The present invention relates to a polishing apparatus and, more particularly, to a polishing apparatus which arranges a polishing pad on the outer surface of an elastic dome formed on a base, thereby polishing lenses with various shapes.
Conventionally, to polish, using a polishing apparatus, the concave surface of a lens cut into a spherical or toric surface shape by an NC-controlled curve generator, a polishing pad is bonded to a metal polishing jig having a convex surface almost conforming to the shape of the concave surface to be polished. The polishing jig and lens are relatively slid while pressing the polishing pad against the concave surface to be polished.
In this polishing method, however, various polishing jigs must be prepared in accordance with the shapes of the concave surfaces of lenses to be polished. For, e.g., a toric lens for correcting astigmatism, there are 3,000 to 4,000 kinds of toric surfaces (part of a surface obtained by rotating an arc about an axis that is present in the same plane as that of the arc and does not pass through the center of the curvature of the arc), so a corresponding number of polishing jigs must be prepared. This increases the manufacturing cost of polishing jigs. In addition, a large storage space is necessary, and management thereof is cumbersome.
Not only a spherical surface or a toric surface but also a concave surface having a complex shape such as an aspherical surface (part of a surface of revolution whose curvature continuously changes from the apex to the periphery) shape, an atoric surface (a surface having principal meridians which have different curvatures and are perpendicular to each other, and the section of at least one principal meridian is a non-circular surface) shape, or a free-form surface shape of, e.g., a progressive-power lens may be formed. Such a concave surface cannot be polished by the conventional polishing method using a polishing jig.
As a method of solving these problems, for example, a polishing apparatus and polishing jig described in Japanese Patent Laid-Open No. 2000-117604 are known. This polishing apparatus comprises a holding tool which holds an object to be polished, a polishing jig having a flexible sheet that is expanded to a dome shape by a fluid pressure, and a polishing pad bonded to the surface of the flexible sheet. The surface to be polished in the object to be polished is polished by an abrasive supplied between the polishing pad and the surface to be polished along a trackless polishing locus in which the polishing locus shifts little by little for each revolution in accordance with the left-and-right/fore-and-aft movement of the holding tool and the swiveling movement of the polishing jig.
In polishing, the curvature of the dome is changed by changing the internal pressure of the flexible sheet. When the concave surface is a toric surface, and curvatures in directions perpendicular to each other are largely different, a spherical dome may not be able to cope with such a concave surface. In this case, presser jigs are pressed against the flexible sheet near the two end portions in one of the directions perpendicular to each other in the flexible sheet, thereby suppressing expansion of the sheet by the fluid pressure. Since the dome can have different curvatures in directions perpendicular to each other, a surface almost similar to the toric surface of the object to be polished can be obtained.
When the curvature of the dome is changed by the fluid pressure and presser jigs, one jig can cope with concave surface shapes in a wide range. For this reason, different polishing jigs need not be prepared in accordance with the shape of the concave surface. Hence, the number of polishing jigs can be greatly decreased.
In the polishing jig described in Japanese Patent Laid-Open No. 2000-117604, however, the peripheral portion of the flexible sheet is sandwiched and fixed by the disk-shaped fixing jig main body and a press jig which has a flat circular ring shape having the same diameter as that of the fixing jig main body. A sealed space is formed between the fixing jig main body and the flexible sheet, and the flexible sheet is expanded to a dome shape by the fluid pressure. In addition, the pair of presser jigs for suppressing the expansion of the sheet are attached onto the press jig so as to freely move in the radial direction of the dome. When the concave surface of a lens is to be polished by relatively sliding the polishing jig and lens which are kept in contact with each other, the lens must be prevented from touching the press jig and presser jigs located aside near the polishing surface of the polishing jig and, more particularly, the presser jigs.
When the sliding distance for polishing is shortened to keep the lens from the press jig or presser jigs, a large lens cannot be polished. To ensure a sufficient sliding distance, the polishing surface area is made much larger than the lens surface. In this case, the polishing jig becomes bulky. Additionally, when the dome curvature for the large polishing surface is increased, the polishing jig becomes considerably high. Also, to polish lenses with various diameters and concave surface shapes by one polishing jig using presser jigs, the size of the polishing jig must be set on the basis of the largest diameter lens to be polished. This also increases the polishing jig size. If the polishing jig is bulky, the weight and moment of inertia become large. This may impede the swiveling movement of the polishing jig.
Furthermore, in the above-described polishing jig, the polishing pad must be bonded to the dome surface by an adhesive because of the structure of the jig itself. Attaching/detaching the polishing pad is time-consuming.
To polish the concave surface of a lens, the curvature of the dome portion must almost equal the curvature of the lens. To do this, the polishing jig described in Japanese Patent Laid-Open No. 2000-117604 sets the curvature of the dome portion by the internal pressure of the flexible sheet. However, if the deformation amount of the dome portion is large with respect to the pressure variation amount, the pressure is hard to adjust in accordance with the curvature. In addition, when the flexible sheet degrades due to a change over time, the correlation between the pressure and the curvature changes. Hence, even when the pressure is kept unchanged, no desired curvature can be obtained.
The lens holding tool used together with the above-described polishing jig is generally formed from a lens holder unit and a low-melting alloy (to be also referred to as an alloy layer hereinafter).
As a typical conventional lens holding tool, a tool described in, e.g., U.S. Pat. No. 5,421,770 is known. FIG. 34 shows this lens holding portion. Referring to FIG. 34, reference symbol A denotes a lens holder unit; B, a lens; and C, an alloy layer. The lens holder unit A has a recess portion D in a surface a opposing the lens B. The recess portion D has, at its outer periphery, a step E that rises at an acute angle. A plurality of hollows F are vertically formed in the recess portion D. The step E prevents removal of the lens holder unit A from the alloy layer C. The hollows F prevents rotation of the lens holder unit A with respect to the alloy layer C. For these reasons, the lens holder unit A and alloy layer C are firmly connected.
When the lens B is held by the lens holder unit A and alloy layer C, the lens B deforms due to the influence of heat of the alloy layer C or shrinkage of the alloy layer C in hardening, as is known (Japanese Patent Laid-Open No. 7-116950).
Conventionally, however, plastic lenses for glasses are formed using a diethylene glycol bisallylcarbonate-based resin (n=1.50) that is a most general-purpose plastic lens material. In addition, a semifinished lens (a lens in which only the first refractive surface is optically finished) is designed to be thick. For these reasons, when the lens is fixed to the lens holder unit through the hardened low-melting alloy, the influence of heat or shrinkage of the alloy layer is small.
However, since lens materials have high refractive indices, and semifinished lenses become thinner recently, the influence of heat and shrinkage of the alloy layer increases. It is therefore urgently necessary to improve the lens holding tool. More specifically, an urethane- or epithio-based resin having a refractive index of 1.55 to 1.75 is used as a lens material in place of the diethylene glycol bisallylcarbonate-based resin having a refractive index of 1.5. In addition, to meet the requirement for reducing the material cost and saving the resources, the semifinished lens is thinned to reduce the cut amount on the concave surface side. Then, the influence of heat and shrinkage of the alloy layer becomes large. Especially, a semifinished lens for a minus-power lens is greatly influenced by heat and shrinkage of the alloy layer because the lens is thin at its center. Moreover, the alloy layer of the conventional lens holding tool has a large amount because of the above-described structure that increases the connection strength between the alloy layer and the lens holder unit. Hence, the influence of heat and shrinkage of the alloy layer is large.
In Japanese Patent Laid-Open No. 7-116950 described above, a bottom plate is inserted into the space between the lens holder unit and the lens to reduce the amount of alloy, thereby preventing deformation due to shrinkage at the time of hardening. However, even when the amount of alloy is reduced, the influence of heat and shrinkage may still remain because the central thickness of the alloy layer changes depending on the type of lens.
Conventionally, only the central portion of a lens is held by the alloy layer. Hence, the strength of lens at the central portion where the alloy layer is present is different from that at the outside portion. If the concave surface is cut or polished in this holding state, polishing marks may be formed on the concave surface at a portion corresponding to the boundary between the portion with the alloy layer and the portion without the alloy layer on the convex surface side.
To cut a concave surface in the preprocess of the lens polishing process, an NC-controlled curve generator is generally used. However, when the concave surface of a lens is cut using the curve generator, a process step (undulation) is formed on the cut surface due to backlash.
More specifically, the tool (turning tool) for cutting a lens moves vertically and horizontally and makes a complex movement with inflection points. When the turning tool is moved using a ball screw, and the direction of rotation of the ball screw changes, a process step M having a size of several μm is formed near an inflection point due to, e.g., backlash generated by the play of the ball screw, as shown in FIGS. 35A and 35B. Even when the turning tool is moved using a linear motor, a similar process step is formed due to, e.g., a delay in control when the moving direction reverses. The process step M must be removed in the next polishing step to obtain a concave surface having a desired curvature. FIG. 35A shows a state during polishing using a polishing pad P. FIG. 35B shows a state after polishing. Reference symbol S denotes a concave surface of a lens; and T, a balloon member (to be described later) of the polishing jig.
However, when a surface is polished using the polishing jig which expands a sheet by a fluid pressure to form a dome-shaped surface, the polishing surface is elastic. For this reason, even when the concave surface S is polished using the relatively soft polishing pad P (made of, e.g., a non-woven fabric) that is conventionally used in a metal polishing jig, as shown in FIG. 35A, the polishing pad and dome-shaped surface follow the shape of the process step M, as shown in FIG. 35B. For this reason, the process step M cannot be completely removed. The process step M with a size of about 1 to 2 μm still remains. In this case, when the polishing time is prolonged, and a polishing margin corresponding to the process step is added to the normal polishing margin, the process step M can be removed. However, since the surface must be polished more than necessity, the polishing time becomes long. In addition, the outer appearance quality and optical accuracy of the lens degrade.