Abstract:
Apparatus and methods are disclosed for applying electromagnetic force (EMF) to flanging and hemming metal panels to form a hemmed panel assembly. First and second apparatus and method steps use a translating EMF coil to flange a sheet and to finish a hem by progressive non-planar bending. An alternate embodiment uses EMF hemming for difficult-to-hem portions of a flange and optionally precedes this with conventional hemming of straight hem portions.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Patent Application No. 60/562,853 filed Apr. 15, 2004. 
    
    
     TECHNICAL FIELD 
     This invention relates to hemming the edges of inner and outer body panels to form a hemmed assembly having closed edges and to flanging of outer panels prior to hemming. More particularly, the invention relates to electromagnetic (EMF) flanging and hemming apparatus and methods. 
     BACKGROUND OF THE INVENTION 
     Electromagnetic (EMF) forming uses very high-current pulses in a specially designed electrical coil to generate magnetic fields, which impart opposing magnetic fields in a highly electrically conductive metal workpiece, such as an aluminum alloy or steel. With the coil held in a fixed position, the repulsive magnetic forces act upon the workpiece causing it to deform at very high strain rates. Metals deformed at these very high strain rates can exhibit “hyperplasticity,” a level of plastic ductility well beyond what the material is capable of during conventional forming, e.g. flanging and hemming, operations. 
     Roller hemming uses a solid wheel, driven and controlled by a robot (or other device) to gradually bend a 90° flange to a closed hem position as it traverses the perimeter of a panel. The roller hem usually requires two to three passes around the panel to completely bend the flange to the closed, flat hem position. 
     An advantage of the roller hem method compared to conventional hemming is the alternate strain path through which the flange is bent. Conventional hemmers deform the flange through a “plane strain bending” path, which is very severe and can cause cracking failure when hemming aluminum panels, especially panels stamped from AA6111. The roller hem method imparts a component of strain in the direction of the hem line, different from “plane strain bending,” that allows AA6111 to be flat hemmed without cracking. 
     SUMMARY OF THE INVENTION 
     This invention combines concepts from the technologies of electromagnetic force (EMF) forming and roller hemming to provide a method for flanging and hemming sheet metal panels. 
     In this invention, the solid roller of the roller hemming concept is replaced with an electromagnetic coil designed to force the sheet metal flange to bend around the hemline to the closed hem position. A robot, or other device, can drive the electromagnetic coil with translation and rotation around the part contour as required. The combined advantages of non-plane strain bending and hyperplasticity may be realized to avoid cracking failure in aluminum panels. 
     In an alternative embodiment, electromagnetic forces may be used to flange and/or hem a curved or otherwise shaped “difficult to hem” portion of a longer hem wherein the other portions of the hem could be flanged or hemmed by conventional hemming apparatus and methods. The electromagnetic forces could be applied by a stationary coil fitted in a conventional hemming machine and performing plane strain bending assisted by hyperplasticity of the formed material, or the forces could be applied by a traveling coil as previously mentioned to include the advantages of non-plain strain bending. 
     In another alternative embodiment, the very large electromagnetic forces would be managed by employing a rigid stationary electromagnetic hemming anvil, in which the forming coils would remain stationary, and the sheet metal components would be moved progressively through them, with rapidly repeating electromagnetic pulses forming the complete hem. 
     Benefits to be realized from the invention include: 
     Flexible Manufacturing—non-product specific tooling can be created to flat hem many different products. 
     Preservation of class—A surface quality—the electromagnetic forming process requires no direct contact with the workpiece. 
     Non-plane strain bending—the alternate strain path enables greater bending plasticity to avoid cracking in AA6111 aluminum panels. 
     Improved hem quality—electromagnetic (EMF) forming enhances the ductility of metals, which can enable greater bending strains and sharper hems to attain the “jewel” effect at the hemline. 
     Elimination of the conventional flanging process—EMF may be used to flange and hem panels from 180° open to the closed, flat hem condition. The hold-down fixture of the inner panel could be used to provide the support needed to establish the break line of the hem. Alternatively, the outer edge of the inner panel could also be used to wrap the outer flange around the inner panel, creating a tight, flat, crisp hem appearance. 
     The robotic end effector or stationary anvil-type flanging/hemming base could include two or more EMF coils in series to flange and hem the outer panel in a single pass. Multiple coils would each bend the flange a controlled amount. 
     These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1-5  are simplified isometric cross-sectional views of an electromagnetic force (EMF) flanging apparatus illustrating steps in the EMF flanging of a panel sheet in preparation for hemming; 
         FIGS. 6-9  are simplified isometric cross-sectional views of an EMF hemming apparatus illustrating steps in the EMF hemming of a panel; and 
         FIGS. 10-13  are simplified isometric cross-sectional views of the fixtures and workpieces for EMF hemming of a panel with complex curvature and illustrating steps in the EMF hemming method. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Manufacture of hemmed panel assemblies commonly involves a series of manufacturing steps, including forming, flanging and hemming. The hemming process begins with individual metal sheets that are cut, surface treated as desired and formed by known processes, such as by drawing or stamping, into three dimensional panels ready to be assembled into a panel assembly. These steps do not form part of the apparatus and method of the present invention, although they may be combined with this invention to form a hemmed panel manufacturing process. 
     This invention is directed to apparatus and methods used in flanging and/or hemming steps involving electromagnetic forming of hemmed panel assemblies. The following exemplary embodiments and steps incorporate various related concepts of flexible EMF flanging and hemming, as shown in the drawings. 
     Outer Panel Flanging 
     An initial step directed to electromagnetic (EMF) flanging of an outer panel for hemming is illustrated in  FIG. 1 . The figure shows the initial setup wherein an electromagnetic coil  10  is positioned close to a sheet metal flange  12  extending from an outer panel  14  made from steel or aluminum alloy, as an example. The flange  12  is to be bent from a 180° open position to a flanged position of 90° open. 
     The outer panel  14  is supported by suitable tooling, such as anvil  16 , and is retained by hold-down tooling  18 , which provides a rigid support against which the flange will be bent and which establishes the flange radius. The EMF coil  10  is part of end-of-arm-tooling supported and driven by a robot or other device (not shown). 
     Electromagnetic forces are used in this invention to deform the sheet to produce a flange along the periphery of a formed outer closure panel, and, in a subsequent step, to further deform the flange to join inner and outer panels with a hem. A very high current pulse from a capacitor bank, not shown, is passed through the coil  10  held in proximity to the workpiece. The current pulse results in a high magnetic field around the coil. 
     The magnetic field induces eddy currents  20  in the workpiece as shown in  FIG. 2  and an associated secondary magnetic field. The magnetic fields of the coil and of the workpiece are opposite in sign so that an electromagnetic repulsive force  22  causes the deformation of the workpiece as shown in  FIGS. 3-5 . In this example, the electromagnetic force  22  bends the sheet metal flange to the 90° open position as shown in  FIG. 5 . The exact design, shape and electrical characteristics of the coil depend on the specific flange material and geometry. 
     Electromagnetic deformation takes place at very high strain rates, on the order of 10 3  (in/in)/s, or greater. Metals, such as aluminum alloys, characterized by relatively poor formability in conventional forming processes, e.g. stamping, exhibit enhanced ductility when electromagnetically formed at very high rates. This “hyperplasticity” is usually accompanied by reduced springback and a decreased tendency for wrinkling. 
     As the flanging operation proceeds, the EMF coil is moved by the robot or other device in the direction of arrow  24  along the perimeter of the panel, as shown in  FIGS. 3-4 , to bend the flange to the 90° open position as shown in  FIG. 5 . As the coil moves along the panel, the 180° open flange  12  is progressively bent to the finished 90° open position of  FIG. 5 . 
     Finish Hemming 
     Referring now to  FIGS. 6-9 , there is shown a second apparatus and method for applying the EMF concept to the steps of flanging and hemming together of metal panels into a panel assembly. These figures illustrate a simplified apparatus and method for the hemming step. 
     After a panel is flanged, either by EMF flanging or by conventional flanging methods, the EMF hemming method can be used to hem the panel assembly.  FIG. 6  shows the apparatus including a support or anvil  16  supporting the outer panel  14  with its upstanding flange  12 . An inner panel  25  is positioned against the main portion of the outer panel  14  with an outer edge  26  engaging the open flange  12 . Hold-down tooling members  28  clamp the panels against the anvil  16  to hold the panels in assembly. An EMF coil  10 , supported by a robot or other device, not shown, is positioned initially opposite one end of the flange  12   
     As described with respect to the flanging step, when current is pulsed through the EMF coil  10 , eddy currents  20 , shown in  FIG. 6 , result in electromagnetic forces  22  acting on the flange as shown in  FIGS. 7-9 . These forces act as the coil is traversed from one end of the flange to the other to bend the sheet metal in a non-plain strain manner. 
     As the hemming operation proceeds, the EMF coil  10  is moved in the direction of arrow  24  by the robot or other device along the perimeter or edge of the panel to bend the 90° open flange  12  to the flat hem position as shown in  FIGS. 7 and 8 . As the coil moves along, the 90° open flange  12  is progressively bent to a finished, flat hem  30 .  FIG. 9  shows the final position of the EMF coil as the hemming operation is finished. 
     In this embodiment, the EMF flanging and hemming procedures are distinct and can be applied together or independently to flange and/or hem sheet metal panel subassemblies. This EMF sheet bending procedure is “similar” to roller hemming concepts in the way that the sheet metal flange is progressively bent. By bending the flange in this way, the material deformed at the hemline goes through a “non-plane strain” bending path, avoiding plane strain bending (which is the worst case for extreme deformation—leading to failure by cracking in some aluminum alloys). The non-plane strain bending path provides more bending strain and enables flatter hemming with tighter radii in aluminum sheet metal panels without cracking along the hem line. 
     Alternative Embodiment 
     In another exemplary embodiment of the EMF hemming concept, an EMF coil could be incorporated within a traditional hemming device and specifically used to flatten hem areas or features that are very difficult to hem conventionally. One such difficult-to-hem area is shown schematically in  FIG. 10 . 
     In this embodiment, the outer panel  32  has complex curvature in the flange area to accommodate a design feature. The inner panel  34  is shown slightly away from the married position wherein the outer edge  36  of the inner panel would engage the inside of curved flange  38  as well as of adjacent straight flanges  40 . The length (height) of the flange  38  in the difficult-to-hem area is usually cut much shorter than the flanges  40  immediately adjacent opposite ends of the difficult-to-hem area. The flange  38  length must be short in order to avoid splitting (of a stretch flange) or wrinkling (of a compression flange) during the conventional hemming procedure. 
     Whether a flange is a compression flange or a stretch flange depends on the complex curvatures of the outer panel  32  in the difficult-to-hem areas. When these areas have tight radii of curvature, the flanges must be very short (or narrow) and occasionally do not completely cover the edge  36  of the inner panel  34  after hemming. This situation does not provide a desirable appearance and may allow for water leakage if the hem adhesive does not provide a tight seal. 
     In accordance with the invention, a conventional hemming device (not shown) could be used for hemming the “simple” flange areas  40 , while an EMF coil, not shown, would be used to hem the difficult-to-hem flange  38 . The EMF coil could be mounted on the conventional hemmer and driven by a slide or other mechanism (not shown) to move into close proximity to the flange  38  for hemming. 
     Because EMF makes use of “hyperplasticity” to deform the sheet metal, it can be used to successfully hem flanges that would be considered too long (or wide) for conventional hemming. The hyperplastic deformation can resist splitting of stretch flanges and can inhibit wrinkling of compression flanges. As a result, the flange length in difficult-to-hem areas can be made longer, as shown by the curved flange  42  of  FIG. 11 , in order to assure adequate sealing of the hem. 
     During hemming, the outer panel  44  is supported by an anvil  46  and the inner panel  34  is in the married position for hemming and has the longer curved flange  42 . 
       FIG. 12  illustrates one possible hemming sequence for this application with the hold down fixtures represented by numeral  28 . In this case, a conventional hemmer, not shown, could hem the “simple” flanges  40 , leaving the difficult-to-hem flange section  42  in the open position as shown by the cross-sectional views  12 A,  12 B,  12 C taken in planes  48 ,  50 ,  52  of each flange area.  FIGS. 12A and 12C  show their flanges  40  folded over to the finished flat hem position, while  FIG. 12B  shows the central difficult-to-hem flange  42  still in the 90° open position. 
     Finally, the EMF coil, not shown, would be moved into position to flat hem the difficult-to-hem flange  42 , with the longer flange length, without wrinkling or splitting as shown in  FIG. 13  and cross section  13 A. The operation of the EMF coil, not shown in this embodiment, may be like that of coil  10  previously described. The coil may be designed to travel along the length of the flange where non-plane strain bending of the flange is desired or necessary, or the coil could be configured to the shape of the flange section  42 , to bend this section in a single fold. 
     While the invention has been described by reference to certain preferred embodiments, it should be understood that numerous changes could be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the disclosed embodiments, but that it have the full scope permitted by the language of the following claims.