An opthalmic lens is typically manufactured from a lens blank which has a previously formed spherical surface on a first side. An optic surface is formed on the second side of the lens blank by cutting or grinding the appropriate shape into the blank surface. The exact shape to be cut or ground is determined based on the curvature of the first surface in conjunction with the required prescription. As is well known, for opthalmic lenses the second surface is concaved and may be spherical or toric. A toric lens has two different radii of curvature in planes which are perpendicular to each other The second radius of curvature in a toric lens is generally known as the cylinder correction.
Opthalmic lenses are frequently manufactured from lens blanks made of glass, polycarbonate, or a material known as CR39.TM. available from PPG, Industries. CR39.TM. and polycarbonate lens blanks may be surfaced by milling or cutting the blank to remove material. Glass, however, may only be surfaced by grinding the blank, typically using a diamond grinding tool.
It is common practice to produce opthalmic lenses using manually-operated grinding machines. With such machines, the lens blank is "blocked", and mounted to a support. A cup-shaped tool typically having a diameter of 3 inches to 4 inches, is mounted to the grinding machine and rotated about its axis as the tool is swept past the stationary lens. The cup-shaped tool may be tilted with respect to the lens so as to approximate the desired radius of the lens in the plane being cut. The tool is usually swept past the lens manually or by a hydraulic feed so as to generate the approximate curve on the lens surface. When the lens blank is made of polycarbonate or CR39.TM. the cup-shaped tool may include cutting blades disposed about the lip of the cup so as to form the lens surface by cutting the material from the blank. When a glass blank is used, a cup-shaped grinding tool is utilized having diamonds adhered to the lip of the cup.
The typical lens surfacing machine which utilizes a cup-shaped tool incorporates various linear way systems, frequently in combination with rotational way systems. The combination of rotational way systems and linear way systems is used to tilt the cup-shaped tool out of plane with respect to the lens blank and to sweep the tool past the lens in a predetermined path to form the desired surface.
In order to produce the wide range of radii of curvature as is necessary to be able to form all or most of the desired surfaces, without having to maintain a complete inventory of different cup-shaped tools, one of each possible radius, lens surfacing machines are designed to utilize a limited number of different cup-shaped tools to approximate all of the different radii by tilting the cup-shaped tool out of Plane with respect to the lens blank. In order to achieve the desired tilt and approximate the required radius, linear way systems and rotational way systems are incorporated together in various geometries.
A major problem with lens surfacing machines utilizing tilted cup-shaped tools to approximate the desired radii of the lens is that the desired radius is only approximated, and elliptical error is introduced into the surface. The elliptical error exists as excess material which must be removed in subsequent operations, such as lapping. Lapping requires additional equipment, tooling, time and labor.
In order to reduce the amount of elliptical error created by such lens surfacing machines, way systems have been incorporated in the machines in various arrangements. Precision way systems, typically incorporating precision slides, ballscrews and preloaded nuts, are expensive to manufacture and maintain. They can require time consuming and complex setup and alignment procedures for each lens being generated. This procedure is very labor intensive and requires a relatively high degree of operator skill in order to adjust the equipment and process as necessary to produce precision opthalmic lenses.
During the generation of the lens surface, debris, in the form of dust and chips, is produced when cutting polycarbonate or CR39.TM. lens blanks. When glass lens blanks are ground, a cutting fluid must be used to cool and lubricate the lens. The accuracy of way systems can be seriously degraded by the presence of such contamination, including cutting fluids. Way systems are also very difficult to protect directly against such contamination. For these reasons, typical prior art lens surfacing machines incorporate a flexible bellow disposed about the lens blank and the rotating cup-shaped tool to contain all of the cutting debris or cutting fluid used during the surfacing operation. The bellow is attached at either end to the respective non-rotating supports of the tool and the lens blank. Proper installation and operation of the bellow requires additional time to be expended by the operator.
Another drawback to these lens surfacing machines is the time required for changeover, setup and alignment when switching between polycarbonate or CR39.TM. lens blanks and glass lens blanks. The difference between the cup-shaped cutting tool for the respective materials is that an additional setup procedure must be followed in order to calibrate the machine to the new tool. Although quick change systems are available, changeover still requires a significant amount of time and effort. Improper setup due to changeover results in errors in the generation of the lens surface.
An alternative to the commonplace cup-shaped tool lens surfacing machines is one in which a spherical, ball nose mill is utilized in conjunction with two precision way systems and a rotating lens blank. In this system, the rotating spherical cutting tool is mounted at an angle on a precision linear way system which moves in a first direction. The lens blank is mounted and rotated about its axis, which is supported by a second linear way system that moves in a second direction which is perpendicular to the movement of the first precision linear way system. A computer controls the displacement of both linear way systems with respect to the rotating lens blank, so as to generate the surface on the lens blank.
This system has all of the drawbacks present with linear way systems of the other lens surfacing systems. It includes the expensive precision slides, ballscrews and preloaded nuts. As mentioned previously, such linear way systems require significant maintenance and lubrication. Setup and alignment can also take significant amounts of time. Additionally, the way system cannot be easily protected against contaminates or the cutting fluid used in grinding glass lens blanks. Such a system can therefore not be easily used to generate surfaces on glass lens blanks.
Thus, there is a need for a lens surfacing apparatus and method which eliminates elliptical error to generate a precision surface which need only be polished to produce an optical finish. There is a need for a system which avoids the drawbacks of precision slides and the associated cost of manufacture, and also avoids the difficulty in protecting such slides. The system needs to include the capability of cutting or grinding all suitable materials, including but not limited to polycarbonate, CR39.TM. and glass, while maintaining accuracy and incorporating easy maintenance. The system also needs to be capable of being quickly changed over to handle the various types of materials, such as glass and polycarbonate or CR39.TM.. The system should not require any particular skill to operate, and should minimize the cycle time necessary to form the lens surface.