blocking tool with processed lenses is put back on to the table in its place. At Station IV the blocking tool with processed lenses is heated until the block reaches the melting temperature of the pitch. At Station I, the blocking tool with processed lenses is still at the melting temperature of the pitch and the processed lenses are quickly removed from the lens seats by means of a suction cup 28, Fig. 5, on the end of a vacuum pipe 29. As the removal of the lens blanks from the' blocking tool consumes very little time, the blocking' tool remains sufficiently hot so that a new batch of precoated lens blanks can be blocked into the seats without having to reheat the blocking tool. By using a very high cycle frequency on the induction heating coil, i.e. 400,000 cycles per second on the generator to which the heating coil is connected, a skin heating effect is obtained, whereby the heating of the blocking tool is confined to the portion of the blocking tool adjacent its curved surface containing the lens seats instead of heating the entire mass of the blocking tool. Thus, heat from the outside portion of the blocking tool can dissipate into the body of the blocking tool by conduction so that pitch of a blocked lens will more rapidly cool and set, and there will be less chance for it to change position while the pitch is setting; also, the more rapid heating of the seats during unblocking means that the lenses themselves will remain cooler and will be less subject to any deleterious effects which might result from their becoming heated.