Abstract:
A tool and process utilizing the tool for forming a compressed-radius hem on a sheet metal assembly is provided. A concave portion on the tool is formed by at least two surfaces that together define a cavity. A flange on the perimeter of an outer panel is initially formed to an approximate perpendicular angle and is then formed to about 45°. In a final forming step, an intermediate portion of the flange is engaged by the cavity and is formed into a compressed-radius hem.

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The present invention relates to a process and tool for forming a compressed-radius hem on an outer panel to join it to an inner panel to form a sheet metal assembly. 
     2. Background Art 
     Hemming is a production process for joining an outer panel to an inner reinforcement panel. Conventional hemming processes are accomplished by bending a flange of the outer panel back onto the inner panel. Normally, a three-step process is used. In the first step, the outer panel is flanged with a perimeter portion being formed to extend substantially perpendicularly relative to the body of the outer panel. In the second step, the panel is pre-hemmed wherein the flange is formed to an acute angle of approximately 45° to extend inwardly over a perimeter portion of the inner panel. In the third step, the panel is finally hemmed wherein the flange is formed to engage the inner panel and extend parallel to the body of the outer panel and perimeter portion of the inner panel. 
     Several different types of tools are used to perform hemming processes including reciprocating ram presses, tabletop hemming tools, and roll forming tools that may be manipulated by a robot. In conventional roll formed hem operations, a first pass is required to bend a 90° flange to an intermediate angle of about 45°. The hem is closed in a second pass. 
     Recent developments in the field of hem forming have led to the development of reduced radius hems that improve the appearance of the fit of adjacent panels by reducing the perceived margin between adjacent panels. In conventional hem forming processes, the hem radius is controlled by the thickness of the inner and outer panels. 
     One problem with reduced radius hems is a tendency of the resulting hem to fracture near the tip of the hem if the part and tools are not properly aligned. One example of this hemming method and tooling is disclosed in U.S. Pat. No. 6,257,043 to Wiens and in Publication No. US 2001/0029766A1 that both relate to producing reduced radius hems. The final hem tool includes a flat section and an inclined section that can produce a hem with a reduced radius when compared to a conventional flat hem. In automotive manufacturing, for example, there are tolerances allowed both as to the relative location of the inner panel, outer panel, and the hem tool. These tolerances may result in variances with respect to the hem tool and flange location that are not a problem with conventional flat hemming techniques because the flat hem tool is not sensitive to inboard/outboard alignment of the tool relative to the flange. In the Wiens patent, if the final hem steel is too far inboard, the hem tip may be distorted and will either fail or result in an unattractive hem. 
     The use of a curved forming tool for forming a hem is disclosed in U.S. Pat. No. 6,000,118 to Biernat et al. that relates to a reciprocating ram press tool forming a sealed edge joint. The Biernat patent does not disclose a tool or method for producing a compressed-radius hem. 
     The disadvantages and shortcomings of the prior art are addressed by Applicant&#39;s invention as summarized below. 
     SUMMARY OF INVENTION 
     According to one aspect of the present invention, a tool is provided for forming a compressed-radius hem on a sheet metal assembly comprising an inner panel having an outwardly extending flange and an outer panel having a bendable flange. The bendable flange is initially located generally perpendicular to an outer peripheral portion of the outer panel. The tool cooperates with a supporting surface on which the outer panel and inner panel are located. According to one embodiment of the invention, a roller having a cylindrical surface is used to bend the flange inwardly toward the surface of the outwardly extending flange of the inner panel. The roller also has a concave portion extending from a first circumferential line at the intersection of the concave portion and the cylindrical surface to a second circumferential line axially spaced from the first circumferential line. The concave portion is formed by at least two surfaces that together define a cavity relative to a chord extending between the first and second circumferential lines. 
     According to other aspects of the invention, the surfaces defining the cavity may include two or more partially conical surfaces that lie in two different coaxial conical sections that are coaxial with the cylindrical surface. Three, four, or more conical surfaces may be provided. The surfaces defining a cavity may also include a conical surface and a curved surface that is contiguous with the conical surface and also coaxial with the cylindrical surface. The curved surface may be located between the conical surface and the cylindrical surface and may be contiguous with both surfaces. 
     According to additional aspects of the invention, the cylindrical surface may be oriented to contact an inner portion of the flange as it is pressed against the outwardly extending flange of the inner panel while at least one of the surfaces forming the concave portion engages an intermediate portion of the flange that extends from a bight portion of the flange to the inner portion. A bend in the flange between the inner portion of the flange and the intermediate portion of the flange is preferably engaged by the roller near the first circumferential line. The concave portion preferably applies force in a direction normal to the intermediate portion of the flange. The cylindrical surface preferably applies force to the inner portion of the flange to flatten the flange against the outwardly extending flange of the inner panel. 
     According to yet another aspect of the invention, the same cylindrical surface used in the final hemming step may be used in the pre-hemming step to initially bend the flange from its initial generally perpendicular orientation to extend at an angle of about 45° and partially over the outwardly extending flange of the inner panel. A concave portion in one embodiment may have an outer circumference that is greater than the cylindrical surface. In yet another embodiment, the concave portion may have an outer circumference that is less than the circumference of the cylindrical surface. 
     According to another aspect of the invention, a tool for forming a hem on a sheet metal assembly comprising an inner panel having an outwardly extending flange and an outer panel having a bendable flange comprises a support on which the outer panel and inner panel are located and a hem tool having a first surface that is parallel to the supporting surface, a second surface obliquely angled relative to the first surface and extending towards the supporting surface, and a third surface obliquely angled relative to the second surface and extending towards the supporting surface wherein the third surface is less oblique relative to the first surface than the second surface. 
     According to another aspect of the invention, the first surface is oriented to contact an inner portion of the flange as it is pressed against the outwardly extending flange of the inner panel while the second surface engages an intermediate portion of the flange that extends from a radiused portion to the inner portion. A bend in the flange between the inner portion of the flange and the intermediate portion of the flange may be engaged by the intersection of the first section and second section. The forming tool may be either a roller or a reciprocating ram press die. 
     According to another aspect of the invention, a method of hemming an outer metal panel having a perimeter flange and an inner metal panel together comprises the steps of placing the inner panel and outer panel together on a supporting surface. Forming a perimeter flange of the outer panel to extend generally perpendicularly relative to the body of the outer panel. In a pre-hemming pass, the perimeter flange is formed to an acute angle relative to the body of the outer panel with a roller. The perimeter flange of the outer panel is then formed in a final pass into engagement and over a perimeter portion of the inner panel. The roller has a first forming surface that is parallel to the perimeter portion of the inner panel and a second forming surface that is contiguous with the first forming surface and is oriented at a first oblique angle relative to the first forming surface. In the final hemming pass the roller may be oriented with its axis of rotation generally parallel to an intermediate portion of the perimeter flange. In this way, the bending force is focused on the intermediate portion of the flange so that the overlapping portions of the inner and outer panels are allowed to slide more easily in a parallel direction as the hem is formed in the final hemming pass. 
     The method may also include providing a third forming surface that is spaced from the first forming surface and contiguous with the second forming surface. The third forming surface may be oriented at a second oblique angle relative to the first forming surface that is less oblique than the first oblique angle. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a schematic representation of a robotic roll hem machine; 
     FIG. 2 is a schematic view showing a first pre-hem pass of a conventional roll hemming tool; 
     FIG. 3 is a schematic representation of a second, final hemming pass of a compressed-radius roll hemming tool made according to the present invention; 
     FIG. 4 is a schematic representation of a second, final hemming pass of an alternative embodiment of a compressed-radius roll hemming tool made according to the present invention; 
     FIG. 5 is a schematic view of a pre-hem pass of a compressed-radius roll hemming tool having a partially curved surface; 
     FIG. 6 is a schematic view showing a second, or final, pass of the compressed-radius roll hemming tool shown in FIG. 5; 
     FIG. 7 is a schematic view showing a compressed-radius hem tool having a flat forming surface and two straight sections disposed at two different oblique angles; 
     FIG. 8 is a schematic view showing a compressed-radius hem tool having a flat forming surface and two straight sections disposed at two different oblique angles; 
     FIG. 9 is a schematic view showing a compressed-radius hem tool having a flat forming surface and three straight sections disposed at different oblique angles with the first section being substantially longer than the other two sections; 
     FIG. 10 is a schematic representation showing the forces applied in a final finish hem step using a compressed-radius tool having a partially curved surface; 
     FIG. 11 is a schematic representation of a compressed-radius forming tool having a frustoconical surface and showing the force being applied in a direction normal to the intermediate portion of the flange; 
     FIG. 12 is a schematic representation of a compressed-radius forming tool having a partially curved surface showing in phantom potential outboard movement of the roller; 
     FIG. 13 is a schematic view showing the relative height of the inclined section being 25% of the total stack height; 
     FIG. 14 is a schematic view showing the relative height of the inclined section being 100% of the stack height; 
     FIG. 15 is a schematic view showing a hem tool for a reciprocating ram press having a flat hemming surface and a single angled surface with the tool being misaligned too far inboard; 
     FIG. 16 is a schematic view showing the flexibility of the compressed-radius hem tool with respect to the relative position of the hem tool and the flange; 
     FIG. 17 is a schematic representation demonstrating the relative height of the inclined section being 25% of the stack height; 
     FIG. 18 is a schematic view showing the relative height of the inclined section being 100% of the stack height; 
     FIG. 19 is a schematic view showing a reciprocating ram press compressed-radius hem tool having a flat hemming surface and two obliquely angled straight sections; 
     FIG. 20 is a schematic view showing a reciprocating ram press compressed-radius hem tool having a flat hemming surface and three obliquely angled sections; and 
     FIG. 21 is a schematic view showing a reciprocating ram press compressed-radius hem tool having a flat hemming surface and three obliquely angled sections. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1, a robotic hemming machine  10  is shown to include an articulated arm  12  that manipulates a hem forming tool  14 . An inner panel  16  and an outer panel  18  are shown disposed on a supporting surface  20 . The outer panel  18  is shown in the condition that it would be after a first pre-hemming operation. 
     Referring now to FIG. 2, a cylindrical bending tool  14  is shown making a pre-hemming pass to bend a bending flange  24  of the outer panel  18  over an outwardly extending flange  24  of the inner panel  16 . The precise degree of bending is not critical and could range from 30° to 60°. Forty-five degrees is nominally stated since it is halfway between the initial perpendicular orientation of the flange  26  that again may vary upon tooling requirements and 0° which is the orientation of the end of the bending flange  26  after the final hemming pass. Prior art roll form hemming tools generally include a cylindrical surface as shown by hem forming tool  14 . Hem forming tools made according to the present invention may include a cylindrical portion that functions in the pre-hemming step in a similar manner to hem forming tool  14 . 
     Referring now to FIG. 3, a hem forming tool made according to one embodiment of the present invention is generally referred to by reference numeral  30 . The tool  30  has a cylindrical surface  32  that may be used in the pre-hemming step to form the flange  26  to approximately 45° and in the final hemming step to form the flange  26  onto the outwardly extending flange  24 . A first oblique surface  34  is a flat surface extending radially outwardly at an oblique angle from the cylindrical surface  32 . A second oblique surface  36  extends radially outwardly at an oblique angle from the first oblique surface  44 . The first and second oblique surfaces  34  and  36  together form a concave portion that is used to form the compressed-radius hem  38 . 
     Referring now to FIG. 4, another embodiment of the compressed-radius forming tool  40  is shown to include a cylindrical surface  42  that is used to form the pre-hemming operation and first and second oblique surfaces  44  and  46  that together form a concave portion that engages the bending flange  26  in the final hemming step. The tool  40  is mounted on and rotates around a rotatable shaft  48  that performs the hemming operations. Shaft  48  may be supported on its articulated arm  12 . 
     Referring now to FIGS. 5 and 6, yet another embodiment of the hem forming tool  50  of the present invention is shown. The tool  50  includes a cylindrical surface  52  that is used in both the pre-hem and final hem passes. The pre-hem pass is shown in FIG. 5 while the final hemming pass is shown in FIG. 6. A partially curved surface  54  and a conical surface  55  are provided to bend the bending flange  26  of the outer panel  18  over the outwardly extending flange  24  of the inner panel  16  during the final hemming step as shown in FIG.  6 . The partially curved surface  54  and conical surface  55  together define a concave portion that engages the bending flange  26 . 
     Referring now to FIGS. 7 and 8, two different styles of the tool  30  are shown with FIG. 7 illustrating the surfaces of the tool  30  while FIG. 8 illustrates differently proportioned surfaces of the tool  30 ′. In each, a cylindrical surface  32 ,  32 ′ is shown for use in the pre-hemming pass and the final hemming pass. A first oblique surface  34  is shown in FIG. 7 while in FIG. 8 surface  34 ′ is substantially greater in length to provide additional tooling alignment flexibility. A second oblique surface  36  and  36 ′ are shown in FIGS. 7 and 8, respectively. The first oblique surfaces  34 ,  34 ′ and the second oblique surfaces  36 ,  36 ′ together form a concave portion. A chord C relative to which the concave portion is concave is shown as a dashed line. 
     Referring now to FIG. 9, another alternative embodiment of the tool  60  is shown to include a cylindrical surface  64  and a first oblique surface  64 , second oblique surface  66 , and third oblique surface  68  of sequentially greater radial extent. It should be understood that additional oblique surfaces could be added within the spirit and scope of the invention. However, each separate surface on the forming tool may require additional machining steps and entail additional tooling costs. As the number of surfaces increases, the extent to which the surfaces may extend the length of the compressed-radius hem flange may be increased and the margin of error for alignment of the tool with the flange may be increased. 
     FIGS. 10 and 11 feature force diagrams that illustrate two different directions that force may be applied to the bending flange  26 . In FIG. 10, the pre-hemming forces are applied in the direction shown by force arrows F to the bending flange  26 . The bending flange  26  is shown to include an inner portion  72  that is pressed against the outwardly extending flange  24  and an intermediate portion  74  that extends from the inner portion  72  to a radiused portion  76 . In this embodiment and in the embodiment of FIG. 3, the principal force arrow F applies a downward force on the inner portion  72  of the flange  26 . 
     Referring now to FIG. 11, force arrow F shows the preferred direction to apply force to the intermediate portion  74  of the flange  26  so that it reduces the tendency of the inner portion  72  of the flange  26  to resist shaping the intermediate portion  74  into the desired configuration. This advantage is also obtained in the embodiments shown in FIGS. 4-6 in which the roller axis is angled relative to the flange  24 . 
     Referring now to FIG. 12, the tool  70  is shown properly aligned with the flange  26  in solid lines and in phantom shown in a maximum outboard position in which an acceptable compressed-radius hem could be formed in the final hemming pass. However, between position shown in solid lines and position shown in phantom lines, a final hem could be formed with the tool  70 . 
     Referring now to FIG. 13, another embodiment of the tool  80  is shown wherein a cylindrical surface  82  and oblique surface  84  are provided. In this embodiment, the stack height of the tool  80  represented by the difference in the minimum and maximum radius of the oblique surface  84  is approximately 25% of the stack height represented by the thickness of the hem illustrated by the arrows in the center portion of the drawing. 
     Referring now to FIG. 14, the concept of the present invention is shown wherein by providing additional oblique surfaces  64 ,  66 , and  68 , the stack height indicated by the arrows on the left side of FIG. 14 is equal to a stack height of the finished hem represented by the drawings in the center of FIG.  14 . In accordance with the tool  60  shown in FIG. 14, considerable flexibility can be provided in forming a compressed-radius hem having a radiused portion, or bight,  76 , intermediate portion  74 , and inner portion  72 . 
     Referring now to FIG. 15, a reduced radius hem tool  88  for a reciprocating press is shown. The reduced radius hem tool  88  includes a flat forming surface  90  and an inclined forming surface  92  that are designed to engage and form a final hem after the flange  26  has been processed in a pre-hemming step as shown in FIG. 2 as described above. A corner  94  can cause a deformation  96  in the flange  26  if the tool and inner and outer panels  16  and  18  are not properly aligned prior to the final hemming press step. The deformation  96  can cause a panel to be rejected or result in body panels having a poor appearance. 
     Referring now to FIG. 16, a compressed-radius hem tool  100  is shown in conjunction with a compressed-radius hem formed on an inner and outer panel assembly wherein the bending flange  26  is bent over the outwardly extending flange  24  of the inner panel  16 . The compressed-radius hem tool  100  includes a flat forming surface  102  and first, second and third inclined forming surfaces  104 ,  106 , and  108  that are each oriented at an oblique angle relative to the flat forming surface  102 . The inclined forming surfaces  104 ,  106 , and  108  are sequentially less oblique. Stated otherwise, the angular orientation of the first, second and third inclined forming surfaces increases to provide an increased radius on the forming tool  100  as the sections increase in distance from the flat forming surface  102 . The series of inclined forming surfaces provide a concave portion and allow greater manufacturing tolerances wherein the tool  100  may be aligned with the bending flange  26  in a range of positions from that shown in solid lines in FIG. 16 to the dotted line position shown in FIG.  16 . This greater degree of manufacturing tolerances results in increased workmanship and higher quality compressed-radius hems. 
     Referring now to FIG. 17, a reduced radius hem tool  88  that includes a flat forming surface  90  and inclined forming surfaces  92  and  93  that are oriented to contact the bending flange  26 . The arrows on the left side of FIG. 17 illustrate the reduced stack height of approximately 25% compared to the stack height illustrated by the arrows in the center of FIG.  17 . This reduction in stack height should be compared with the stack height achieved by the compressed-radius hem tool  100  shown in FIG.  18 . 
     In FIG. 18, the arrows at the left side of the figure illustrate a stack height to the stack height corresponding to the arrows shown in the center of FIG. 18 wherein the stack height is equal to three thicknesses of the material forming the hem. The three inclined surfaces  104 ,  106 , and  108 , combine to define a concave portion and result in a greater degree of flexibility in the manufacturing process. 
     Referring now to FIG. 19, another embodiment of the compressed-radius hem tool  110  for reciprocal presses is shown that includes a flat forming surface  112 , a first inclined forming surface  114 , and second inclined forming surface  116  that provide a concave portion relative to the chord C shown as a dashed line. For greater manufacturing tolerances, a compressed radius hem tool  100  or  100 ′ as shown in FIGS. 20 and 21, respectively, may be provided. 
     In FIG. 20, the compressed-radius hem tool  100  includes a flat forming surface  102  and first, second, and third inclined forming surfaces  104 ,  106 , and  108  that are each obliquely angled relative to the flat forming surface  102  but are of decreasing degrees of obliqueness relative thereto. Surfaces  104 ,  106 , and  108  define a concave portion relative to the chord C. 
     In FIG. 21, a similar compressed-radius hem tool  100 ′ is shown that has a flat forming surface  102  and the same number of inclined forming surfaces, but the first inclined forming surface  104  is substantially elongated in comparison to the other inclined forming surfaces  106  and  108  to provide an increased degree of manufacturing tolerance when compared to the embodiment shown in FIG.  20 . 
     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.