Abstract:
A method and tool for forming a hem flange. A larger radius bend is initially formed when a pre-hem flange is formed. A sharper radius bend is subsequently formed when the flange is formed. After the flange is formed, conventional hem forming methods and tools may be used to complete formation of a hem that may be used to join inner and outer panels.

Description:
BACKGROUND OF INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a tool for forming a hem flange.  
           [0003]    2. Background Art  
           [0004]    Vehicle body panels such as deck lids, hoods, doors and the like frequently include inner and outer panels that are secured together by means of a hem that extends about their periphery. Such body panels have traditionally been manufactured from steel sheet metal panels. Steel is very ductile and is easily formed in a hem forming operation. Increasingly, automotive manufacturers are turning to aluminum to obtain weight savings for vehicle body panels. Aluminum alloys offer a high strength/low weight alternative to steel.  
           [0005]    Aluminum does not, however, have the same degree of ductility and resistance to work hardening offered by steel. Forming a hem on a sheet metal body panel made of aluminum is more difficult than forming the same hem with steel due to aluminum&#39;s reduced ductility in comparison to steel. One proposed solution to this problem was to form a larger radius hem when making body panels of aluminum sheet metal. Larger radius hems result in lower fit and finish ratings because larger radius hems cause gaps between door closure panels and their openings appear larger. Further, the low ductility of aluminum may cause tears or splits starting from the outer surface of a hem. Tears and splits result in high part rejection rates and unacceptable scrap rates.  
           [0006]    Substantial work hardening may occur during the hem flange formation process. The hem flange formation process is the initial step in forming a hem wherein a peripheral portion of a blank or drawn part is bent to about 90°. Forming a 90° bend in an aluminum sheet around a relatively tight radius causes substantial amounts of deformation. This amount of strain results in splits and even tears as the hem flange is further formed in pre-hem and final hem forming steps.  
           [0007]    These and other problems are addressed by applicants&#39; invention as summarized below.  
         SUMMARY OF INVENTION  
         [0008]    According to one aspect of the present invention, a method of forming a hem flange for securing a sheet metal outer panel to an inner panel is provided. The method comprises retaining a sheet metal blank on a first die with a peripheral portion of the sheet metal blank initially extending outboard of a first flange forming portion. The first flange forming portion has a first supporting portion, a first flange pre-forming surface, and a first radiused portion. A first bender is moved in a direction normal to the initial sheet metal blank surface and into engagement with the peripheral portion of the sheet metal blank. The first bender bends the peripheral portion over the first radiused portion and against the flange pre-forming surface to form a sheet metal blank with a partially formed flange. The sheet metal blank including the partially formed flange is secured to a second die with the partially formed flange of the sheet metal blank initially extending outboard of a second flange forming portion. The second flange forming portion of the second die has a second supporting portion, a second flange forming surface, and a second radiused portion. The second radiused portion has a tighter radius than the first radius. A second bender die is moved into engagement with the partially formed flange to form the partially formed flange against the second flange forming surface in and around the second radiused portion by moving at an angle relative to the sheet metal blank surface that is at least partially directed toward the second flange forming surface.  
           [0009]    According to another aspect of the method of the present invention, a method of forming a hem flange is provided wherein a sheet metal part having a body portion and a flange portion is clamped in a hem flange forming tool with the body portion clamped in the hem flange tool and the flange portion extending outboard of the tool. The flange portion is bent to form a relatively larger radius bend in the sheet metal part between the main body portion and the flange portion by contacting the flange portion with a movable tool. The flange portion is then further bent to form the larger radius bend into a sharp radius bend with the flange portion being disposed at a right angle relative to the body portion by moving the movable tool from a position outboard of the tool inwardly toward the tool.  
           [0010]    According to another aspect of the invention, a flanging tool for forming a hem flange is provided. The flanging tool includes a first pre-form die having a flange pre-forming surface comprising a blank supporting surface, a flange pre-forming surface, and a first radius therebetween. A first flanging punch having a first clamping portion holds the sheet metal blank on the first pre-form die and a first outer portion of the first flanging punch moves relative to the first pre-form die. The sheet metal blank has a peripheral portion that extends outboard of the first pre-form die and the first clamping portion. The first outer portion of the first flanging punch engages the peripheral portion of the sheet metal blank and forms a pre-form flange from the peripheral portion. A first flange die is provided that has a flange forming surface comprising a second blank supporting surface, a flange pre-form forming surface, and a second radius therebetween. A second flanging punch has a second clamping portion that holds the sheet metal blank with the pre-form on the first flange die. A second outer portion moves relative to the first flange die and the second clamping portion. The pre-form flange of the sheet metal blank initially extends outboard of the second blank supporting surface and clamping portion, wherein the second outer portion engages the pre-form flange and forms a flange on the blank from the pre-form flange.  
           [0011]    According to other aspects of the invention, the bending steps may be performed by a flange tool or may also be formed by a roll former or hammer. The pre-form may be formed by a flange tool moving perpendicular to the plane of the sheet metal blank or may alternatively be moved in an angular direction relative to the sheet metal blank surface. Bending the pre-form flange may be performed by a tool that moves at an angle to or perpendicular to the surface of the sheet metal blank so that a compressive force is applied to the stretched surface of the sheet metal blank. The wide radius of the pre-form flange may be 2 to 3 mm or, stated another way, may be two to three times the thickness of the metal forming the blank. The sharp radius may be formed to 0.5 mm or, stated another way, may be the radius of one-half the thickness of the sheet metal forming the blank. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0012]    [0012]FIGS. 1A through 1D are schematic cross-sectional views of final flange forming steps that illustrate a flange forming process for forming a flange according to the present invention;  
         [0013]    [0013]FIGS. 2A through 2D are schematic cross-sectional views that illustrate further processing steps used to form a flat hem after initial forming of the flange according to the present invention;  
         [0014]    [0014]FIGS. 3A through 3D are schematic cross-sectional views that illustrate a one-step flange forming process wherein a flange forming tool is moved at an acute angle relative to the sheet metal blank to be formed;  
         [0015]    [0015]FIGS. 4A through 4D are schematic cross-sectional views that illustrate a flange forming process wherein a roll forming tool is used to form a hem flange in a sheet metal blank according to the present invention;  
         [0016]    [0016]FIGS. 5A through 5D are schematic cross-sectional views that illustrate a process for forming a hem flange in a sheet metal blank using a hammer or other impact tool according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0017]    Referring now to the drawings, FIGS. 1A through 1D are related as steps in a process for forming a flange on a panel for a hem. In FIG. 1A pre-flange tool  10  is shown to include a lower die  12  and an upper die  14 . Upper die  14  includes a clamping portion  16  and a bending member  18 , or flanging punch. The clamping portion  16  clamps the sheet metal blank  20  to the lower die  12  during pre-flange process. Bending member  18  acts on a peripheral portion of the sheet metal blank  20 . Bending member  18 , as indicated by the directional arrow “A”, wipes the sheet metal blank  20  to form it against the lower die  12 . Lower die  12  includes a supporting portion  22  that supports the sheet metal blank  20  and a large radius portion  24  over which the sheet metal blank  20  is initially formed to impart a larger radius bend in the sheet metal blank  20 . A forming portion  26  of the lower die  12  is a surface against which the sheet metal blank  20  is formed as the bending member  18  forces it against the lower die  12 .  
         [0018]    Referring now to FIG. 1B, the same pre-flange tool  10  is shown at a point in the cycle wherein the bending member  18  has completed forming the sheet metal blank  20  against the large radius  20  and forming portion  26  of the lower die  12 . A partial flange  28  is shown as it is initially formed in the pre-flange tool  10 .  
         [0019]    Referring now to FIG. 1C, two alternative embodiments of a flange tool  30  are shown. On the left side of FIG. 1C, a flange tool  30  is provided with a lateral bender  32  that moves in a lateral direction as shown by the directional arrow “B”. On the right side of FIG. 1C, an alternative embodiment is shown wherein the flange tool  30  is provided with an angularly moved bender  34  and moves in the direction of the arrow “C” as indicated. In each embodiment shown in FIG. 1C, a sharp radius  36  is formed on the lower die  38 . Clamping member  40  clamps the sheet metal blank with a partial flange  42  between the clamping member  40  and lower die  38 . The lower die  38  includes a supporting portion  44  that supports the sheet metal blank  42  as shown with the partially formed flange spaced from a flange forming portion  46  of the lower die  38 . The lateral bender  32  or angularly moved bender  34  engages the partial flange of the sheet metal blank  42  and drives it with radial pressure being applied to the stretched surface into engagement with the flange forming portion  46 .  
         [0020]    As shown in FIG. 1D, the angularly moved bender  34  is shown driving the sheet metal blank with partial flange  42  into engagement with the flange forming portion  46  causing it to follow the sharp radius  36  on the lower die  38 .  
         [0021]    In this way, a flange suitable for hemming in conventional hem forming processes is provided that has a sharp radiused flange formed in two steps. In the first step, the sheet metal blank  20  is formed around a relatively large radius  24  that applies a lower level of strain to the sheet metal blank and enables the process to be applied to aluminum alloy having limited ductility. After the pre-flanging operation, the partially formed flange is formed by means of either lateral bender  32  or the angularly moved bender  34  that apply a compressive force to the stretched side of the flange as the flange is formed around the sharp radius  36 . The compressive force applied to the flange improves its ductility characteristics and minimizes any tendency of the material to split or otherwise show signs of strain as it is formed around sharp radius  36 .  
         [0022]    Referring now to FIGS. 2A through 2D, a conventional hem forming process is shown wherein a panel having a sharp radius flange is formed to provide a flat hem in a pre-hemming and final hemming tool. Referring now to FIG. 2A, pre-hem tool  50  is shown with an outer panel  52  and an inner panel  54 . A flange bender  56  is shown above the distal end of the hem flange while a clamping element  58  holds the outer panel  52  and inner panel  54  against the base  60 .  
         [0023]    Referring now to FIG. 2B, the pre-hem tool  50  is shown forming the pre-hem on the outer panel  52 . The hem flange is formed to an approximate 45° angle relative to the outer portions of outer panel  52 .  
         [0024]    Referring now to FIG. 2C, a final hem tool is shown operating on the outer panel  52  and inner panel  54  that are clamped by clamping element  58  into engagement with the base  60 . The hem bender  66  in FIG. 2C is shown as it initially contacts the hem flange. In FIG. 2D, the hem bender  66  is shown completing the formation of the hem joint  68 . Hem joint  68  is a flat hem that joins the outer panel  52  and inner panel  54  together with a tight radius hem joint  68 .  
         [0025]    Referring now to FIGS. 3A through 3D, an alternative embodiment of the invention is shown wherein a one step flange forming tool  70  is provided. The one step flange forming tool  70  acts on a sheet metal blank  72  that is captured between a lower die  74  and a clamping element  76 . A peripheral portion of the sheet metal blank terminates in a distal end  78 . The distal end  78  is contacted by a radiused corner  80  moving in the direction indicated by directional arrow “D” that causes the sheet metal blank to bend around a bending radius  82 .  
         [0026]    As shown in FIG. 3B, the radiused corner  80  contacts the distal end  78  and applies a force in the direction reflected by the directional arrow “E”.  
         [0027]    Referring now to FIG. 3C, the point of contact transitions from the radiused corner  80  to the forming surface  84  with the tool moving in the direction indicated by arrow “F” as the flange is formed until it forms an over-bend  86  as shown in FIG. 3C. The over-bend  86  has a larger radius then the bending radius  82 .  
         [0028]    Referring now to FIG. 3D, the final flange  88  is shown with the forming surface  84  driving the final flange  88  in the direction indicated by arrow “G” into contact with the flange forming surface  90 . In the latter stages of the bending process reflected by FIGS. 3C and D, a compressive force is applied to the stretched surface of the flange as it is driven to form a sharp radius flange.  
         [0029]    Referring now to FIGS. 4A through D, an alternative embodiment of the one step flange forming tool  70 ′ shown. In FIGS. 4A through D, similar reference numerals are used to refer to components that are similar to those shown at FIGS. 3A through D. The one step flange forming tool  70 ′ uses a roll former  92  to form the flange in the sheet metal blank  72 . A lower die  74  and clamping element  76  clamps a sheet metal blank  72  in place during the flange forming process. The roll former  92  initially contacts the distal end  78  of the sheet metal blank  72 . The roll form is shifted on its rolling axis to the angular orientation shown in FIG. 4B. This causes distal end  78  of the sheet metal blank  72  to bend to a large radius that is gradually increased by rotating the axis of the roll former  92  to the position shown in FIG. 4C. The radius of the bend is gradually increased until it forms an over bend  86  as shown in FIG. 4C. Referring now to FIG. 4D, the roll former applies a compressive force to the flange as it is driven against the flange forming surface  90  forming the final flange  88  that has a sharp radius following the contour of the sharp radius on lower die  74 .  
         [0030]    Referring now to FIGS. 5A through 5D, another alternative embodiment of a one step flange forming tool  70 ″ is shown that is intended for low volume production applications. In this embodiment, a hammer  94  is used to form the sheet metal blank  72 . The hammer  94  initially contacts the distal end  78  of the peripheral portion of a sheet metal blank  72  initially shown in FIG. 5A. Referring now to FIG. 5B, the hammer is shown forming a large radius around the bending radius  82  formed on the lower die  74 . The hammer  94  applies a force on the sheet metal blank then gradually forms an over bend  86  as shown in FIG. 5C. The hammer  94  is then used to apply forming pressure to form the final flange  88  against the flange forming surface  90  with the final flange  88  being formed around the bending radius  82 .  
         [0031]    While the best mode for carrying out the invention has been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims.