Abstract:
A novel method is provided for forming a clear hole through a composite plate. At least two plates can be joined together to form a wall of a trailer. The following steps are taken to perform the method: A punching apparatus is advanced through an extension skin of a first composite plate and a first skin of a second composite plate, thereby compressing the core material of the second composite plate and forming a first slug. The apparatus is withdrawn therefrom. The composite plates are flipped over. The apparatus is advanced through a second skin of the second composite plate, forming a second slug. The apparatus is further advanced through the core material of the second composite plate, thereby ejecting the first slug, a portion of the core material of the second composite plate and the second slug to form a hole through the extension skin and the second composite plate. The apparatus is then withdrawn from the second composite plate. The hole is then re-punched and re-withdrawn in order to remove any excess core material which may have expanded into the hole. A rivet can be easily placed through the hole to join the plates together. The re-punching and re-withdrawing step can be eliminated from the method by providing a shoulder on the apparatus. The shoulder will remove any excess core material which may expand into the hole prior to the apparatus being withdrawn from the second composite plate.

Description:
BACKGROUND OF THE INVENTION 
     This invention is generally directed to a novel method of punching a composite plate. 
     One prior art method of punching a hole in a composite plate, which is formed of first and second skins with a core sandwiched therebetween, was performed by punching a hole through the plate with a single punch by using a punching apparatus. Due to the spring back qualities of the core material, the core material entered into the hole formed by the punch after the punch was removed from its engagement with the plate. Therefore, the hole created by the punch was not clear and a rivet could not be placed therethrough without first completely clearing the hole by using separate, special tools. 
     The special tools are used to drill out or remove the excess core material within the hole. These tools are expensive and the process of removing the excess core material is labor intensive. 
     Another prior art method which is disclosed in this inventor&#39;s U.S. Pat. No. 5,774,972, and which is commonly owned by the Assignee herein, uses a novel double punching method to punch the composite plate. This method eliminates the need for special tools to drill out or remove the excess core material within the hole. A problem has been found with this method in that, at times, when the punch is driven completely through the first overlaying skins, the core and the second skin, after breaking the first skins and driving them through the core, the first skins sometimes slipped to the side of the punch, causing the resulting hole to be improperly aligned. 
     The present invention resolves the problems found in the prior art. The present invention provides a novel method for punching a composite plate which eliminates the step of drilling out or removing excess core material within the hole by using a separate tool and eliminates improper alignment of the resulting hole. In addition, the method of the present invention presents several other advantages and features including the provision of a novel joint structure which will become apparent upon a reading of the attached specification. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     A general object of the present invention is to provide a novel method of punching a composite plate to provide a hole through the composite plate which is free of interfering material. 
     An object of the present invention is to provide a novel method of providing a clear hole through a composite plate without using a separate tool to remove excess material from the hole. 
     Briefly, and in accordance with the foregoing, the present invention discloses a novel method for forming a clear hole through a composite plate. The composite panel is twice punched by a punching apparatus to form the clear hole. At least two composite plates can be joined together through the clear hole to form a wall panel, such as may be used in a trailer. 
     Each composite plate is formed from first and second metal skins having a resilient plastic core sandwiched therebetween. The second skin of one plate and the first skin of the adjacent plate each include an integrally formed skin extension which extends past the end of the respective cores and overlaps the respective first or second skin on the adjacent plate. 
     To perform the method, the following steps are performed: The composite plates are placed adjacent to each other such that the ends of the cores abut against each other and the skin extensions overlap the respective first or second skin of the adjacent composite plate. Thereafter, the punching apparatus is engaged with the plate to be punched to compress the plate core material. 
     The punching apparatus punches through the extension skin of one plate and the first skin of the adjacent plate, forming a first slug which is slightly pushed into the core material of the adjacent plate. The punching apparatus is then withdrawn from its engagement. The composite plates are flipped over so that the area of the broken extension and first skins are in the direct path of the punching apparatus. Next, the punching apparatus is engaged with the plate to compress the core material. The punching apparatus then punches through the second skin of the plate, forming a second slug which is pushed into the core material of the adjacent plate. The punching motion is continued to punch entirely through the adjacent plate and the extension skin, thereby ejecting the first slug, a portion of the core material and the second slug of the plate, completing the formation of a hole through the plate and the extension skin. The punching apparatus is then withdrawn from its engagement and the core is allowed to expand, due to the spring back qualities of the resilient core material, into the hole. Next, the punching apparatus is re-engaged with the second skin of the plate and compresses the core. The punching apparatus re-punches through the plate and the extension skin in the same location as the punched hole to shear any excess material from within the hole. The punching apparatus is then re-withdrawn from its engagement with the composite plate. After re-punching through the same hole area, the hole is clear and free of any interfering material. A rivet can be easily placed through the hole to join the plates together. 
     An enlarged shoulder can also be added to the punching apparatus to eliminate the re-punching and re-withdrawing steps from the method. During the withdrawing of the punching apparatus after creating the hole, the shoulder on the punching apparatus will shear any excess core material that has expanded in the hole behind the shoulder from within the hole. 
     Of course, a composite plate can be punched by using the novel method described herein to form a hole clear of any interfering material by first punching the second skin, then flipping the plate over, and then punching the extension and first skins. Also, a composite plate having only first and second skins and a core can be punched by using the novel method described herein to form a hole clear of any interfering material. That is, a composite plate, without the provision of an overlapping skin can be punched by using this method. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The organization and manner of the structure and operation of the invention, together with further objects and advantages thereof, may best be understood by reference to the following description, taken in connection with the accompanying drawings, wherein like reference numerals identify like elements in which: 
     FIGS. 1-11 are cross-sectional views that display the method of punching a clear hole through a composite plate as described in the first embodiment. 
     FIGS. 12-19 are cross-sectional views that display the method of punching a clear hole through a composite plate as described in the second embodiment. 
     FIG. 20 is an enlarged, cross-sectional view of the clear hole formed by the punching apparatus; 
     FIG. 21 is a cross-sectional view of the composite plate with a rivet, shown in elevation, inserted through the clear hole formed by the punching apparatus; 
     FIG. 22 is a cross-sectional view of adjacent composite plates attached together by rivets to form a joint; and 
     FIG. 23 is a perspective view of a trailer in which the punched composite plate is used to form a side wall of the trailer. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     While the invention may be susceptible to embodiment in different forms, there is shown in the drawings, and herein will be described in detail, specific embodiments with the understanding that the present disclosure is to be considered an exemplification of the principles of the invention, and is not intended to limit the invention to that as illustrated and described herein. 
     The present invention presents a novel method of punching a composite plate  20  to form a hole  22  through the plate  20  which is clear and free of interfering materials in the hole  22 . No separate tools, as are necessary in prior art methods of forming a hole through a composite plate, are required to clear out the hole  22  after the composite plate  20  is punched by the methods described herein. 
     The composite plate  20  formed in accordance with the present invention can be used to form a wall panel in a trailer  24 , shown in FIG.  23 . The trailer  24  is generally comprised of a floor  26 , a roof  28 , a front wall  30 , a pair of opposite side walls  32  (only one of which is shown), rear cargo doors (not shown), a landing gear  34 , and an undercarriage assembly  36 . 
     Each side wall  32  of the trailer  24  is formed from a plurality of composite plates shown in the drawings as  20   a - 20   f.    
     The structure of each composite plate  20   a - 20   f  is described with respect to composite plate  20   a  with the understanding that the other composite plates are identically formed. Composite plate  20   a  is formed from a core  38   a  sandwiched between a first skin  40   a  and a second skin  42   a . The skins  40   a ,  42   a , are preferably formed of full hard, high strength, high tension, galvanized steel. Preferably, the first skin  40   a  is formed from G 60  (60 grams/meter) galvanized steel and the second skin  42   a  is formed from G 90  (90 grams/meter) galvanized steel. The core  38   a  is preferably made of a light-weight, resilient plastic material, such as high density polyethylene (HDPE) or polypropylene. The skins  40   a ,  42   a  may be adhesively bonded or otherwise affixed to the core  38   a . A skin extension  44   a  is integrally formed with the respective first skin  40   a  or second skin  42   a  of the composite plate  20   a  which extends beyond the end of the core  38   a . The skin extension  44   a  is used to overlap the respective skin of the adjacent composite plate. 
     FIG. 22 shows the preferred construction of joined or spliced adjacent composite plates  20   a ,  20   b  in the final form which are used to form the side walls  32  of the trailer  24 . Such a construction of joined or spliced adjacent composite plates  20  is disclosed in U.S. Pat. No. 4,940,279, which disclosure is herein incorporated by reference. The ends of the plates  20   a ,  20   b  abut directly against one another. The skin extensions  44   a ,  44   b  form an overlap joint  46  for joining the adjacent plates  20   a ,  20   b  together. As shown, the skin extension  44   a  which is integral with the second skin  42   a  of composite plate  20   a  overlaps the second skin  42   b  of composite plate  20   b , and the skin extension  44   b , which is integral with the first skin  40   b  of composite plate  20   b , overlaps the first skin  40   a  of composite plate  20   a . The skin extensions  44   a ,  44   b  seat tightly against the respective first and second skins  42   b ,  40   a.    
     Preferably, each composite plate  20   a - 20   f  is rectangular having a height greater than its width. Each composite plate  20   a - 20   f  (without the respective skin extension, for example  44   a ,  44   b ) is approximately forty nine inches in length. Each skin  40   a ,  42   a ;  40   b ,  42   b  and thus each skin extension  44   a ,  44   b  is preferably nineteen thousandths of an inch in thickness. The overall thickness of each composite plate  20   a - 20   f  is approximately two hundred and thirty thousands of an inch. 
     To join or splice adjacent composite plates  20   a - 20   f  together to form the side wall  32  of the trailer  24 , the novel method of the present invention described herein is used. For ease and clarity in describing the present invention, the punching method is described with respect to the composite plate  20   a  and the skin extension  44   b  which overlaps the first skin  40   a  of the composite plate  20   a , except where the composite plate  20   b  and second skin extension  44   a  are specifically described. The composite plates  20   a ,  20   b  are punched in the area of the overlap joint  46 , that is, through the composite plate  20   a  or  20   b  and the respective skin extension  44   b ,  44   a  to form a hole  22 . Preferably, in the particular embodiment disclosed, the holes  22  formed through the composite plates  20   a ,  20   b  are one and a quarter inches apart from each other along a four foot plate. A first embodiment of the method is shown in FIGS. 1-11 and a second embodiment of the method is shown in FIGS. 12-19. For ease and clarity in describing the present invention, the punching method is described with respect to the composite plate  20   a  and the skin extension  44   b  which overlaps the first skin  40   a  of the composite plate  20   a , except where the composite plate  20   b  and second skin extension  44   a  are specifically described. It is to be understood that the method of the present invention can be used to punch a composite plate that does not include a skin extension to provide a clear hole through the composite plate. In addition, the method of the present invention can be used to punch other forms of joints between composite plates. 
     Attention is now drawn to the method of punching the plates in FIGS. 1-11. 
     The punching apparatus  48  includes a top structure  50  and a bottom structure  52 . The composite plate  20   a  is placed between the structures  50 ,  52  during the novel punching process described herein. 
     The top structure  50  generally includes a press ram  54 , a punch holder  56 , a punch  58  having a shaft  59 , a spring  60  having a predetermined spring constant and a stripper plate  62 . The top structure  50  can be moved upwardly and downwardly relative to the bottom structure  52 . 
     The punch holder  56  and punch  58  are fixedly attached to the press ram  54  and extend downwardly therefrom toward the bottom structure  52 . The punch  58  shaft  59  of the may have any desired diameter, but in the embodiment disclosed, the shaft  59  preferably has a diameter of approximately two hundred and sixty-five thousands of an inch. The punch  58  also has a tip  63 , which in the preferred embodiment, is curved. However, it would be apparent to those of ordinary skill in the art to shape the tip in other forms in order to serve the same purpose that the curved tip of the preferred embodiment serves as described herein. 
     One end of the spring  60  is connected to the punch holder  56  and the other end is connected to an upper surface of the stripper plate  62 . The punch  58  is positioned through the middle of the spring  60  and is aligned with a bore  64  through the stripper plate  62 . The stripper plate  62  includes an embossment  66  thereon which extends downwardly from a plate engaging surface  68  and encircles the bore  64  in the stripper plate  62 . The embossment  66  is approximately twenty thousands of an inch (slightly less than the thickness of one skin of the composite plate). Other dimensions for the embossment  66  and skins of the composite plate may be used depending on the application. 
     The bottom structure  52  generally includes a die holder  70 , a die  72  and a press frame  74 . The bottom structure  52  is stationary. 
     The die holder  70  is attached to, and extends upwardly from, the press frame  74 . The die  72  is seated within the die holder  70 . The die holder  70  and the die  72  each have a passageway  76 ,  78 , respectively, therethrough which are aligned with each other. The press frame  74  includes a chute  80  therethrough which is in communication with the passageways  76 ,  78  through the die holder  70  and die  72 . The passageways  76 ,  78  through the die holder  70  and die  72  are aligned with the bore  64  through the stripper plate  62 . 
     In FIG. 1, the punching apparatus  48  is shown disengaged from the composite plate  20   a  which is to be punched. The composite plate  20   a  is placed on top of the bottom structure  52  of the punching apparatus  48 , and thus below the top structure  50  of the punching apparatus  48 . 
     As shown in FIG. 2, the punching apparatus  48  is engaged with the composite plate  20   a  and the skin extension  44   b , but prior to the punch  58  being passed through the skin extension  44   b  and the first skin  40   a  of the composite plate  20   a . The lower surface of the second skin  42   a  of the plate  20   a  rests on the die holder  70  and die  72 . The stripper plate  62 , with the embossment  66 , is brought into contact with the upper surface of the skin extension  44   b  by moving the top structure  50  downwardly toward the bottom structure  52 . 
     As the press ram  54  pushes the stripper plate  62  down onto the skin extension  44   b , pressure is applied to the composite plate  20   a  which causes the core  38   a  of the composite plate  20   a  to compress. For example, the punching apparatus  48  applies 6,000 pounds of pressure on the composite plate  20   a . The embossment  66  presses against the skin extension  44   b  and causes a portion  82   b ,  84   a  of each of the skin extension  44   b  and the first skin  40   a  and a portion of the core  38   a  to slightly deform around the embossment  66 . The inward deformation of portions  82   b ,  84   a  of the skin extension  44   b , and the first skin  40   a  are exaggerated for clarity in showing the deformation in FIGS. 2-11 and  20 - 22 . If the composite plate  20   a  is being punched through a portion where the skin extension  44   b  is not present or if a portion of the overlap joint  46  which is formed by the composite plate  20   b  is being punched, the embossment  66  presses against the first skin  40   a ,  40   b , respectively, of the composite plate  20   a ,  20   b.    
     To punch the composite plate  20   a , as shown in FIG. 3, the press ram  54  continues its downward movement which causes the punch  58  to punch through and break the skin extension  44   b  and the first skin  40   a  once the spring constant of the spring  60  is overcome. After the punch  58  breaks through the skin extension  44   b  and the first skin  40   a  of the composite plate  20   a , the curved punch tip  63  forms a slug  86 . Due to the tip  63  of the punch  58  being curved, the slug  86  that is formed is also curved. The slug  86  is formed of a broken piece  88   b  of the skin extension  44   b  and a broken piece  90   a  of the first skin  40   a  of the composite plate  20   a . The slug  86  is compressed into the core  38   a  by the punch  58 . The punch  58  is used only to break the skin extension  44   b  and first skin  40   a  to form a hole  92  through the skin extension  44   b  and first skin  40   a  and depress a slug  86  slightly into the core  38   a . The slug  86  has a diameter that is smaller than that of the hole  92  due to its curved nature. Since the slug  86  has a smaller diameter than the hole  92 , the ejection of the slug  86  from the composite plate  20   a , as described herein, will meet less resistance than if the diameter of the slug  86  were substantially equivalent to the diameter of the hole  92 . Because the slug  86  is ejected from the composite plate  20   a  with little resistance, there is less opportunity to possibly damage the composite plate  20   a  or the skin extension  44   b  during the ejection of the slug  86 . 
     As the punch  58  passes through the skin extension  44   b  and the first skin  40   a , the punch  58  deforms the skin extension  44   b  and the first skin  40   a  slightly by causing the portion  82   b ,  84   a  of each of the skin extension  44   b  and the first skin  40   a  around the break caused by the punch  58  to bend slightly inward toward the core  38   a.    
     Thereafter, the punch  58  is removed from its engagement through the skin extension  44   b  and the first skin  40   a  of the composite plate  20   a , such that the pressure from the top structure  50  is removed, leaving the hole  92  through the skin extension  44   b  and the first skin  40   a  as shown in FIG.  4 . As the press ram  54  moves upwardly, the spring  60  expands. 
     As shown in FIG. 5, after the punch  58  and the stripper plate  62  are withdrawn from engagement with the composite plate  20   a , the composite plates  20   a ,  20   b  are flipped over so that the second skin  42   a  of the composite plate  20   a  is presented to the punch  58 . The composite plate  20   a  is positioned such that the hole  92  in composite plate  20   a  sits over the passageways  76 ,  78  of the die holder  70  and the die  72  respectively and is aligned with the punch  58 . 
     As shown in FIG. 6, the punching apparatus  48  is engaged with the second skin  42   a  of the composite plate  20   a , but prior to the punch  58  being passed through the second skin  42   a  of the composite plate  20   a . The upper surface of the skin extension  44   b  of the plate  20   b  rests on the die holder  70  and die  72 . The stripper plate  62 , with the embossment  66 , is brought into contact with the lower surface of the second skin  42   a  by moving the top structure  50  downwardly toward the bottom structure  52 . 
     As the press ram  54  pushes the stripper plate  62  down onto the second skin  42   a , pressure is applied to the composite plate  20   a  which causes the core  38   a  of the composite plate  20   a  to compress. The embossment  66  presses against the second skin  42   a  of the composite plate  20   a  and causes a portion  94   a  of the second skin  42   a  and a portion of the core  38   a  to slightly deform around the embossment  66 . The inward deformation of portion  94   a  of the second skin  42   a  is exaggerated for clarity in showing the deformation in FIGS. 6-11 and  20 - 22 . 
     The press ram  54  continues its downward movement, as shown in FIG. 7, which causes the punch  58  to punch through and break the second skin  42   a  once the spring constant of the spring  60  is overcome. As the punch  58  passes and breaks through the second skin  42   a , the punch  58  deforms the second skin  42   a  slightly by causing the portion  94   a  of the second skin  42   a  around the break caused by the punch  58  to bend slightly inward toward the core  38   a.    
     After the punch  58  breaks through the second skin  42   a  of the composite plate  20   a , a second skin slug  95  is formed. The core  38   a  is then compressed and displaced outwardly from the punch  58  as the punch enters the core  38   a . As the punch  58  passes through the core  38   a  of the plate, a slug  96  is pushed through the core  38   a . The slug  96  is formed of the second skin slug  95 , the slug  86  and a portion  97  of the core  38   a  therebetween. Due to the tip  63  of the punch  58  being curved, the second skin slug  95  that is a part of slug  96  is also curved. Therefore, the slug  96  has a diameter that is smaller than that of a hole  98  that is formed by the punch  58  due to the curved nature of the slug  96 . Since the slug  96  has a smaller diameter than the hole  98 , the ejection of the slug  96  from the composite plate  20   a , as described herein, will meet less resistance than if the diameter of the slug  96  were substantially equivalent to the diameter of the hole  98 . Because the slug  96  is ejected from the composite plate  20   a  with little resistance, there is less opportunity to possibly damage the composite plate  20   a  or the skin extension  44   b  during the ejection of the slug  96 . When the punch  58  reaches the first skin  40   a  of the composite plate  20   a  the core  38   a  has been displaced outward around the punch penetration area. 
     As the punch  58  passes through the first skin  40   a  and the skin extension  44   b  where the hole  92  was formed, the slug  96  is deposited into the passageway  78  through the die  72 . The slug  96  passes through the passageways  76 ,  78  in the die holder  70  and die  72  and then downwardly through the chute  80  in the press frame  74  to a collection area (not shown). 
     Thereafter, the punch  58  is removed from its engagement through the composite plate  20   a . As the press ram  54  moves upwardly, the spring  60  expands. The punching apparatus  48  is disengaged from the composite plate  20   a  such that the punch  58  and the stripper plate  62  are withdrawn from engagement with the composite plate  20   a  and the hole  98  is formed, as shown in FIG.  8 . Due to the resiliency of the core material, a portion  100  of the core material springs back into the hole  98  when the punch  58  is removed in such a manner so as to partially block the punched hole  98  through the plate  20   a ,  20   b , respectively. Therefore, at this time, a rivet cannot be placed through the hole  98  due to the core material  100  which interferes with the hole  98 . If the method used in the prior art were employed here, a separate tool would now be used to drill out the interfering material  100  in the punched hole  98 . 
     The inner wall  102  of hole  98  is convex such that the interfering core material  100  protrudes inwardly toward the center of the hole  98  after the punching apparatus  48  has punched entirely through the composite plate  20   a ,  20   b . For example, the interfering core material  100  can form a minimum diameter in the hole  98  of one hundred and eighty-five thousands of an inch when the punch  58  has a diameter of two hundred and sixty-five thousands of an inch. Of course, the amount of springback is dependent on the amount of pressure placed on the composite plate  20   a ,  20   b  when the plate  20   a ,  20   b  is compressed by the punching apparatus  48  and the thickness of the core material. 
     To clear the area through the hole  98  so that a rivet may be passed therethrough, the composite plate  20   a  is punched again by the punching apparatus  48  through the same area as where the hole  98  is first punched. As shown in FIG. 9, the stripper plate  62 , with embossment  66 , is once again pressed against the second skin  42   a  to compress the core  38   a  of the composite plate  20   a . When the stripper plate  62  applies pressure to the composite plate  20   a , the core  38   a  is again compressed and core material around the hole  98  is moved into the hole  98  due to the pressure on the composite plate  20   a  by the punching apparatus  48 . Depending on the amount of compression on the core  38   a , more core material than just the interfering material  100  may be moved into the hole area. At this point, the punch  58  is not engaged with the composite plate  20   a.    
     Thereafter, as shown in FIG. 10, the punch  58  is passed through the composite plate  20   a  in a manner similar to that as described hereinabove. Of course, the punch  58  does not have to penetrate through the skin extension  44   b  and the first and second skins  40   a ,  42   a  because the punch  58  is being passed through the same hole  98  already formed by the previous punch. When the punch  58  passes through the plate  20   a , the punch  58  shears the core material  100  that interferes with the punch  58  as it penetrates through the hole  98  for the second time. The slug  104  that is formed by this punch passes through the passageways  76 ,  78  and through the chute  80  to the collection area. 
     Next, as shown in FIG. 11, the punching apparatus  48  is withdrawn from the composite plate  20   a , as described hereinabove. Some of the core material may spring back into the hole area, however, it is not sufficient to interfere with the clear hole  22  formed through the plate  20   a.    
     Depending on the amount of material moved into the hole area because of the pressure placed on the core  38   a , while the pressure from the top structure  50  is being placed on the composite plate  20   a  and after the punch  58  has completely penetrated the plate  20   a , the core material around the hole area may be generally concave, that is, the inner wall of the hole  22  is concave outwardly from the center of the hole  22 . If this occurs, when the pressure on the composite plate  20   a  by the punching apparatus  48  is removed and the core material  38   a  springs back into the hole area, the core material  38   a  does not spring back far enough so as to enter into the hole  22 . 
     Attention is now directed to the second embodiment of the method shown in FIGS. 12-19 which is used to form a hole  22 ′ which is clear of interfering material. The punching apparatus  48 ′ used in this embodiment of the method is identical to that of the first embodiment, except that the end of the punch  58 ′ has an enlarged shoulder  107 ′ at the end thereof. That is, the punch  58 ′ includes a shaft  59 ′ which has an enlarged shoulder  107 ′ of a larger diameter than the shaft  59 ′ at the end thereof. The same reference numerals are used to denote like elements in the first and second embodiments, with a prime being provided after the reference numbers denoted in the second embodiments. 
     In FIG. 12, the punching apparatus  48 ′ is shown disengaged from the composite plate  20   a ′ which is to be punched. The composite plate  20   a ′ is placed on top of the bottom structure  52 ′ of the punching apparatus  48 ′, and thus below the top structure  50 ′ of the punching apparatus  48 ′. 
     As shown in FIG. 13, the punching apparatus  48 ′ is engaged with the composite plate  20   a ′ and the skin extension  44   b ′, but prior to the punch  58 ′ being passed through the skin extension  44   b ′ and the first skin  40   a ′ of the composite plate  20   a ′. The lower surface of the second skin  42   a ′ of the plate  20   a ′ rests on the die holder  70 ′ and die  72 ′. The stripper plate  62 ′, with the embossment  66 ′, is brought into contact with the upper surface of the skin extension  44   b ′ by moving the top structure  50 ′ downwardly toward the bottom structure  52 ′. 
     As the press ram  54 ′ pushes the stripper plate  62 ′ down onto the skin extension  44   b ′, pressure is applied to the composite plate  20   a ′ which causes the core  38   a ′ of the composite plate  20   a ′ to compress. For example, the punching apparatus  48 ′ applies 6,000 pounds of pressure on the composite plate  20   a ′. The embossment  66 ′ presses against the skin extension  44   b ′ and causes a portion  82   b ′,  84   a ′ of each of the skin extension  44   b ′ and the first skin  40   a ′ and a portion of the core  38   a ′ to slightly deform around the embossment  66 ′. The inward deformation of portions  82   b ′,  84   a ′ of the skin extension  44   b ′ and the first skin  40   a ′ are exaggerated for clarity in showing the deformation in FIGS. 13-22. If the composite plate  20   a ′ is being punched through a portion where the skin extension  44   b ′ is not present or if a portion of the overlap joint  46 ′ which is formed by the composite plate  20   b ′ is being punched, the embossment  66 ′ presses against the first skin  40   a ′,  40   b ′, respectively, of the composite plate  20   a ′,  20   b′.    
     To punch the composite plate  20   a ′, as shown in FIG. 14, the press ram  54 ′ continues its downward movement which causes the punch  58 ′ to punch through and break the skin extension  44   b , and the first skin  40   a ′ once the spring constant of the spring  60 ′ is overcome. After the punch  58 ′ breaks through the skin extension  44   b ′ and the first skin  40   a ′ of the composite plate  20   a ′, the curved punch tip  63 ′ forms a slug  86 ′. Due to the tip  63 ′ of the punch  58 ′ being curved, the slug  86 ′ that is formed is also curved. The slug  86 ′ is formed of a broken piece  88   b ′ of the skin extension  44   b ′ and a broken piece  90   a ′ of the first skin  40   a ′ of the composite plate  20   a ′. The slug  86 ′ is compressed into the core  38   a ′ by the punch  58 ′. The punch  58 ′ is used only to break the skin extension  44   b ′ and first skin  40   a ′ to form a hole  92 ′ through the skin extension  44   b ′ and first skin  40   a ′ and depress the slug  86 ′ slightly into the core  38   a ′. The slug  86 ′ has a diameter that is smaller than that of the hole  92 ′ due to its curved nature. Since the slug  86 ′ has a smaller diameter than the hole  92 ′, the ejection of the slug  86 ′ from the composite plate  20   a ′, as described herein, will meet less resistance than if the diameter of the slug  86 ′ were substantially equivalent to the diameter of the hole  92 ′. Because the slug  86 ′ is ejected from the composite plate  20   a ′ with little resistance, there is less opportunity to possibly damage the composite plate  20   a ′ or the skin extension  44   b ′ during the ejection of the slug  86 ′. 
     As the punch  58 ′ passes and breaks through the skin extension  44   b ′ and the first skin  40   a ′, the punch  58 ′ deforms the skin extension  44   b ′ and the first skin  40   a ′ slightly by causing the portion  82   b ′,  84   a ′ of each of the skin extension  44   b ′ and the first skin  40   a ′ around the break caused by the punch  58 ′ to bend slightly inward toward the core  38   a′.    
     Thereafter, the punch  58 ′ is removed from its engagement through the skin extension  44   b ′ and the first skin  40   a ′ of the composite plate  20   a ′, such that the pressure from the top structure  50 ′ is removed, leaving the hole  92 ′ as shown in FIG.  15 . As the press ram  54 ′ moves upwardly, the spring  60 ′ expands. 
     As shown in FIG. 16, after the punch  58 ′ and the stripper plate  62 ′ are withdrawn from engagement with the composite plate  20   a ′, the composite plates  20   a ′,  20   b ′ are flipped over so that the second skin  42   a ′ of the composite plate  20   a ′ is presented to the punch  58 ′. The composite plate  20   a ′ is positioned such that the hole  92 ′ in the composite plate  20   a ′ sits over the passageways  76 ′,  78 ′ of the die holder  70 ′ and the die  72 ′ respectively and is aligned with the punch  58 ′. 
     As shown in FIG. 17, the punching apparatus  48 ′ is engaged with the second skin  42   a ′ of the composite plate  20   a ′, but prior to the punch  58 ′ being passed through the second skin  42   a ′ of the composite plate  20   a ′. The upper surface of the skin extension  44   b ′ of the plate  20   b ′ rests on the die holder  70 ′ and die  72 ′. The stripper plate  62 ′, with the embossment  66 ′, is brought into contact with the lower surface of the second skin  42   a ′ by moving the top structure  50 ′ downwardly toward the bottom structure  52 ′. 
     As the press ram  54 ′ pushes the stripper plate  62 ′ down onto the second skin  42   a ′, pressure is applied to the composite plate  20   a ′ which causes the core  38   a ′ of the composite plate  20   a ′ to compress. The embossment  66 ′ presses against the second skin  42   a ′ of the composite plate  20   a ′ and causes a portion  94   a ′ of the second skin  42   a ′ and a portion of the core  38   a ′ to slightly deform around the embossment  66 ′. The inward deformation of portion  94   a ′ of the second skin  42   a ′ are exaggerated for clarity in showing the deformation in FIGS. 17-22. 
     The press ram  54 ′ continues its downward movement, as shown in FIG. 18, which causes the punch  58 ′ to punch through and break the second skin  42   a ′ once the spring constant of the spring  60 ′ is overcome. As the punch  58 ′ passes and breaks through the second skin  42   a ′, the punch  58 ′ deforms the second skin  42   a ′ slightly by causing the portion  94   a ′ of the second skin  42   a ′ around the break caused by the punch  58 ′ to bend slightly inward toward the core  38   a.    
     After the punch  58 ′ breaks through the second skin  42   a ′ of the composite plate  20   a ′, a second skin slug  95 ′ is formed. The core  38   a ′ is then compressed and displaced outwardly from the punch  58 ′ as the punch enters the core  38   a ′. As the punch  58 ′ passes through the core  38   a ′ of the plate, a slug  96 ′ is pushed through the core  38   a ′. The slug  96 ′ is formed of the second skin slug  95 ′, the slug  86 ′ and a portion  97 ′ of the core  38   a  ′ therebetween. Due to the tip  63 ′ of the punch  58 ′ being curved, the second skin slug  95 ′ that is a part of the slug  96 ′ is also curved. Therefore, the slug  96 ′ has a diameter that is smaller than that of a hole  98 ′ that is formed by the punch  58 ′ due to the curved nature of the slug  96 ′. Since the slug  96 ′ has a smaller diameter than the hole  98 ′, the ejection of the slug  96 ′ from the composite plate  20   a ′, as described herein, will meet less resistance than if the diameter of the slug  96 ′ were substantially equivalent to the diameter of the hole  98 ′. Because the slug  96 ′ is ejected from the composite plate  20   a ′ with little resistance, there is less opportunity to possibly damage the composite plate  20   a ′ or the skin extension  44   b ′ during the ejection of the slug  96 ′. When the punch  58 ′ reaches the first skin  40   a ′ of the composite plate  20   a ′ the core  38   a ′ has been displaced outward around the punch penetration area. 
     As the punch  58 ′ passes through the first skin  40   a ′ and the skin extension  44   b ′ where the hole  92 ′ was formed, the slug  96 ′ is deposited into the passageway  78 ′ through the die  72 ′. The slug  96 ′ passes through the passageways  76 ′,  78 ′ in the die holder  70 ′ and die  72 ′ and then downwardly through the chute  80 ′ in the press frame  74 ′ to a collection area (not shown). The punch  58 ′ creates the hole  98 ′. 
     While the punch  58 ′ is passing through the plate  20   a ′ and the skin extension  44   b ′, depositing the slug  96 ′ into the passageway  78 ′, a portion  100 ′ of the core material  38   a ′ springs back, behind the shoulder  107 ′, into the hole  98 ′ created by the punch  58 ′ due to the resiliency of the core material. The core material  38   a ′ is able to expand into the hole  98 ′, behind the shoulder  107 ′, due to the difference in the diameters of the shoulder  107 ′ and the shaft  59 ′ of the punch  58 ′. The diameter of the shoulder  107 ′ is greater than the diameter of the shaft  59 ′ of the punch  58 ′. Therefore, the diameter of the hole  98 ′ is the substantially equivalent to that of the shoulder  107 ′, and larger than the diameter of the shaft  59 ′ of the punch  58 ′. Thus, when the shaft  59 ′ of the punch  58 ′ is positioned within the hole  98 ′, there is extra space within the hole  98 ′ not occupied by the shaft  59 ′ of the punch  58 ′, where the core material  38   a ′ may expand to. 
     As shown in FIG. 19, when the punch  58 ′ is removed from its engagement through the composite plate  20   a ′, the portion  100 ′ of the core material  38   a ′ that sprung back into the hole  98 ′ comes into contact with the shoulder  107 ′ that is provided for on the punch  58 ′. As the punch  58 ′ is removed from engagement with the composite plate  20   a ′, the shoulder  107 ′ of the punch  58 ′ shears the portion  100 ′ of the core material  38   a ′ that interferes with the shoulder  107 ′ as the shoulder  107 ′ again passes through the hole  98 ′. The slug  104 ′ that is formed by the shoulder  107 ′ during the disengagement of the punch  58 ′ can then be removed by suitable means. 
     After the punch  58 ′ is withdrawn from the composite plate  20   a ′, some of the core material  38   a ′ may spring back into the hole area, however, it is not sufficient to interfere with the clear hole  22 ′ formed through the plate  20   a ′. 
     Depending on the amount of material moved into the hole area because of the pressure placed on the core  38   a ′, while the pressure from the top structure  50 ′ is being placed on the composite plate  20   a ′ and after the punch  58 ′ has completely penetrated the plate  20   a ′, the core material around the hole area may be generally concave, that is, the inner wall of the hole  22 ′ is concave outwardly from the center of the hole  22 ′. If this occurs, when the pressure on the composite plate  20   a ′ by the punching apparatus  48 ′ is removed and the core material  38   a ′ springs back into the hole area, the core material  38   a ′ does not spring back far enough so as to enter the hole  22 ′. 
     FIG. 20 shows the clear punched hole formed by each embodiment of the method through the composite plate  20   a  and the skin extension  44   b . For convenience in explanation, FIGS. 20,  21 , &amp;  22  are shown and described using the reference numerals of the first embodiment. The inner wall  106  of the hole  22  formed after the last punch in accordance with the present method is generally straight. The portions  82   b ,  84   a ,  94   a  of the skins  44   b ,  40   a ,  42   a  around the hole area are deformed (shown exaggerated) and the portion of the core  38   a  around the hole area is compressed. It is to be understood that the inner walls of the holes through other portions of the composite plates  20   a ,  20   b  that do not have the skin extension  44   b  are also generally straight after being punched in accordance with the novel method described herein. 
     Thereafter, as shown in FIG. 21, a rivet  108 , which is formed of an elongated shank  112  having a first end  113  and a second end  114 , and a head  110  at the first end  113  of the shank  112 . The rivet  108  may easily passed through the clear, punched hole  22  in the composite plate  20   a  (or plate  20   b ). Because of the formation of the downward deformation of the first skin extension  44   b  (or the first skin  40   a ,  40   b  if that portion is punched) by the embossment  66 , the edges of the rivet head  110  sit beneath the upper surface of the skin extension  44   b  (or the first skin  40   a ,  40   b ). This prevents or at least minimizes the possibility of an article snagging the edge of the rivet head  110  which could cause the rivet head  110  to shear off of the shank  112 . As shown in FIG. 22, the second end  114  of the rivet  108  is swaged and enlarged or upset to secure the rivet  108  to the plate  20   a.    
     If any irregularities are formed along the inner wall  106  of the hole  22  during the punching process, a space will be formed between the rivet shank  112  and the inner wall  106  of the hole  22 . When the end  114  of the rivet  108  is swaged and enlarged or upset to secure the rivet  108  to the composite plate  20   a , the rivet shank  112  expands to fill any such spaces so as to provide a tight uniform fit between the rivet shank  112  and the inner wall  106  of the punched hole  22 . 
     Depending on the amount of pressure placed on the composite plate  20   a ,  20   b  by the punching apparatus  48 , the inner wall of the hole  22  after being punched, may be slightly convex or concave. If the inner wall  106  of the hole  22  is convex, this aids in forming a tight fit between the rivet shank  112  and the core  38   a . If the inner wall  106  of the hole  22  is concave, such that the inner wall  106  is undercut the first and second skins  40   a ,  42   a  of the plate  20   a , when the rivet end  114  is swaged, the rivet shank  112  expands so as to fill any space between the inner wall  106  of the hole  22  and the rivet shank  112 . 
     It is also envisioned that a second punching apparatus could be used in the methods described in the first and second embodiments. The use of a second punching apparatus could obviate the need for flipping over the composite plate and realigning the previously formed hole therein with the punch. The composite plate could either be manually moved to the second punching apparatus and realigned, or the composite plate could rest on a conveyor belt which could move the composite plate from the first punching apparatus to the second punching apparatus, thereby automatically aligning the second punch with the previously formed hole. The use of a second punching apparatus could also obviate the need for resetting the distance that the first punching apparatus will punch through. Since the first punching apparatus punches a shorter distance for the first punch than it does for the second and, if necessary, third punch, the first punching apparatus could punch only the shorter distance while the second punching apparatus could punch the longer distance, i.e., completely through the composite plate. 
     While preferred embodiments of the present invention are shown and described, it is envisioned that those skilled in the art may devise various modifications of the present invention without departing from the spirit and scope of the appended claims.