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CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on provisional application 60/469,848 filed May 13, 2003 and entitled “Vision Panel Frame for Fire Door”, and claims the benefit thereof. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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     BACKGROUND OF THE INVENTION 
     This invention relates generally to a vision panel providing a window in a fire door. More particularly, the present invention provides a vision panel providing simplified assembly. 
     Vision panels for fire doors provide a window through the fire door that preserves the integrity of the fire door by retaining an approved wire glass even after prolonged exposure to fire and possible thermal shock caused by the glass being sprayed with a fire hose after heating. 
     Currently available vision panels provide frame halves that may fit into the opening of the door from either side of the door. The frame halves have flanges which abut the front and rear faces of the door and inner sash members extending in from the flanges and which come together about a pane of glass to support the glass therebetween. Assembly of the vision panel can require two people, one to hold one frame half against one side of the door, while the second person from the other side of the door positions the glass and installs screws holding the frame halves together. In some designs the screws must pass through the door itself requiring accurate drilling of holes and raising the risk of splintering the door skin material. 
     U.S. Pat. No. 4,550,542, filed Aug. 9, 1984, and assigned to the assignee of the present invention and hereby incorporated by reference, describes an improved vision panel frame providing simplified installation by using bolts passing wholly through the precut window opening and a spring clip that may be used to hold one-half of the frame in position while the installer works from the other side of the door. 
     The spring clip in the &#39;542 patent has a finger at one end engaging a portion of a sash member of the frame half to be retained, and a hook at the other end that may grip the edge of the window opening cut in the door opposite the installed frame. A second frame half is then installed over this hook covering it from view. 
     Care must be taken when tightening the screws holding the frame together to provide sufficient force to compress the sash against the glass so that should the glass become molten, the edges of the sash will embed themselves in the glass to hold it in position. Too much force in tightening the screws holding the frame together, however, can distort the metal frame halves. 
     Separate support elements may be placed underneath the glass pane during installation to support it above the lower sill of the opening centered within the sash plates. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a number of improvements over the above-described vision panel that greatly simplify assembly, reduce the number of parts that an installer must contend with, and provides superior control over the compressive force with which the glass is held reducing the chance of mis-installation or distortion of the frame unit. 
     More specifically, the present invention provides a fire resistant vision panel for assembly in an opening passing through a door, the vision panel, including first and second flanges sized to frame the opening on either side of the door, and sash elements extending into the opening from each of the first and second flanges to capture a transparent pane therebetween. At least one retention member is attached to the first flange and extends into the opening to grip a sill surface of the opening to retain thereby the first flange and its sash element in position for assembly. At least, one fastener is adapted to draw the first and second flanges and the sash elements together against the pane. 
     Thus it is one object of at least one embodiment of the invention to simplify installation by eliminating the need for the installer to have a helper holding the flange on one side of the door while the flange on the other side of the door is installed or the need to contend with a separate spring clip. 
     It is yet another object of at least one embodiment of the invention to provide a retention member that may attach directly to the flange rather than to a face of the door, so as to provide a lower profile frame that does not need to accommodate a hook or the like. 
     The retention member may have a hole in its inner end and may include a nail adapted to pass through the hole into a core material of the sill surface. 
     It is thus another object of at least one embodiment of the invention to provide a positive engagement of the frame and the sill surface that readily resists accidental dislodgement. 
     It is yet another object of at least one embodiment of the invention to provide an engagement method that may be installed using commonly available tools and intuitive fastening methods. 
     The second flange may include at least one hole for receiving the fastener therethrough and the retention member may further include a socket receiving an end of the fastener after it has passed through the hole. 
     Thus, it is another object of at least one embodiment of the invention to provide that the retention member perform double duty, both to hold the first frame in place, and to provide a point of attachment between the two frame elements. 
     It is another object of at least one embodiment of the invention to provide a simple method of eliminating fastener holes on the first frame portion by displacing the fastener receiving holes into the interior of the frame. 
     It is another object of at least one embodiment of the invention to shorten the length of necessary fasteners and, hence, their difficulty in alignment of the fastener and holes by moving the point of attachment for the fastener toward the face of the door. 
     The socket may be attached to the retention member by a spring element allowing movement of the socket toward the second flange against a spring biasing force. 
     Thus, it is another object of at least one embodiment of the invention to provide a spring-like compression between the frame halves such as improve the retention of the glass in normal and molten state. 
     The spring element may be a cantilever tab extending across the axis of the threaded fastener that deflects with increasing engagement of a threaded fastener. 
     It is thus another object of at least one embodiment of the invention to provide an extremely simple mechanism for providing the necessary spring biasing in flexure of the tab. 
     The threaded fastener may include a non-threaded section limiting the engagement of the threaded fastener with the socket. 
     It is thus another object of at least one embodiment of the invention to provide a simple method of avoiding over compression of the frame halves while ensuring sufficient compression is obtained. The installer simply turns the threaded fastener until the socket has been drawn wholly off of the threaded portion of the fastener, providing the necessary compressive force, but limiting further compression. 
     These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view in perspective showing the vision panel in a typical fire door installation; 
         FIG. 2  is an exploded fragmentary perspective view of the vision panel as it is assembled in the opening of the door showing retention members used to retain frame halves of the vision panel during the installation process; 
         FIG. 3  is a cross sectional view taken along lines  3 - 3  of  FIG. 1  of a lower sill area of an opening in the door showing the elements of the frame halves and the retention member in position per  FIG. 2 ; 
         FIG. 4  is a fragmentary perspective view of the inner end of the retention member showing portions that allow for holding the retention member against the sill and for receiving the threaded fasteners which attach the frame halves together; and 
         FIG. 5  is a cross-sectional view taken along line  5 - 5  of  FIG. 4  showing flexure of a tab of the retention member receiving the threaded fastener such as limits compressive forces. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIGS. 1 and 2 , the vision panel  10  of the present invention may be used with a conventional fire door  11 ; the fire door having a front face  12  and rear face  14  typically composed of a skin material such as a wood or steel panel sandwiching a core material  16  which may, for example, be honeycombed paper, foam, composite board, or cementations material. 
     The vision panel  10  includes a front frame half  18  and rear frame half  20 , respectively, that may fit within an opening  22  that is generally rectangular in shape having been cut through the front face  12 , the core material  16 , and the rear face  14 . The opening  22  defines four sill surfaces  26  formed by the core material  16  and positioned at a top, bottom, left, and right of the opening  22 . 
     Referring now to  FIGS. 2 and 3 , each of front frame half  18  and rear frame half  20 , include flanges  23  which abut the front face  12  and rear face  14 , respectively, to frame the opening  22  on either side. The flanges  23  may, for example, be eighteen or twenty gauge sheet metal and covered with veneer, paint or polyester powder coating, or may be stainless steel. 
     Extending into the opening  22  at the inner edge of the front frame half  18  and rear frame half  20  are sash elements  24 . 
     Opposed inner edges of the sash elements  24  terminate in finger portions  28  having sharp edges  30  which press inward against either side of a pane glass  32  as is described in U.S. Pat. No. 4,550,542. Softening of the glass  32  during a fire allows the edges  30 , under proper compression, to embed themselves in the glass  32  preventing the glass  32  from falling out. The glass  32  may be a quarter inch wire glass panel or other transparent material. No glazing compound, tape, or intumescent is required. 
     Flanges  23  of front frame half  18  include holes  34  centered along each side of the front frame half  18  formed by the flanges  23  within the perimeter of the opening  22  thus allowing free passage of threaded fasteners  36  therethrough into the opening  22  to engage portions of the rear frame half  20  as will be described below. Threaded fasteners  36  may be one-way fasteners or other tamper proof designs. 
     The flanges  23  and sash element  24  may be folded from a single strip of material, miter cut, and attached by weldments  38  shown in  FIG. 2  at the corners to provide a rugged single unit for assembly. 
     Referring now to  FIG. 3 , a retention arm  40  may attach at an inner surface of the flanges  23  of the rear frame half  20  by spot welding or the like at midpoints along the length of each flange  23  opposite the holes  34  of the front frame half  18 . The retention arm  40  may be folded from sheet metal, and when spot welded to the inner surface of the flanges  23 , adds a thickness less than required by a hook retention system allowing flanges  23  of front frame half  18  and rear frame half  20  to more closely conform to the front face  12  and rear face  14  of the door providing an extremely low profile vision panel  10 . 
     Referring to  FIGS. 3 and 4 , the retention arms  40  may extend into the opening  22  between the sash element  24  and the sill surfaces  26  providing on centrally facing surfaces a ledge  25  supporting edges of the glass  32  to center the glass  32  within the opening  22 . A distal end  42  of the retention arm  40  removed from rear frame half  20  provides a foot element  44  extending from the distal end  42  toward the sill surface  26  and providing a foot  46  abutting the sill surface  26  and having a hole  48  therethrough. The foot  46  supports the ledge  25  of the retention arm  40  to remain essentially parallel to the sill surface  26 . 
     A nail  50  may be used to fasten the foot  46  to the core material  16  of the sill surface  26  preventing movement of the foot  46  along the sill surface  26  such as would allow the rear frame half  20  to slip out of the opening  22 . Note that the nail  50  need only resist this sliding motion and, thus, need not be retained strongly against removal from the core material  16 . Nails  50  may be installed in each of the retention arms  40  or any opposed pair of retention arms  40 . 
     The distal end  42  of the retention arm  40  also includes a downwardly extending tab  52  that does not contact the sill surface  26  to any substantial degree but is held in cantilevered fashion from the remainder horizontal portion of retention arm  40 . This tab  52  includes a threaded hole  54  which may be pre-threaded or threaded by a self-tapping action of threads  56  of the threaded fasteners  36 . 
     Referring to  FIGS. 2 and 4 , the front frame half  18  and rear frame half  20  may be retained together by the threaded fasteners  36  having their threaded portions engaging holes  54  in corresponding retention arms  40  positioned opposite each of the holes  34  with the heads of the threaded fasteners  36  exposed through the flanges  23  of front frame half  18  and capturing the front frame half  18  beneath them. 
     Referring now to  FIG. 5 , the threaded fasteners  36  include a non-threaded portion  58  between the head and the threads  56 . Tightening the threaded fastener  36  causes a flexure of the tab  52  toward the front frame half  18  causing the threaded portion of the hole  54  to move onto the non-threaded portion  58  preventing further tightening of the front frame half  18  and rear frame half  20 . The flexure  60  of the tab  52 , nevertheless, provides a continual spring biasing of the front frame half  18  and rear frame half  20  together. 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.

Summary:
An improved vision panel provides a nail in retention method for holding half the panel in place during installation using inwardly extending retention arms that also provide threaded fasteners for attaching the halves of the frame together about the door, support for the glass pane, and when used with special fasteners having partially non-threaded shanks, a compression limiting spring biasing of the frame elements together.