Patent Publication Number: US-6668612-B2

Title: Device for holding a sheet metal blank in a forming press

Description:
TECHNICAL FIELD 
     This invention relates to machines designed to hold sheet metal blanks between upper and lower forming press tools. More specifically, this invention relates to devices designed to secure the sheet metal blank between the forming press tools in connection with superplastic, or other high temperature, forming. 
     BACKGROUND OF THE INVENTION 
     Superplastic metallic alloys, such as certain alloys of aluminum, magnesium and titanium, are relatively ductile and can undergo substantial tensile deformation in the presence of low shaping forces. Sheet metal superplastic alloys can be shaped by a variety of processes and can be formed into the complex shapes characteristic, for example, of automotive body panels. 
     Forming presses have been used to manufacture vehicle body parts using superplastic aluminum alloys, such as very fine-grained AA5083. Often these presses comprise concave upper and convex lower tool members where a sheet metal blank is inserted and initially balanced on the convex lower member. The forming tools have been pre-heated and maintained at a suitable superplastic forming temperature, e.g., 800° F., for the sheet metal blank. The blank itself may have been preheated to this temperature or will simply be heated in the forming press. And since the forming process is gradual and complex, the hot blank needs to be held in proper position between the forming tools. 
     As the upper concave tool member lowers and closes over the convex lower tool member during press operation, the sheet metal blank must be secured near its edges so that it is properly formed between the tools. Robots can be used to initially place the sheet metal blank on the lower tool in the hot environment. Presently, there exist blank holding devices that temporarily locate the edges or corners of the blank prior to upper tool closure. As disclosed in U.S. Pat. No. 6,085,571 entitled Gravity-Operated Blank Loading Device by Brinas et al., four loading devices, for example, are provided and used inside the press to position the four corners of a rectangular blank between the open tool members. However, these devices release the blank during tool closure. For this reason, there remains a need for a mechanism functional at press operating temperatures to hold the edges of the blank during tool closure for proper part manufacture. Unwanted movement of the blank leads to mal-formed parts and waste. 
     It is an object of the present invention to provide a blank loading device that is capable of positioning and securing the sheet metal blank to achieve proper alignment in the press that is maintained through out the entire press forming process. 
     SUMMARY OF THE INVENTION 
     The present invention provides a device used to position and securely hold a sheet metal blank in a sheet metal forming press operated at high temperatures. The forming press comprises an upper tool member having a concave surface and a lower tool member having a convex surface complementary to that of the upper tool member. The sheet metal blank is initially positioned in the forming press often by resting the sheet metal blank on the highest portion of the convex surface of the lower tool member. The sheet can be balanced, for example, by a gravity operated blank loading device much like that described in the &#39;571 patent. This invention provides a robust mechanism for gripping and securing near the edge of the blank so that it does not move out of position during tool closure. 
     In accordance with the present invention, sheet metal gripping mechanisms are suitably located at the margin(s) of the upper and lower forming tools. Generally, the forming tools are rectangular in outline and the subject gripping mechanisms would be attached to opposite sides of each tool. Where the inserted blank is balanced on the top of the lower convex tool, the subject gripping mechanisms are preferably located above and below these locations at the sides of the tools. The gripping mechanism carried on the upper tool is positioned to engage the margin of the sheet metal blank as soon as the tool is lowered from its open to its forming position. Preferably, this upper gripping mechanism comprises a block or beam with a knife-edge on its bottom surface. The edge is shaped to progressively engage and deform the sheet metal blank to grip it against a complementary gripping surface on the lower tool underlying the sheet. 
     As the upper tool lowers, the knife-edge engages the blank but the forming tool continues to drop. The upper knife-edged gripping mechanism must be capable of sliding up the side of the closing upper tool member in order to suitably maintain its grip on the edge of the sheet metal blank. Preferably, the gripping beam or block on the upper tool has sufficient weight or mass so that the knife-edge maintains a tight grip on the edge of the blank as it is undergoing substantial deformation between the working tools. In one embodiment, the gripping beam slides in slotted brackets mounted on the edge of the forming tool. The gripping mechanism comes into contact with the margin of the sheet metal blank and stops. The upper tool continues to lower and the gripping tool slides along its surface as it lowers. The weight of the gripping tool and the friction of the lowering upper tool contribute to the gripping force of the mechanism. Usually there will be at least two or even four such gripping devices attached to the ends of the upper tool member and each gripping device can include one or more knife-edges. 
     Underlying each of the upper tool gripping mechanisms is a complementary gripping surface located on the lower tool. In a preferred embodiment, each of these lower gripping surfaces remains stationary and are mounted so that they can bear the pressing force of the upper gripping tool and the blank. While the upper surface of the lower gripping device is generally flat like the blank margin that it is engaging, the device may contain a channel underlying the knife-edge of the upper mechanism so that sheet metal can be forced by the knife-edge into the channel to mechanically secure the blank. The channel also prevents the knife-edge from impacting the hard surface of the block, thus prolonging the life of the knife-edge blade. The sides of the channel in the lower gripping surface may be “V” shaped corresponding to the shape of the knife-edge in the upper gripping surface. 
     An important feature of the invention is the way in which the upper gripping mechanism is supported on the upper forming tool. As described above, it is necessary that the gripping mechanism be capable of sliding up the side of the forming tool as the forming tool closes. In one embodiment, the upper gripping mechanism is carried on a beam that spans the side of the upper tool and slides in slotted vertical brackets. In another embodiment, the upper knife-edged gripping device is carried on a block, which slides on two vertical rods fixed to the side of the upper tool. In still another embodiment, the upper gripping mechanism can be supported by both a rod and a slotted bracket. These features will be described in more detail below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A-1C show a sequence of side view press forming positions during press operation illustrating a first embodiment of the guide mechanism. 
     FIG. 2 is a top view of a part of the embodiment shown in FIGS. 1A-1C showing the two slidable brackets and horizontal plate configuration. 
     FIG. 3 is a side view of the knife-edged engagement tool used in all three embodiments. 
     FIGS. 4A and 4B is a perspective view of a second embodiment of the invention showing the positions of the upper and lower tool members of the forming press prior and subsequent to press operation. 
     FIG. 5 is a cut-off prospective view of the dual rod configuration of the guide mechanism mounted to the end surface of the upper tool member demonstrating a third embodiment of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     This invention provides an improved design of a gravity operated sheet metal blank loading device comprising a guide mechanism used to securely position a sheet metal blank during high temperature press operation. FIG. 1A provides an end view of a first embodiment of the present invention. A blank loader  10  has a counter-balanced arm  11  with a support hand  48  in which a panel of sheet metal  12  is placed and held. The sheet metal blank  12  is balanced on the highest portion  51  of the convex forming surface  24  of the lower tool member  16  using one or more blank loaders  10 . The counter-balanced arm  11  with its pivot  50  can rotate and release the blank  12  during closing movement of the upper tool  14 . 
     The forming press (not shown) includes an upper tool member  14  and a lower tool member  16 . The concave surface  22  of the upper tool member  14  comprises a shape suitable for superplastic forming the desired part. The lower tool member  16  includes a convex surface  24 , where such surface complements that of the concave surface  22 . Surfaces  22 ,  24  define the shape of the desired part, e.g., body panels, automotive deck lids, tail gates, or the like. 
     A two-part gripping apparatus is mounted to the ends of the upper and lower forming tools  14 ,  16  is shown in FIGS. 1A-1C and  2 . The upper gripping tool, which is mounted to end surface  52  of upper tool member  14 , comprises a horizontal cross-beam  26 , where the ends  42 ,  44  of cross beam  26  are supported in slotted vertical brackets  18 ,  20 . The top view of the brackets  18 ,  20  is shown in FIG.  4 . In the open position of upper tool  14  (FIG.  1 A), horizontal beam  26  rests at the bottom ends of brackets  18 ,  20 . Rotational movement of the horizontal beam  26  is restricted by the two brackets. Attached to the center of the bottom of beam  16  is a knife-edge engagement tool  28 , for engaging and securing the sheet metal blank  12  between the upper and lower tool members  14 ,  16 . Engagement tool  28  is a metal block with sufficient weight to press on and hold the edge of blank  12 . As seen in FIGS. 1A-1C and  3 , the bottom portion of tool  28  is tapered at surfaces  72  to a knife-edge  30 . For purposes of this description, knife-edge  30  comprises a “V” shape. As best seen in FIG. 3 side surface  84  of knife-edge  30  is curved for more progressive contact of edge  30  with a blank  12 . Side surface  86  is similarly shaped. 
     As seen in FIG. 1A, horizontal beam  26  hangs at the lowest ends of brackets  18 ,  20 . In this open position of upper tool  14 , knife-edge  30  is suspended above the margin of the portion of blank  12  lying on the highest point  51  of convex tool surface  24 . 
     Immediately below the same portion of blank  12  is a gripping block  32  with a blank gripping surface member  34  complementary to knife-edge  30  mounted on end surface  54  of lower tool member  16 . Gripping surface member  34  has surfaces  75  that slope away from engagement with blank  12 . Surface  34  also has a channel  36  underlying knife-edge  30  for good gripping engagement with blank  12 . The upper sides  80 ,  82  of generally rectangular channel  36  taper outwardly and complementary to sides  84 ,  86  of the upper knife-edge tool. 
     The shape of surface member  34  allows knife-edge  30  of the tool  28  to press an edge of blank  12  into the sloped sides  80 ,  82  of channel  36  in secure gripping engagement without piercing or damaging the lower member  16  of the forming press. Surface member  34  is secured to the lower member  16  by means of a support block  40 . 
     FIGS. 1A-1C show a sequence of positions of the high temperature forming tools during operation. The sheet metal blank  10  can be pre-heated before being placed between the forming tools  14 ,  16 . Alternatively, an unheated sheet metal blank  12  can be placed between forming tools  14 ,  16 . The sheet metal blank  12  used in this illustration is made of an aluminum alloy that is heated to a temperature between 800 and 1050° F. for superplastic forming. 
     Initially, the forming tools are in a press-open position (FIG.  1 A). A sheet metal blank  12  is placed and balanced on the highest portion  51  of convex surface  24  of the lower tool member  16 . The sheet metal blank  12  is temporarily positioned and held by one or more blank loaders  10 . The ends  42 ,  44  of horizontal beam  26  rests on a pre-formed ledge (not shown) at the bottom ends of brackets  18 ,  20 . 
     FIG. 1B shows the position of the upper tool  14  as it has moved from its press-open position to an initial engagement position with blank  12 . The engagement tool  28  pinches the sheet metal blank  12  at its margin, where it was resting on the lower tool member  16 . Once the engagement tool  28  pinches the sheet metal blank  12  against surface member  34 , its motion stops as the upper tool member  14  progresses in the downwards direction. As shown in FIG. 2, the space between the brackets  18 ,  20  and beam  26  permits the closing upper tool  14  to slide past beam  26  and engagement tool  28 . 
     As the upper tool member  14  reaches its press-close position (FIG.  1 C), the beam  26  has slid to the top portion of brackets  18 ,  20 , but engagement tool  28  maintains its hold on blank  12 . The sheet metal blank  12  has been completely formed and shaped at this point according to the shape of the upper and lower surfaces  22 ,  24  of tool members  14 ,  16 . As the upper tool member  14  drops, the blank loader  10  releases the blank  12  as the counter-balanced arm  11  tips by means of pivot  50  in conformance with the downward movement of the upper tool member  14  and support hand  48  releases sheet metal blank  12 . The lower tool member  14 , however, remains stationary during the entire process. 
     A second embodiment is shown in FIGS. 4A and 4B. FIG. 4A is a perspective view of the forming tools at the press-open position. The top  90  and bottom  92  portions of the forming press carry the upper and lower forming tools  14 ,  16 . A portion of a sheet metal blank  12  is shown held by support hand  48  connected to the counter-balanced arm  11  of the blank loader (not shown) suspended between the open tools  14 ,  16 . The remainder of blank  12  is cut away so that convex surface  24  of the lower tool member  16  can be seen. Surface  24 , is shaped to form the desired part. 
     The gripping apparatus shown in FIGS. 4A and 4B is mounted on the end surfaces  52 ,  54  of the upper tool member  14  and lower tool member  16 , respectively. This gripping apparatus comprises a guide rod  98  held by means (not shown) on the end surface  52  of the upper tool member  14 . Engagement block  108  is attached to a horizontal beam  104 , which provides additional weight to the apparatus. Block  108  is slidably attached to guide rod  98  and horizontal beam  104  slides in bracket  94 . To prevent horizontal beam  104  from twisting or rotating about guide rod  98 , the end  132  of the horizontal beam  104  is placed within the slot of bracket  94 , which is mounted to the end surface  52  of the upper tool member  14 . Thus, the end  132  of the horizontal beam  104  secured from rotation about rod  98  by the bracket  94 . The bottom portion of rod  98  provides support (not shown) for the horizontal beam  104  at the press-open stage of the forming process. A knife-edged engagement tool is mounted underneath horizontal beam  104 . This knife-edge engagement tool is not visible under horizontal beam  104  in perspective views of FIGS. 4A and 4B. However, this is suitably just like knife-edge engagement tool  28  shown in FIGS. 1A-1C and  3  and described in connection with these figures. A like guide rod  98  and gripping apparatus can also be mounted on the opposite end surface of the upper and lower tool members  14 ,  16 . 
     As upper tool member  14  drops and approaches the press-close position as shown in FIG. 4B, the knife-edge tool presses the sheet metal  12  (not shown in FIG. 4B) into channel  36 . Thus, beam  104  with its knife-edge and block  40  secure sheet metal blank  12  between tool members  14 ,  16 . Once the knife-edge has secured sheet metal blank  12 , beam  104  slides upwards guided by bracket  94  and guide rod  98 . 
     FIG. 5 provides a cut-off perspective view of the upper tool member  14  and top portion  90  of the forming press of another embodiment of the present invention. The gripping apparatus shown comprises two guide rods  126 ,  128  connected to an upper block by means (not shown) that is mounted to surface  52  of upper tool member  14 . The two guide rods  126 ,  128  support a horizontal beam  120  such that rotational movement of beam  120  is prevented. The horizontal beam  120  includes two weighted blocks  122 ,  124 . It also carries underlying engagement tool (not shown) on board of guide rod  128 . Following the same process as describe above, as upper tool member  14  drops, guide rods  126 ,  128 , slide through horizontal beam  120  and engagement tool knife-edge (not shown) secures and holds a sheet metal blank against a complementary gripping surface. 
     While the invention has been described in the context of the preferred embodiments, it is not intended to be limited to the above description, but rather only to the extent set forth in the following claims.