Abstract:
A deep-drawing press includes a one-piece drawing cushion, an arrangement of cushion cylinders and a control unit. The overall performance of the press is increased by the possibility of using both large and small tools. The position of the drawing cushion is controlled during the forming process.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation of International Application No. PCT/EP00/08047, filed Aug. 17, 2000, the disclosure of which is incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to a deep-drawing press with a table drawing cushion and a tool arrangement comprising at least one upper die secured to a press ram, at least one blank holder supported by cushion bolts, and at least one lower die in connection with a press table.  
           [0003]    Deep-drawing presses comprising one-piece drawing cushions and one or a plurality of cushion cylinders are known in the prior art, according to FIG. 10, primarily for the purpose of forming by means of large tools. However, these deep-drawing presses only permit a small off-center load during the forming process.  
           [0004]    In addition, a multi-point drawing cushion can be used in connection with a big tool, the piston rods of the cushion cylinders acting directly on the blank holder of the tool, analogously to the cushion bolts. If, however, a plurality of tools is provided above a one-piece cushion, compensating bolts for load compensation must be used in the case of an off-center application of the load. In many cases, however, there is no room for such compensating bolts. When a plurality of smaller tools is used, also the necessary hold-down force will normally be lower than in cases in which one big tool is used.  
           [0005]    The prior art also discloses, according to FIG. 11, the use of a two-piece drawing cushion for receiving a plurality of tools and/or smaller tools. This configuration necessitates that the number of cushions and the center-to-center distances between the cushions corresponds to those of the tools. When a two-piece drawing cushion for receiving a big tool is used, it is often impossible, due to the separating groove, to equip all bolt rows with cushion bolts.  
         BRIEF SUMMARY OF THE INVENTION  
         [0006]    It is therefore an object of the present invention to improve a deep-drawing press of the type mentioned at the outset, in such a way that, in combination with a one-piece drawing cushion, big as well as small tools can be used without subjecting the cushion guide means to unacceptable loads. Further, an improved method for deep-drawing sheet-metal workpieces is achieved by an improved blank-holder function in any case.  
           [0007]    In the case of a deep-drawing press of the type mentioned at the outset, this object is achieved in accordance with the present invention by the features that a plurality of outer cushion cylinders is adapted to be brought into engagement with a one-piece drawing cushion and that these outer cushion cylinders are arranged at least in corner areas of the drawing cushion.  
           [0008]    When sheet-metal workpieces are formed by means of big tools, these outer cushion cylinders only contribute a small percentage of the total hold-down force. They primarily serve to compensate tilting movements in the operative system and to control the position of the upper dead center of the cushion. In addition, they cause the pre-acceleration of the cushion prior to the forming process and the ejection of the workpiece after the forming process.  
           [0009]    According to a preferred embodiment of the deep-drawing press according to the present invention, the one-piece drawing cushion is implemented such that at least one cushion cylinder is arranged in the central area below the drawing cushion, in such a way that it presses against the drawing cushion with a slight pre-tensioning pressure. When the deep-drawing press is equipped with a big tool, these inner cylinders take up most of the forming force required. When smaller tools are used, the inner cylinders may, however, be excluded from force transmission by reducing the pre-tensioning pressure.  
           [0010]    According to a further advantageous embodiment of the present invention, a control unit is provided which, in response to distance and pressure signals of the tool arrangement, causes an application of pressure to the inner and outer cushion cylinders. The pressure signals from all cylinder chambers of the cushion cylinders as well as the measured values of the cushion and ram paths are compared with the set values for position and tilt detection. The valves of the cushion cylinders are then controlled in accordance with the load distribution which is to be achieved over the drawing cushion, and for the purpose of minimizing the elastic deformation.  
           [0011]    In accordance with another advantageous embodiment of the present invention, it is also possible to use smaller tools in the boundary area of the deep-drawing press, e.g., in a tool arrangement used as a transfer tool. Depending on the hold-down force required, only the outer cushion cylinders have, in this case, pressure applied thereto so as to produce an off-center cushion force.  
           [0012]    The above-mentioned object is achieved according to the present invention as far as the method is concerned by providing the possibility of applying pressure either to the outer cushion cylinders alone or to both the outer and the inner cushion cylinders in combination. This permits processing of the sheet-metal workpieces in various travelling directions. Long sheet-metal workpieces, for example, can be processed in the longitudinal direction of the deep-drawing press by means of 2-3 tools with an off-center cushion force.  
           [0013]    Alternatively, 2-6 smaller tools, arranged in a rectangular mode of arrangement, can be used for executing the forming process in the transverse direction of the deep-drawing press.  
           [0014]    In the case of a possible deviation from parallelism between the press table and the press ram, the outer and inner cushion cylinders have, in accordance with the method according to the present invention, pressure applied thereto in such a way that a total force is produced parallel to the press table or the press ram.  
           [0015]    It is, however, also possible to control the total force in a different way, e.g., such that it is not parallel to the press table or the ram, and to vary in this way the pressure distribution among the blank holder faces, whereby the flange draw-in will be influenced. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0016]    The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. In the drawings:  
         [0017]    [0017]FIG. 1 is a demand diagram for toolings in a deep-drawing press;  
         [0018]    [0018]FIG. 2 is a side elevation view of a transfer tool with eccentric application of a small force;  
         [0019]    [0019]FIG. 3 is a side elevation view similar to FIG. 2 of a transfer tool with a higher hold-down force;  
         [0020]    [0020]FIG. 4 is a side elevation view of an arrangement with a big tool and a strong forming force;  
         [0021]    [0021]FIG. 5 is a three-dimensional perspective representation of a press table having a drawing means integrated therein;  
         [0022]    [0022]FIG. 6 is an exploded view of a press table according to FIG. 5;  
         [0023]    [0023]FIG. 7 is a sectional view of the press table of FIG. 5;  
         [0024]    [0024]FIG. 8 is a schematic diagram of a drawing cushion with sensors for the pressure p, the cushion path ski and the ram path sst;  
         [0025]    [0025]FIG. 9 is schematic diagram for a control unit for the press with drawing cushion;  
         [0026]    [0026]FIG. 10 is a side elevation view of a prior art press with one-piece drawing cushion; and  
         [0027]    [0027]FIG. 11 is a side elevation view of a prior art press with at least one small drawing cushion for transfer tools. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]    [0028]FIG. 1 shows a survey of the various toolings above the press table  6 , which will be explained in detail hereinbelow. FIGS. 1 a ) to  1   c ) show toolings with a large tool  5 , a central cushion force K being produced in FIG. 1 a ), an off-center cushion force K which is variable over the cushion stroke being produced in FIG. 1 b ), and an influenceable load distribution K on the drawing pins being produced in FIG. 1 c ). FIG. 1 d ) shows a tooling with 2-3 smaller tools  5  for long, narrow parts and passage in the longitudinal direction with an off-center cushion force K to be produced for one tool. FIG. 1 e ) shows a tool arrangement comprising 2-6 smaller tools  5  in a rectangular mode of arrangement for deep-drawing in the transverse direction of the deep-drawing press with a cushion force K for 1-2 tools. In all these cases, outer cushion cylinders  9  used for taking up the predominantly eccentrically occurring cushion force K are provided, preferably in the corner areas of the drawing cushion  8  (which is not visible FIG. 1, because it is located below the press table  6 , but is shown in other Figures), as will be explained in detail in the following.  
         [0029]    An embodiment of a transfer tool in which only a comparatively low forming force is required is shown in FIG. 2. A sheet-metal workpiece  3 , supported by a blank holder  4 , is formed by pressing the upper die  2 , which is fixed to the press ram  1 , against the lower die  5 . In this case, the outer cushion cylinders  9  are used not only for controlling the position of the drawing cushion  8  but also for providing the whole hold-down force required.  
         [0030]    If the hold-down force required is higher, additional inner cushion cylinders  10  can have pressure applied thereto, as shown in FIG. 3.  
         [0031]    When large sheet-metal workpieces are formed by means of large tools  2 ,  5  in accordance with FIG. 4, the inner cushion cylinders  10  provide most of the hold-down force, whereas the outer cushion cylinders  9  effect position control as well as the pre-acceleration of the cushion.  
         [0032]    In FIG. 5, the lower area of a deep-drawing press for deep-drawing sheet-metal workpieces is shown, this lower area consisting of a press table  6  and an arrangement of outer and inner cushion cylinders  9 ,  10 . Both the outer and the inner cushion cylinders  9 ,  10  have pressure applied thereto via neighbouring compressed-gas containers  12  in a highly dynamic manner.  
         [0033]    As can be seen from FIG. 6, the drawing cushion unit  8  is guided in the press table by the outer and inner cushion cylinders  9 ,  10 . The outer cushion cylinders  9  are connected by a cardanic connection to the drawing cushion  8  by means of bolts  13  through openings in the support  11 .  
         [0034]    The sectional view in FIG. 7 shows the structural design comprising four outer cushion cylinders  9  and three inner cushion cylinders  10 . The inner cushion cylinders  10  need not be fixedly connected to the drawing cushion  8 . If necessary, they are pressed against the drawing cushion by a suitable pre-tensioning pressure.  
         [0035]    [0035]FIG. 8 shows schematically the arrangement for force and load control in the form of a sequence control by measuring the distance at the ram  1  by means of distance measuring elements  13 , as well as the distance and the pressure at all cushion cylinders  9 ,  10 , by means of distance measuring elements  14  and pressure measuring elements  15 .  
         [0036]    As can be seen in the control unit  16  according to FIG. 9, the pressure signals p ascertained, the measured values of the drawing cushion path ski and the measured values of the ram path sst serve, after a comparison with set values, to control the stroke control unit, the tilt control unit in the x-direction and the tilt control unit in the y-direction. They are also used for the purpose of synchronization. The values ascertained are, in turn, used for controlling the valves of the cushion cylinders  9 ,  10 , so as to achieve the necessary position of the drawing cushion  8  as well as the necessary hold-down force.  
         [0037]    On the basis of the individual control possibilities of the stroke control unit and of the tilt control units in the x- and y-directions, a total force of the drawing cushion parallel to the press table and/or the press ram can be produced, even in the case of an inclined position of said press table or press ram in the z-direction. The possibility of controlling the stroke control unit and the tilt control units individually also permits a control of the draw-in of the sheet-metal workpieces by producing a total force of the drawing cushion which is not parallel to the bed or ram.  
         [0038]    It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.