Abstract:
A method for producing a complex sheet metal part includes clamping a border region of the metal sheet between a die ring of an upper tool part of a pressing tool and a downholder of a lower tool part of the pressing tool; moving the upper tool part further downwards thereby pulling the metal sheet over extended punch inserts of a punch of the lower tool part, thereby generating local material reserves of the metal sheet; moving the upper tool part further downwards while actively retracting the punch inserts, thereby releasing the local material reserves; and forming the metal sheet between the die and the punch using only the local material reserves.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of German Patent Application, Serial No. 10 2016 005 902.8, filed May 13, 2016, pursuant to 35 U.S.C. 119(a)-(d), the content of which is incorporated herein by reference in its entirety as if fully set forth herein. 
       BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to a method for producing a complex formed sheet metal part by forming a metal sheet by means of a pressing tool and a pressing tool suited therefore. 
         [0003]    The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention. 
         [0004]    Complex formed sheet metal parts, in particular those for use in vehicle body construction, differ from simple formed sheet metal parts, for example such as a bowl or a trough, in that these sheet metal parts have a more complex shape, which typically includes different surface areas and in particular also further design features, for example design or styling edges and/or stiffening edges with small radii. Because there are limits regarding the feasibility of forming, production of such complex formed sheet metal parts is often difficult and cost-intensive. Special problems are encountered when great drawing depths are to be achieved under serial production conditions. 
         [0005]    It would therefore be desirable and advantageous to provide an improved method and an improved tool for producing a complex formed sheet metal part, in particular a large-area formed sheet metal part (for example of &gt;0.5 m 2 ) with which a great drawing depth can be achieved under serial production conditions while ensuring a flawless quality of the parts. 
       SUMMARY OF THE INVENTION 
       [0006]    According to one aspect of the present invention a method for producing a complex sheet metal part said method includes inserting a metal sheet into an open pressing tool, said pressing tool comprising a lower tool part which includes a punch, a downholder surrounding the punch and multiple punch inserts, and an upper tool part which includes a die and a die ring surrounding the die; closing the tool by moving the upper tool part downwards thereby clamping a border region of the metal sheet between the die ring and the downholder; with the punch inserts being in an extended position, moving the upper tool part further downwards so that the die ring displaces the downholder downwards and the metal sheet is pulled over the extended punch inserts, thereby generating local material reserves of the metal sheet; forming a region of the metal sheet between the die ring and the punch by moving the upper tool part further downwards into a position in which the die ring and the downholder have reached respective lower movement dead points, wherein during the moving of the upper tool part into said position the punch inserts are actively retracted, thereby releasing the local material reserves; and forming another region of the metal sheet between the die and the punch by moving the die further downwards until the die reaches a lower movement dead point, wherein during the forming of the other region a peripheral region of the metal sheet is clamped between the die ring and the downholder and between the die ring and the punch so as to prevent flow of metal sheet material from the border region of the metal sheet so that only the local material reserves are used for the forming of the other region. 
         [0007]    The process can also be performed with inverse kinematic. In this case the upper tool part and the lower tool part are arranged inversely and are correspondingly configured. 
         [0008]    The method according to the invention enables production of a complex sheet metal part with great drawing depth under conditions of serial production in a pressing plant, in particular also with high frame depth, and flawless quality. The term great drawing depth in particular means a drawing depth (=drawing path in working direction) of at least 200 mm, preferably at least 250 mm and in particular at least 300 mm. Within a closing or working movement the formed sheet metal part can essentially be finally formed starting from a plan metal plate, so that at least no additional forming operations that significantly change the drawing depth and/or the shape are required. In a press line the number of required drawing stages can thus be kept low or can even be reduced. 
         [0009]    The term active backwards movement of the punch inserts means that the punch inserts are not passively displaced or urged backwards as in the state of the art, but are actively moved back. This can for example be accomplished by a mechanical coupling of the corresponding punch inserts with the downholder so that the punch inserts are moved back by the downholder or are moved or inserted into the punch recesses provided therefore. This can be performed synchronously or asynchronously (i.e., in a defined sequence and/or with different speeds), in order to thereby control the punch contact, i.e., contact between the metal sheet and the punch surface or the effective surface of the punch, in a targeted manner. The tool or the pressing tool can also have punch inserts which are moved or urged back passively in a conventional manner. 
         [0010]    According to another advantageous feature of the present invention, at least one of the punch inserts is configured for generating in the metal sheet or formed sheet metal part a design edge with a small radius. The complex sheet metal part that can be generated with the method according to the invention can thus also have at least one distinctive design edge in spite of the significant drawing depth. The outer edge radius of the design edge is preferably only a few millimeters in size. This design edge is free of skid and impact lines because the method according to the invention prevents that the sheet moves relative to the punch during the drawing process. 
         [0011]    According to another aspect of the present invention a pressing tool, in particular deep drawing tool, for producing a complex sheet metal part, includes a first tool part including a punch, a displaceable downholder surrounding the punch, and multiple movable punch inserts, wherein the downholder is mechanically coupled (i.e., a by means of a mechanics) with at least one of the punch inserts, so that a displacement of the downholder causes active retraction of the at least one punch insert; and a second tool part including a die and a die ring surrounding the die 
         [0012]    The pressing tool according to the invention can thus be configured so that between the downholder and at least one of the punch inserts, preferably all punch inserts, a mechanical coupling is present by which during the displacement and in particular during the downward movement of the downholder (i.e., during the forming or drawing process as described above) the respective punch insert or the respective punch inserts is/are actively retracted, i.e., moved or inserted into the punch recess/es provided therefore. 
         [0013]    According to another advantageous feature of the present invention, the punch inserts can be arranged in the pressing tool so as to enable generating a material reserve (as explained above) in all critical regions at the beginning of the drawing process. 
         [0014]    According to another advantageous feature of the present invention, at least one of the punch inserts, in particular the punch insert that is coupled with the downholder, is additionally resiliently supported, for example by means of a gas pressure spring. Preferably all punch inserts are additionally resiliently supported. 
         [0015]    According to another advantageous feature of the present invention, at leas one of the punch inserts can be configured for a point contact (=small-area contact) with the metal sheet to be formed. At least one of the punch inserts can be configured for a line contact with the metal sheet to be formed, in particular for forming a design edge with a small radius (as described above). 
         [0016]    According to another advantageous feature of the present invention, the die can also have at least one movable die insert which is situated opposite a punch insert, in particular for forming a design edge and which is configured with a corresponding counter contour so that the metal sheet can be formed between the punch insert and the die insert in the manner of an embossment. In analogy to a punch insert such a die insert is arranged in a die recess and is also preferably resiliently supported, for example by means of a gas pressure spring. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0017]    Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which 
           [0018]      FIGS. 1A-D  show respective schematic sectional views of a pressing tool according to the invention, illustrating steps of the method according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0019]    Throughout all the Figures, same or corresponding elements may generally be indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the figures are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted. 
         [0020]      FIG. 1A  shows a schematic illustration of a pressing tool according to the invention. The pressing tool  100  has a lower tool part  110  and an upper tool part  120 . The pressing tool is a deep drawing tool, which is installed in a not shown press, wherein the lower tool part  110  is arranged on the pressing table and the upper tool part  120  is fastened on the press table. During lowering of the press table the upper tool part  120  performs an upward movement or work movement with an essentially continuous downward movement. 
         [0021]    The lower tool part  110  has a punch  130  a down holder or sheet holder  140  which surrounds the punch  130 , and multiple movable punch inserts  131 ,  132  and  133 . The punch inserts  131 ,  132  and  133  are arranged in punch recesses. The punch insert  131  is configured for a line contact and the punch inserts  132  and  133  for a point contact. The upper tool part  120  has a die  150  and a die ring or counter holder  160 , which surrounds the die  150 . The tool  100  is configured for a drawing depth of at least 200 mm. in the following the drawing process is described. 
         [0022]    A metal sheet M in the form of a plan metal plate (metal sheet blank) is inserted into the opened tool  100  and the tool  100  is closed by downward movement of the press ram or the upper tool part  120 . Hereby a border region of the metal sheet M is clamped between the die ring  160 , which precedes the die  150 , and the downholder  140 , as shown in  FIG. 1 a   . The punch inserts  131 ,  132  and  133  are situated in the extended positions or starting positions in which they protrude over the punch surface or the effective surface. 
         [0023]    During the further downward movement of the upper tool part  120  the die ring  160  displaces the downholder  140  downwards. This means that the downholder  140  is moved downward together with the die ring  160  and hereby moves relative to the stationary punch  130 . The downholder  140  can be supported in a known manner by a drawing pillow which belongs to the press or the tool or the like, as illustrated with the shown springs. Hereby the metal sheet M is pulled over the punch inserts  131 ,  132  and  133  whereby in the metal sheet M local material reserves are generated. This is shown in  FIG. 1B . The punch inserts  131 ,  132  and  133  are for example resiliently supported by means of gas pressure springs as illustrated with the shown springs  200 , so that these push against the metal sheet M with a defined force. 
         [0024]    Simultaneously a design edge with small radius is formed into the metal sheet M by means of the center punch insert  131  and the opposite die insert  151 , for which purpose the punch insert  131  and the die insert  151  are provided with corresponding forming sections at their respective end sides. Also the die insert  151  is for example resiliently supported with a gas pressure spring as illustrated with the symbolically shown spring  200 . 
         [0025]    During the further downward movement of the upper tool part  120  the die ring  160  and the downholder  140  are moved to their lower dead point as shown in  FIG. 1C . At the same time the punch inserts  131 ,  132  and  133  are active retracted, i.e., they are not retracted by being displaced, whereupon the metal sheet M can successively contact the punch surface or effective surface of the punch  130  in the region of the punch inserts  131 ,  132  and  133 . In this way a controlled punch contact is achieved. In addition the local material reserves are released for further forming. Also simultaneously the metal sheet M is already formed in an overlap region between the die ring  160  and the punch  130  for which purpose the die ring  160  and the punch  130  are configured with corresponding effective surface areas, 
         [0026]    The backward movement or retraction of the punch inserts  131 ,  132  and  133  is performed by a mechanical forced coupling with the downholder  140 . For this purpose the punch inserts  131 ,  132  and  133  are for example fastened on a frame or a carrier plate  180  which is for example carried along or moved along by means of catches  190  arranged on the downholder  140  when the downholder is moved past a defined downholder position (or past a defined draw path position). The frame  180  is for example resiliently supported on the base plate (not shown) which belongs to the lower tool part  110 , as illustrated with the shown springs  200 . 
         [0027]    When the downholder  140  has reached its lower dead point (see  FIG. 1C ) the punch inserts  131 ,  132  and  133  are also located in their retracted end positions. Depending on the desired goal the end sides of the punch inserts are situated behind the punch surface (as shown in  FIG. 1 c   ), end flush with the punch surface or still protrude over the punch surface. The retraction and the end positions of the punch inserts can be adjusted individually so that also asynchronous punch insert movements are possible. Different from the shown vertical movement directions of the punch inserts  131 ,  132  and  133 , also oblique movement directions are possible, for example by using sliders. 
         [0028]    By further downward movement of the press punch ram or the upper tool part  120  the trailing die  150  is now moved to its lower dead point, wherein the metal sheet M or the formed sheet metal part is finally formed in the middle region (with the forming being already completed in the outer region). This means the die ring  160  first reaches its lower dead point, then the die  150  continues to move until reaching its lower dead point and thereby forms the middle region. Due to the border-side clamping of the metal sheet M between the die ring  160  and the punch  130  no sheet metal material or substantially no sheet metal material can flow from the outside towards the inside, i.e., toward the center region, so that the metal sheet M is formed between the die  150  and the punch  130  in the manner of an embossment by only using the local material reserves that have been generated beforehand by means of the punch inserts  131 ,  132  and  133 , wherein the resiliently supported die insert  151  is retracted or is displaced upwards. As an alternative it can be provided that the middle punch insert  131  is not moved backwards and thus continues to push against the metal sheet M or against the die insert  151 . 
         [0029]      FIG. 1D  shows the end state in which the downholder  140 , the die ring  160  and the die  150  are in their lower movement dead points. Forming of the metal sheet or sheet meal workpiece M is completed. After opening the tool  100  by lifting or upwards movement of the press ram or the upper tool part  120  the formed sheet metal part can be removed. By means of the springs the tool parts move back into the starting arrangements or positions as shown in  FIG. 1A  so that the next metal sheet M can be inserted and formed.