Abstract:
An apparatus for forming blanks having two thicknesses separated by a blank step includes a forming tool defining a binder surface that is characterized by a tool step, a first region on a first side of the tool step, a second region on a second side of the tool step, and a bead segment having a surface contiguous with the second region. The bead segment is configured to pinch material from a blank into any gap created between the tool step and the blank step caused by misalignment of the blank in the tool, thereby ensuring an effective seal to form the blank against a forming surface.

Description:
TECHNICAL FIELD 
     This invention relates to forming tools having binder surfaces characterized by a step and a bead segment with a surface that is contiguous with the portion of the binder surface on one side of the step. 
     BACKGROUND OF THE INVENTION 
     Metal forming tools used in fluid forming operations, such as superplastic forming (SPF), quick plastic forming (QPF), and sheet hydroforming, typically include a first portion that defines a fluid pressure chamber and a second portion that defines a forming surface. A metal blank is placed between the first and second portions of the forming tool such that a first side of the blank is in fluid communication with the chamber and a second side of the blank faces the forming surface. Binder surfaces grip the blank along its periphery and provide a seal between the pressure chamber and the blank. Fluid pressure is introduced into the chamber, which acts on the first side of the metal blank, causing the blank to deform so that the second side contacts, and assumes the shape of, the forming surface. 
     Certain blanks are characterized by regions of different thicknesses. For example, a tailor welded blank includes at least two sheet elements that are welded together at respective edges thereof, forming a weld region. A tailor welded blank is thus characterized by regions of different thicknesses. In order to effect a seal across the tailor welded seam, i.e., at the transition from a first thickness to a second thickness, a tool binder surface must have a step to accommodate the thickness difference. When placing the tailor-welded blank on the forming tool it is essential to precisely locate the seam in the blank to the step in the tool to maintain a pressure-tight seal for the pressure chamber. 
     SUMMARY OF THE INVENTION 
     An apparatus for forming blanks having two thicknesses separated by a blank step is provided. The apparatus includes a forming tool defining a binder surface. The binder surface is characterized by a tool step to accommodate the blank step, a first region on a first side of the tool step, a second region on a second side of the tool step, and a bead segment having a surface that is contiguous with the second region. In the event that a blank is misaligned when it is positioned in the tool such that a gap exists between the tool step and the blank step, the seal bead segment pinches the blank to force material into the gap, thereby ensuring an effective seal despite the gap caused by the misalignment of the blank. 
     Accordingly, the apparatus provided herein improves upon the prior art by increasing the tolerance in positioning a multiple-thickness blank, such as a tailor welded blank, inside the tool while ensuring an effective seal between the blank and the binder surface, and correspondingly ensuring adequate pressure in a pressure chamber to form the blank against a forming surface. 
     The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic, cross-sectional view of a metal forming apparatus including first and second forming tools having first and second binder surfaces, respectively; 
         FIG. 2  is a schematic, side view of the metal forming apparatus of  FIG. 1 ; 
         FIG. 3  is a schematic, perspective view of a portion of the first and second forming tools; 
         FIG. 4  is a schematic, perspective view of the first forming tool; 
         FIG. 5  is schematic, perspective view of a portion of the first and second binder surfaces; 
         FIG. 6  is a schematic, top view of a portion of the first binder surface; and 
         FIG. 7  is a schematic, cross-sectional view of a portion of the first and second tools. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , a metal forming apparatus  10  for fluid forming blanks is schematically depicted. The apparatus  10  may be configured for any fluid forming process within the scope of the claimed invention, such as superplastic forming, quick plastic forming, sheet hydroforming, etc. The apparatus  10  includes an upper tool  12  and a complementary lower tool  14 . The forming tool  12  defines a surface  18  that has been cast and machined into the tool body. The surface  18  defines a cavity  20  having an opening  22 . The forming tool  12  also defines a binder surface  24  that circumscribes the surface  18  and the cavity opening  22 . Binder surface  24  includes a seal bead  26  and shallow valleys  28 , one on each side of bead  26 . 
     Forming tool  14  is characterized by a surface  30  that defines a cavity  34  having an opening  38 . The tool  14  also defines a binder surface  42  circumscribing the surface  30  and the cavity opening  38 . The tools  12 ,  14  are movable with respect to one another from an open position, as shown in  FIGS. 3 and 5 , to a closed position, as shown in  FIGS. 1 and 2 , in which the binder surface  24  of tool  12  opposes binder surface  42  of tool  14  such that the binder surfaces  24 ,  42  cooperate to grip the periphery of a metal blank  44 . Blank  44  is comprised of a metal suitable for fluid forming. Those skilled in the art will recognize various metals that are suitable for fluid forming. 
     In the embodiment depicted, surface  18  is a forming surface, and cavity  34  is a pressure chamber. During operation of the metal forming apparatus  10 , pressurized fluid, such as water, air, nitrogen, or argon gas, is introduced into the cavity  34  through an inlet  46  formed in the tool  14 . The blank  44  is characterized by opposing sides  50 ,  54 , and spans the opening  38  of the cavity  34  such that one side  50  of the blank  44  is in fluid communication with the cavity  34 . The pressure of the fluid from the cavity  34  acts on side  50  of the blank  44 , causing the blank  44  to deform such that side  54  of the blank contacts, and assumes the shape of, the forming surface  18 . Tool  12  defines an outlet  58  through which air in the cavity  20  can exit as it is displaced by the movement of the sheet  44  into the cavity  20  and against the forming surface  18 . 
     As understood by those skilled in the art, the binder surfaces  24 ,  42  must sufficiently grip the entire periphery of the blank  44  to create a pressure-tight seal with the blank  44  to prevent fluid from leaking from the cavity  34 . That is, the blank  44  must seal the entire opening  38  of the cavity  34  in order to prevent fluid from leaking from the cavity  34 . 
     Alternatively, and within the scope of the claimed invention, surface  30  may be a forming surface, cavity  20  may be a pressure chamber, element  46  may be an outlet, and element  58  may be an inlet. 
     Referring to  FIGS. 2 and 3 , wherein like reference numbers refer to like components from  FIG. 1 , the blank  44  is a tailor welded blank having a first portion  62  and a second portion  66 . The second portion  66  is thicker than the first portion  62 . Side  50  of the blank  44  is characterized by a step  70  at the weld seam. The step  70  separates a first region  74  of surface  50 , which is formed by the thin portion  62  of the blank  44 , and a second region  78  of surface  50 , which is formed by the thick portion  66  of the blank  44 . 
     Correspondingly, the binder surface  42  of the tool  14  is characterized by a step  82  to accommodate the difference in thickness between the first and second portions  62 ,  66  of the blank  44 . The step  82  separates a first region  86  of the binder surface  42  and a second region  90  of the binder surface  42 . The first region  86  is configured to contact, and form a seal along, the peripheral portion of the thick portion  66  of the blank  44 . The second region  90  is more protuberant than the first region  86 . The second region  90  is configured to contact, and form a seal along, the peripheral portion of the thin portion  62  of the blank  44 . In the embodiment depicted, regions  86 ,  90  are substantially flat, but may have contours within the scope of the claimed invention, e.g., to accommodate a contoured blank. 
     In the embodiment depicted, the step  70  at the weld seam of the blank  44  is characterized by a surface that is perpendicular to surfaces  50 ,  54 . If the step  70  of the blank  44  is not in contact with the step  82  of the binder surface  42  when the tools  12 ,  14  are closed, as shown in  FIG. 2 , then a gap  92  is formed therebetween, which will allow pressure from the cavity  34  to leak therethrough. 
     Referring to  FIG. 4 , wherein like reference numbers refer to like components from  FIGS. 1-3 , the binder surface  42  is characterized by two steps  82  that are on opposite sides of the cavity  34  from one another. The binder surface  42  includes two bead segments  94  that protrude from the steps  82 . Each of the bead segments  94  includes a surface  96  that is contiguous with the second region  90  and forms a peninsular portion thereof. The first region  86  surrounds the bead segment  94  and surface  96  on three sides thereof. The bead segment  94  and surface  96  are narrower than the second region  90 . In the embodiment depicted, no part of the bead segment is more protuberant than the second region  90 . Surface  96  in the embodiment depicted is flat and is coplanar with region  90 ; however, surface  96  may be contoured within the scope of the claimed invention, e.g., to accommodate a contoured blank. 
     Referring to  FIGS. 4-6 , each bead segment  94  is characterized by an inner lateral side  98  and an outer lateral side  100  opposite from the inner lateral side  98 . The first region  86  extends on both sides  98 ,  100  of the bead segment. Surface  102  of the bead segment interconnects the surface  96  and the first region  86 . 
     The binder surface  42  further defines a valley portion  106  that is between, and that interconnects, the first region  86  with the lateral side  98  of the bead segment  94 . The binder surface  42  also defines valley portion  10  that is between, and that interconnects, the first region  86  with the lateral side  100  of the bead segment  94 . At least part of each of the valley portions  106 ,  110  is less protuberant than the first region  86 . 
     Referring to  FIG. 7 , wherein like reference numbers refer to like components from  FIGS. 1-6 , the sealing bead  26  has a cusp-shaped cross-section. As used herein, the term “cusp” refers to the shape formed by two interconnected arcs, which may or may not intersect each other. For example, and within the scope of the claimed invention, a cusp may be defined by two arcs that intersect at a point, two arcs connected by a flat surface, etc. In the embodiment depicted, each valley  28  cooperates with a respective lateral side of the bead  26  to define a respective arc that is interconnected by surface  114 . 
     The cusp-shaped seal bead  26  may be suitably formed by machining the binder surface  24  using two offset spherical cutters (not shown) moved in suitably-spaced parallel paths. The bead  26  is cut in a suitable path, typically a linear path, around the periphery of the tool  12  as necessary to enclose and sealingly engage the perimeter of the blank  44 . More specifically, each of the offset cutters forms a respective one of the valleys  28  and lateral sides of the bead  26 , such that each of the valleys  28  cooperates with a respective side of the bead to define a respective arc in cross section, as shown in  FIG. 7 . After the two spherical cutting tools have traced their respective paths in the binder surface  24  to form the valleys  28 , the arcs intersect each other at a point; a final flat cutting tool (not shown) is supplied both to remove the tip of the cusp and to provide a truncated cusp with the flat surface  114  that is more protuberant than the remainder of surface  24 . 
     Similarly, bead segment  94  may also be cusp-shaped in cross section, with the valley  106  and side  98  cooperating to define a first arc and valley  110  and side  100  cooperating to define a second arc, and surface  96  being a flat surface interconnecting the first and second arcs, as shown in  FIG. 7 . It should be noted that one of the valleys  106 ,  110  may be omitted in some applications. It should also be noted that, in the embodiment depicted, surface  96  is wider than surface  114 , and valleys  106 ,  110  are larger than valleys  28 . It may be desirable for the bead  26  to be offset from the bead segment  94  toward the inside of the tool  14  to reduce or eliminate tearing of the blank during forming, i.e., the centerline of sealing bead  26  is not aligned with the centerline of the bead segment  94 . It should also be noted that, although the regions  86 ,  90  and the bead segment  94  are integrally formed in the tool  14  in the embodiment depicted, various features of a binder surface may be formed by separate pieces within the scope of the claimed invention. 
     The operation of the metal forming apparatus  10  is described with reference to the forming of a tailor welded blank  44 ; however, the metal forming apparatus  10  may be advantageously employed in the formation of any blank characterized by regions of different thicknesses. 
     While the best modes for carrying out the invention have 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 within the scope of the appended claims.