Method for lathing a lens

A method of lathing a lens, especially a contact lens, involves attaching the cylindrical blank of lens material to a first block of machineable plastic material, lathe cutting a desired first lens surface in the cylindrical blank, attaching the machined first lens surface of the blank to a second block, lathe cutting to remove the block of machineable plastic material, and lathe cutting a desired second lens surface in the blank. Both the first and second blocks may be adhered to the lens blank with an adhesive curable by ultraviolet radiation, and a two-piece block, composed of a head section and a body section, is disclosed.

BACKGROUND OF THE INVENTION 
The present invention relates to improved methods and apparatus for lathe 
cutting lenses, especially contact lenses, from a blank or button. 
A conventional method of manufacturing lenses, especially contact lenses, 
involves lathing the lens from a cylindrical blank of polymerized lens 
material (such cylindrical blanks commonly referred to as a "button"). The 
buttons may be cut initially from rod stock or sheets, or individually 
cast in cups using a curable liquid monomeric composition. 
While it is possible in some cases to insert the button directly into the 
lathe collet, it is more typical to first attach the button to a separate 
metal pin or "block", the opposite end of the block being configured for 
removable insertion into the collet of the lathe. The button is typically 
adhered to the block with an adhesive or special type of wax called 
"pitch" (with the block thus commonly referred to as a "pitch block" in 
the art). While the block and button are turned, the desired concave (or 
"base") curve is lathed into the exposed end of the button opposite the 
adhered end. Subsequently, this semi-finished button is deblocked from 
this first base curve metal block, for example, by removing the pitch 
adhering the button to the metal block. 
Then, this partially-finished button is adhered to a second metal block 
such that the lathe cut base curve of the button is adhered to the second 
metal block, again typically with pitch, while attempting to maintain 
proper alignment vis-a-vis the second metal block and the cut base curve. 
This front curve metal block is mounted to a lathe collet for lathing the 
convex (or "front") curve from the button. Subsequently, the lens must be 
deblocked from the front curve metal block. 
These operations may also involve intermediate lathing operations, for 
example, cutting a desired precision thickness or cutting an edge chamfer 
in the button, prior to lathing the front curve. Also, typically these 
operations will involve lens edging and polishing operations on the lathe 
cut surfaces. 
The multiple mounting and removal operations of the button to and from the 
blocks during the overall lathing process consume a significant amount of 
time and are prone to error, especially in maintaining the necessary axial 
alignment between the base curve block and the front curve block. Also, 
deblocking of the button or lens from the metal blocks to remove pitch 
requires the undesirable use of solvents or liquid suspension media as 
well as adds to the manufacturing cycle time. 
SUMMARY OF THE INVENTION 
This invention provides an improved method of lathing a lens, especially a 
contact lens, from a cylindrical blank. According to a first embodiment, 
the method comprises: attaching the cylindrical blank of lens material to 
a block of machineable plastic material; mounting the block in a lathe 
collet and lathe cutting a desired first lens surface in the cylindrical 
blank; attaching the machined first lens surface of the blank to a second 
block while the blank remains adhered to the block of machinable plastic 
material; mounting the second block in a lathe collet, and lathe cutting 
to remove the block of machineable plastic material; and lathe cutting a 
desired second lens surface, opposite the first lens surface, in the 
blank. 
According to various preferred embodiments, both the first block of 
machineable plastic material and the second block are adhered to the lens 
blank with an adhesive curable by ultraviolet radiation. 
According to various other preferred embodiments, the second block is 
composed of a head section and a body section, the head section comprising 
a convex upper surface for adhering to the concave base curve of the 
blank, and the body section comprising an elongated cylindrical body. A 
lower portion of the head section is releasably securable in an upper 
portion of the body section. The body section can be separated from the 
head section with the lens being retained on the head section, with the 
body section being recycled for additional cycles. Subsequently, the lens 
can be separated from the head section of this second block, for example, 
by immersion in a heated aqueous bath. 
The invention also provides a method of preparing a cylindrical blank of 
lens material for lathe cutting a lens therefrom comprising attaching to 
the lens blank a block of plastic material with an adhesive curable by 
ultraviolet radiation, said block being transparent to ultraviolet 
radiation, and transmitting ultraviolet radiation through the block to 
cure the adhesive; and, following lathe cutting a desired first lens 
surface in the blank, attaching to the cut first lens surface of the blank 
a second block of plastic material with an adhesive curable by ultraviolet 
radiation, said second block being transparent to ultraviolet radiation, 
and transmitting ultraviolet radiation through this second block to cure 
the adhesive. 
The invention permits precise axial alignment of the second block with 
respect to the lens blank having a first curve lathe cut therein. 
Additionally, the invention avoids deblocking operations that require a 
solvent and are more time intensive.

DETAILED DESCRIPTION OF VARIOUS PREFERRED EMBODIMENTS 
Referring now to the drawing figures, FIG. 1 illustrates cylindrical blank 
1 of lens material. Such cylindrical blanks for lathing contact lenses are 
commonly referred to as "buttons". The cylindrical blanks may be cut 
initially from rod stock or sheets of material, or individually cast in 
cups, from a curable monomeric composition. For the illustrated embodiment 
involving lathing of contact lenses, blank 1 will typically have a 
diameter of about 10 to 18 mm. 
Attached to blank 1 is first block 2. For the described preferred 
embodiment, first block 2 is composed of a transparent, machineable 
plastic material. First block 2 is intended for insertion in the collet of 
a lathe for lathing a first lens surface in blank 1. It is possible that 
this first block could be formed as an integral part of the blank, i.e., 
first block and the blank would be molded from the same material as an 
integral, one-piece article. However, it was found that in such cases, 
compression forces on the first block, when mounted in the lathe collet, 
have a tendency to distort the blank portion thus affecting the precision 
of subsequent lathe cutting operations. Thus, it is preferred that first 
block 2 is a separate article from blank 1, a suitable material being 
polymethylmethacrylate or copolymers thereof. 
As seen in FIG. 1, first block 2 may merely have the shape of a cylindrical 
disk. As discussed below, ultimately, first block 2 will be removed from 
blank 1 by lathe cutting away block 2. Accordingly, the size of block 2 
should be selected so that it has sufficient surface area for mounting in 
the lathe collet, but excess bulk should be avoided to avoid 
inefficiencies and excess time required to lathe away the block. A 
suitable size is a cylindrical disk having a diameter of about 6 to 10 mm 
and a height (or thickness) of about 2 to 5 mm, more appropriately a 
diameter of about 8 mm and a height of about 2.5 mm. 
According to preferred embodiments, block 2 is adhered to blank 1 with an 
adhesive 3 curable by exposure to ultraviolet (UV) radiation. Accordingly, 
a metered amount of liquid adhesive may be applied to the blank, block 2 
is then applied to the adhesive and blank, and while maintaining a 
compressive force UV radiation is directed to cure the adhesive. It is 
preferred that block 2 is transparent and that the UV radiation is 
directed through the block rather than directed through the blank, as it 
is possible that such exposure of the blank to UV radiation could 
undesirably alter properties of the polymeric material constituting the 
blank. And since UV-curable adhesives absorb UV radiation, this 
arrangement ensures the blank is exposed to minimal UV radiation. The 
adhesive should be selected such that it has sufficient strength to 
withstand compressive and shear forces in subsequent processing steps. A 
suitable adhesive is available under the tradename Loctite 363 from 
Loctite Corporation. 
As illustrated in FIG. 2, block 2 is mounted in lathe collet 5, and a first 
desired lens surface 6 is lathe cut in blank 1 via cutting tool 8. For the 
illustrated embodiment, lens surface 6 is a concave, base curve surface 
(although it is within the scope of aspects of the invention for this 
initial lathing operation to involving cutting the convex, front curve 
lens surface). While the blank and block are mounted in the lathe collet, 
additional cuts can be made to blank 1. For example, the periphery 10 of 
blank 1 can be cut to the desired final lens diameter and a second 
peripheral cut can be made to form a blocking diameter 11 and blocking 
shoulder 12 so that the blank assumes a shape along the lines of FIG. 3. 
For the illustrated embodiment where lens surface 6 is a concave, base 
curve surface, it is preferred that the lathe collet includes a dead-stop 
collet. More specifically, surface 7 of block 2 abuts an internal stop in 
the lathe collet, thus permitting precise control of the depth of the base 
curve cut, i.e., the thickness 13 of the blank (measured from surface 7 to 
the apex of curve 6, as shown in FIG. 3) following the base curve cut is 
precisely known for subsequent lathing operations. Typically, thickness 13 
will range from about 4 to 8 mm, more typically about 4 to 6 mm. 
Following this lathing operation, surface 6 can be polished, if desired or 
necessary, to a desired final optical quality finish. In the case where 
the lathe cutting of surface 6 is relatively higher quality, a dry 
polishing operation can be performed, for example, using a polishing pad 
with an polishing agent such as aluminum oxide incorporated therein. In 
the case where the lathe cutting is of relatively lower quality, a more 
vigorous polishing operation can be performed, for example, a wet 
polishing operation using a polishing agent such as aluminum oxide 
suspended in a silicone oil. It is preferred to use a dry polishing 
operation. 
As seen in FIG. 4, blank 1 is now mounted to second block 15. According to 
preferred embodiments, and further illustrated in FIG. 5, block 15 is 
composed of two pieces, head section 16 comprising convex upper surface 17 
extending from cylindrical body 18, and body section 19 comprising an 
elongated cylindrical body 20, these two pieces being releasably securable 
to each other. For example, the lower portion 18 of the head section may 
be tapered providing an interference fit with a corresponding taper of the 
upper portion 21 of the body section. Body section 19 may include a 
longitudinal slot 23 for alignment with a lathe collet for lathing 
operations discussed below. A suitable material for head section 16 is 
polymethylmethacrylate or copolymers thereof, and a suitable material for 
body section 19 is polycarbonate or copolymers thereof, polycarbonate 
having a slight resiliency for accepting and holding lower portion 18 of 
the head section. 
Similar to the mounting of the first block, it is preferred that the second 
block 15 is adhered to the lens blank with a UV-curable adhesive 25. For 
this blocking procedure, it is important that the block 15 is properly 
aligned with respect to the blank. More specifically, both blank 1 with 
first block 2 still attached thereto, as well as block 15, should be 
maintained in alignment with respect to one another while adhesive 25 is 
cured therebetween to adhere these two pieces. An illustrative procedure 
is as follows. 
Blank 1 is preferably supported vertically so that surface 6 of the blank 
is in the upward position. For example, first block 2 may nest in a 
support and retained against this support by applying a vacuum. Block 15 
can be retained in a metallic tube such that surface 17 is in the downward 
position and the tube has a slightly larger inside diameter than the 
largest outside diameter of block 15. By supplying compressed air evenly 
around block 15 to effect an air bearing, block 15 can be made to center 
within the tube. A metered amount of liquid adhesive is applied to surface 
6 of the blank, and the blank is lifted vertically by its support so that 
adhesive 25 contacts surface 17 of block 15 held in the tube. While 
waiting a few seconds for the adhesive to flow into a uniform film between 
surfaces 6 and 17, the air bearing in the tube is activated to center 
block 15. Then, UV radiation is directed to cure the adhesive. It is noted 
that such a procedure serves to center the blank and block 15 with respect 
to one another, as well as to avoid axial tilt between the two pieces. It 
is again preferred that block 15 is transparent and that the UV radiation 
is directed through the block rather than directed through the blank, so 
as to avoid exposing the blank to UV radiation. The adhesive should be 
selected such that it has sufficient strength to withstand compressive and 
shear forces in subsequent processing steps, however, the adhesive should 
be selected so that the lens can be removed from the adhesive in 
subsequent operations. A suitable adhesive is available under the 
tradename Loctite 3751 from Loctite Corporation. 
Body section 19 is inserted in the collet 30 of a lathe for lathing a 
second lens surface in blank 1. Prior to machining the desired second lens 
surface of the blank (opposed to surface 6), first block 2 is lathe cut 
away as shown schematically in FIG. 6; it may also be desired to reduce 
the overall thickness of blank 1 to a predetermined thickness with the 
cutting head of the lathe during this cut. Also, at this point, a chamfer 
cut (not shown in FIG. 6) can then be made in the periphery of the lens, 
for example, in the case where the final front, convex surface of the lens 
will include a curve peripheral to the central optical zone or when this 
front lens surface has a toric central optical zone and a ballasted 
peripheral zone. Collet 30 can be designed to receive alignment slot 23 of 
body section, so that in the case that a rotationally asymmetric surface 
or a surface offset from the center of the blank is being cut, the 
rotational position of the blank can be controlled in combination with the 
position of the lathe cutting head. It is noted that the removal of second 
block 2 by a lathing operation eliminates time-consuming deblocking 
methods required for prior methods involving a metal block. 
Now, the blank is ready for lathing the final desired lens surface, which, 
for the described embodiment, is the convex front lens surface. This cut 
is shown schematically by dashed line 31 in FIG. 6. (In the case where the 
initial lathed lens surface was the convex, front curve, the head section 
of second block may include a concave upper surface for adhering to the 
convex lens surface.) Second block 15 can remain in collet 30 for this 
operation, or second block 15 can be mounted in the collet of another 
lathe set-up for this operation. It is noted that slot 23 again may be 
used to register the rotational position of the blank with respect to the 
lathe collet, especially where this final lathing operation involves 
cutting a rotationally asymmetric curve such as a toric surface. 
Following this lathing operation, the convex surface of the lens can be 
polished to a desired final optical quality finish, as well as rounding of 
the front peripheral edge of the lens, if desired or necessary. As 
mentioned previously, in the case where the lathe cutting is relatively 
higher quality, the aforementioned dry polishing operation can be 
performed; in the case where the lathe cutting is of relatively lower 
quality, a more vigorous polishing operation can be performed, for 
example, a wet polishing operation. Any such edging or polishing 
operations can be performed while second block remains inserted in the 
lathe collet, or alternately, second block can be inserted in a spindle of 
the edger/polisher. It is especially preferred to use a dry polishing 
operation to avoid the need to use solvents or liquid suspension media for 
cleaning residuals from wet polishing of the lens. 
The final stages of the process involve separating the finished lens from 
block 15. According to the described preferred embodiments, first, body 
section 19 is removed from head section 16 with the lens remaining adhered 
to the head section. This can be done by gripping the body section, for 
example, in a collet, and inserting a plunger through elongated 
cylindrical body 20, the plunger contacting head section 16 to disengage 
head section 16 from body section 19. The body section can be recovered 
for reuse in subsequent operations if desired. Then, the lens is removed 
from the head section. This is preferably accomplished by immersing the 
lens and head section in a heated bath, for example, in a heated saline 
solution where the lens is extracted and hydrated. Soft hydrogel lens 
materials typically swell upon hydration, and it has been found that such 
swelling of the lens, and the fact that the adhesive bond is stronger to 
the block than to the lens, is adequate to release the lens from the 
adhesive and head section without damaging the lens. For this operation, 
the block and lens can be retained in a basket when immersed in the bath 
so as to facilitate recovery of the lens once released from the second 
block. The second block can be discarded. It is noted that this deblocking 
operation is less labor intensive than prior methods involving lens blanks 
adhered to a block with pitch, while also avoiding the need for solvents 
or liquid suspension media to remove the pitch. Further, it is noted that, 
in the case a dry polishing operation has been used to polish the lens 
surface, any residues from this dry polishing operation will be washed 
from the lens during the extraction and hydration, thereby further 
avoiding the need for a solvent or a liquid medium to clean such residues 
from the lens. 
Following recovery of the lens, the lens can be subjected to conventional 
post-lathing operations, such as inspection and packaging. 
The above-described preferred embodiment relates primarily to methods 
involving lathing a contact lens button having an initial cylindrical 
shape. However, aspects of the invention involving the second block of the 
invention are also applicable to methods involving a semi-finished contact 
lens blank, i.e., methods where the initial contact lens blank has one 
molded-in lens surface, and subsequently the opposite surface of the 
button and periphery are lathe cut while this semi-finished blank is 
mounted to the described second block, to obtain an article with a final 
contact lens shape. Various other alternate embodiments and variations of 
the present invention will be evident to one skilled in the art.