CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present application is related to U.S. Application Serial Number ______, filed ______, by Stephen C. Meissner, and entitled, “Machine For Polishing The Surface Of A Work Piece;” U.S. application Ser. No. 10/241,144, filed Sep. 11, 2002, by Stephen C. Meissner, and entitled, “Dual Motion Polishing Tool.” 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention relates generally to the field of optical manufacturing processes, and in particular to polishing of optical surfaces. More specifically, the invention relates to a high-precision polishing tool for polishing an optical quality surface onto a substrate.  
         BACKGROUND OF THE INVENTION  
         [0003]    In manufacturing of optical components, lenses, molds and the like, preliminary operations, such as grinding or diamond turning, are performed to generate an optical surface on a raw blank of material. These preliminary operations provide the general form of the component, but leave surface defects that include turning grooves, cutter marks, and sub surface damage. A final polishing step is required to remove these surface and sub-surface defects. Polishing is typically accomplished in a variety of ways depending upon the material and the form of the surface, i.e., plano form, spherical form, or aspherical form.  
           [0004]    Skilled artisans will appreciate that work piece surfaces having either a plano and spherical form are typically polished using “full-aperture” or “full-surface” tools. Full aperture tools tend to cover over 80% of the work piece surface during polishing. These tools are constructed in a variety of ways, including traditional “pitch” and more recent pad-type. “Pitch” polishing tools are comprised of a soft flow-able material, such as pitch or bees wax, which is used to create a mold of the optical surface.  
           [0005]    Referring to FIG. 1, a typical prior art full aperture “pitch” polishing tool  10  has a plurality of grooves  12  to aid in the movement of polishing fluid at the interface of work piece surface (not shown). Polishing tool  10  has a support surface  14  attachable to a shank  16 . Shank  16  defines an arbor for holding the polishing tool  10  for operation in a working unit. In operation, polishing tool  10  is held against the work piece (not shown) with an applied force and the two components, i.e., work piece and polishing tool  10 , are moved relative to one another in the presence of a free abrasive polishing compound, such as cerium oxide, to achieve polishing.  
           [0006]    Referring to FIG. 2, a typical full-aperture pad polishing tool  20  has of a pad mounting surface  22  for receiving a polishing pad  24  thereon. The polishing pad  24  is typically attached to the pad mounting surface  22  via an adhesive or via friction grip as disclosed in U.S. Pat. No. 4,274,232 issued to Wylde on Jun. 23, 1981, titled “Friction Grip Pad.” 
           [0007]    Those skilled in the art will appreciate that polishing aspheric surfaces using full-aperture tools involves much iteration to rebuild or reshape the polishing tool slowing the polishing process considerably. Therefore polishing of aspheric surfaces is commonly restricted to sub-aperture methods using existing ring-tools or small-area tools. Sub-aperture methods using ring-tools or small-area tools rely on a polishing tool that contacts less than 50% of the work piece surface at one time. Ring tools, as disclosed in U.S. Pat. No. 4,768,308 issued to Leland G. Atkinson, III, et al. on Sep. 6, 1998, titled “Universal Lens Polishing Tool, Polishing Apparatus And Method Of Polishing,” have a diameter that is comparable to or larger than the radius of the work piece and contact the work piece surface over an area that is much larger than that for a small-area tool. Small-area tools contact only a small area of the work surface at a time and create an interfacial contact area that is on the order of 99% smaller than the area of the work piece surface.  
           [0008]    Moreover, it is well known that sub-aperture small-area tools may be outfitted with a variety of polishing head shapes, including spherical (as shown in FIG. 3), but may also include conical, cylindrical, and flat along with a polishing pad. In FIG. 3, a sub-aperture tool  30  includes an arbor  32  fixedly attached to a spherical polishing head  34 . It should be noted that the spherical polishing head  34  may be substituted with one of the aforementioned polishing heads of a different geometrical shape. Sub-aperture ring-tools may be considered a variation on the small-area tool with the polishing head being of ring-shaped configuration with surface contact during polishing being from 3% to 50% of the work piece surface.  
           [0009]    It is further known that sub-aperture tools are commonly made from materials, such as felt, wood, cast iron, lead, and woven polymers, that allow free-abrasive particles to become imbedded within the material so that relative motion is generated between the abrasive particles and the part. Such materials allow free abrasive particles to imbed themselves within this carrier, allowing the tools to wear which is a detriment when trying to control material removal precisely. The concept of a compliant tool that does not allow free abrasive particles to imbed themselves and therefore is resistant to wear, provides advantage in precision polishing.  
           [0010]    One precision polishing method, Elastic Emission Machining (EEM) uses this concept where a polishing tool (with a spherical or flat configuration) is made from an elastic solid. This tool is precisely controlled to maintain a minute gap between itself and the part surface within a temperature-controlled bath of free-abrasive slurry. The tool is rotated at high speed and is driven to traverse the part surface creating a hydrodynamic bearing at the tool-part interface gap. This situation allows abrasive particles to be projected against the surface being conditioned with sufficient energy to cause elastic penetration and subsequent precision material removal.  
           [0011]    As with any method that uses a rotating tool, the area at the tool&#39;s center of rotation remains stationary—creating a “dead zone.” As the radial distance from the center of rotation increases, the tangential velocity also increases. Therefore, when polishing with sub-aperture tools, the greatest removal and subsequent benefit comes from a contact point radially distant from the tools center of rotation. For spherical or conical polishing tools, the tool must be tipped at an angle to provide needed tangential velocity. In order to generate productive velocities for polishing, spherical and conical polishing tools either need to be rotated at very high speeds or tipped at a large angle or a combination of the two. When very small tools are used, a combination of the two is required to gain maximum benefit.  
           [0012]    Referring to FIGS.  4 - 7 , limitations exist when attempting to polish deep concave surfaces, where a large angle would restrict the polishing capability of the tool. For example, using a tool with a spherical tip, an impractical angle of nearly 60 degrees would be required to achieve the same surface speed as a toroidal tip of the same overall diameter. FIG. 4 illustrates this example, where the arbor  32  of the spherical tipped tool interferes with the curved surface  40  of the part  42 . Similar limitations exist for cylindrical and “flat” sub-aperture tools. As depicted in FIGS. 5, 6, and  7 , cylindrical and “flat” sub-aperture tools  46  having arbor  32  and polishing surface  48  do not conform to curved surfaces  40  of the work piece or part  42  well and therefore restrict their application for polishing these surfaces. The cylindrical sub-aperture tool  46  allows minimum tilt while allowing maximum tangential velocity to be achieved, however, the contact area is crescent-like and tends to be non-uniform in this configuration, which is detrimental to precision polishing. If used without any tilt, benefits are realized from the maximized diameter, but fluid flow is restricted and the ability to polish severe or steep aspheres is limited.  
           [0013]    Therefore, the need persists in the art for a precision polishing element for polishing optical surfaces without adversely affecting the quality of the surface.  
         SUMMARY OF THE INVENTION  
         [0014]    It is, therefore, an object of the invention to provide a precision polishing tool capable of uniformly polishing optical surfaces.  
           [0015]    Another object of the invention is to provide a sub-aperture tool that minimizes tilt limitations and conforms to the surface of the work piece to be polished.  
           [0016]    The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, a precision polishing element for polishing a work piece surface has a compliant, toroidal polishing member mountable to a support member. The compliant, toroidal polishing member has an active polishing surface extended uniformly beyond a peripheral surface of the support member for engaging the work piece surface.  
           [0017]    The tool disclosed provides greater advantage for free-abrasive type polishing operations, like EEM, due to the toroidal geometry. The toroidal shape allows shallow contact angles to be used, providing significant practical advantage for polishing steep concave surfaces. The toroidal shape is also advantageous due to the low axial profile, while providing radial distance to maximize tangential speed for material removal. The shape of the toroid also provides a uniform geometry that allows for polishing fluid to be transported along the circumference, providing the necessary flow required for polishing. In addition, the contact area generated at the tool-part interface tends to be very uniform and consistent, appearing oval in shape, which is essential for deterministic polishing. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    The above and other objects, features, and advantages of the present invention will become more apparent when taken in conjunction with the following description and drawings wherein identical reference numerals have been used, where possible, to designate identical features that are common to the figures, and wherein:  
         [0019]    [0019]FIG. 1 is an isometric view of a prior art polishing tool;  
         [0020]    [0020]FIG. 2 is an isometric view of a prior art polishing tool with a polishing pad;  
         [0021]    [0021]FIG. 3 is an isometric view of a prior art sub-aperture polishing tool with a spherical polishing head;  
         [0022]    [0022]FIG. 4 shows the interference condition when a spherical sub-aperture polishing tool used in prior art, angled at nearly 60 degrees from vertical to achieve satisfactory surface speed for productive material removal, engages a concave surface;  
         [0023]    [0023]FIG. 5 shows a sub-aperture pitch/pad polishing tool used in prior art, engaged with a concave surface;  
         [0024]    [0024]FIG. 6 shows a sub-aperture pitch/pad polishing tool used in prior art, engaged with a convex surface;  
         [0025]    [0025]FIG. 7 shows a sub-aperture cylindrical polishing tool used in prior art, engaged with a convex surface;  
         [0026]    [0026]FIG. 8 illustrates the first example of the embodiment of the compliant toroidal polishing tool according to the invention;  
         [0027]    [0027]FIG. 9 is a close up view of the toroidal polishing tip of the compliant toroidal polishing tool according to the invention;  
         [0028]    [0028]FIG. 10 illustrates the second example of the embodiment of the polishing tool according to the invention;  
         [0029]    [0029]FIG. 11 is an exploded view of the third example of the embodiment of the polishing tool according to the invention;  
         [0030]    [0030]FIG. 12 is an exploded view of the fourth example of the embodiment of the polishing tool according to the invention;  
         [0031]    [0031]FIG. 13 is a close up view of the compliant toroidal tip;  
         [0032]    [0032]FIG. 14 is a cross-sectional view of the polishing tip showing a circular cross section of the compliant toroidal tip;  
         [0033]    [0033]FIG. 15 is a cross-sectional view of the polishing tip showing an oval cross-section of the compliant toroidal tip with the long side of the oval aligned perpendicular to the arbor axis;  
         [0034]    [0034]FIG. 16 is a cross-sectional view of the polishing tip showing an oval cross-section of the compliant toroidal tip with the long side of the oval aligned parallel to the arbor axis;  
         [0035]    [0035]FIG. 17 is a cross-sectional view of the polishing tip showing a square cross-section with rounded corners of the compliant toroidal tip;  
         [0036]    [0036]FIG. 18A illustrates a typical clearance condition when a compliant toroidal polishing tool is used in service; and  
         [0037]    [0037]FIG. 18B illustrates the polishing tool of the invention in contact with the work piece. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0038]    Turning now to the drawings, and in particular to FIGS.  8 - 13 , the compliant toroidal polishing element  50  of the invention is illustrated. Broadly defined, compliant toroidal polishing element  50  includes a support member, or mounting arbor  52 , and a compliant toroidal polishing tip  54  mounted to the mounting arbor  52 . Compliant toroidal polishing tip  54  preferably has a circumferential active polishing surface  57  extending substantially symmetrically beyond the peripheral surface of the mounting arbor  52  for engaging the curved surface  40  of work piece  42 . Work piece  42  may comprise a variety of materials including metallic materials, ceramic materials, and vitreous materials. According to FIGS.  9 - 12 , the mounting arbor  52  may be constructed simply as a solid (as shown) or hollow cylinder. Referring to FIG. 11, the mounting arbor  52  is preferably constructed with additional features, such as a locating shoulder  51  for advantage in tool holding and repetitive placement. Mounting arbor  52  has a distal end  56  with support surface  58  for supporting compliant toroidal polishing tip  54 . Upon support surface  58 , a centering boss  60  may be mounted which aids in providing concentric alignment of toroidal polishing tip  54  during attachment. If provided, centering boss  60  projects normally from support surface  58  of distal end  56 . Toroidal polishing tip  54  of toroidal geometry is then attached centrally to the support surface  58 , whereby the toroidal polishing tip  54  is concentric with the diameter of the mounting arbor  52 . According to FIG. 13, the toroidal polishing tip  54  may have an alignment port  70  concentric with its outside diameter intended to mate with centering boss  60  (FIG. 12) if provided on mounting arbor  52  to provide concentric alignment and to aid in attachment.  
         [0039]    Attachment of the toroidal polishing tip  54  to the arbor  52  may be accomplished in a variety of ways including adhesive, chemical, thermal, or mechanical bonding. Once joined, the compliant toroidal polishing tool  50  of the invention is ready for use.  
         [0040]    Referring again to FIGS. 9 and 10, an important feature of compliant toroidal polishing element  50  is the compliant toroidal tip  54 . The compliant toroidal tip  54  may be manufactured from a variety of compliant polymers such as, but not limited to, polyurethane, chloroprene, fluorocarbon, fluorosilicone, ethylene propylene, and nitrile. We prefer using nitrile because of its compliant properties. For advantageous sub-aperture deterministic polishing in the presence of free-abrasive lapping compound, the hardness of the compliant toroidal polishing tip  54  needs to be in the range of 40-95 on the Shore A scale.  
         [0041]    Referring to FIGS.  14 - 17 , the shape of toroidal polishing tip  54  may deviate from a pure toroid and still provide advantage in polishing. According to FIG. 14, toroidal tip  54  has a substantially circular shape. According to FIG. 15, toroidal tip  54  has a substantially horizontally oriented oval shape. According to FIG. 16, toroidal tip  54  has a substantially vertically oval shape. According to FIG. 17, toroidal tip  54  has a substantially square shape with rounded corners  59 . If used, centering boss  60  should be made so arbor material does not extend beyond the boundary of the toroidal polishing tip  54  to avoid risk of damage to part surface when polishing.  
         [0042]    According to FIG. 18A, in operations, compliant toroidal polishing element  50  is mounted in a rotary polishing machine  80  for polishing a work piece. As shown in FIG. 18B, the polishing surface  57  engages curved surface  40  of work piece  42  for polishing. An important advantage of compliant toroidal polishing element  50  is that a clearance  64  is formed between the mounting arbor  52  and the curved surface  40  of the work piece  42 . Experience suggests that compliant toroidal polishing element  50  should be angled at about 30 degrees from a centerline extending normally through the curved surface  40  for most efficient operation. According to our experience, this arrangement produces satisfactory surface speed for productive material removal during polishing.  
         [0043]    The invention has been described with reference to a preferred embodiment. However, it will be appreciated that variations and modifications can be effected by a person of ordinary skill in the art without departing from the scope of the invention.  
       Parts List  
       [0044]    [0044] 10  full-aperture pitch polishing tool  
         [0045]    [0045] 12  grooves  
         [0046]    [0046] 14  support surface  
         [0047]    [0047] 16  shank  
         [0048]    [0048] 20  full-aperture pad polishing tool  
         [0049]    [0049] 22  pad mounting surface  
         [0050]    [0050] 24  polishing pad  
         [0051]    [0051] 30  sub-aperture tool  
         [0052]    [0052] 32  arbor  
         [0053]    [0053] 34  spherical polishing head  
         [0054]    [0054] 40  curved surface  
         [0055]    [0055] 42  work piece  
         [0056]    [0056] 46  sub-aperture tool  
         [0057]    [0057] 48  polishing surface  
         [0058]    [0058] 50  compliant toroidal polishing element  
         [0059]    [0059] 51  locating shoulder  
         [0060]    [0060] 52  mounting arbor  
         [0061]    [0061] 54  polishing tip  
         [0062]    [0062] 56  distal end  
         [0063]    [0063] 57  polishing surface  
         [0064]    [0064] 58  support surface  
         [0065]    [0065] 59  rounded corners  
         [0066]    [0066] 60  centering boss  
         [0067]    [0067] 64  clearance  
         [0068]    [0068] 70  alignment port  
         [0069]    [0069] 80  rotary polishing machine
A precision sub-aperture polishing tool has a compliant, toroidal polishing member mountable to a support member. A circumferential portion of the polishing member extends uniformly beyond the peripheral surface of the support member and forms a clearance with the work piece surface during polishing operations.