US3383808A - Lens block - Google Patents

Description

y 1968 V J. DESHAYES ETAL 3,383,808

22 FIG."

INVENTORS Jean Des/Wye;

United States Patent 3,383,808 LENS BLOCK Jean Deshayes, Vincennes, France, and Waldemar Stoppaehcr, White Plains, N.Y., assignors to La Lunette de Paris, Inc., New York, N.Y., a corporation of New York Filed May 25, 1965, Ser. No. 458,726 5 Claims. (Cl. 51-216) ABSTRACT OF THE DISCLOSURE A lens block assembly is formed by placing an intermediate member on top of a mold, the upper surface of which intermediate member seats a lens blank with a clearance defined between said upper surface and the lens blank. The intermediate member is provided with a passage which provides fluid communication between the clearance and the mold cavity below the lower surface of the intermediate member. A low melting alloy is poured into the mold cavity and fills the clearance, the passage and the cavity below the lower surface of the member. The solidification of the alloy results in a unitary solid lens block assembly which includes as part thereof the lens blank firmly adhered within the assembly.

The present invention relates generally to the blocking of lens blanks and specifically to the provision of improved lens blocks especially adapted for holding plastic lens blanks so that they can be chucked in machines for generating and polishing their surfaces and for grinding their edges. 7

The manufacture of ophthalmic lenses requires a series of operations which, among others, include grinding and polishing operations. For these latter operations, it has been the practice heretofore to provide individual blocks formed of cast iron, steel, aluminum, or the like, and to attach each of the lens blanks to a respective block with a blocking medium such as pitch or other suitable adhesive. However, it has been found that lens blocks of this type are relatively impractical and inaccurate to manufacture, require separate and precision assemblies with the blanks and, in order to be reusable, the blocks must be thoroughly cleaned upon completion of each surfacing operation to remove the pitch or other blocking medium which remains on their lens mounting surfaces after the lens blanks are removed therefrom. The cleaning of such lens blocks involves a tedious and time consuming operation which, in combination with the initial cost of the blocks, increases the overall cost required to manufacture optical lenses.

While the foregoing device was usable in the manufacture of glass lenses, it left much to be desired when used with plastic lenses. More specifically, for plastic lenses it was imperative that a low melting pitch be used and the melting point of pitch used in the manufacture of glass lenses was 300 F., not sufficiently low to prevent injury to a plastic lens.

Plastic lenses may be cast in unfinished form with both surfaces thereof left to be ground to prescription, or may be cast in semifinished form with one surface thereof left to be ground to prescription. On the other hand, semifinished glass lenses are made by having one surface thereof finished by industrial means, leaving the other surface to be ground to prescription. The semifinished plastic lens is sent to an optical laboratory for finishing. While glass lens machines are used for this purpose, the techniques must, of necessity, be different because the workability of plastic is different from glass.

The art of blocking glass lens blanks by using preformed blocks to which the lens is secured by pitch has been replaced by the improved process in accordance 3,3335% Patented May 21, 1968 ice with which the lens blank is held in position on top of a mold and a heated l-ow-rnelting-point alloy is flowed into the mold under one side of the lens blank, which alloy is then allowed to cool adhering itself to the blank. The mold is then cooled and the mold cavity is shaped so that the alloy cools and solidifies to the shape of a lens block.

In the manufacture of a lens blank, there is no serious problem involved in providing a finished surface to one of the lens blank surfaces. There is, however, a serious problem involved in holding the semi-finished blank at the finished surface to permit working on the other surface. The described improved device solves this problem by easting the lens block onto the previously finished surface.

The improved device for blocking a lens blank, which involves casting the block directly to a lens blank precoated for adhering purposes, has many advantages over the use of preformed blocks which are adhesively secured to the lens blanks. In the first place, the cost is greatly reduced because the cast low-melting-alloy blocks can be remelted and used over and over again. In addition, there is provided, in effect, a new block each time that the lens is blocked for a surfacing operation.

However, despite the various advantages of the improved device, it has been found that it is satisfactory only for glass lenses and not for plastic lenses. More specifically, despite the fact that the improved device utilizes cooling means to absorb and dissipate the heat from the molten metal which is cast to a plastic lens, it has been found that, during the cooling operation, a significant amount of heat is transmitted from the molten metal to the lens. While, in the case of a glass lens, this heat has no significant effect, in the case of a plastic lens, the heat dissipated by the molten metal during the cooling thereof causes certain strains which are set into the plastic lens. This strain is such that when the plastic lens is removed from the block, the strain is released, affecting the curvature of the lens sufficiently to distort the optical properties thereof. As a result, after the worked lens surface has been ground to the specifications of the prescription, such surface will, after the lens is unblocked, change its curvature as a result of the release of the stress. In order to overcome such distortion, it has been attempted to compensate therefor by over or under grinding as the case may be, but such efforts have proved to be both burdensome and inaccurate.

In view of the foregoing, it is an object of the present invention to obviate the disadvantages of the directly cast blocking method when utilized in connection with plastic lenses.

It is another object of the present invention to retain all the advantages inherent in the directly cast blocking method for plastic lenses to the same extent that such advantages apply to glass lenses.

It is another object of the present invention to provide a means for blocking plastic lenses by means of a directly cast blocking operation which eliminates the generation of stress in the plastic lens resulting from dissipation of heat from the molten metal during the cooling thereof.

It is another object of the present invention to obviate the necessity of over or undergrinding plastic lenses to compensate for anticipated distortion caused by heat stress therein.

It is another object of the present invention to reduce the amount of molten metal required to form lens blocks of the prior art.

It is another object of the present invention to reduce the amount of heat dissipated during the cooling of the lens block.

It is a further object of the present invention to reduce the time required to cool the lens block.

Other and further objects and advantages of the present invention will be readily apparent to one skilled in the art from a consideration of the following specification taken in connection with the appended drawings.

In the drawings, which illustrate the best mode presently contemplated for carrying out the invention:

FIG. 1 is a view in elevation, and partly in section, of a directly cast lens block pursuant to the prior art;

FIG. 2 is a fragmentary view, in cross section, and illustrates the mold cavity into which the molten metal is introduced to form the directly cast lens block;

FIG. 3 is a fragmentary view partially in section and illustrates the mold cavity with a lens blank positioned thereon in accordance with the prior art method;

FIG. 4 is a view in elevation, and partially in section, of an intermediate lens block member pursuant to the present invention;

FIG. 5 is a view similar to FIG. 2, and illustrates the intermediate member of FIG. 4 positioned in the mold cavity and a lens blank mounted on the intermediate member;

FIG. 6 is a composite view of the lens and block pursuant to the present invention;

FIG. 7 is a view, partially in section, taken on line 7-7 of FIG. 3;

FIG. 8 is a view taken on line 8-8 of FIG. 5;

FIG. 9 is a fragmentary view of a prior art lens blocking apparatus which can be utilized in practicing the method of the present invention;

FIG. 10 is a view taken on line 10-10 of FIG. 9; and

FIG. 11 is a fragmentary sectional view, on an enlarged scale, taken on line 11-11 of FIG. 9.

In accordance with the previously described process of blocking a lens blank, the lens blank is held in position on top of a mold, and a heated low-melting-point alloy is flowed into the mold under one side of the lens blank, and is then allowed to cool adhering itself to the blank. The mold is shaped so that the alloy will cool to the shape of a lens block. This prior art method, and an apparatus for blocking lenses in accordance with said method, are illustrated and described in U.S. Patent No. 3,049,766, issued Aug. 21, 1962 to L. A. Buckminster, and FIGS. 1 through 3, and 10 therein illustrate said prior art apparatus.

Pursuant to said Buckminster patent, provision is made for a molding apparatus generally indicated by the reference numeral 10 (FIG. 9). Mounted directly on said molding apparatus is a suitable sighting device 12 provided with an eye piece. Mounted beneath the sighting device is a mold 14 (FIG. 2). The lens blank 16 is seated on top of the mold cavity 18 on a resilient ring 20, which surrounds the mold cavity and which constitutes a wall to contain the molten alloy. The lens, as here shown, has a lower or convex spherical surface 22 which has already been worked, and an upper or concave surface 24, which has yet to be worked. The seating ring is mounted so that it can be tilted to any angle to adjust the blank for the desired amount of prism and prism axis, and part of the mold 14 itself is rotatably adjustable about an axis disposed at right angles to the axis of tilt of the ring, so that the lens block can be molded for the desired axis position.

The lens blank 16 is adapted to be held on the seating ring 20 by a resilient ring 26 (FIG. 11) mounted on the bottom of an axially-adjustable tube 28 that forms part of the sighting device 12. The required adjustments for axis and prism are made, and the tube 28 of the sighting device 12 is lowered to bring the resilient ring 26 into engagement with the upper surface 24 of the lens blank 16.

Mounted in the mold 14 for axial reciprocation therein is a piston 30 which carries three pins 32, 34 and 36 that extend into the mold cavity 18. These pins are in line diametrically of the mold. The two outside pins 32 and 36 are conical in conformation and serve to form, in

the molded lens block 38 (FIG. 1), conical recesses 40 for centering the lens on a generating or polishing machine if a toric lens is to be ground. The center pin 34 is cylindrical and serves to form a circular recess 42 in the molded block 38 by which the block can be chucked in a spherical grinding machine if a spherical lens surface is to be ground.

For supplying the molten metal to the mold 14, a hopper is mounted in a column 44 on which the sighting device 12 is secured. The hopper is intended to receive chunks of metal alloy, including previously used lens blocks. The hopper has a heater secured to it for melting down the chunks of metal. Reciprocally mounted in the column 44 beneath the hopper is a tank or container for receiving molten metal from the hopper. The tank is provided with a nozzle 46 through which the metal may flow into the mold through an inlet channel 48. A manually operable normally closed valve (not illustrated) is provided to prevent flow of metal from the tank into the mold. A manually operable lever (not illustrated) is provided for moving the nozzle 46 into the opening 48 in one side of the mold 14; and then the valve is opened to permit flow of the molten metal from the tank into the mold cavity 18 to fill the mold cavity. Means is provided for circulating water or other coolant around the mold, when the pouring operation is completed, to solidify the molten mass and thereby form the lens block 38 which adheres to the lens blank 16. The tank and its nozzle are then retracted to withdraw the nozzle from the mold. The piston 30 is then moved upwardly in the mold (the lens holding pins having been retracted) to strip the now-completed lens block 38 from the mold 14.

The material used for forming the lens block 33 is low melting point, electrically conductive alloy made, for example, of a mixture of bismuth, lead, tin, indium and cadmium. The alloy which is preferably used has a melting point ranging between 117 F. and F. Pieces of the alloy, and of previously used lens blocks, are dumped into the hopper and melted down to provide the material for the lens blocks.

It will be noted that there is no problem of matching the convex lens surface 22 and the spherical surface 50 of lens block 38. In this connection the spherical convex surface 22 of the lens blank always has a good fit with the circular resilient seating ring 20. In filling the mold cavity 18, the molten metal always assures that the block surface 50 is complementary to the lens surface 22. In order to assure good adherence of lens block 38 to the lens blank surface 22, the latter is provided with a coating which increases adherence between the solidified block and the lens.

The described method of forming the cast lens block 38 is eminently suitable for use with glass lens blanks 16, where the convex spherical surface 22 has already been worked by grinding and polishing it to the required specifications with the finished surface 22 cast to the spherical surface 50 of the block. The unfinished surface 24 is readily available to be worked on. After the molded block 38 has served its purpose, it can readily be removed from the lens blank, for example, by putting it into a small vice and squeezing on the block to cause it to break away clear of the lens blank. The removed block can be melted down again and used in molding another block for another lens. Thus, each time a lens blank is blocked by the described process, a new clean, unmarred, accurate block is provided.

If the lens blank 16 is formed of glass, the transfer of heat from the molten metal during the cooling thereof will have no significant effect on the lens. However, if the lens 16 is made of plastic, it has been found that during the cooling of the molten metal, the heat transferred from the metal to the lens blank causes stresses in the plastic lens blank. This is true despite the fact that the apparatus 10 has a water cooling system as sullicient heat nevertheless is transmitted to the lens blank and causes stresses therein. Consequently, when the block is removed from the blank, these stresses are released and cause distortion of the finished and worked surfaces of the plastic lenses.

Pursuant to the present invention, the advantages of the directly cast lens block 38 are retained while the undesirable distortion of the plastic lens blanks has been eliminated. Heretofore, it was necessary to compensate for such distortion when the plastic lens was ground and such proved to be both cumbersome and inaccurate. The necessity for such compensation has, in accordance with the invention, been dispensed with.

In accordance with the basic inventive concept of the present invention, provision is made for the utilization of an intermediate lens block element or member 52 (FIG. 4) which is preferably formed of aluminum or other suitable metal. As best shown in FIGS. 5, 6 and 8, member 52 is provided with a dished upper surface '54 which is encompassed by a raised circumferential rim 56. Its lower surface 58 is provided with a circular recess 60. Four canals 62 extend from the lower surface 58 to the upper surface 54 and are inclined inwardly toward the center of the upper surface. The raised rim 56 is provided with three notches 64 which are spaced 120 apart. These notches are similar to the single notch 66 (FIG. 7) provided in the previously mentioned seating ring 20. In the prior art embodiment illustrated in FIGS. 1-3, notch 66 was utilized as a flashing notch to indicate the complete filling of cavity 18 with the molten metal. When the metal flashed through notch 66, the mold cavity 18 was completely filled. However, in accordance with the present invention, the seating ring 20 is eliminated. The intermediate block element 5-2 is disposed directly within the cavity 18 of mold 14, as best shown in FIG. 5. The lower circumferential step 68 on the member 52 has a snug fit against the wall 70 of cavity 18. The piston 30 is retracted as much as possible into mold 14 to permit for the insertion of member 52 into the mold cavity. The piston is then returned until its center circular pin 34 abuts the lower surface 58 of the member 52. It will be noted that the conical pins 32 and 34 underlie the circular groove 60 in the intermediate member 52. The precoated plastic lens blank 16 is then placed on the intermediate member. It will be noted that the lower spherical surface 22 of the lens blank is seated on the raised circular rim '56 of the intermediate member in the same position that lens 16 in FIG. 3 is seated on the raised circular rim 21 of seating ring 20. Therefore, in both cases, the lens is automatically centered on the underlying raised circular rim.

Furthermore, it will be noted that due to the raised rim 56, there is defined a clearance 69, as best seen in FIG. 5, between the worked lower surface 22 of the lens blank and the confronting dished surface 54 of the intermediate member 52. In addition, it will be noted that the canals 62 provide communication between mold cavity 18 and clearance 69. Furthermore, it will be noted that the canals 62 are disposed normally, and not tangentially, with respect to lens surface 22.

With the member 52 abutting piston pin 34, as shown in FIG. 5, tube 28 of eyepiece 12 is moved downwardly until clamping ring 26 is engaged against the worked upper surface 24 of a plastic lens blank 16, as disposed in FIG. 5. Thereafter, nozzle 46 is inserted into opening 48 and the molten metal is introduced into mold cavity 18. When the metal flashes, as indicated at 72 in FIG. 10, through the three notches 64 in rim 56 of member 52, the mold cavity 18 is filled with the molten metal and the supply of the latter to the cavity is discontinued.

After the molten metal has cooled, the plastic lens 16 is secured to the lens block which is generally indicated by the reference numeral 74. The lens block 74 includes the intermediate member '52, a layer 76 of metal which filled clearance 69 between member 52 and lens 16, and a metallic base portion 78. Portion 78 is provided with four integral extensions or projections 80 which extend from base 78 through canals 62 integrally into layer 76. Base portion 78 also has a raised circular ridge 82 which extends into the circular recess 60 in intermediate member 52, and three diametrical recesses 84, 86 and 88 complementary to pins 32, 34 and 36. Recesses 84 and 88 are conical and underlie ridge 82 and the circular recess 86 is backed by the lower surface 58 of the intermediate member 52.

The lens block 74 provided with the lens 16 thereon is readily removed or stripped from the mold 14, in the same manner as described in the Buckminster patent. The conical recesses 84 and 88 are adapted to receive the prongs of a vacuum chuck or the like in the same manner as conical recesses 40 in block 38 and the circular recess 86 is adapted to receive the spindle of a finishing machine in the same manner as circular opening 42. The raised groove 82 serves as a cushion behind recesses 84 to asborb stresses from the pins of the vacuum chuck. In addition the engagement of ridge 82 in recess 60 pro vents the shearing of the base portion 78 with respect to the intermediate member 52. "Furthermore, since the raised portions 80 extend normally relative to lens 16 and not tangentially thereto, there is also provided a better distribution of the stresses applied by the finishing machines.

It will be noted that the lens block 74 provides many advantages over lens block 38. Due to the use of the novel intermediate member 52, the quantity of molten metal required to form block 74 is drastically reduced from the quantity of molten metal required to form block 38. This will be readily apparent from a comparison of FIGS. 1 and 6.

The relatively large bulk of metal constituting block 38 required the use of a proportionately large amount of hot molten metal. During the prior art cooling operation much of the heat of the molten metal dissipated into the lens blank 16. With a glass blank 16, this was no problem. However, such large heat dissipation sets up internal stresses and strains in a plastic lens blank 16 so as to make the prior art method impossible of use with a plastic lens. In accordance with the present invention, there is a tremendous reduction in the amount of molten metal that is to be used. This results from the fact that the intermediate member 52 replaces much of the bulk of block 38. Consequently, it will be noted that the present invention results in a large reduction in the amount of heat to be dissipated by the cooling molten metal. The reduction in the quantity of molten metal not only reduces the amount of heat dissipated thereby but provides for the greater portion of said heat to be absorbed by the metal member 52 and leaving only the minimal amount of heat dissipated by the thin molten metal layer 76 to be absorbed by lens 16. As a result, the amount of heat applied to plastic lens 16 from the relatively small amount of molten metal introduced into mold cavity 18 is so small as to have practically no effect on plastic lens 16. Consequently, there is substantially no resulting distortion of the plastic lens. It will, therefore, be apparent that the metal insert 52 of the present invention in addition to greatly reducing the quantity of heated molten metal which is to be utilized in the block, also itself serves to absorb some of the heat dissipated from the smaller amount of molten metal. The consequent amount of heat directly applied to the plastic blank 16 is insufficient to create internal stress or strain therein.

Pursuant to another advantage of the present invention, the time required to cool block 74 is greatly reduced from the time heretofore required to cool block 38. In actual practice, it has been found that whereas the cooling mechanism of apparatus 14) required two minutes to cool block 38, it requires only 30 seconds to cool and solidify block 74. This large reduction in cooling time results directly from the large reduction in the quantity of molten metal required for use with the metal insert 52 of the present invention. Such reduction in cooling time is therefore seen to decrease the production time and cost.

Pursuant to another advantage of the present invention, the volume of clearance 69 can be predetermined by predetermining the curvatures of dished surface 54 of member 52 and lower surface 22 of lens 16 so as to provide a relatively thin evenly distributed layer 76 of molten metal on lens blank 16 to practically eliminate heat stresses therein. From the prior description, it will be apparent that the thickness of layer 76 is directly related both to the amount of heat dissipated and the cooling time. The thinner the layer 76 and the more equal distribution thereof provides advantages in both of these areas.

At the end of the finishing operation, block 74 can be removed from lens 16, in the same manner that block 38 can be removed therefrom, or in any other suitable manner.

From the foregoing, it will be readily apparent that all of the advantages of the present invention flow from the utilization of an intermediate insert 52 to replace most of the molten metal heretofore required to fabricate a lens block so as to adapt the use of lens blocks, which heretofore had been limited to the fabrication of glass lenses, to the fabrication of plastic lenses also.

While the present invention has been illustrated and described in connection with the fabrication or finishing of a lens blank having one spherical surface, it will be understood that it is not limited thereto. The basic concept of the present invention, namely the utilization of the intermediate block member 52, can equally well be used to block completely unfinished lens blanks having two planar surfaces or semi-finished lens blanks whose finished surface is not spherical but is instead toric or of some other configuration.

It will be understood that various changes and modifications may be made within the structure of block 74 or the method of the present invention without, however, departing from the basic inventive concept thereof, as set forth in the appended claims.

Having thus described our invention, what we claim and desire to secure by Letters Patent is:

1. A lens block assembly comprising:

a member having opposing surfaces;

a lens blank seated on one of said surfaces and defining an enclosed clearance therewith of substantially uniform width which is smaller than the thickness of said lens blank;

a solidified molten metal adhered to the other surface of said member and extending through said member into said clearance;

said metal filling said clearance and being adhered to said lens blank and said one surface.

2. A lens block assembly comprising:

a member having opposing surfaces;

a lens blank seated on one of said surfaces and defining an enclosed clearance therewith of substantially uniform width which is smaller than the thickness of said lens blank;

a solidified molten metal adhered to the other surface of said member and extending through said member into said clearance;

said metal filling said clearance and being adhered to said lens blank and said one surface;

said metal at said other surface of said member defining a block on which the lens blank may be chucked in a machine for operating on the lens blank.

3. A lens block assembly as in claim 2, wherein a plurality of channels are defined in said member and solidified metal extends through said canals normal to said lens blank.

4. A lens block assembly as in claim 2, wherein the other surface of said member has a recess defined therein and said solidified molten metal extends into said recess.

5. A lens block assembly as in claim 2, wherein said member is provided at said one surface with a raised rim on which said lens blank is centered.

References Cited UNITED STATES PATENTS Re. 19,015 12/1933 Hill 51277 X 1,599,801 9/1926 Vreeland 164-112 2,352,178 6/1944 Bolsey 51216 X 2,465,153 3/1949 Fritzsche 51-216 X 3,049,766 8/ 1962 Buckminster.

HAROLD D. WHITEHEAD, Primary Examiner.

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