BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field  
           [0002]    The present invention relates to a method of temporarily adhering a stack of sheets together to facilitate drilling a hole through the stack of sheets.  
           [0003]    2. Related Art  
           [0004]    A stack of sheets may be adhered together around a periphery of each sheet using an adhesive such as a glass cloth impregnated with epoxy. Hole burring occurs as a drill passes through the stack. Additionally, each sheet separates from an adjacent sheet as the drill passes between the sheets. This results in the whole stack of sheets expanding. An alternate method includes the permanent application of cured epoxy over the entire surface of each sheet in the stack. The thickness of the epoxy between each sheet results in excessive thickness of the overall stack as well as composites which will contain the separated sheets when they are processed into composites. A method of drilling a stack of sheets that overcomes these problems plus others is desired.  
         SUMMARY OF THE INVENTION  
         [0005]    A first general aspect of the present invention provides a structure comprising:  
           [0006]    a stack comprising a plurality of sheets such that each successive sheet is coupled with a removable adhesive between each sheet; and  
           [0007]    an opening extending through the plurality of sheets within the stack.  
           [0008]    A second general aspect of the present invention provides a structure comprising:  
           [0009]    a plurality of stacks wherein each successive stack is coupled with a removable adhesive to an intermediate layer between each stack wherein each intermediate layer prevents burr formation; and  
           [0010]    wherein each stack comprises a plurality of sheets such that each successive sheet is coupled with a removable adhesive between each sheet.  
           [0011]    A third general aspect of the present invention provides a method comprising:  
           [0012]    forming a stack by stacking a plurality of sheets;  
           [0013]    applying an adhesive between each successive sheet;  
           [0014]    applying a vacuum to the plurality of sheets;  
           [0015]    heating the plurality of sheets to melt the solid adhesive;  
           [0016]    applying a first pressure to the sheets;  
           [0017]    cooling the plurality of sheets; and  
           [0018]    forming an opening through the plurality of sheets; and  
           [0019]    separating the sheets.  
           [0020]    A forth general aspect of the present invention provides a method comprising:  
           [0021]    forming a stack by stacking a plurality of sheets;  
           [0022]    applying a thin layer of water between each successive sheet;  
           [0023]    applying a pressure to the sheets; and  
           [0024]    forming an opening through the plurality of sheets. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    For an understanding of the present invention, reference should be made to the following detailed description taken in connection with the accompanying drawings wherein:  
         [0026]    [0026]FIG. 1 illustrates a first embodiment of the present invention including a side view of a laminated structure including a stack comprising a plurality of sheets including a removable adhesive between each sheet;  
         [0027]    [0027]FIG. 2 illustrates a second embodiment of the present invention including a side view of a laminated structure including a plurality of stacks including a plurality of layers separating each stack;  
         [0028]    [0028]FIG. 3 illustrates a third embodiment of the present invention including an exploded side view of another embodiment of a laminated structure including a plurality of blotter sheets and a plurality plates added to a stack;  
         [0029]    [0029]FIG. 4 illustrates the laminated structure of FIG. 3 positioned within a press apparatus for forming a rigid stack;  
         [0030]    [0030]FIG. 5 illustrates the rigid stack of FIG. 4 with a burr free hole drilled through each sheet of the stack;  
         [0031]    [0031]FIG. 6 illustrates a plan view of a hole within a sheet of the stack; and  
         [0032]    [0032]FIG. 7 illustrates a flow chart of a method for producing a burr free hole in each sheet of the stack.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]    [0033]FIG. 1 illustrates a schematic view of a laminated structure  10  including a stack  12  in accordance with a first embodiment. The stack  12  includes a plurality of sheets  14 A- 14 L. A removable adhesive  16 A- 16 K is applied between each sheet  14 A- 14 L, respectively. The sheets  14 A- 14 L may comprise any suitable material (e.g., copper, invar, copper-invar-copper, etc.). Invar is an iron-nickel alloy. The removable adhesive  16 A- 16 K may comprise any suitable material (e.g., fructose, sucrose, water soluble polymers, water, etc.). The material may comprise any suitable form (e.g., powered fructose, a liquid, a liquid solution, etc.).  
         [0034]    [0034]FIG. 2 illustrates a laminated structure  10 A including a stack  12 A in accordance with a second embodiment. The stack  12 A includes a plurality of stacks  12 C,  12 D,  12 E, and  12 F interspersed with a plurality of layers  18 A- 18 E. Each stack  12 C,  12 D,  12 E and  12 F includes a plurality of sheets  14  with removable adhesive placed between each sheet  14 . Also, the removable adhesive  16  is placed between each layer  18 A- 18 E and each adjacent sheet  14 . Each layer  18 B and  18 C prevents chip build up within drill bit flutes  21  by providing an interruption in the sheet  14  being drilled. Each layer  18 A- 18 E comprises any suitable soft material (e.g., impregnated and laminated epoxy/glass, phenolic/paper laminate, aluminum, etc.). The formation chips are broken off each time the drill bit  20  passes from one material to a dissimilar material. For example, if each sheet  14  is copper, and each layer  18 A- 18 E is (aluminum or paper/phenolic), then any tendency for chip build up would be interrupted each time the drill bit  20  passes from copper to (aluminum or paper/phenolic), or from (aluminum or paper/phenolic) to copper. Additionally, the first layer  18 A keeps the drill bit from wandering as it enters the stack  12 A of the laminated structure  10 A. A soft material, such as aluminum, assists the drill bit  20  in starting a hole  102  at a desired location without wandering about a top surface  22  of the layer  18 A.  
         [0035]    [0035]FIG. 3 illustrates a laminated structure  10 B including a stack  12 B in accordance with a third embodiment. The stack  12 B includes a plurality of sheets  14 . The removable adhesive  16  is applied between each sheet  14 . The stack  12 B includes a first surface  24  and a second surface  26 . A first surface  28 A of a first layer  18 F is coupled with the removable adhesive  16  to the first surface  24  of the stack  12 B. A first surface  30 A of a second layer  18 G is coupled with the removable adhesive  16  to the second surface  26  of the stack  12 B. A first surface  32 A of a first foil  34 A contacts a second surface  28 B of the first layer  18 F. A first surface  36 A of a second foil  34 B contacts a second surface  30 B of the second layer  18 G.  
         [0036]    A first surface  38 A of a first plate  50 A contacts a second surface  32 B of the first foil  34 A. A first surface  40 A of a second plate  50 B contacts a second surface  36 B of the second foil  34 B. A first surface  42 A of a third plate  50 C contacts a second surface  38 B of the first plate  50 A. A first surface  44 A of a fourth plate  50 D contacts a second surface  40 B of the second plate  50 B. The plates  50 A- 50 D may include any suitable material, such as stainless steel.  
         [0037]    The laminated structure  10 B includes a first blotter pad  60 A and a second blotter pad  60 B. The first blotter pad  60 A includes at least one blotter sheet  62 , and the second blotter pad  60 B includes at least one blotter sheet  62 . The blotter sheets  62  may include any suitable compressible material, such as paper. The blotter pads  60 A and  60 B absorb any misalignment between a pair of pressure surfaces  90 A and  90 B (FIG. 4) and the stack  12 B, so that a uniform pressure is applied to the stack  12 B.  
         [0038]    A first surface  64 A of the first blotter pad  60 A contacts a second surface  42 B of the third plate  50 C. A first surface  66 A of the second blotter pad  60 B contacts a second surface  44 B of the fourth plate  50 D. A second surface  64 B of the first blotter pad  60 A contacts a first surface  66 A of a fifth plate  50 E. A second surface  66 B of the second blotter pad  60 B contacts a first surface  68 A of a sixth plate  50 F. The plates  50 E and  50 F may include any suitable material, such as stainless steel.  
         [0039]    [0039]FIG. 4 illustrates the laminated structure  10 B of FIG. 3 positioned within a press apparatus  92 . The press apparatus  92  includes a first pressure head  94 A, a second pressure head  94 B, a vacuum source  96 , and a heat source  98 . The first pressure head  94 A and the second pressure head  94 B provide a compressive force  100  to the laminated structure  10 B. A second surface  66 B of the fifth plate  50 E contacts the pressure surface  90 A of the first pressure head  94 A. A second surface  68 B of the sixth plate  50 F contacts the pressure surface  90 B of the second pressure head  94 B.  
         [0040]    [0040]FIG. 7 illustrates a flow chart of a method for producing a burr free hole  102  in each sheet  14  of the stack  12 B (FIGS. 5 and 6). Step  104  includes stacking a plurality of sheets  14  to form the stack  12 B. Step  106  includes applying a removable adhesive  16  between each successive sheet  14  in the stack  12 B. Step  108  includes applying the removable adhesive  16  between the first layer  18 F and the first surface  24  of the stack  12 B. As mentioned above, the removable adhesive  16  may comprise any suitable material (e.g., fructose, sucrose, water soluble polymers, water, water solutions, etc.). The following example describes using the removable adhesive  16  in the form of solid powered fructose. Step  110  includes applying the removable adhesive  16  between the second layer  18 G and the second surface  26  of the stack  12 B. Step  111  includes forming the laminated structure  10 B by applying the foil sheets  34 A,  34 B the  50 A- 50 F, and the blotter pads  60 A and  60 B to the stack  12 B, as illustrated in FIG. 7. Step  112  includes applying a vacuum to the laminated structure  10 B using the vacuum source  96 . Step  114  includes heating the laminated structure  10 B to melt the removable adhesive  16 .  
         [0041]    Step  116  includes applying the compressive force  100  to the laminated structure  10 B. For fructose, the heat source  98  provides a temperature of about 50° C. to 200° C. and a first compressive force  100  applied to the laminated structure  10 B creates a pressure of about 25 to 75 psig. After about 10 to 20 minutes, the fructose particles start to soften, and the pressure is increased to about 200 to 400 psig. The increase in pressure is delayed to ensure that every fructose particle has been softened to prevent denting of the sheets  14 . The pressure and temperature are held for about 5 to 15 minutes and then shut off and the laminated structure  10 B is cooled while maintaining pressure. When the stack is cool, the pressure is removed and the laminated structure  10 B is removed from the press apparatus  92 .  
         [0042]    In step  118 , the laminated structure  10 B is cooled to an ambient temperature. In step  119 , the foil sheets  34 A,  34 B, the blotter pads  60 A,  60 B, and the plates  50 A- 50 F are removed from the laminated structure, and the stack  12 B and the attached layers  18 F and  18 G are removed from the press apparatus  92 . In step  120 , a drill bit  20  drills the hole  102  through the stack  12 B and the layers  18 F and  18 G, as illustrated in FIG. 5. In step  122 , the stack  12 B and the layers  18 F and  18 G are dipped into a liquid solution  132  to dissolve the fructose. For fructose, the liquid solution  132  may include an ethylene glycol/water mixture. In step  124  the removable adhesive is heated in the liquid solution  132  temperature of about 130° C. to about 150° C. for about 5 minutes to melt the removable adhesive  16 . In step  126 , the sheets  14  are separated from each other using any suitable device, such as tweezers, clamping devices, etc. In step  128 , each sheet  14  is rinsed with any suitable liquid, such as, distilled water and/or a mild detergent solution. In step  130  each sheet  14  is dried using a gas, such as hot dry air. Alternatively, the sheets  14  may be separated by placing the stack  12 B in a steam autoclave  134 . In the steam autoclave  134 , pressurized steam is driven between each sheet  14 . The sheets  14  are then separated, rinsed in water and dried. FIG. 6 illustrates a plan view of the sheet  14  having formed a burr free hole  102 .  
         [0043]    While embodiments of the present invention have been described herein for purposes of illustration, many modifications and changes will become apparent to those skilled in the art. For example, the removable adhesive  16 A- 16 K may comprise a thin layer of water. The surface tension of the thin layer of water between each sheet  14 A- 14 L of the laminated structure  10 A holds the laminated structure  10 A together while the hole  102  is drilled. Each sheet  14 A- 14 L is peeled off the laminated structure  10 A and rinsed in additional water to remove any material particles. Then, each sheet  14 A- 14 L is dried. Accordingly, the appended claims are intended to encompass all such modifications and changes as fall within the true spirit and scope of this invention.
The present invention provides a method of temporarily adhering a stack of sheets together to facilitate drilling a hole through the stack of sheets. The method includes using a temporary adhesive that prevents burring while drilling a hole through the stack.