Patent Publication Number: US-4928435-A

Title: Apparatus for working curved surfaces on a workpiece

Description:
This application is a continuation, of now abandoned application Ser. No. 06/865,419 filed May 21, 1986. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an apparatus for precisely working aspheric surfaces having mirror surfaces of metals, ceramic materials, glass materials, and so on. 
     2. Description of the Prior Art 
     High precision mirror surface processing of a mold for a plastic lens is conventionally performed by cutting or grinding with a computerized numerical control (CNC) lathe or curve generator and after which polishing is performed by a polishing machine or by hand works. Another conventional method for processing a mold for a plastic lens is by processing the material to be formed as the mold by single point diamond turning. 
     On the other hand, hard metals or ceramics are used as a mold for a glass lens. The mold made of hard and brittle materials such as ceramics or glasses is conventionally worked by moving a diamond grinder, which rotates at a high speed, along a desired working shape to simultaneously work the material into a desired configuration with a mirror surface. 
     When a free curved surface of a mold is worked by CNC milling or by CNC electrical discharge machining, cutting, grinding or electrical discharge machining is performed, control is provided with CNC positioning in accordance with predetermined working data. The curved surface worked by those methods has a surface roughness of the order of several micron meters and a mirror surface is not obtained. The process tolerance of the processed surface is on the order of several micron meters which is defined by the accuracy of the processing machines. As a result, a finishing process is required to obtain a mirror surface. Usually, the finishing process is performed by polishing the workpieces by hand with observing the processed shape. Instead of the hand polishing, automatic polishing methods for curved surfaces are sometimes introduced. 
     However, the conventional methods have several defects. When the cutting or grinding is performed, the worked surface has a surface roughness on the order of a micron meter and the shape accuracy is also in the range of a micron meter. Therefore, it is necessary to polish the workpiece to obtain an accuracy on the order of sub-micron meter by an operator&#39;s hand polishing which requires operator&#39;s skill and inefficiency. The electrical discharge machining has the same defects. 
     The diamond cutting and diamond grinding can obtain the surface roughness and shape accuracy on the order of a sub-micron meter which is able to realize processing a mirror surface. However, these methods require a finishing process to eliminate trace marks of the tool. As described above, the conventional methods require a preworking process for obtaining a desired shape and a finishing process for obtaining a mirror surface. Furthermore, the grindstone and the rotating mechanism thereof in a grinding process and the polisher in polishing process are difficult to miniaturize in terms of their sizes, and it is difficult to grind or polish a minute region on the curved surfaces. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an apparatus for working aspheric surfaces on a workpiece which is able to shape a free curved surface and finish the same as a mirror surface simultaneously and easily. 
     It is another object of the present invention to provide an apparatus for working aspheric surfaces on a workpiece which makes it possible to form the curved surface as a mirror surface on the order of a sub-micron meter in roughness. 
     It is a further object of the present invention to provide an apparatus for working aspheric surfaces on a workpiece which allows a minute area to be accurately worked. 
     According to the present invention, an apparatus is provided which comprises a spindle having a tool at one end thereof, means for making a central axis of the spindle parallel with the normal direction at every point on a surface which is to be fabricated on a workpiece such that the center axis of the spindle coincides with the normal direction at a point on the assumed aspherical surface to be fabricated where the center axis of the spindle intersects the assumed aspherical surface, means for driving the spindle and tool, and means for contacting the tool to the workpiece with a constant pressure. The tool is preferable a flat type polisher or a spherical type polisher. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described in detail by way of illustrative example with reference to the accompanying drawings, in which: 
     FIG. 1 is a perspective view of an apparatus for working curved surfaces on a workpiece according to the present invention; 
     FIG. 2 is a front view, partly in cross section, of a working head portion of the apparatus of FIG. 1; 
     FIG. 3 is a fragmentary front view of the working head portion of the apparatus of FIG. 1; 
     FIGS. 4A to 4C are cross-sectional side views explanatory of a process of working a workpiece according to the present invention; 
     FIG. 5 is a front view, partly in cross section, of another embodiment of the working head portion of the apparatus according to the present and invention; 
     FIGS. 6 and 7 are respective explanatory front views of third and fourth embodiments of the working head portion of the apparatus according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows an arrangement of an apparatus for working curved or aspheric surfaces on a workpiece. Reference numeral 1 designates an X-axis mechanism which determines the location of a workpiece 4 in a horizontal or X-axis direction. The X-axis mechanism 1 comprises a fixed plate 41 and a slide plate 42 slidably mounted on the fixed plate 41. A rotary table 2 is rotatably mounted on the slide plate 42 of the X-axis mechanism 1. On the rotary table 2, a work chuck 3 is provided for supporting the workpiece 4 thereon in such a manner that the axis of rotation of the workpiece 4 coincides with the axis of rotation of the rotary table 2. A tool 5, such as a polisher, is mounted on a lower end of a rotatable spindle 6. The spindle 6 is freely slidable along the direction of the axis of rotation thereof (hereafter called the R-axis) with a slide means 7 slidably mounted on an R-axis mechanism 9. The R-axis mechanism 9 is provided on one surface of a θ-axis mechanism 10 in order to rotationally shift the spindle 6 around a θ-axis which is perpendicular to the R-axis. On the opposite surface of the θ-axis mechanism 10, a balancer 11 is provided for well balancing the θ-axis mechanism 10 to the rotationally shifted spindle 6. A roller 12 rotates along a guide face 14 of the balancer 11 to rotate the balancer 11 about the θ-axis, whereby the θ-axis mechanism 10 is rotated about the θ-axis. A rotary encoder 13 detects the angular location of the θ-axis mechanism 10. The θ-axis mechanism 10 is contained in a housing 16 which is movable in a vertical direction, hereinafter called the (Z-axis direction) along a screw 17 of a Z-axis mechanism 15. 
     Referring to FIG. 2, a spindle rod 61 of the spindle 6 is supported rotatably and movably in the axial direction thereof (R direction) in a casing 21. On the lower end of the spindle rod 61, a plate polisher 51 housing convex surfaces as the tool 5 is provided. The polisher 51 is made of cast iron, tin, pitch or polyurethane. On the top end of the spindle rod 61, a base plate 62 is provided which is disposed in a cavity in the casing 21. A compressed spring 8 is mounted between the base plate 62 of the spindle 6 and a lower part of the slide means 7. The spring 8 holds the spindle rod 61 and polisher 51 in constant pressure. The base plate 62 is rotated by a capstan 28 which is mounted on an output shaft 27 of a motor 26. The casing 21 is rotated by the θ-axis mechanism 10 for adjusting the axial direction R of rotation of the spindle rod 61 to coincide with a direction which is normal to an arbitrary point P on a surface 4a, which is an assumed surface to be fabricated by working the surface 4b of the workpiece 4. In FIG. 2, the axial direction R is inclined by an angle θ from the vertical direction. 
     Referring now to FIG. 3, operation of the apparatus will be explained. In FIG. 3, the polisher 51 is a slender one, but the plate polisher as shown in FIG. 2 may be also preferable. 
     In FIGS. 2 and 3, working data for coordinates X, Z, and angle θ for working the surface 46 into the desired shape 4a, which are calculated from a preworked shape of the workpiece 4, are stored in a control portion not shown in the figures. In operations, the R-axis mechanism 9 adjusts the location of the polisher 51 in such a manner that the top of the polisher 51 lies on the axis of rotation of the θ-axis mechanism 10. At the same time, the θ-axis mechanism 10 adjusts its rotational location in angular position to coincide the axial direction of the spindle rod 61 with the normal direction at a point P on an aspherical surface of the workpiece 4 in accordance with to the working data x, Z and θ of the point P. After the adjustment, the polisher 51 is placed in contact with the preworked surface 4b of the workpiece 4 in compression state to release the compression spring 8. In this state, the spindle rod 61 rises and falls in response to the shape errors of the preworked surface 4b of the workpiece 4. When the motor 26 is driven, the spindle rod 61 and the polisher 51 are rotated through the capstan 28. At the same time, the workpiece 4 is rotated by rotating the rotary table 2. As a result, the workpiece 4 is worked by the aid of grinding particles and working liquid. 
     FIGS. 4A to 4C designate working steps. FIG. 4A shows a preworked state. The preworked surface 4b of the workpiece 4 has an uneven shape compared to the desired surface 4a to be worked. When the polisher 51 is rotated in a clockwise direction as shown by an arrow with the grinding particles 30 such as diamond particles, the preworked surface 4b of the workpiece 4 is worked as shown in FIG. 4B. In this step, the size of the grinding particles is comparatively large because of the uneven surface of the workpiece 4. After the uneven shape of the workpiece 4 has been removed, scratches 31 left on the worked surface are removed by working with smaller grinding particles 32 as shown in FIG. 4C. During the working, the polisher 51 and the spindle rod 61 fall with dead weight against the elastic force of the compressed spring 8, whereby working is performed by constant pressure of the polisher 51 against the workpiece. 
     As described above, shape formation and mirror finishing of the workpiece are simultaneously or continuously processed in the same working manner. These operations are successively repeated on other points or areas of the workpiece 4. 
     Referring now to FIG. 5 another embodiment of the invention will be explained. The embodiment employs a spherical polisher 52 having a hemispherical polishing surface for working a concaved aspherical surface 4C. The concave workpiece 4 is mounted on the work chuck 3 which is rotated in the direction of an arrow A by the rotary table 2 mounted on the X-axis mechanisms. The spherical polisher 52 is arranged on a lower end of the spindle rod 61, and rotated in the direction of arrow on B by a motor formed with a rotor 71 and a stator 72. The concave workpiece 4 and the spherical polisher 52 are arranged relatively to work aspherical surfaces. The spindle rod 61 and the spherical polisher 52 are supported by the compressed spring 8, and the spherical polisher 52 is pushed against the surface of the concave workpiece 4 in the normal direction at every point on a surface which is to be worked. When the spherical polisher 52 and the concave workpiece 4 are rotated with a suspension of grinding particles, the workpiece 4 is worked. The worked amount is controlled by relative moving speed of the spherical polisher 52 and the workpiece 4, pushing pressure of the spherical polisher 52, the number of the rotations of the spherical polisher 52 and the workpiece 4, and the size of the grinding particles. The spherical polisher 52 is made of metals of cast iron or tin, or soft or elastic materials such as polyurethane, cloth, or pitch. When the spherical polisher 52 is made of metals, the contact area on the workpiece 4 is very small, and stabilized lapping may be carried out. When the spherical polisher 52 is made of soft or elastic material, it is possible to lap in accuracy on the order of less than 0.01 micron meter. 
     FIG. 6 shows an example of working at a center position of the concave workpiece 4. The axial direction of the spindle rod 61 is inclined at an angle α from a vertical line passing through the working point which corresponds to the contact point of the spherical polisher 52 on the workpiece 4, the axial direction being shifted a distance ra from the center axis of the spherical workpiece 4 such that an axis passing through the contact point and a center of curvature of the hemispherical polishing surface coincides with a normal direction at a point on an assumed aspherical surface to be fabricated where the axis intersects the assumed aspherical surface. The working point has a certain speed of rotation which produces a high working force although the moving speed of the working point of the workpiece is zero. 
     FIG. 7 shows an example of working at a center position of a convex workpiece 4. In this case, a flat polisher 53 is employed instead of the spherical polisher 52 in FIG. 6. The flat polisher 53 is provided in such a manner that the flat surface of the flat polisher 52 makes a right angle with the rotational axis of the spindle rod 61. The working point of the flat polisher 53 is shifted by a distance β from the center of the flat polisher 53. The axis of rotation of the flat polisher 52 is adjusted in a parallel relation with the normal direction at the working point of the convex workpiece 4. Thus, the flat polisher 53 works the workpiece 4 from the normal direction of the worked point of the workpiece 4. The worked point of the workpiece 4 of FIG. 7 is the center or the top portion of the convex contour of the workpiece 4, where the moving speed is substantially zero while the workpiece 4 is rotating. Therefore, if the center of the flat polisher 53 contacts the worked point of the workpiece 4, no working is performed because of no rubbing. The offsetting of the axis of the flat polisher 53 from the working point by a distance β avoids this problem. 
     The embodiments of FIGS. 6 and 7 are practicable to work a center position of the rotated workpiece 4 where the moving speed is zero. 
     As described above, the apparatus of the present invention makes it possible to work a desired aspherical shape on a workpiece with high accuracy by controlling the apparatus with data of X, Y, - and the preworking shape. The spindle 6 having the tool 5 is supported by the compressed spring 8 so that contact or push pressure of the tool 5 against the workpiece 4 is adjustable between zero and the dead weight of the supported spindle portion by the location of the R-axis mechanism 9. This means that the contact or push pressure of the tool 5 is substantially constant in small variation of tool location. According to the present invention, a ripple in or unevenness of the surface to be polished is small, so that the tool 5 is contacted to the workpiece 4 in constant pressure which allows the production of a surface roughness on the order of 0.01 micron. 
     The spindle 6 may be swung or vibrated instead of rotation thereof keeping its axial direction coincident or parallel with the normal direction at the point to be worked. In this case, the motor 26 and capstan 28 in FIG. 2 may be replaced by a vibration mechanism such as a clank driving mechanism. 
     The apparatus of the present invention is useful and effective to make a mold for plastic lenses, mirror polishing of ceramic or glass lenses, and so on.