Abstract:
Forming apparatus includes a binder with novel flow lock beads that engage edge portions of a sheet metal blank during metal drawing. In an initial step, the beads draw inner portions of the edge portions into a die cavity to partially form a component. In a second step, the beads are inserted into bead recesses in an opposing die, locking the edge portions in place and causing stretching of the metal in a final portion of the drawing stroke. Stresses in the resulting component are relieved by the stretching process which may be selectively used in forming a component with varying draw depths across its length.

Description:
TECHNICAL FIELD 
     This invention relates to a method and apparatus for drawing high strength steel in a conventional forming press. 
     BACKGROUND OF THE INVENTION 
     It is known in the art of metal forming to form deep drawn three dimensional sheet metal structures for automobile body components and the like using a conventional forming press and drawing dies working with low carbon steels of an adequate gauge or thickness. 
     In order to provide lighter structures for forming or supporting body components, it has long been desired to utilize high strength steels of thinner gauge to replace the components currently formed from low carbon steel. However, the use of high strength steels has not previously been successfully accomplished where significant drawing of the metal is required since the resulting product is deformed due to internal stresses in the high strength steel which are not relieved in the forming process. 
     SUMMARY OF THE INVENTION 
     The present invention provides an apparatus and method for successfully drawing high strength steel to form relatively light weight structural members for automobile bodies and the like with a high degree of accuracy in the resulting component. The invention involves a two-step process wherein a blank, which may be flat or preformed to a desired shape, is first partially drawn in a forming press while binder edge portions of the blank are drawn across flow restricting beads in a press binder to allow drawing of metal into a die cavity by an associated punch to form a partially completed structure. The process is completed by forcing the beads further into associated die cavities to lock the binder edges of the blank so that the final portion of the forming process is limited to working the metal of the blank by stretching it into the final shape, sufficient working being accomplished to develop yielding of the metal which avoids subsequent warping or distortion of the finished component upon removal from the die. 
     The forming apparatus includes mating upper and lower dies, the lower die assembly including a binder having novel flow lock beads. In an initial predetermined portion of the stroke of the press, the beads are positioned to apply a restraining force while allowing metal from the binder edges to flow into the die cavity. Upon further movement of the press toward the final forming position, the beads are forced upward into associated  10  cavities. This locks the edges of the blank against further flow, causing the final forming step to stretch the metal so that it yields and forms a smooth drawn structure with stresses relieved to avoid subsequent distortion of the part. 
     In a preferred embodiment, the apparatus includes a drawing die with a mating punch and binder, the die and binder having opposing binder edge engaging portions. The die includes a bead recess opposing a bead carrying pocket in the binder. At least one adjustable bead is received in the pocket, having an initial metal restraining position. Transfer means in the binder move the bead to a second position during relative motion of the die and punch to lock the associated binder edge of the blank in position for final forming and working of the metal to a smooth stress relieved component. 
    
    
     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 
     In the drawings: 
     FIG. 1 is a semi-schematic pictorial view illustrating elements of associated upper and lower dies according to the invention and a drawn component formed thereby; 
     FIG. 2 is a fragmentary longitudinal cross-sectional view through a binder portion of the lower and upper dies in their closed position; 
     FIG. 3 is a transverse cross-sectional view through the dies showing the original blank in position for forming; 
     FIG. 4 is a view similar to FIG. 3 but showing the upper die lowered to clamp the blank against the binder; 
     FIG. 5 is a view similar to FIG. 4 showing the die lowered to a partially formed condition of the blank with the beads positioned for subsequent upward movement; and 
     FIG. 6 shows the dies at the completion of the drawing stroke with the beads raised to lock the binder edge portions in place while the final portion of the forming step is being completed. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIG. 1 of the drawings, numeral  10  schematically indicates an assembly of an upper die  12 , a lower die  14  and a formed engine cradle member  16  disposed between the dies. The formed cradle member  16  includes a U-shaped longitudinal beam portion  18  extending the length of the component and having varying width and depth dimensions over its length. Lateral binder edge portions  20  extend outward from the edges of the U-shaped channel and terminate in upturned binder lock portions  22 . 
     The upper die  12  includes a central die cavity  24  for receiving and forming the beam portion  18  of the cradle member. Bead recesses  26  are spaced laterally on either side of the die cavity  24  and extend generally parallel therewith. The die  14  has a lower surface  28  which is arched to provide the slightly bowed shape of the formed cradle member  16 . 
     The lower die  14  includes a central punch  30  having a U-shaped upper edge  32 , configured to form the inner surface of the U-shaped portion of cradle member  16 . The punch also includes a lower end  34  and an intermediate portion carrying guide shoes  36  that in turn slidingly engage guide plates  38  received in a binder  40  carried in the lower die  14  for guiding relative motion of the punch within the binder. 
     The binder carries a plurality of transfer means including transfer pins  42  threadably engaging movable beads  44  which are laterally spaced on opposite sides of the punch upper edge  32  and are adapted to be received in the bead recesses  26  of the upper die. 
     FIGS. 2-6 illustrate the structure of the die assembly  10  of FIG. 1 together with additional structural features. A die shoe  48  is mounted on the base of a conventional forming press, not shown. The die shoe  48  includes a shelf  50  that supports the lower end  34  of the die punch  30 . A nitrogen manifold  52  is attached to the bottom of the shelf  50 . Pneumatic cylinders  54  are fixed in threaded pockets  56  of the manifold  52  and have pistons  58  which extend upward through openings in the shelf  50  to support the binder  40  that surrounds the punch  30 . 
     The separate beads  44  are received in slots  60  which are formed in the binder and have stepped upper surfaces  62  that are engaged by bottom edges of the individual beads  44  when in their lower positions, as shown in FIGS. 3-5. The transfer pins  42  extend up through cylindrical openings  64  from enlarged pockets  66  containing return springs  68 . The springs are compressed in the pockets  66  against heads  70  on the lower ends of the transfer pins  42 . The springs  68  urge the transfer pins downward so as to normally retract the beads  44  against the upper surfaces  62  of the binder. The pneumatic cylinders  54  always bias the binder  40  upward toward its initial position, shown in FIG. 3, but the binder is forced downward by engagement of the upper die until the heads  70  of the transfer pins engage the shelf  50 . 
     FIGS. 3-6 show sequential steps in the movement of the upper die and binder components during drawing of the cradle member  16 . In FIG. 3, the upper die  12  is shown fully raised for placement of a high strength sheet metal blank  74  on the curved upper surface  76  of the binder. The blank may be precurved, if desired, to fit the upper surface of the binder but instead is preferably flat and held in place by retractable edge retainers, not shown. After placement of the blank  74 , the upper die is lowered in a continuous stroke which first reaches the position shown in FIG. 4 where the upper die lower surface  28  engages the sheet metal blank  74  and clamps the blank against the curved upper surface  76  of the binder. Since the beads  44  are held by the transfer pins in a position extending slightly above the surface of the binder, the beads  44  extend a small amount up into recesses  26  of the upper die. Thus, the binder edge portions  20  of the blank are deformed slightly upwardly and are restrained between the opposing surfaces of the binder and upper die by a predetermined force, as well as by the slight deformation of the edge portions  20 . 
     FIG. 5 illustrates a further downward motion of the upper die which causes the punch  30  to enter partially into the die cavity  24 , forcing the sheet metal upward into the cavity. During this process, metal from the binder edge portions  20  is drawn across the tops of beads  44  and partially into the die cavity to allow drawing to take place without excessively stressing the metal. Also, at this point, the heads  70  of the transfer pins have been lowered with the binder so that the heads  70  engage the shelf  72  of the die shoe. 
     The transfer pins  42  and the attached beads  44  are thus bottomed out so that when the upper die  12  moves further downward to complete its stroke, the binder moves down with it but the beads  44  do not. Instead, the beads move upward relative to the downwardly moving binder  40 . The beads, thus enter the recesses  26  and bend over the outer edges of the binder edge portions  20  to form the upwardly extending lock portions  22 . This locks the flanges in place so that the final motion of the upper die and binder downward to the position shown in FIG. 6 completes drawing of the high strength sheet metal by stretching the metal sufficiently to work the metal beyond its yield point and thereby reduce retained stresses in the metal. This stretching process also smoothes out wrinkles in the metal itself and thus provides a very clean and stress free sheet metal member which retains its shape without distortion when the upper die is retracted and the part may be removed. 
     If desired, when forming components with variable draw depths or for other reasons as appropriate, portions of the blank flanges  20  may be cut away in advance so that they are drawn past the beads in the drawing step and are not locked in place as are other portions of the blank. For example, note that binder lock portions  22  are formed only on the shallower drawn end portions of the cradle member  16 . These end portions are locked by the beads  44  during final drawing of the component  16 , as previously described. The central portions of the binder edge portions of the blank  20  are cut away so that they are not locked by the beads. This is desirable because the deeper draw in the central portion of the component  16  works the metal adequately without additional stretching of the metal. If desired, the same result may be obtained by omitting the movable beads from the parts of the binder where locking of the edge portions  20  is not desired. 
     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.