Abstract:
A method for edge finishing glass sheets. Glass sheets are separated into desired sizes, after which the edges of the glass sheets are finished using first grinding wheels to grind the edges, followed by polishing wheels to round off the ground edges by contacting and moving the edges of the glass sheet against stationary rotating grinding and polishing wheels which are each oriented approximately parallel to the major surface of the glass sheet.

Description:
FIELD OF THE INVENTION 
     The invention relates to a method and apparatus for finishing the edges of glass sheets, particularly sheets for use in flat panel displays. 
     BACKGROUND OF THE INVENTION 
     The manufacturing process of flat panel display substrates requires specific sized glass substrates capable of being processed in standard production equipment. The sizing techniques typically employ a mechanical scoring and breaking process in which a diamond or carbide scoring wheel is dragged across the glass surface to mechanically score the glass sheet, after which the glass sheet is bent along this score line to break the glass sheet, thereby forming a break edge. Such mechanical scoring and breaking techniques commonly result in lateral cracks about 100 to 150 microns long, which emanate from the score wheel cutting line. These lateral cracks decrease the strength of the glass sheet and are thus removed by grinding the sharp edges of the glass sheet. The sharp edges of the glass sheet are ground by a metal grinding wheel having a radiused groove on its outer periphery, with diamond particles embedded in the radiused groove. By orienting the glass sheet against the radiused groove, and by moving the glass sheet against this radiused groove and rotating the diamond wheel at a high RPM (revolutions per minute), a radius is literally ground into the edge of the glass sheet. However, such grinding methods involve removal of about 100 to 200 microns or more of the glass edge. Consequently, the mechanical scoring step followed with the diamond wheel grinding step creates an enormous amount of debris and particles. 
     In addition, in spite of repeated washing steps, particles generated during edge finishing continue to be a problem. For example, in some cases particle counts from the edges of glass sheets prior to shipping were actually lower than subsequent particle counts taken after shipping. This is because the grinding of the glass sheets resulted in chips, checks, and subsurface fractures along the edges of the ground surfaces, all of which serve as receptacles for particles. These particles subsequently would break loose at a later time, causing contamination, scratches, and sometimes act as a break source in later processing. Consequently, such ground surfaces are “active”, meaning subject to expelling particles with environmental factors, such as, temperature and humidity. The present invention relates to methods for reducing these “lateral cracks” and “micro-checking” caused by grinding, thereby forming a glass sheet having edges that are more “inactive”. 
     Laser scoring techniques can greatly reduce lateral cracking caused by conventional mechanical scoring. Previously, such laser scoring methods were thought to be too slow and not suitable for production manufacturing finishing lines. However, recent advances have potentially enabled the use of such methods in production glass finishing applications. Laser scoring typically starts with a mechanical check placed at the edge of the glass. A laser with a shaped output beam is then run over the check and along a path on the glass surface causing an expansion on the glass surface, followed by a coolant quench to put the surface in tension, thereby thermally propagating a crack across the glass in the path of travel of the laser. Such heating is a localized surface phenomenon. The coolant directed behind the laser causes a controlled splitting. Stress equilibrium in the glass arrests the depth of the crack from going all the way through, thereby resulting in a “score-like” continuous crack, absent of lateral cracking. Such laser scoring techniques are described, for example, in U.S. Pat. Nos. 5,622,540 and 5,776,220 which are hereby incorporated by reference. 
     Unfortunately, unbeveled edges formed by laser scoring are not as durable as beveled edges, due to the sharp edges produced during the laser scoring process. Thus, the sharp edges still have to be ground or polished as described herein above. An alternative process has been to grind the edges with a polishing wheel made from a soft material, such as, a polymer, in order to smooth out the flat sharp edges formed by the scoring process. However, the polishing process often gives rise to a phenomenon that is known in the industry as an “edge roll”, where during the finishing of an edge having a flat surface, the surface tends to roll over and form an associated radius. 
     In light of the foregoing, it is desirable to design a process to finish an edge of a glass sheet that curbs prospective chips, checks and subsurface fractures along the edge. Also, it is desirable to provide a process that allows a smaller amount of glass removal and yet maintain the edge quality. Furthermore, it is desirable to design a process that increases the speed of finishing an edge of a glass without degrading the desired strength and edge quality attributes of the glass. Also, it is desirable to provide a technique that provides an edge without blended radiuses. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a method for finishing the edges of glass sheets comprising the steps of chamfering the top and bottom of each of the edges of the glass sheet to form chamfered planes while reducing the overall width of each of the edges by not more than 35 microns, and where the angle between each of the chamfered planes and the adjacent major surface of the glass sheet is less than 40 degrees, preferably approximately 30 degrees. The method further comprises rounding each edge formed by the intersection of each of the chamfered planes and the original edge of the glass sheet. One such embodiment involves moving the edges of the glass sheet over at least one rotating grinding wheel having at least one v-shaped groove in the grinding surface and one rotating polishing wheel having a flat polishing surface, each of the grinding and polishing surfaces being oriented such that each of the grinding and polishing wheels are parallel to the major plane of the glass sheet. In a preferred embodiment, the v-shaped groove in the grinding surface of the grinding wheel is embedded with diamond particles, whereas the polishing surface of the polishing wheel is sufficiently soft so that formation of a concave beveled edge is avoided. Also, a preferred embodiment, each of the grinding wheels have a surface speed that is greater than the surface speed of each of the polishing wheels. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a perspective view of a process in accordance with the present invention. 
     FIG. 2A illustrates a partial cross-sectional view illustrating the grinding process illustrated in FIG.  1 . 
     FIG. 2B illustrates a partial cross-sectional view of the grinding process illustrated in FIG.  1 . 
     FIG. 2C illustrates a partial cross-sectional view of the grinding process illustrated in FIG.  1 . 
     FIG. 3A illustrates a partial cross-sectional view of the polishing process illustrated in FIG.  1 . 
     FIG. 3B illustrates a partial cross-sectional view of the polishing process illustrated in FIG.  1 . 
     FIG. 3C illustrates a partial cross-sectional view of the polishing process illustrated in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention generally provides a method for grinding and polishing the edges of a sheet of glass, in particular, a flat panel display glass sheet. According to the present invention, the sheet of glass is held in place by securing means and the sheet of glass is conveyed on a conveyer system as shown in FIG.  1 . FIG. 1 illustrates a preferred embodiment of the invention in which a plurality of grinding wheels and polishing wheels are used to finish the edges of a glass sheet. FIG. 1 shows a glass sheet designated generally by reference numeral  10  being conveyed on a conveyer system in the direction of arrow  15  while at least one edge of the glass sheet  10  is being ground and polished by the set of grinding wheels  20 A and  20 B and polishing wheels  30 A and  30 B. The major surface  19  and  23  of each of the grinding wheels  20 A and  20 B, respectively, and the major surface  33  and  29  of each of the polishing wheels  30 A and  30 B, respectively, are positioned parallel to the major surface  16  of the glass sheet  10 . In the embodiment shown in FIG. 1, the grinding wheels  20 A and  20 B, each rotate in opposite directions. Specifically, grinding wheel  20 A rotates in a counterclockwise direction, whereas grinding wheel  20 B rotates in a clockwise direction. Similarly, polishing wheels  30 A and  30 B each rotate in opposite directions. Specifically, polishing wheel  30 A rotates in a counterclockwise direction, whereas polishing wheel  30 B rotates in a clockwise direction. 
     As shown in FIG. 1, the grinding surface  21  of the grinding wheel  20 B contacts one of the edges  14  of the glass sheet  10 , whereas the grinding surface  22  of the grinding wheel  20 A contacts an opposite edge  12  of the glass sheet  10 . Similarly, the polishing surface  32  of the polishing wheel  30 A contacts the edge  12  of glass sheet  10 , whereas the polishing surface  31  of the polishing wheel  30 B contacts the edge  14  of the glass sheet  10 . In the preferred embodiment, each of the grinding wheels  20 A and  20 B and each of the polishing wheels  30 A and  30 B rotate simultaneously. Moreover, opposing edges  12  and  14  are simultaneously ground and polished in the preferred embodiment. In particular, each of the edges  12  and  14  first contact the grinding surfaces  22  and  21  of the grinding wheels  20 A and  20 B, respectively, and then the ground edges next contact the polishing surfaces  32  and  31  of each of the polishing wheels  30 A and  30 B, respectively. Also, as shown in FIG. 1, each of the grinding wheels  20 A and  20 B are spaced apart from each of the polishing wheels  30 A and  30 B, with grinding wheel  20 A and polishing wheel  30 A being positioned proximate to each other on one edge  12  of the glass sheet  10 , and with grinding wheel  30 A and polishing wheel  30 B being positioned proximate to each other on the other edge  14  of the glass sheet  10 . 
     Furthermore, in the preferred embodiment, each of the grinding wheels  20 A and  20 B and each of the polishing wheels  30 A and  30 B are stationary, whereas, the glass sheet  10  is moved in the direction of arrow  15 , so that each of the edges  12  and  14  are first ground and then polished. FIGS. 2A-2C show the details of one of the edges  12  being ground, whereas, FIGS. 3A-3C show details of the edge  12  being polished after the edge  12  has been ground. FIG. 2A shows a partial cross-sectional view of the grinding surface  22  of the grinding wheel  20 A. As shown, the grinding surface  22  has at least one V-shaped groove  24  on the outer periphery, where a radial line passing through the center of the V-shaped groove  24  forms an angle θ with the V-shaped groove  24 . The angle θ is in a preferred embodiment approximately between 15 and 40 degrees, most preferably, approximately 30 degrees. Although FIG. 2A shows only a single V-shaped groove  24 , as shown in FIG. 1, the grinding wheels  20 A and  20 B each can have a plurality of V-shaped grooves  24 , and in a preferred embodiment, each of the grinding wheels  20 A and  20 B have six V-shaped grooves  24 . As shown in FIG. 2A, the edge  12  of the glass sheet  10  is aligned with the V-shaped groove  24 . Specifically, the edge  12  has a flat region  12 C located between a pair of corner regions  12 A and  12 B respectively. As shown in FIG. 2B, the edge  12  is inserted into the V-shaped groove  24  such that only the pair of comer regions  12 A and  12 B contact the V-shaped groove  24 , whereas, the middle portion of the flat region  12 C does not contact the grinding surface  22  of the grinding wheel  20 A. As the comer regions  12 A and  12 B are chamfered by the V-shaped groove  24 , the pair of comer regions  12 A and  12 B are transformed into a pair of ground beveled regions  12 D and  12 E, respectively, as shown in FIG.  2 C. Also as shown in FIG. 2C, each of the rounded beveled regions  12 D and  12 E form an angle θ with the top surface  16 A and the bottom surface  16 B, respectively, of the glass sheet  10 . In a preferred embodiment, the angle θ is approximately between 15 and 40 degrees, and most preferably, approximately 30 degrees. As shown in FIG. 2C, the middle portion of the flat region  12 C of the edge  12  remains the same shape as before grinding, since this portion of the edge  12  is not contacted by the grinding wheel  20 A. 
     The ground edge  12  next contacts the polishing surface  32  of polishing wheel  30 A, as shown in FIG.  3 A. As shown in FIG. 3A, the polishing surface  32  of polishing wheel  30 A is substantially flat. Furthermore, the polishing surface  32  is sufficiently soft so that formation of a concave beveled edge on the edge  12  is avoided. As shown in FIG. 3B, as the ground edge  12  contacts the polishing surface  32  of the polishing wheel  30 A, the polishing surface  32  becomes depressed in conformity with the shape of the ground edge  12 . In this manner, each of the sharp interfaces that the ground beveled regions  12 D and  12 E form with the flat region  12 C is substantially rounded, as represented by  12 F and  12 G shown in FIG.  3 C. The edge  14  of glass sheet  10  is rounded and polished simultaneously with edge  12  in a similar manner as described herein above, but instead with grinding wheel  20 B and polishing wheel  30 B. 
     In another aspect, the invention provides a method of finishing an edge  12  of a glass sheet  10  having a thickness not greater than approximately 3 mm. The method comprises the steps of chamfering the top surface  16 A and the bottom surface  16 B of the edge  12  of the glass sheet  10  to form chamfered planes  12 D and  12 E while reducing the overall width of the edge  12  by not more than approximately 35 microns. Moreover, the angle θ between each of the chamfered planes  12 D and  12 E and the adjacent major surfaces  16 A and  16 B of the glass sheet  10  is approximately less than 40 degrees. The method further comprises the step of next rounding the edge  12  formed by the intersection of each of the chamfered planes  12 D and  12 E, and the original edge  12 C of the glass sheet  10 . The chamfering step comprises contacting the top surface  16 A and the bottom surface  16 B of the edge  12  of the glass sheet  10  with at least one rotating grinding wheel  20 A that has a grinding surface  22  with at least one V-shaped groove  24 , where the grinding surface  22  is parallel to the major surface  16  of the glass sheet  10 . Furthermore, the rounding step comprises contacting the top surface  16 A and the bottom surface  16 B of the edge  12  having chamfered planes  12 D and  12 E with at least one rotating polishing wheel  30 A that has a polishing surface  32  that is sufficiently soft so that formation of a concave chamfer on the edge  12  is avoided. The angle θ formed by each of the chamfered planes  12 D and  12 E with the adjacent top surface  16 A and the bottom surface  16 B of the glass sheet  10  is preferably approximately 30 degrees each. 
     Accordingly, the edge finishing process of the present invention removes not more than approximately 35 microns from each edge of the glass sheet, which improves the strength of the glass sheet as well as the edge quality since less micro cracks are generated in the process. Moreover, the angle θ formed by each of the chamfered planes is preferably approximately 30 degrees, which takes into account any lateral shifts of the glass sheet due to the grinding equipment conveying inaccuracies. 
     The finishing method further comprises first conveying the glass sheet  10  on a conveyer system that includes a plurality of wheels  18  (shown in FIG.  1 ). The conveyor system conveys the glass sheet  10  between each of the rotating grinding wheels  20 A and  20 B and each of the rotating polishing wheels  30 A and  30 B. Furthermore, the conveying step includes securing glass sheet  10  onto the conveyer system by a set of belts  17  that are partially shown in FIG.  1 . The conveying step further includes first cutting the glass sheet  10  to size by forming at least a partial crack in the glass sheet  10  along a desired line of separation, and leading the crack across the glass sheet  10  by localized heating by a laser, and moving the laser across the sheet to thereby lead the partial crack and form a second partial crack in the desired line of separation and breaking the glass sheet  10  along the partial crack. Preferably, the grinding wheels  20 A and  20 B rotate faster than the polishing wheels  30 A and  30 B. In a preferred embodiment, each of the grinding wheels rotate at approximately 2,850 RPMs, whereas each of the polishing wheels rotate at approximately 2,400 RPMs. Moreover, the surface speed of each of the grinding wheels  20 A and  20 B is greater than the surface speed of each of the polishing wheels  30 A and  30 B. Also, in a preferred embodiment, the glass sheet  10  is conveyed at a feed rate of approximately 4.5 to 6 meters per minute. In a preferred embodiment, the diameter of each of the grinding wheels  20 A and  20 B is less than or equal to the diameter of each of the polishing wheels  30 A and  30 B. 
     In a preferred embodiment, the grinding wheels  20 A and  20 B employed in the invention are metal bonded grinding wheels, each having six recessed grooves, each of the grooves being embedded with diamond particles. The diamond particles have a grit size in the range of approximately 400 to 800, preferably about 400. Further, each of the grooves of the grinding wheels  20 A and  20 B employed in the invention are approximately 0.7 mm wide. Moreover, preferably, the grinding wheels  20 A and  20 B each have a diameter of 9.84 inches and a thickness of about one inch. The glass sheet  10  is conveyed at a feed rate of 4.5 to 6 meters per minute. Further, the surface speed of each of the grinding wheels  20 A and  20 B is approximately 7,338 sfpm (surface feet per minute), whereas, the surface speed of each of the polishing wheels  30 A and  30 B is approximately 5,024 sfpm. The polishing wheels  30 A and  30 B employed in the invention each comprise an abrasive media dispersed within a suitable carrier material, such, as a polymeric material. The abrasive media may be selected, for example, from the group consisting of Al 2 O 3 ; SiC, pumice, or garnet abrasive materials. Preferably, the particle size of the abrasive media is equal to or finer than 220 grit, more preferably equal to or finer than 180 grit. Examples of suitable abrasive polishing wheels of this sort are described, for example, in U.S. Pat. No. 5,273,558, the specification of which is hereby incorporated by reference. Examples of suitable polymeric carrier materials are butyl rubber, silicone, polyurethane, natural rubber. One preferred family of polishing wheels for use in this particular embodiment are the XI-737 grinding wheels available from Minnesota Mining and Manufacturing Company, St. Paul, Minn. Suitable polishing wheels may be obtained, for example, from Cratex Manufacturing Co., Inc., located at 7754 Arjons Drive, San Diego, Calif.; or The Norton Company, located in Worcester, Mass. In addition the preferable diameter of each of the polishing wheels  30 A and  30 B is approximately 8.0 inches and the thickness is about one inch. 
     Although the invention has been described in detail for the purpose of illustration, it is understood that such detail is solely for that purpose and variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention which is defined by the following claims.