Abstract:
To hold the edges of a sheet metal blank during stamping or forming, such as stretch forming, a flange of the blank is clamped between faces of one of the dies and a binder which form a lockbead in the flange that restrains movement of the flange during the subsequent forming operation. A double V lockbead having three alternately angled sharp bends of near 90 degrees and one or two connecting bends totaling near 90 degrees provides improved locking performance, especially where the clamping surfaces of the die are sloped from horizontal. The legs or sides of the beads may have unequal lengths. The bend angles may be varied, in appropriate cases, within a reasonable range of from 75 to 120 degrees more or less as desired. Also, an additional angle or two, forming a third V could be used. The lockbead forming shapes are machined into the mating die and binder to form and retain the lockbead during the metal forming process.

Description:
TECHNICAL FIELD 
     This invention relates to lockbeads and to lockbead retaining and forming means for restraining the edges of sheet metal blanks against inward motion during forming of the blanks into stampings in a forming press. 
     BACKGROUND OF THE INVENTION 
     It is known in the art relating to press forming of a sheet metal blank into a stamping, in particular stretch forming processes, to provide a lockbead around edges of the blank to hold the blank edges against inward motion during forming of the blank in a press. A conventional lockbead comprises a trapezoidal or nearly rectangular protrusion of the blank material upward or downward from the associated flange or edge portion of the blank. 
     The bead is usually formed in the press by engagement of the blank by one of upper and lower dies in the press and an associated binder (also called a cushion ring or blank holder). These members have opposed blank holding surfaces for engaging opposite sides of the blank adjacent its edges or periphery in what may be called the outer flange area of the blank. One of the opposing die and binder members is provided with a protruding bead of rectangular, square or trapezoidal cross section lying generally parallel to the blank edge. The other of the members has formed therein a mating bead recess or slot into which the protruding bead of the first member extends when the blank holding surfaces of the members are forced against opposite sides of the blank. This forms a nearly square, rectangular or trapezoidal bead in the blank running along its edges and trapped in the mating beads of the associated die and binder members. 
     A conventional bead, formed by a square protrusion forcing the blank into a parallel sided slot with small clearance, forms a nearly square sided bead in the blank having four sequential bends in the bead of nearly 90 degrees each, but slightly less because of the clearance required for bead forming. In stretch forming of sheet metal blanks, for example, the bead is usually capable of restraining the blank edges against inward motion if the angles of the four bends total 340 degrees or more (for example 85 degrees for each bend). This works well for horizontally disposed blank edge flanges but is less or not at all satisfactory where the blank edges are disposed at an angle, such as 45 degrees from horizontal. In such a case, the requirement that the slot edges remain vertical for entry of the protruding bead during a vertical motion of the press causes the bends in the bead to be substantially less than 90 degrees. The resulting sum of the bends then falls to significantly less than 340 degrees and sometimes less than 300 degrees, resulting in inadequate restraint of the flanges, or blank edges, against slipping, or inward motion during forming. 
     SUMMARY OF THE INVENTION 
     The present invention provides a novel form of double V or similar form of lockbead and lockbead retaining and forming means. Lockbeads and their retaining means according to the invention provide restraint which is less dependent upon clamping surface angle and is able to provide 340 degrees or more of total bending while using less of the blank edge material than a conventional lockbead. In a preferred embodiment, the double V lockbead includes three alternately angled sharp bends forming in cross section the double V configuration. Two additional sharp bends of about 45 degrees each connect the double V bead with the adjacent portions of the clamped flange of the blank. The sum of the five angles thus totals at least 340 degrees, nearly 90 degrees each from the three bends of the double V and nearly 45 degrees each for the two connecting bends. Because of their shape and the possibility of using slightly shorter legs between the bends than with a conventional bead, the double V bead uses less material of the blank flanges for bead formation. Thus, a substantial saving of sheet metal is accomplished which provides cost savings for the double V bead as opposed to conventional beads. 
     In various alternative embodiments, the bend angles may vary and total more or less than 340 degrees as is required for the particular application, material and forces developed in the forming process being used. For example the sum of the alternately angled and connecting bends could be as little as 300 degrees with the alternately angled beads contributing only 225 degrees to the total. Each of the alternately angled bends could vary in a range of from 75 to 120 degrees. If desired, the double section could be increased by adding an additional bend, or two which would form a triple V bead. Then lesser bend angles might be adequate for forming loads or greater loads could be accommodated. In the unusual case of near vertical blank holding surfaces, the double V bead of this invention could still be used but one of the connecting bends would approach 90 degrees while the other side of the double V would have a nearly straight, connection. The legs or sides of the V elements may have unequal lengths but negative die angles or back draft cannot exist. 
     As to the lockbead retaining means or beads formed in one die and an associated binder, forming the bead surfaces with 90 degree angles would allow cutting the beads with a standard end mill. However, other bead surface angles can also be formed which could range from 105 to 60 degrees, comparable to the 75 to 120 degree bend angles mentioned above and considered reasonable for normal use. Other angles may, of course, be appropriate for various particular forming conditions. 
     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 fragmentary cross-sectional view of a single action forming press including dies and a binder ring with a lockbead illustrative of the prior art; 
     FIGS. 2 and 3 are schematic views showing prior art lockbeads applied to horizontal and angled clamping surfaces; 
     FIGS. 4 and 5 are schematic views showing upwardly and downwardly projecting lockbeads of the present invention applied to horizontal clamping surfaces; 
     FIG. 6 is a cross-sectional view showing an upwardly projecting lockbead of the present invention applied to angled clamping surfaces; and 
     FIGS. 7-9 are line drawings comparing the application of the present invention and prior art lockbeads to forming of a compartment lid panel. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIG. 1 of the drawings in detail, numeral  10  generally indicates a single action forming press having a movable upper platen  12  and stationary lower platen  14 . The upper platen  12  is vertically movable and is shown in its lower position. An upper die  16  is mounted to the upper platen  12 . 
     The lower platen  14  mounts a lower die assembly  18  including a lower die shoe  20 . A fixed lower punch cap  22  is carried on an upper surface of the lower die shoe  20 . A movable cushion retainer ring or binder  24  surrounds the lower punch cap  22  and the upper portion of the lower die shoe  20 . The binder  24  is supported on pneumatic cylinders  25  which are operable to move the binder vertically and provide a controlled resistance to downward motion thereof. A sheet metal blank formed into a stamping  26  is illustrated clamped between the upper die  16  on the upper side and the lower punch cap  22  and the surrounding binder  24  on the lower side of the blank. The stamping  26  is shown in its finish formed condition wherein the upper die has been fully lowered to form the blank by engagement of its upper and lower surfaces with corresponding lower and upper surfaces of the upper and lower dies. 
     Outward of the punch cap  22 , the upper die  16  and the binder  24  below are provided with opposing clamping surfaces  28 ,  30 , respectively. An outer edge portion or flange  32  of the sheet metal blank or stamping  26  is clamped between the surfaces  28 ,  30  and is locked against inward lateral motion by a lockbead to be described in connection with FIG.  2 . 
     Referring now to FIG. 2, there is shown a flange  34  or outer edge portion of a sheet metal blank formed into a stamping  36 . The flange  34  is clamped between an upwardly facing clamping surface  38  of a binder and a downwardly facing clamping surface  40  of an upper die. The clamping surface  40  includes a downwardly projecting bead  42  of square cross section extending into a straight sided groove  44  in the binder clamping surface  38 . The flange  34  of stamping  36  is formed with a bead  46  having an approximately square cross section that follows the contours of the downwardly protruding bead  42 . The cross-sectional configuration has sides which are not completely parallel by reason of a small clearance between the protrusion  42  and the groove  44  to allow for forming of the bead  46  without tearing of the blank. Preferably, the clearance is about equal to one sheet metal thickness on each side of the die bead  42 . 
     The contours of the protruding bead  42  and groove  44  and the resulting bead  46  formed in the metal blank or stamping  36  represent conventional prior art construction previously used for restraining inward motion of the flanges or outer edges of blanks during their forming by various processes, such as stretch form stamping, in die presses of various types. In this embodiment, the inner and outer corners of the protrusion  42  are formed with 90 degree angles for ease of machining. The upper corners of the groove  44  are also formed with 90 degree angles. The resulting bead  46  in the sheet metal flange  34  has a slightly off square shape by reason of the small clearance provided between the protrusion  42  and groove  44  so that the bead  46  includes four bends including two outward bends  48 ,  50  and two inward bends  52 ,  54 , each of which forms a bend angle of slightly less than 90 degrees for a total bend angle for the four bends of slightly less than 360 degrees. 
     It should be understood that the terms bend angle and bend refer to the angle (here slightly less than 90 degrees) through which the metal is bent at each of the respective bends as opposed to the included angles of the bent legs of the bead which form included angles of slightly greater than 90 degrees. Thus, in general, reference to the configuration of the sheet metal bead will be by bend angles whereas reference to the machined angles of protrusions and grooves in the corresponding die and binder members will be by reference to included angles of the machined surfaces. 
     Referring now to FIG. 3 of the drawings, there is shown a modified form of prior art lockbead wherein a flange  56  of a sheet metal blank formed into a stamping  58  is clamped between an upper clamping surface  60  of a binder and an opposing lower clamping surface  62  of an upper die, surfaces  60  and  62  being oriented at approximately a 45 degree slope with respect to the horizontal. Again, the lower clamping surface  62  includes a downwardly protruding bead  64  having parallel vertical sides which cooperates and extends into a vertically sided groove  66  in the associated binder surface  60 . 
     Bringing together of the surfaces  60 ,  62  against the flange  66  deforms the flange into a bead  68  which includes four bends  70 ,  72 ,  74 ,  76 . Because of the angle of the clamping surfaces  60 ,  62 , two of the bends form bend angles of less than 90 degrees at bends  70 ,  72 , while the remaining bends  74 ,  76  are radiused to avoid metal tearing. The result is a less than satisfactory clamping restraint of the metal during the forming process since the total of the bend angles of the bead equals substantially less than a desirable 340 degrees. 
     It should be noted at this point in reference to FIG. 1, that the slightly sloped clamping surfaces  28 ,  30  of the die  16  and binder  24  shown there result in a less distorted form of prior art lockbead than that shown in FIG.  3 . In FIG. 1, the protrusion from the upper die has a trapezoidal cross-sectional configuration capable of forming bend angles in the associated blank or stamping flange  32  approximating 85 degrees and therefore adequate to restrain the flange against inward motion during forming of the associated blank into a desirably configured stamping. However, as the slopes of clamping surfaces increase, as in FIG. 3, the prior lockbead configuration becomes inadequate. 
     Referring now to FIG. 4, there is shown a first embodiment of double V lockbead formed in accordance with the present invention. Here a sheet metal blank formed into a stamping  78  includes a flange  80  clamped between parallel upper and lower clamping surfaces  82 ,  84 , respectively, of an associated binder and upper die respectively. The lower clamping surface  84  of the upper die is formed with double V recesses creating three alternating sharp angles  86  of preferably 90 degrees each and outwardly connected with the horizontal lower clamping surface  84  by connecting sharp angles  88  of 45 degrees each which together provide a total angular sum of 360 degrees. The opposing upper clamping surface  82  is formed with mating upward protrusions forming alternate sharp angles each of 90 degrees, the protrusions being connected with the upper clamping surface  82  by 45 degree angles  92 . 
     When the flange  80  is compressed between these surfaces  82 ,  84 , a double V bead  94  results which includes centrally three alternately angled bends  96  forming a double V configuration. These bends approximate but are slightly less than 90 degrees because of the clearance (about 0.5 mm) provided between the sheet metal and the angled surfaces to avoid the need for spotting or fitting of the machined die members. The double V bead is then connected by connecting bends  98  to the adjacent horizontal surfaces  82 ,  84 , the connecting bends being approximately 45 degrees or slightly less. Again, the total of the bend angles is nearly 360 degrees, which has been shown to form more than an adequate restraint for the flanges or outer edges of blanks being formed in an associated die press. It should be noted that, because of the double V form as well as the shorter lengths of the legs of the double V bead, the bead  94  itself occupies or requires a smaller portion of the flange  80  to form the bead. For example, comparing the prior art embodiment of FIG. 2 with the inventive embodiment of FIG. 4, the width of the flange required for formation of the double V bead is less by 10 mm than that required for formation of the prior art square bead. Thus, a substantial saving of material is obtained by use of the improved double V bead. 
     Referring now to FIG. 5, a double V bead arrangement similar to but reversed from that of FIG. 4 is illustrated. Without numerical references, it is merely noted that the double V recesses or grooves are formed in the binder while the mating double V protrusions are formed in the upper die with the corresponding dimensions of the angles and bends being the same as described in connection with FIG.  4 . The operation and capabilities of the inverted bead of FIG. 5 are thus the same as those of the described bead of FIG.  4 . 
     Reference is now made to FIG. 6 wherein a double V bead application is disclosed which corresponds in general nature to the application of a prior art bead between sloping clamping surfaces as illustrated in FIG.  3 . In FIG. 6, a sheet metal blank formed into a stamping  100  has a flange  102  formed at a 45 degree angle from the horizontal and clamped between a correspondingly sloping upper clamping surface  104  of an associated binder  106  and a lower clamping surface  108  of a mating upper die  110 . The lower surface  108  is formed with three alternating sharp angles  112  of 90 degrees each, forming a double V recess connected with the sloping surface  108  by connecting 45 degree sharp angles  114 . In like manner, the binder  106  includes dual protrusions having alternating 90 degree sharp angles  116  with connecting 45 degree angles  118 . The sides of the angled portions extend alternately vertically and horizontally so that the vertically moving die may be engaged with the stationary or movement resisting binder  106 . The resulting embossment  120  forms a double V bead having three alternately angled bends  122  of nearly 90 degrees connected at their edges by near 45 degree connecting bends  124 . Thus, the sum of the sharp bend angles forming the bead is slightly less than 360 degrees and in excess of 340 degrees, as is considered desirable for restraining the blank. The double V bead according to the invention therefore provides significantly improved clamping abilities as compared to the modified prior art bead of FIG.  3 . 
     It is noted here that should there be a need in an unusual case to form a clamping surface which is more highly sloped, a double V bead of the sort described in FIG. 6 may still be utilized. However, as the slope approaches vertical, the upper connecting bend  124  will be increased while the lower connecting bend  124  will be decreased. The legs or sides of the V beads may have unequal lengths but negative die angles or back draft must be avoided. In this case, however, the total of the alternating and connecting bends will still approximate 360 degrees or slightly less, so that the total clamping ability is not diminished by the increased slope of the clamping surfaces. 
     Referring now to FIGS. 7-9, there is shown in schematic form the application of the present invention as compared to prior art restraining beads to the forming of a compartment lid outer panel, indicated by numeral  126 . Panel  126  is provided with a flange  128  which has a multi-directional orientation extending from a relatively smaller slope at the right end of FIG. 7, also shown in FIG  8 , and a relatively steeper slope of about 45 degrees at the left end of FIG. 7, also shown in FIG.  9 . At its right end, which forms the lower rear edge of the trunk lid, as shown in FIGS. 7 and 8, the flange is restrained by a double V bead  130 , replacing the modified prior art bead  132  which has somewhat less restraining force. On its left end, as shown in FIG.  7  and in FIG. 9, a double V bead  134 , formed according to the invention, replaces a pair of prior art beads  136 , which would be required to provide a restraining force equal to that of the double V bead on the relatively highly sloped surface of the flange  128 . 
     As should be apparent, a novel double V bead arrangement has been presented which provides increased flange restraining force under various conditions of flange orientation from horizontal to near vertical in place of a prior art bead shown to be inadequate for use with flange clamping surface orientations beyond 30 degrees from horizontal. It should be noted that modifications of the double V bead could involve variations in the bend angles of the bead within reasonable ranges appropriate to the particular application and might also include the addition of one or more alternately angled bends which, in the case of two additional bends, would form a triple V bead that adds one additional V to the double V bead arrangement disclosed. The legs or sides of the beads may have unequal lengths which enhance the die machining process or prevent prohibitive negative draft angle. 
     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.