Tool for polishing optical surfaces, method for processing optical surfaces of plastic lenses

Abstract

Tool for polishing an optical surface, comprising a base body (1) having an effective surface (3) facing the optical surface, an elastic intermediate layer (4) arranged on the effective surface of the base body and a polishing medium carrier (6) arranged on the elastic intermediate layer (4), characterized in that the elastic intermediate layer (4) extends radially beyond the effective surface (3) of the base body (1) and that the polishing medium carrier (6) extends radially beyond the elastic intermediate layer (4).

Description

[0001] The invention relates to a tool for polishing an optical surface, comprising a base body having an effective surface facing the optical surface, an elastic intermediate layer arranged on the effective surface of the base body, and a polishing agent carrier arranged on the elastic intermediate layer. The invention further relates to the use of such a tool for processing the optical surfaces of spectacle lenses made of plastic, as well as a method for processing such plastic lenses.

[0002] Spectacle lenses are typically manufactured from blanks by machining the prescription surfaces during the prescription manufacturing process. After this machining step, the optical properties of the lens are defined by the resulting surface shape. The machined surface is then polished to achieve a microscopically smooth surface and the desired optical properties of the lens.

[0003] DE 10 2007 026 841 A1 and DE 10 2005 010 583 A1 disclose a polishing disc for a tool for the fine finishing of optically effective surfaces, particularly on spectacle lenses. DE 603 ​​12 475 T2 discloses a surface finishing tool for an optical surface. US 5 095 660 A discloses a polishing device for polishing an optical lens.

[0004] Polishing typically involves using a polishing tool whose polishing surface, formed by a polishing medium carrier, is approximately adapted to the shape of the lens surface to be polished. This approximate adaptation to the shape of the lens surface is manageable with reasonable effort for polishing spherical or toric prescription surfaces. However, the proportion of high-precision freeform surfaces in spectacle lenses is increasing significantly; these are usually generated using diamond tools in a turning process on CNC-controlled machines.

[0005] Aspherical or point-symmetric surfaces and freeform surfaces exhibit curvatures that vary across the surface. During the polishing process of such freeform surfaces, the polishing tool moves across at least a portion of this irregularly curved surface. Therefore, the polishing tool must be able to adapt its flexural stiffness or elasticity to the respective local curvature in such a way that the polishing pressure across the contact area remains as constant as possible. Only then is a predictable, constant material removal achieved, and the polished surface is polished uniformly. If this is not ensured, the surface or the topography of the freeform surface will be deformed, and its optical quality will be impaired. Conversely, local irregularities resulting from the machining process, such as grooves, waves, or center defects, should be completely removed.

[0006] The invention is based on the objective of creating a tool and a method with the aforementioned properties that allows for rapid, cost-effective and optically high-quality polishing of optical surfaces, especially of plastic lenses.

[0007] In the tool according to the invention, this problem is solved by the elastic intermediate layer projecting radially beyond the effective surface of the base body and by the polishing agent carrier projecting radially beyond the elastic intermediate layer.

[0008] First, some terms used within the scope of the invention will be explained. The tool according to the invention serves for the fine finishing of an optical surface, in particular for polishing such an optical surface. This is a process in which no or no significant change is made to the shape of this surface.

[0009] The base body serves to connect the tool to a machine tool, in particular a CNC-controlled system. For this purpose, it usually has a corresponding mounting.

[0010] The base body has an effective surface that, during use, faces the optical surface of the glass being processed. The effective surface is therefore the surface of the base body through which the force required for processing is transferred to the optical surface of the glass.

[0011] An elastic intermediate layer is arranged on the working surface of the base body. This is preferably an elastic foam. Due to the elastic deformability of this layer, the polishing surface of the tool according to the invention, formed by the polishing medium carrier (which will be explained later), can adapt to a certain extent to the geometry of the glass being processed.

[0012] A polishing agent carrier is arranged on the elastic intermediate layer. This is a part of the tool according to the invention that comes into direct contact with the optical surface of the glass to be processed.

[0013] According to the invention, the surfaces of the polishing medium carrier, the elastic intermediate layer, and the working surface of the base body facing the optical surface to be processed are successively reduced, meaning that the polishing medium carrier has a radial overhang relative to the elastic intermediate layer, and the elastic intermediate layer has a radial overhang relative to the base body. The radial direction is the direction that lies in the plane or tangential plane of the optical surface to be processed, i.e., approximately perpendicular to the axis of rotation of the tool. In this context, an overhang means that an axially acting force of the base body no longer acts directly on the overhanging area of ​​the elastic intermediate layer, and correspondingly, an axial force of the elastic intermediate layer no longer acts directly on the overhanging area of ​​the polishing medium carrier.

[0014] The technological objective in polishing plastic lenses is to create a microscopically smooth surface structure free of waviness. This involves eliminating both the grooves created by the turning process during machining and any optically disruptive waviness on the surface. Such waviness inevitably occurs to a certain extent during the turning process with diamond tools, for example, due to inaccuracies in the diamond cutting edge contour, influences from the machine control system, and / or minor inaccuracies in mechanical components of the lathe, such as the bearings of the machine axes. Furthermore, a surface defect typical of this process develops in the center of the optical surface produced by turning, primarily due to the near-zero cutting speed at this point.Such a surface defect in the center of the lens is often exacerbated by the fact that, in practice, the position of the turned diamond is not always perfectly aligned. Typical characteristics of such optically relevant waviness in diamond-turned lenses are amplitudes in the range of 0.5 to 3 µm and wavelengths of approximately one to several millimeters. A central defect as mentioned above typically has a height of approximately 0.5 to 3 µm and a diameter of approximately 1 to 3 mm.

[0015] The inventive design of the tool with the described “double overhang” makes it possible to provide a tool that, on the one hand, has a large active surface (a large area of ​​the polishing medium carrier that comes into contact with the optical surface of the glass during use) and, on the other hand, has a good adaptability of this active surface to the optical surface of the glass, even if it is an irregularly shaped optical surface, for example a freeform surface.

[0016] In contrast, in the prior art such freeform surfaces are machined with polishing tools that are smaller than the optical surface (diameter e.g. less than 50 mm), as these adapt more easily to the optical surface to be machined than larger tools.

[0017] The use of tools with a relatively large active surface area, made possible by the tool according to the invention, allows for significantly faster processing of an optical surface and thus a shortening of the polishing process.

[0018] For a high-quality polishing result from an optical perspective, it is advantageous if the polishing force exerted by the tool on the glass decreases towards the outer edge of the tool's surface, ideally approaching zero continuously. Otherwise, visible spiral structures can form on the polished glass, which degrade the surface quality.

[0019] The overhang of the elastic intermediate layer present in the tool according to the invention, extending over the effective surface of the base body on the one hand and of the polishing medium carrier over the elastic intermediate layer on the other, enables a good approximation of the ideal of a continuous decrease in polishing force towards the edge. The polishing force acting in the center of the tool (in line with the drive axis of the machine tool) approaches zero at the edge of the tool, since only the unsupported, generally flexible polishing medium carrier is present there, upon which the axially acting polishing force hardly acts in this edge region.During rotation of the tool, this edge area receives only a small force component in the direction of the optical surface to be machined due to components of the centrifugal forces, especially if it is a concave optical surface, and also a small force component that depends on the bending stiffness and elasticity of the polishing medium carrier used.

[0020] In the transition area where the polishing medium carrier is only supported by the elastic intermediate layer, this elastic intermediate layer results in a lower force being applied in the axial direction (polishing force), which depends essentially on the material properties of this elastic intermediate layer.

[0021] The forces acting in the axial direction in this transition area can be increased by placing an additional layer, such as a tear-resistant polyurethane film, between the base body and the elastic intermediate layer (foam), for example by gluing.

[0022] This increases the flexural stiffness of the elastic intermediate layer on this side, while at the same time not fundamentally changing the elastic material properties relevant for the polishing process.

[0023] A stronger application of axially acting polishing force only occurs in the central area of ​​the tool, which is fully or essentially fully supported by the working surface of the base body.

[0024] Preferably, according to the invention, the working surface of the base body has a surface curvature that is at least approximately adapted to a glass optical surface to be processed (i.e., designed approximately as a counter-surface). This allows for a relatively uniform force transmission to the optical surface to be processed. The working surface can, in particular, be spherical or toric. Accordingly, a plurality of tools must be provided for different surfaces to be processed (e.g., convex or concave curved surfaces).

[0025] An exact design of the working surface as a counter-surface, for example to freeform surfaces of spectacle lenses, is not necessary; the design according to the invention allows a sufficiently uniform force transmission with a continuous decrease in the polishing force towards the edge of the tool, so that a good surface quality can be achieved with low material removal and the polishing defects described above do not occur or only occur to an insignificant extent.

[0026] Preferably, the overhang of the elastic intermediate layer beyond the effective surface of the base body is 2-10 mm in the radial direction, and more preferably 3-8 mm. Likewise, the overhang of the polishing medium carrier beyond the elastic intermediate layer is preferably 2-10 mm in the radial direction, and more preferably 3-8 mm. This overhang in the edge region of the tool improves its adaptability to the geometry of the optical surface being machined and also reduces the polishing force towards the edge of the tool, which is important for the optical quality of the polished surface.

[0027] The effective diameter of the polishing medium carrier (measured diametrically from edge to edge of the polishing medium carrier in its maximum radial extent, including the overhanging edge regions) is preferably 40–80 mm, more preferably 50–70 mm. This preferred embodiment is a tool with a relatively large diameter, particularly suitable for processing plastic lenses, and thus a relatively large effective polishing surface, enabling rapid processing of the optical surface. The design according to the invention, with the radially projecting or overhanging areas of the elastic intermediate layer and the polishing medium carrier, allows plastic lenses to be processed with a tool that is very large in relation to the optical surface being processed, without impairing the quality of the optical surface.The invention thus combines the advantage of large tools in terms of efficiency and short processing times with the advantage of smaller tools in terms of adaptability to different shapes of the optical surfaces to be processed and in terms of a largely homogeneous pressure distribution over the surface to be processed during the processing operation.

[0028] The diameter of the base body's effective surface is preferably 50–85% of the effective diameter of the polishing medium carrier. Further preferred ranges are 60–70%. The resulting overhang of the polishing medium carrier beyond the edge of the base body provides the advantages described above regarding adaptability to the geometry of the optical surface to be polished and reduction of the polishing force towards the edge of the tool. Both the effective surface of the base body and the polishing medium carrier are preferably substantially circular to facilitate uniform polishing during the usual rotational movement of the tool. The polishing medium carrier may have interruptions, openings, or recesses at its circumferential edge, as will be described in more detail below.

[0029] The elastic intermediate layer preferably comprises a foam, more preferably a foam with a static modulus of elasticity of 0.1-0.5 N / mm². 2 , preferably 0.2-0.4 N / mm 2 Such a foam supports the desired distribution of the polishing force with a decrease towards the edge of the tool. The static modulus of elasticity is measured according to DIN 53513 at the upper limit of the static application range; the values ​​apply to a form factor q=3 and a material thickness of 25 mm. Suitable materials include, for example, mixed-cell polyurethanes, such as Sylomer® foams from Getzner Werkstoffe GmbH, preferably Sylomer® SR42.

[0030] The thickness of the elastic intermediate layer (in the axial direction) can preferably be between five and 15 mm, more preferably between seven and 13 mm. For example, it can be 10 mm.

[0031] According to the invention, the polishing agent carrier can be a foam, preferably a foam with a density of 0.4-0.7 g / cm³, more preferably 0.5-0.6 g / cm³. 3 The Shore A hardness is preferably between 80 and 95, and more preferably between 85 and 95. Suitable polishing media carriers are available, for example, from Universal Photonics under the name LP Unalon®. These are microcellular polyurethanes. The foams can be unfilled or filled with suitable abrasives such as metal oxides (e.g., corundum, cerium oxide, zirconium oxide), diamond, boron nitride, or the like. Suitable polishing media carriers include, for example, LP-57 (unfilled, density 0.51 g / cm³). 3 , Shore A hardness 88) or GR-35 (filled with zirconium oxide, density 0.59 g / cm³) 3 , Shore A hardness 90). These preferred variants are relatively hard polishing agent carriers.

[0032] In the prior art, plastic lenses are typically polished using soft, fibrous, or felt-like materials as a polishing medium carrier. Surprisingly, it has been shown that, according to the invention, the use of an unusually hard polishing medium carrier is possible, with which a very good smoothing effect against waviness can be achieved while simultaneously minimizing material removal. With these polishing medium carriers, preferred according to the invention, significantly higher polishing forces and relative speeds can be used when processing plastic lenses compared to conventional soft polishing medium carriers due to the considerably lower coefficient of friction, without overheating, mechanical overload, or breakdown of the lubricating film. The porosity of the surface serves as a lubricant reservoir.The use of a polishing medium carrier with a preferred density and Shore A hardness for machining the optical surfaces of plastic lenses may therefore warrant separate protection, regardless of the specific design of the tool. The plastic lenses to be machined can consist of polyurethane or polycarbonate materials. Particularly suitable plastic materials for machining include, for example, allyl diglycol carbonates such as CR-39® from PPG Industries or polyurethanes such as the MR® series from Mitsui Chemicals, for example, MR-7 or MR-8.

[0033] If the optical surface of plastic lenses treated with the polishing media mentioned here does not achieve the desired properties with regard to its microscopic structure, a further polishing step can be performed using a soft, felt-like, or fibrous coating material corresponding to the known state of the art. A suitable tool for such a fine polishing step is fundamentally constructed in the same way as the one described here.

[0034] According to a preferred embodiment of the invention, a carrier film can be additionally arranged between the polishing medium carrier and the elastic intermediate layer. The purpose of this carrier film is to reinforce the polishing medium carrier in order to increase its stability, particularly in the area overhanging the elastic intermediate layer. This carrier film can, for example, be a tear-resistant polyurethane film. Such an additional carrier film can contribute to giving the tool the desired shear, compressive, and tear resistance, as well as the robustness and service life desired under production conditions.

[0035] The polishing medium carrier can be formed as a solid surface, meaning it acts on the optical surface to be processed with a closed (preferably circular) area. According to a further embodiment, the polishing medium carrier can have openings. These openings can be, for example, openings, slots (especially slots extending radially from the edge to a central area), or an edge design that deviates from a circular ring, such as a serrated or wavy edge. The openings can serve as a reservoir for polishing medium and, particularly with a corresponding edge design, contribute to a decrease in the polishing force towards the edge of the tool.Particularly in such a design of the polishing medium carrier with perforations, the carrier film arranged between the polishing medium carrier and the elastic intermediate layer can significantly contribute to giving the tool the desired mechanical properties and sufficient stability.

[0036] The invention further relates to the use of a tool as described above for processing optical surfaces of plastic lenses. The processed plastic lenses preferably consist of the materials already described in more detail above, which can be processed particularly well with a tool according to the invention.

[0037] The invention further relates to a method for processing optical surfaces of plastic lenses, comprising the steps of: a) providing a tool according to the invention, b) Processing an optical surface of the plastic glass with the following process parameters: - Effective polishing force: 60-110 N, preferably 70-95 N, - mean relative velocity between optical surface and tool 3-6 m / s, preferably 4-5 m / s.

[0038] The aforementioned relative speed ranges allow the processing time for a polishing operation to be significantly reduced, preferably to a period of 1 minute or less, and more preferably 30 seconds or less.

[0039] An embodiment of the invention is explained below with reference to the drawing. The drawing shows: Fig. 1: Schematic representation of an axial section through a tool according to the invention; Fig. 2: A top view of the tool from the side of the polishing medium carrier.

[0040] A tool according to the invention has a substantially rotationally symmetrical base body 1, which has a receptacle 2 for a corresponding holder of a machine tool, a machining robot, or the like. The tool can be set into rotation via the receptacle 2, and a force can be exerted in the axial direction (in the direction of the rotational symmetry axis of the tool or the base body 1).

[0041] The base body 1 has an effective surface 3 pointing towards the optical surface to be machined, which in this embodiment is convexly curved and has a diameter of 42 mm. Thus, the tool of this embodiment serves to machine concavely curved optical surfaces.

[0042] An elastic intermediate layer 4 is applied to the effective surface 3 of the base body 1, preferably bonded to it. In the exemplary embodiment, it has a diameter of 50 mm and an axial thickness of 10 mm. The material of this elastic intermediate layer 4 is Sylomer® SR42. The static modulus of elasticity of this material, determined according to the method described above, is 0.282 N / mm². 2 .

[0043] The elastic intermediate layer 4 thus extends radially 4 mm beyond the effective surface 3 of the base body 1 at its edge.

[0044] A tear-resistant PU film (D44, Getzner) is applied, preferably glued, to the end face of the elastic intermediate layer 4 facing away from the effective surface 3. In the exemplary embodiment, it has a diameter of 58 mm and a thickness of 1.0 mm.

[0045] The polishing agent carrier 6 is attached to the carrier film 5, preferably by adhesive. In the exemplary embodiment, GR 35 is used as the polishing agent carrier, the properties of which have already been described in more detail above.

[0046] In this embodiment, the largest diameter of the polishing medium carrier 6 is 58 mm, so that radially the polishing medium carrier 6 is flush with the carrier film 5. The carrier film 5 and the polishing medium carrier 6 project radially beyond the elastic intermediate layer 4 by 4 mm on each side.

[0047] The polishing medium carrier 6 has six openings, which extend radially from the edge towards the center as slots 7. The radial extent of each slot 7 from the edge towards the center is approximately 20.5 mm, and the circumferential width is approximately 2 mm. Thus, a circular segment with a diameter of approximately 17 mm remains free of openings in the center of the polishing medium carrier 6.

[0048] The complete process of manufacturing a spectacle lens using a tool according to the invention is explained below by way of an example.

[0049] A blank spectacle lens made of CR-39 material is provided, the shape of which already meets the optical requirements. The blank is mounted on a fixture suitable for machining in a CNC machine. This can be done by blocking it onto a block or clamping it in a suitable holding device.

[0050] In the next step, the desired recipe surface is generated using a milling and / or turning process.

[0051] In the next step, polishing is carried out according to the inventive method using the polishing tool described above in the exemplary embodiment. Poly Pro All Format (Satisloh) is used as the polishing medium. The effective polishing force is 70-95 N, and the average relative speed between the processed optical surface and the polishing tool is 4-5 m / s. The polishing time is between 15 and 25 s, during which five irregular pivoting movements with continuously changing end positions are performed.

[0052] The service life of a tool according to the invention when carried out in such a method according to the invention is approximately 200 glasses.

[0053] The processed glass has an optical surface without zones, streaks, or the like. The Ra value (average roughness according to DIN EN ISO 4287-1998) is in the range of approximately 6 nm; the roughness can be further reduced by a subsequent coating process with hard lacquer.

[0054] Optionally, an additional fine polishing process can be added. For example, a fibrous polishing pad (e.g., crystal from DAC) can be used for approximately 10 seconds. This results in a surface roughness Ra of approximately 4 nm. Reference symbol list 1 Basic body 2nd recording 3 Effective area 4 Intermediate layer 5 carrier film 6 Polishing medium carriers 7 openings / slots

Claims

[1] Tool for polishing an optical surface, comprising a base body (1) having an active surface (3) facing the optical surface, an elastic intermediate layer (4) arranged on the active surface of the base body and a polishing agent carrier (6) arranged on the elastic intermediate layer (4), characterized by that the elastic intermediate layer (4) projects radially beyond the active surface (3) of the base body (1) and that the polishing agent carrier (6) projects radially beyond the elastic intermediate layer (4). [2] Tool according to claim 1, characterized by that the effective surface (3) of the base body (1) has a spherical or toric surface curvature. [3] Tool according to claim 1 or 2, characterized by that the elastic intermediate layer (4) projects radially beyond the active surface (3) of the base body (1) by 2 to 10 mm, preferably 3 to 8 mm. [4] Tool according to one of claims 1 to 3, characterized bythat the polishing agent carrier (6) projects radially beyond the elastic intermediate layer (4) by 2 to 10 mm, preferably 3 to 8 mm. [5] Tool according to one of claims 1 to 4, characterized by that the effective diameter of the polishing agent carrier (6) is 40 to 80 mm, preferably up to 70 mm. [6] Tool according to one of claims 1 to 5, characterized by that the diameter of the effective surface (3) of the base body (1) is 50-85%, preferably 60-70%, of the effective diameter of the polishing agent carrier (6). [7] Tool according to one of claims 1 to 6, characterized by that the elastic intermediate layer (4) has a static modulus of elasticity of 0.1 to 0.5 N / mm 2 , preferably 0.2 to 0.4 N / mm 2 has. [8] Tool according to one of claims 1 to 7, characterized by that the thickness of the elastic intermediate layer (4) is 5 to 15 mm, preferably 7 to 13 mm. [9] Tool according to one of claims 1 to 8, characterized by that the polishing agent carrier (6) is a foam with a density of 0.4 to 0.7 g / cm 3 , preferably 0.5 to 0.6 g / cm 3 is. [10] Tool according to one of claims 1 to 9, characterized by that the polishing agent carrier (6) is a foam with a Shore A hardness of 80 to 95, preferably 85 to 95. [11] Tool according to one of claims 1 to 10, characterized by that a carrier film (5) is additionally arranged between the polishing agent carrier (6) and the elastic intermediate layer (4). [12] Tool according to one of claims 1 to 11, characterized by that the polishing agent carrier (6) has openings (7). [13] Use of a tool according to one of claims 1 to 12 for machining optical surfaces of plastic lenses. [14] Use according to claim 13, characterized bythat the plastic lenses consist of a plastic material selected from the group consisting of polyurethanes and polycarbonates. [15] Method for processing optical surfaces of plastic lenses, comprising the steps: a) providing a tool according to one of claims 1 to 12, b) Processing an optical surface of the plastic lens with the following process parameters: - effective polishing force: 60-110 N, preferably 70-95 N, - average relative speed between optical surface and tool 3-6 m / s, preferably 4-5 m / s.

Images