Abstract:
A method for creating a concave section of glass from a glass substrate having flat surfaces includes the use of a grinding wheel and a turntable. The method includes the steps of securing the glass substrates to the turntable. The turntable is then spun to create a turntable axis of rotation. The grinding wheel is rotated about a wheel axis of rotation such that the wheel axis of rotation is perpendicular to the turntable axis of rotation. The grinding wheel and the turntable move relative to each other along the turntable axis of rotation. The glass substrate is ground by the grinding wheel contacting the glass substrate to create the concave section of glass while the glass substrate is rotating about the wheel axis of rotation and moving relative to the turntable.

Description:
This patent application is a division of U.S. patent application Ser. No. 12/697,348 filed Feb. 1, 2010, which in turn claims priority to a provisional patent application having an application No. 61/148,403, filed on Jan. 30, 2009, the specifications of which are incorporated herein expressly by reference. 
    
    
     BACKGROUND ART 
     1. Field of the Invention 
     The invention relates to a glass substrate created to have more than one type of surface. More particularly, the invention relates to a glass substrate to be used in an external rearview mirror of a motor vehicle wherein the glass substrate used to create the mirror surface includes a primary mirror surface and a blind spot mirror surface. 
     2. Detailed Description of the Related Art 
     Blind spot mirrors are common for exterior external rearview mirrors on motor vehicles, wherein the exterior rearview mirror assembly includes a primary reflector, also known as the Main Viewing Glass, and a secondary reflector also known as a Blind Spot Mirror. Automotive manufacturers often provide these blind spot mirrors on the vehicles sold because it is well known that a “blind zone” or “blind spot” exists on the side of most vehicles. 
     In some instances, this secondary mirror is mounted separately from the primary reflector and thus is independently adjustable from the main viewing glass. In other cases, the secondary reflector is mounted on the same carrier, or backing plate, as the primary reflector. In still another embodiment, the blind spot mirror is a concave depression on a second surface of the primary reflector within the exterior rearview mirror. While this last concept has been well established through prior art and has been so, for some time now, this embodiment is not prevalent within the marketplace because of the extreme difficulty realized in manufacturing the concave depression in the glass substrate. 
     SUMMARY OF THE INVENTION 
     A method for creating a concave section of glass from a glass substrate having flat surfaces includes the use of a grinding wheel and a turntable. The method includes the steps of securing the glass substrates to the turntable. The turntable is then spun to create a turntable axis of rotation. The grinding wheel is rotated about a wheel axis of rotation such that the wheel axis of rotation is perpendicular to the turntable axis of rotation. The grinding wheel and the turntable move relative to each other along the turntable axis of rotation. The glass substrate is ground by the grinding wheel contacting the glass substrate to create the concave section of glass while the glass substrate is rotating about the wheel axis of rotation and moving relative to the turntable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Advantages of the invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a side view of an exterior rearview mirror for a motor vehicle incorporating a glass substrate manufactured using the inventive method; 
         FIG. 2  is a partial cross-sectional top view of the exterior rearview mirror shown in  FIG. 1 ; 
         FIG. 3  is an end view of a grinding wheel used by the inventive method and a cross-section of a substrate of glass; 
         FIG. 4  is a side view of the grinding wheel as it grinds the glass substrate; 
         FIG. 5  is a perspective view of the grinding wheel and the glass substrate fixedly secured to a turntable; 
         FIG. 6  is an end view of a grinding wheel used in an alternative embodiment of the invention; 
         FIG. 7  is a side view of the grinding wheel used in an alternative embodiment of the invention; 
         FIG. 8  is a perspective view of the grinding wheel and the turntable used in the alternative embodiment of the invention; 
         FIG. 9  is an alternative embodiment of a grinding wheel used in the second embodiment of the inventive method; 
         FIG. 10  is an end view of the alternative grinding wheel used in the in second alternative embodiment; 
         FIG. 11  is a perspective view of the alternative grinding wheel used in the alternative embodiment of the invention and the turntable associated therewith; 
         FIG. 12  is a third embodiment of the grinding wheel used according to the first embodiment of the inventive method; 
         FIG. 13  is a side view of the third grinding wheel disposed adjacent the glass substrate; 
         FIG. 14  is a perspective view of the third grinding wheel used according to the first inventive method; and 
         FIG. 15  is a side view of a polishing wheel used in a method for polishing a concave surface on a glass substrate. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention is a method for creating a glass substrate  10  having at least one surface which is complex. The glass substrate  10  is to be used in an exterior rearview mirror assembly, as is generally indicated at  12  in  FIGS. 1 and 2 . The exterior rearview mirror assembly  12  includes a mounting bracket  14 , shown partially cutaway in the Figures which fixedly secures the exterior rearview mirror assembly  12  to a motor vehicle (not shown). A mirror case or housing, generally indicated at  16 , is secured to the mounting bracket  14 . In the embodiment shown, a pivot, graphically represented at  18 , allows the mirror case  16  to pivot with respect to the mounting bracket  14 . The mirror case  16  defines a primary opening  20  that faces rearward with respect to the forward motion of the motor vehicle. The mirror case  16  includes a forward face  22  that wraps around the primary opening  20  to create a mirror frame  24  disposed around the primary opening  20 . The glass substrate  10  that creates the mirror glass is visible through the primary opening  20 . It should be appreciated by those skilled in the art that the exterior rearview mirror assembly  12  may include fewer or more features, depending on the design and package choices and those features do not add or detract from the inventive method. 
     Extending out from the pivot  18  and into the mirror case  16  is a support structure  26 . The support structure  26  is fixedly secured to the mirror case  16 . In many instances, the support structure  26  may be integral with the mirror case  16 . A motor  28  is secured to the support structure  26 . The motor  28  is electrically connected to remote switching mechanisms in the passenger compartment to receive control signals from the driver of the motor vehicle to adjust the orientation of the glass substrate  10  with respect to the position of the driver. The electrical connections between the motor  28 , the remote switching, and power are not shown but are well known to those skilled in the art. 
     A backing plate  30  is fixedly secured to a movable portion of a motor housing  32  that moves with respect to the rest of the motor  28  and the support structure  26 . More specifically, the motor  28  moves the portion of the housing  32  based on the control signals it receives. The backing plate  30  is secured to the portion of the housing  32  that moves such that there is no lost motion therebetween. The backing plate  30  is used to secure the glass substrate  10  to the motor  28  and hold the glass substrate  10  in the desired orientation. 
     The glass substrate  10  is secured to the backing plate  30  with an adhesive (not shown). The glass substrate  10  includes a first surface  34  and a second surface  36 . In the embodiment shown, the first surface  34  is planar and transparent. The second surface  36  is substantially planar and parallel to the first surface  34 . The second surface  36  includes a reflective coating (not shown) that is applied to the glass substrate  10  prior to the adhesive being applied between the backing plate  30  and the second surface  36 . The reflective coating allows the driver of the motor vehicle to see objects rearward of the exterior rearview mirror assembly  12  without having to see the backing plate  30  or the interior  38  of the mirror case  16  and all that it contains. 
     The second surface  36  of the glass substrate  10  is complex in that it has more than one type of viewing surface. More specifically, the second surface  36  includes a primary reflective surface  40  and a secondary reflective surface  42 . The primary reflective surface  40  is planar and parallel to the first surface  34  of the glass substrate  10 . The secondary reflective surface  42  is convex and smaller than the primary reflective surface  40 . The secondary reflective surface  42  is commonly referred to as a blind spot or blind zone mirror. While the secondary reflective surface  42  may be located anywhere with respect to the primary reflective surface  40 , it is shown in the upper left hand corner of the glass substrate  10  as viewed by the driver of the motor vehicle. The secondary reflective surface  42  is created prior to the coating of the second surface  36  with reflective coating. Therefore, it has the same reflective qualities as the primary reflective surface  40 . 
     Once the concave surface  44  has been created, the concave surface  44  is polished. The polishing of the concave surface  44  may include a very fine grit abrasive material. In addition, the step of polishing may include a slurry that may include cerium oxide. The step of polishing may occur before or after the step of removing the glass substrate  10  from the turntable  46 . 
     Referring to  FIGS. 3 through 14 , graphic representations of various preferred embodiments of the inventive method are depicted, wherein similar elements used in the various preferred embodiments of the inventive method are indicated by reference numerals offset from one another by factors of 100. Referring specifically to  FIGS. 3 through 5 , one embodiment of an inventive method is graphically represented. The method used for creating a concave surface  44  in the glass substrate  10  is shown. The concave surface will be used to create the secondary reflective surface  42  once the reflective coating is applied to the second surface of the glass substrate  10 . At this point in the preparation of the glass substrate  10 , there is no reflective coating applied to the second surface  36 . The concave surface  44  is created out of the second surface  36  of the glass substrate  10 . In all of the embodiments discussed herein, the glass substrate  10  will include a first surface  36  and a second surface  36  that are planar and parallel to each other. The concave surface  44  is the only portion of the glass substrate  10  that deviates from the flat first  34  and second  36  surfaces. 
     To create the concave surface  44 , the glass substrate  10  is secured to a turntable  46 . Hooks  48  are used to secure the glass substrate  10  to the turntable  46 . Devices similar to hooks  48  may be used. In addition, a vacuum may be applied to the glass substrate  10  from the turntable  46  to secure the glass substrate  10  thereto. The turntable  46  is spun to create a turntable axis of rotation  50 . As shown in  FIG. 5 , the axis of rotation is defined by an output shaft  52  of a motor  54 , both shown in phantom. It may be appreciated by those skilled in the art that the turntable axis of rotation  50  may be separate and distinct from the axis of rotation for the output shaft  52  and that the turntable  46  may be belt driven or driven through some other linkage that does not directly connect the motor  54  to the output shaft  52 . Arrows  56  graphically represent the rotation of the turntable  56  and, therefore, the glass substrate  10 . The turntable spins about the turntable axis of rotation  50  in a range between four revolutions per minute (rpm) and 100 rpm. In this embodiment, it is contemplated that the turntable  46  spins at approximately 20 rpm. 
     Disposed adjacent the turntable is a pillar  58 . The pillar  58  is stationary with respect to the turntable  46 . The pillar  58  represents a stationary structure from which a grinding wheel  60  will move in relation thereto. It should be appreciated by those skilled in the art that the pillar  58  may be replaced with another structure that provides a support for the movement of the grinding wheel  60  with respect to the turntable  46 . By way of example, the pillar  58  may be replaced with a base located remote from the turntable  46 , whereby a robot arm would extend from the base to position the grinding wheel  60  in the appropriate position with respect to the turntable  46 . 
     The pillar  58  includes two channels  62 ,  64  through which a grinding wheel motor  66  is secured. The channels  62 ,  64  provide the grinding wheel  66  with the ability to move up and down, as represented by arrow  68  (the directions up and down are used for purposes of the relationships shown in the Figures and are not to be interpreted as limiting). The grinding wheel  60  also moves up and down in the directions defined by arrow  68 . The grinding wheel  60  moves, however, along the turntable axis of rotation  50  such that when it is moved far enough, it engages the glass substrate  10  at a point where the turntable axis of rotation  50  intersects the glass substrate  10 . The grinding wheel motor  66  includes an output shaft  70  that is fixedly secured to the center  72  of the grinding wheel  60 . Therefore, the grinding wheel  60  rotates about an axis  74  that is defined by the output shaft  70  of the grinding wheel motor  66 . As it may be appreciated by those skilled in the art, the grinding wheel  60  may be indirectly driven by the grinding wheel motor  66 . 
     The grinding wheel motor  66  spins the grinding wheel  60  in a direction represented by arrows  76 . The grinding wheel is spun at speeds in a range between 3,000 rpm and 5, 000 rpm. In the preferred embodiment, the grinding wheel  60  is spun at a rate of 4,000 rpm. 
     Once the turntable  46  and the grinding wheel  60  are spinning at their appropriate speeds, the grinding wheel  60  is moved downwardly, the grinding wheel  60  engages the glass substrate  10 . The spinning motions of the grinding wheel  60  and the turntable  46  allow the grinding wheel  60  to grind a concave surface  44  out of the second surface  36  of the glass substrate  10 . In this way, the grinding wheel  60 , having a defined radius, produces a concave section of glass having a concave surface  44  with a radius of curvature equal to the radius of the grinding wheel  60 . 
     Referring now to  FIGS. 6 through 8 , wherein elements similar to those described above are offset by 100, the glass substrate  110  is having a concave surface  144  ground out thereof using a grinding wheel  160 . The difference with this method when compared to the method described when referencing  FIGS. 3 through 5  is that the grinding wheel  160  is no longer perpendicular to the glass substrate  110 . In the prior embodiment, the axis of grinding wheel rotation  74  was parallel to the glass substrate  10 . In this embodiment, the axis of grinding wheel rotation  174  is disposed at an angle other than parallel with the glass substrate  110 . And while the grinding wheel  160  engages the glass substrate  110  at the turntable axis of rotation  150 , the grinding wheel  160  does not extend through the turntable axis of rotation  150 . The angle created between the turntable axis of rotation  150  and the axis of grinding wheel rotation  174  is at an acute angle. 
     The grinding wheel  160  and the turntable  146  rotate at speeds comparable to those described above. The advantage of using the grinding wheel  160  at an acute angle with respect to the turntable axis of rotation  150  is that the grinding wheel  160  may have a radius that is smaller than the resulting radius of curvature of the concave surface  144  being created by the grinding wheel  160 . By way of example, the radius of the grinding wheel  160  may be in the range of ⅖ th  the size of the radius of curvature of the resulting concave surface  144 . In this embodiment the grinding wheel  160  moves up and down as represented by arrow  168  in a manner similar to that of the embodiment discussed above. 
     Referring specifically to  FIGS. 9 through 11 , a second alternative embodiment is shown, wherein like elements are offset from the first preferred embodiment by 200. These Figures represent an embodiment similar to those found in  FIGS. 6 through 8  wherein the output shaft  270  for the grinding wheel  260  defines an axis of grinding wheel rotation  274  which is not perpendicular to the turntable axis of rotation  250 . The angle defined between the axis of grinding wheel rotation  274  and the turntable axis of rotation  250  is at an acute angle similar to that described for  FIGS. 6 through 8 . The primary difference between the first alternative embodiment and the second alternative embodiment is that the grinding wheel  260  includes a convex surface  278  resulting in the grinding wheel  260  and output shaft  270  having a profile similar to that of a mushroom. The grinding wheel  260  and the turntable  246  rotate at rotational speeds similar to those described above. The axis of grinding wheel rotation  274  is required to define an angle with respect to the turntable axis of rotation  250  such that a central portion  280  of the convex surface  278  does not engage, abut, or contact the glass substrate  210  while the grinding wheel  260  is grinding the concave surface  244  from the glass substrate  210 . Having the central portion  280  in contact with the glass substrate  210  through the grinding process will compromise the composition of the glass substrate  210  to the point where the glass substrate  210  is not useable and/or destroyed by applying increased pressure at the central portion  280  due to the reduced glass removal which would result from the slower grinding feed rate at that location  280 . 
     Referring to specifically to  FIGS. 12 through 14 , elements similar to those described in the first preferred embodiment are offset by 300. In this embodiment, the output shaft  370  of the grinding wheel  360  is parallel to the glass substrate  310 , and as such, perpendicular to the turntable axis of rotation  350 . And again, the glass substrate  310  is rotated about a turntable  346  while the grinding wheel  360  is rotated about the output shaft  370 . 
     In this embodiment, the grinding wheel  360  includes a convex outer edge  382 . In addition, the grinding wheel  360  has a profile wider than that of the grinding wheel  60  shown in the preferred embodiment ( FIGS. 3 through 5 ). The wider grinding wheel  360  with the convex surface  378  provides for a faster cycle time. 
     Referring to  FIG. 15 , a method of polishing a concave section  444  includes the use of a polishing wheel  484  having a convex polishing pad  486  that complements the concave section  444  of the glass substrate  410 . A slurry of cerium oxide  488  is applied to the glass substrate  410 . The polishing wheel  484  is lowered to the concave section  444  and it is polished. Any convex polishing pad  486  suitable for polishing glass may be used. In the preferred embodiment of the polishing method, a polishing pad offered by Spartan Felt Company under the trademark DuroTex™ is used. Once the convex surface  486  is polished, it is cleaned and prepared to be coated with a reflective coating. 
     The invention has been described in an illustrative manner. It is to be understood that the terminology, which has been used, is intended to be in the nature of words of description rather than of limitation. 
     Many modifications and variations of the invention are possible in light of the above teachings. Therefore, within the scope of the appended claims, the invention may be practiced other than as specifically described.