Patent Publication Number: US-2007105483-A1

Title: Methods and apparatus for discrete mirror processing

Description:
BACKGROUND OF THE INVENTION  
      This invention relates generally to ring laser gyroscopes, and more specifically, to methods and apparatus for processing of discrete mirrors utilized in ring laser gyroscopes.  
      Discrete mirror processing for at least one known ring laser gyroscope involves three processes normally used in optical fabrication, specifically, grinding, lapping, and polishing. The lapping and polishing processes typically utilize a loose abrasive slurry. The loose abrasive slurry comprises multiple abrasive particles dispersed in a liquid medium, for example, water. Although loose abrasive slurries are widely used in the lapping and polishing processes to provide an optically clear surface finish on glass articles, loose abrasive slurries also have many disadvantages associated therewith.  
      For example, such loose abrasive slurries must be periodically analyzed to assure quality and dispersion of the abrasive particles. The equipment associated with the loose abrasive slurry process must also be continually maintained. Maintenance of such equipment is costly due to the labor costs associated with the maintenance. Further, additional equipment is typically incorporated into the slurry process for the preparation, handling and disposal of the loose abrasive slurry mixtures. Also, the cost of maintaining a loose abrasive slurry process is costly in term of raw materials, equipment maintenance, disposal of used slurries and labor. Processes which use slurries are usually very untidy because the loose abrasive slurry splatters easily and is difficult to contain.  
     BRIEF SUMMARY OF THE INVENTION  
      In one aspect, a method for processing glass substrates for utilization as discrete mirrors is provided. The method comprises temporarily mounting a first end of at least one substrate onto a grinding machine plate, engaging a second end of the at least one substrate with a grinding wheel, rotating the plate and grinding wheel in opposite directions, and moving the grinding wheel and the plate toward one another at a rate.  
      In another aspect, a process for fabricating a mirror from a substrate is provided. The method comprises rotating an end of the substrate in a first direction, configuring a grinding wheel with a matrix between nine and twenty-five microns, engaging the end of the substrate with the grinding wheel, and rotating the grinding wheel in a direction opposite that of the substrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a top view illustration of a plurality of glass rods mounted to a grinding machine plate.  
       FIG. 2  is a side view of the plate and glass rods of  FIG. 1 .  
       FIG. 3  is a flowchart describing a glass rod grinding process. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The methods and apparatus described herein provides for the elimination of the loose abrasive slurry lapping process that is currently utilized in the fabrication of high precision optics. Specifically, a grinding process that enables high precision optics to be ground to the same or a better surface finish, with minimal sub-surface damage and clarity as compared to conventional loose abrasive lapping is described. This grinding process provides for the fabrication and modification of glass articles such that the loose abrasive lapping process is eliminated and the glass articles are moved directly to polishing processes.  
       FIGS. 1 and 2  illustrate a plurality of rods  10  mounted to a plate  12 . In one embodiment, plate  12  is of the type that is utilized with a grinding machine. Utilizing the illustrated configuration provides for the fabrication of discrete mirrors and substrates using a multi-speed grinder (e.g., a grinding machine) enabling the elimination of loose abrasive slurry lapping. In an exemplary embodiment, plate  12  is attached to a NAG-250 multi-speed grinder on which is mounted a 9-25 micron metal, metal/resin or resin bond matrix, for example, a nine micron resin bond matrix diamond wheel. In the illustrated embodiment, plate  12  is a circular glass plate. Rods  10  include circular glass rods ranging from, for example, from 0.1 inches to about 3 inches, more specifically, from 0.305 inches to about 2.50 inches in diameter. Glass rods  10  are evenly spaced and temporarily mounted on glass plate  12 . While described in terms of glass rods, it is to be understood that the processes described herein are not limited to such rods. Rather, substrates including, but not limited to, wafers, rods, cubes or other geometrical shapes may be polished as described herein.  
      In one embodiment, when attached to a grinding machine (not shown), glass plate  12  rotates clockwise at a speed of either about thirteen rpm (rough grind) or about four rpm (fine grind) while the grinding tool within the grinding machine rotates counter clockwise at a speed of either about 1000 rpm (rough grind) or about 1400 rpm (fine grind).  
      In addition to the rates of rotation, glass rods  10  (and plate  12 ) are moved towards the grinding wheel at a rate. For example, a rough grind feed rate is about 0.07 mm/min and a fine grind feed rate is about 0.015 mm/min. In these embodiments, a 0.5% coolant and water mixture is used to provide lubrication at the surfaces being ground and further helps to remove glass swarf between the surface of the grinding wheel and ends of glass rods attached to plate  12 .  
       FIG. 3  is a flowchart  100  illustrating a rod grinding process utilizing the equipment and fixtures for grinding as described above. Specifically, glass rods  10  are mounted  102  onto a grinding machine plate  12 . Rods  10  are mounted  102  at an end opposite the end that is to be ground and polished. The ends to be polished are then engaged  104  by a grinding wheel of the grinding machine.  
      The grinding machine plate  12  and grinding wheel are rotated  106  in opposite directions, and the grinding machine plate  12  and grinding wheel are moved  108  toward one another at a specific rate.  
      After the above described grinding process, the surface finish on the glass rods is evaluated with a diamond stylus. One example of such a diamond stylus is commercially available under the trade designation Mahr Pocket Surf available from Deterco, Inc., Houston, Tex. Initial surface finishes or Ra values of the of the ground ends of glass rods  10  typically fall within a range of 0.01 micrometer to 0.30 micrometer, while surface finishes utilizing a nine micron loose abrasive aluminum oxide slurry lapping process after an initial grinding range from 0.28 micrometer to 0.30 micrometer. Sub-surface damage of the ground ends of the glass rods typically fall within 2.87 micrometers to 3.05 micrometers while sub-surface damage of ground ends of glass rods polished using the above described lapping process is between 3.10 micrometers and 3.30 micrometers. The above described grinding process results in substrates and/or glass rods having a surface compatible with polishing processes that result in a polished substrate surface finish of between 1 Angstrom and 3 Angstroms.  
      While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.