Lens generating tool for generating a lens

A tool for generating a prescription surface in a plastic eyeglass lens. A pair of replaceable, cutting elements are mounted on a tool having a spindle. Each cutting element has a solid polycrystalline diamond material forming a cutting edge for progressively shaving a surface on the backside of the lens as the tool is swing back and forth. The surface so generated need only be polished, without the conventional abrasive finishing steps.

BACKGROUND OF THE INVENTION 
This invention is related to a solid polycrystalline diamond cutting tool, 
and a method for generating a prescription surface in a plastic lens used 
in spectacles, and more particularly to a method for generating a surface 
in a plastic lens blank that eliminates the finishing step required after 
the conventional lens grinding step. 
Plastic lens blanks are conventionally formed in an injection molding 
process, either of a polycarbonate or a plastic material commonly known as 
CR-39. Polycarbonate is much tougher than CR-39 and is used where safety 
is a factor in material selection. 
U.S. Pat. No. 3,685,210 which issued Aug. 22, 1972 to Gilbert John Bowen 
shows a prior art method for generating a surface on a lens. 
Most commercial processes employ grinding tools for generating the lens. 
The steps in the conventional grinding process is as follows: 
1. A prescription is delivered to the lens generating lab from the 
optometrist. The prescription is used to set the generating machine. 
2. The front side of a lens blank is coated with a dark blue blocking 
liquid. When dry, the blocking liquid functions to form a temporary bond 
with a support block. The drying time for the liquid constitutes the 
longest step in the generating process. 
3. The lens blank is then mounted in a blocking machine. A metal support 
block is placed on the coated front surface and provides means for 
supporting the lens during the generating, finishing and polishing steps. 
4. The blank with the block is then mounted on the generating machine. The 
generating machine is a standard machine which has been in use for use for 
many years. Typically a grinding tool is employed for forming the backside 
of the lens. The tool is rotated at a high speed while traversing the back 
side of the lens in a series of passes. Each pass removes a layer of 
material. However, the amount of material removed during each pass is 
limited by the nature of the grinding surface. Typically, a grinding tool 
will remove about 2 millimeters per pass, but not limited to. Since as 
much as 12 millimeters of material has to be removed, several passes are 
required. The product of the generating machine is a surface having the 
curvature of the prescription but which is not a finished surface. There 
is a considerable amount of wastage because of the nature of the grinding 
process. 
5. The lens then is transferred to the first of two cylinder finishing 
machines. In the first machine, the lens is finished with a pair of 
abrasive pads in a vibratory process. Each pad is mounted on a metal lap 
and vibrated against the lens, with water employed as a coolant for 45 
seconds to a minute and half. A second abrasive pad is then used, also 
employing water. 
6. The pads are mounted on the lap by an adhesive substance. When the lens 
has been finished, the adhesive pad which forms a tight attachment, must 
then be removed. This is a major time-consuming process. In some large 
shops, some personnel are assigned primarily for peeling the abrasive pads 
from the lap. 
7. The second finishing machine also employs a metal lap on which a thin 
felt-like pad is used for polishing the lens. The polishing step uses a 
slurry with the pad. The final polishing step takes about one to five 
minutes. 
The entire process takes about 18 to 20 minutes and in many cases requires 
a considerable amount of reworking. The felt-like finishing pad is also 
mounted by an adhesive on a metal lap and has to be peeled after each 
lens. 
In some cases a rotating cutting tool has been employed, rather than a 
grinding tool. The cutting tool uses an insert having a generally flat 
surface with a semi-circular cutting edge, however, only a small portion 
of the edge is formed of a solid polycrystalline diamond material. The 
problem is the tool performance deteriorates as the tip becomes chipped or 
worn. Further, it is difficult to cut some curvatures. 
Further, the tool is usually swung in a pivoting motion during each pass. A 
cutting element having a cutting edge only partially formed of a solid 
polycrystalline material will only cut the lens material when swung in one 
direction. It will not cut in a reverse motion which limits the usefulness 
of the generating machine. 
SUMMARY OF THE INVENTION 
The broad purpose of the present invention is to provide an improved tool, 
useful for generating a lens formed either of polycarbonate or CR-39 
plastic. The preferred tool improves the method for forming a lens by 
reducing the finishing steps, reducing scrap lens, and reducing the 
finishing time. The tool has a pair of solid polycrystalline diamond 
inserts. The tool has a spindle adapted to be rotated at several thousand 
RPM such that the cutting elements travel in an annular path of motion. 
Each cutting insert has a flat cutting face formed of a continuous layer of 
solid polycrystalline diamond material. The diamond material, as is well 
known to those skilled in the art, is formed from a powdered material 
which is placed in a press with a catalyst binder, and then cut to shape. 
Each insert has a U-shaped cutting edge and is backed up by a carbide steel 
layer. 
The two cutting elements or inserts are mounted on the tool on opposite 
sides of the axis of spindle rotation, and provide a balanced cutting 
action. 
In the preferred method, the lens blank is first attached to a block and 
then mounted in a conventional lens generating machine. The tool is 
rotated and swung about an axis so the diamond cutting elements traverse 
the back surface of the lens. The tool removes up to about 8 millimeters 
per pass, usually requiring fewer passes than a grinding tool. 
The generated surface is remarkable because the lens can be moved directly 
to the polishing machine, bypassing the time-consuming finishing machines. 
A velveteen pad, mounted on a lap employs a conventional slurry to polish 
the lens. In addition, the reworking of some lens, required by the 
conventional grinding process is virtually eliminated, resulting in 
considerable time and material savings. Further, the inventive process 
takes only about 10 to 12 minutes from blocking through polishing, which 
includes certain savings produced by an improved polishing pad. The 
improved diamond cutting tool will generate a lens either from 
polycarbonate or CR-39 lens material.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the drawings, a preferred tool 10 is illustrated in FIG. 1, 
supporting a pair of identically- shaped cutting elements 12 and 14 for 
rotation about tool spindle axis 16. The tool is mounted in a conventional 
lens generating machine having rotary power means 22. Rotary power means 
22 rotates the tool in the manner well known to those skilled in the art 
of lens generating. 
A commercially-available CR-39 lens blank 24, is releasably connected to 
block 26, and mounted in holder 27, in a stationary position, with respect 
to the tool. 
Cutting elements 12 and 14 are adapted to generate a prescription-defined, 
curved surface 28 on the back side of the lens blank. 
A typical cutting element 12, illustrated in FIGS. 2, 4 and 6, comprises a 
carbide steel body 30 connected by fastener 33 to the tool body. The 
cutting portion of the tool is a solid polycrystalline diamond insert 32, 
backed-up by a carbide steel layer 34. Insert 32 has a planar surface 36 
facing in the direction of arrow 38 which indicates the path of motion of 
the cutting element about axis 16. Insert 32 is about 0.10 cm thick, and 
has a cutting edge 40 formed with about a 5 degree bevel. Cutting edge 40 
is U-shaped with a semi-circular cutting tip 42. The cutting tip can cut a 
broad range of curvatures on the lens blank, can be reground and does not 
have a tendency to chip because the entire face is solid polycrystalline 
diamond material. 
Referring to FIG. 5, cutting elements 12 and 14 are identical. Each is 
adapted to be easily replaced on the tool by removing fastener 33. Cutting 
element 14 has a planar surface 44, corresponding to surface 36 of element 
12. The planar surfaces of the two cutting elements are disposed on 
opposite sides of the tools axis of rotation and is a common plane 48. 
FIG. 3 illustrates an earlier cutting element 100 having a U-shaped 
configuration. However, the cutting edge 102 is formed with an arc less 
than 180 degrees and is off set from the centerline of symmetry 104 of the 
cutting element. Only the area bounded by edge 102 and chord 106 was 
formed of a solid crystalline diamond material. This cutting element had 
limited practical use because when the tool is swung in a pass to shave 
the lens material, the cutting element is only useful for cutting in a 
single direction, but not in the reverse direction. 
Referring to FIG. 1, the two cutting elements are mounted on the tool which 
is then rotated about axis 16. The tool is swung in an arc 60 about a 
pivot point 61 so that initially, cutting element 12 engages the lens and 
progressively shaves the back surface of the blank during a portion of 
each tool rotation. As element 12 traverses the back surface of the blank, 
element 14 provides a follow-up cutting stroke. When the tool has been 
swung in a full pass, it is then swung in the opposite direction so that 
both elements use different portions of their respective cutting edges to 
shave the lens material. 
The two tips provide a balanced cutting means. When the lens surface has 
been generated by the two cutting elements, the lens is then immediately 
ready for polishing. 
In addition, the preferred tool eliminates the use of expensive 
intermediate finishing equipment, and is useful for cutting both 
polycarbonate and CR-39 lens material.