Abstract:
In a method of cutting high-tensile workpieces, in particular sheet metals of martensite steel, the workpiece is subjected to a bending stress prior and during a shearing process. The bending stress causes a substantial component of a tensile force transverse to a desired cutting contour. Tensile stresses are superimposed to the shearing stresses, which favor the beginning of the workpiece separation. The bending stress is preferably selected so that the resulting tensile stress exists at a side facing the cutting tool.

Description:
[0001]    In practice, it is increasingly an objective to separate high-tensile sheet metals that are only a little ductile, or not at all, in particular (martensite) hardened sheet metals (sheet steels), by means of shearing. For example, this is necessary during the manufacture of high-tensile automotive body parts, which are increasingly used. In practice, laser cutting installations are preferably used to separate such high-tensile sheet metals. However, these installations have a limited productivity. 
         [0002]    The attempt to separate such (martensite) hardened sheet metals along predetermined cutting contours by means of punching tools results in extreme wear and tear at the cutting edges of the punching tool. Furthermore, the resulting cutting edges are often imprecise and rough. This disadvantage does not exist when the cutting is performed using laser cutting installations, however, the integration of laser cutting installations into reshaping tools for automotive body parts is at least difficult. Further, reshaping installations, such as step presses set the pace so that only a very short time period is available for performing the cutting processes. 
         [0003]    Therefore, it is an objective of the invention to provide a fast and reliable method of cutting high-tensile sheet metals that provides for an improved lifetime of the used cutting tools. 
         [0004]    This objective is achieved with the method according to claim  1 , as well as with the apparatus according to claim  11 : 
         [0005]    The method according to the invention is based on a conventional shearing process, in which a workpiece is separated by means of introduction of a sufficiently high shearing stress. According to the invention tensile stresses are superimposed to the shearing stresses, which favor the beginning of the workpiece separation. This tensile stress, or a component of the tensile stress, acts transverse to the desired line of bending to be generated. The tensile stress is caused by means of a bending stress of the workpiece. Thereby, the bending stress is preferably selected so that the resulting tensile stress exists at a side facing the cutting tool. 
         [0006]    Through the simultaneous loading of the workpiece to be separated by means of bending stress and shear stress the forming of a separation plane in the workpiece to be separated is substantially aided. A dramatic reduction of the cutting forces as well as a dramatic reduction of the wear and tear of tools results while at the same time improving the quality of the cutting areas. 
         [0007]    Preferably, the bending momentum is set at such a high value that the tensile stress does not yet reach the break limit of the material to be separated. For such a biased material a relatively low shearing stress suffices to start and perform the separation process. 
         [0008]    In a preferred implementation and realization of the invention a sheet metal section on a side of the cutting contour is firmly clamped between a holding-down device and a support surface. The firmly clamped part of the sheet metal is preferably the usable part while the non-clamped part extending beyond the cutting line is preferably the waste piece. The holding-down device is thereby preferably loaded with such a high holding-down force so that the tensile stress introduced into the sheet metal through the bending is drastically reduced in direct vicinity of the cutting contour. It turned out that it is suitable if the ratio between the edge of the holding-down device and the thickness of the sheet metal is about 6. 
         [0009]    The bending of the sheet metal before performing the actual separation process can occur immediately before performing the cutting process. For that purpose, the actual cutting tool can have a projection that touches the sheet metal during the cutting process shortly before the cutting edge of the cutting tool touches the sheet metal. In that case, the distance of the projection from the cutting edge and the time advance with respect to the cutting edge are selected so that the tensile stress in the sheet metal to be separated has at the cutting contour not yet reached the flow stress that is typical for the sheet metal when the cutting edge touches the sheet metal. For that purpose, it turned out as suitable if the distance between the cutting edge and the projection is in a range, which is adjusted to the thickness of the sheet metal and the size of the projection. 
         [0010]    Alternatively, it is possible to implement the pre-bending device separate from the cutting knife. This can be done, for example, with an additional element, which is movable independent from the cutting knife. This element can be used prior to commencing the shearing process for introducing a suitable bending stress and, hence, tensile stress, into the sheet metal. The element in question can be moved in a position-controlled manner as well as in a force-controlled manner. In the first case, it can pre-bend the sheet metal and, then, remain in this position, while the cutting knife separates. Alternatively, the location, i.e., the position of the respective biasing element, can be changed in a controlled manner during the cutting process of the cutting knife. 
         [0011]    In the case of the controlled providing of a force, the pre-bending element is hydraulically adjustable on a case-by-case basis. It is then possible to maintain the pre-bending force constant during the cutting process, or to vary it according to a predetermined time profile, or a profile depending on the position of the cutting knife. All mentioned parameters can be used to optimize the lifetime of tools and the quality of the separation plane to be caused. 
         [0012]    The suitable process can be used in a separate shearing installation as well as in a punching installation, or as part of another forming tool. Accordingly, the apparatus can be implemented as a shearing installation, a punching installation or a part of a forming tool, for example, within a step press for sheet metals. 
         [0013]    When punching thick sheet metals, forces act between the push-rod and the punching tool that vary greatly over time. As long as the material of the workpiece resists the punch a very high force exists, whereby parts of the press are elastically deformed. This concerns the press table, the punching tool, the press supports, the press head piece and, to a certain extent, also the push-rod along the crankshaft and eccentric shaft. When the workpiece gives in under the influence of the punch, the energy stored elastically in the mentioned elements is released in an uncontrolled manner. To better control that the punch suddenly breaks through the workpiece, DE 102 52 625 A1 proposes a system for reducing the cutting punch, in which the tool includes a number of hydraulic cylinders. These can be positioned below, above or on a side of the workpiece. Sensors, such as ultrasound sensors, or sensors that measure the flow speed of the hydraulic fluid flowing out of the hydraulic cylinders, close a valve through which up to now the hydraulic fluid could flow out of the hydraulic cylinders. The hydraulic cylinders are coupled to pressure accumulators, which are under relatively high pressure. Therefore, they generate a high counter force. The force of the punch, which so far has been acting upon the workpiece, is transferred to the hydraulic cylinders when the punches begin to break through the workpiece. 
         [0014]    This approach to the dampening of the cutting shock has been proven true in principle. However, adjusting the sensors for detecting the break through of the workpiece is critical. Further, in an arrangement of the hydraulic cylinders next to the tool a certain cutting shock still exists that is to be reduced further. 
         [0015]    Therefore, it is an objective of the invention to improve the mentioned state of the art. 
         [0016]    This objective is achieved with the press according to claim  20  as well as claim  30 : 
         [0017]    The press has according to claim  1  a device for holding a sheet metal, which presses the tool during the forming process against the lower tool. The lower tool is, for example, a punching tool while the upper tool is, for example, a die. The device for holding a sheet metal is capable of exerting different forces. A control device assigned to the device for holding a sheet metal can influence the force exerted by the sheet metal holding device. 
         [0018]    Usually, the sheet metal holding device includes a holding-down plate, which supports itself against the workpiece. The holding-down plate extends to the immediate neighborhood of the dies (punch dies), and, hence, close to the cut to be made. Thereby, the sheet metal is to be clamped tightly in immediate vicinity of the cut between the holding-down plate and the support tool (punching tool) to achieve a high cutting quality. In the press according to the invention, the sheet metal holding device receives, after the dies break through the workpiece, the force exerted by the push-rod, while it passes through its lower dead center, and temporarily stores the energy released thereby by the push-rod. 
         [0019]    During the return stroke of the push-rod this energy is transferred back to the push-rod, and, therefore, to the drive of the press. Due to the achieved avoidance of the uncontrolled release of the energy elastically stored in the press, the drive of the press as a whole is unburdened, i.e., energy is preserved. Further, the mechanical stress on the press is reduced due to the avoidance of too high a force and sudden changes of force. In addition, the transfer of the force, which is exerted onto the dies up to the breaking through of the workpiece, to the device for holding sheet metals achieves a particularly strong clamping of the workpiece particularly during the break through so that particularly good cutting qualities result. Furthermore, the force can be introduced into the workpiece via the device for holding sheet metals across a particularly large area, and, hence, in a careful manner so that undesired deformations of the workpiece, such as crushing or similar, can be avoided. 
         [0020]    An additional improvement provides claim  30 . It enables in particular to achieve high stroke rates. In the event that the moment the push-rod breaks through the workpiece is recognized, i.e., the moment the workpiece gives in, for example, by monitoring the volume flow of hydraulic fluid released by support devices having hydraulic cylinders, a steep increase of the volume flow occurs the moment the workpiece breaks through. However, the thereby resulting volume flow can actually be below a value, which occurs at high stroke rates when the push-rod reaches the support device, which can be implemented as a device for holding sheet metals. According to the invention, therefore, the control device releases the control of the force provided by the support device only within a predetermined path length of the movement of the push-rod. In this way, errors, which otherwise would lead to a severe malfunction of the press, are safely excluded. 
         [0021]    The predetermined path length advantageously has an adjustable start φ 1  or x 1 . Further, it can advantageously have a variably adjustable end φ 2  or x 2 . Further, its length can be variable and adjustable. 
         [0022]    This allows the press to be adjusted in a simple way to different situations, in particular with respect to the processing speed, or the number of strokes, and the thickness of the sheet metal. 
         [0023]    Advantageously, the device for holding sheet metals or the support device has a hydraulic cylinder that is coupled to a first and a second hydraulic pressure accumulator. Both pressure accumulators have, for example, a movably supported piston having a damped end stop. In the alternative, accumulator devices with membranes may be provided, or storage devices, in which a gas pressure buffer is in direct contact with the hydraulic fluid. Advantageously, both pressure accumulators have different static pressures. The path leading from the hydraulic cylinder to the pressure accumulator having a lower pressure is advantageously regulated by a valve that monitors the fluid flow, i.e., the mass flow, and shuts it down when it exceeds a threshold value. This flow-sensitive valve is an advantageous embodiment of a sensor device for detecting the break through of the workpiece. It indicates when a velocity threshold value of the relative movement between the upper tool (die) and the workpiece is exceeded. Alternative sensor systems can be used, such as acceleration sensors at the push-rod or the die, distance sensors that detect the movement of the push-rod and output a corresponding time-varying signal. The rate of the signal&#39;s change is then determined and used as an indicator for the pressing break through. 
         [0024]    Further, a corresponding device for measuring the distance, or other sensor device can be used for generating a signal, which is then used for setting a trigger window, within which the break through can be expected. Monitoring for detecting the break through occurs then only during this trigger window, while outside this trigger window the support device is passive, or the device for holding the sheet metal exclusively performs its function of holding sheet metals. 
         [0025]      FIG. 17  shows a support for a tool having an intermediate tool plate and an upper tool plate. The holding-down device and the pressure maintaining cylinder  30 , which is also shown in  FIG. 11 , acts as an ejector after traveling the distance delta (A), wherein the holding-down device requires therefore the oil channels  300 . 
       REFERENCE LIST FIG.  15   
       [0000]    
       
         
           
               1  Press, body 
               2  Push-rod 
               3  Holding-down device 
               4  Cutting edges, cutting knives 
               5  Holding-down cylinder 
               6  Sheet metal, workpiece 
               7  Die plate, lower cutting edge 
               8  Oil supply, pump, motor, pressure limiting valve 
               9  High pressure piston storage with end dampening 
               10  Low pressure storage 
               11  Table 
               12  Press frame 
               13  Crankshaft 
               14  Connecting line 
               15  End dampening within the hydraulic storage high pressure 
               16  Flow valve, jetcon valve 
               17  Pressure switch 
               18  Flywheel 
               19  Main motor 
               20  Drive belt 
               21  Oil, fluid 
               22  Nitrogen, low pressure 
               23  Nitrogen, high pressure 
               24  Set back die  1   
               25  Set back die  2   
           
         
       
     
     
    
     
         [0051]    Further details of advantageous embodiments of the invention result from the drawing, the description or the claims. In the drawing, embodiments of the invention are depicted. In the drawing: 
           [0052]      FIG. 1  shows a schematic front view of a sheet metal step press, 
           [0053]      FIG. 2  is a top view of a schematically depicted workpiece, 
           [0054]      FIGS. 3 to 4  show a tool for separating sheet metal parts for the step press according to  FIG. 1  in a schematic vertical cross sectional illustration in various process stages during separating the sheet metal, 
           [0055]      FIGS. 5 to 8  show an alternative embodiment of the tool for the step press according to  FIG. 1  in a schematic vertical cross sectional illustration in various process stages and 
           [0056]      FIG. 9  shows the tool during the separation process in an enlarged, sectional and schematic illustration. 
           [0057]      FIG. 10  shows the press according to the invention in a schematic overview illustration, 
           [0058]      FIG. 11  shows the tool of the press according to  FIG. 1  in a schematic, vertical cross sectional illustration, 
           [0059]      FIG. 13  shows a valve for monitoring a flow of a hydraulic fluid generated by the tool according to  FIG. 2 , and 
           [0060]      FIG. 14  shows diagrams illustrating the dependence of the push-rod stroke from the press angle, as well as from the mass flow displaced from the sheet metal holding apparatus 
       
    
    
       [0061]      FIG. 1  illustrates a step press  1  for forming sheet metal parts with respect to a tool  2  provided for that purpose. The tool  2  is for performing at least one separating process and eventually for additional forming processes. Further, the step press  1  can include additional tools, wherein the workpiece  3  (schematically illustrated in  FIG. 2 ) is then handed from one tool to the other. A transfer device provided for that purpose is not shown. 
         [0062]    The tool  2  has a lower tool mounted stationary on a press table  4 , and an upper tool  7  mounted to the push-rod  6  of the press. The tool  2  opens and closes in accordance with the work cycle of the press, wherein each time one workpiece is processed. When the tool  2  closes, the upper tool  7  touches the lower tool  5 . 
         [0063]    During the processing of the workpiece  3  a cutting process needs to be performed on it, namely along a cutting contour  8  schematically illustrated in  FIG. 2 . The contour can be limited in a straight or curved line. Due to performing the cutting process, the tool  2  has a cutting device  9 , as shown in  FIG. 3 . A tool-clamping device  10  and a cutting knife  11  belong to it. The cutting knife  11  can be referred to as cutting die. It is mounted stationary to the upper tool  7 , or is a part of it. A cutting edge  12  is provided on the cutting knife  11  that determines the form of the cutting contour  8  and corresponds to it. The cutting edge  12  is limited by two surfaces  13 ,  14 , which are preferable positioned to each other at an angle of a little less than 90°, i.e., an acute angle. 
         [0064]    The device  10  for clamping the workpiece is formed by a workpiece support  15  with a surface  16  facing the workpiece  3 , as well as a sheet metal holder  17 . The workpiece support  15  is coupled to the lower tool  5 , or is a part of it. It ends at the cutting contour with a corresponding edge  18 , at which the bordering surfaces form a right angle or a somewhat acute angle with each other. The sheet metal holder  17  is supported by the upper tool  7  and biased against the workpiece  3  by means of, for example, a hydraulic biasing device. During lowering the upper tool  7  the sheet metal holder  17  touches the workpiece  3  before the cutting knife  11  touches the workpiece  3 . Thereby, the sheet metal holder  17  presses the workpiece  3  strongly against the support surface  16 . 
         [0065]    To the cutting apparatus  9  belongs further a pre-bending device  19 , which serves and is configured for providing a bending load to the workpiece  3  before and eventually during the performance of the cutting process. The pre-bending device  19  is in its simplest embodiment, as illustrated in  FIGS. 3 to 6 , formed by a protrusion  20 , which is formed at the cutting knife  11 , or a cutting die protruding towards the workpiece  3 . It extends beyond the surface  14  and the cutting edge  12  towards the workpiece  3  to such an extent that it touches the workpiece  3  before the cutting edge  12 . The protrusion  20  is preferably a rib provided at a predetermined distance a to the cutting edge  12 . Preferably, the protrusion  20  has the same height h along its length, wherein the height h, as indicated in  FIG. 1 , is measured with reference to the cutting edge  12  or a plane that is arranged parallel to the support surface  16  and defined by the upper side of the workpiece  3 , on which the cutting edge  12  rests in  FIG. 3 . 
         [0066]    The ratio of the distance a (l) to the height h (s) is preferably six times the thickness of the sheet metal. 
         [0067]    The protrusion  20  must not necessarily be configured as a rib. It is also possible to provide a number of individual protrusions. Further, they can be individually adjustable in height to allow suitable adjustments depending on the specific form of the cutting contour  8 . For example, it can be suitable to set a different height h in areas in which the cutting contour  8  has a more narrow curvature. 
         [0068]    The tool described so far works as follows: 
         [0069]    During a cutting process the upper tool  7  moves towards the lower tool  5 . Then, the sheet metal holder  17  touches the workpiece  3  and presses it with great force against the surface  16 . Between the sheet metal holder  17  and the area  13  of the cutting knife  11  a small gap remains, which ideally corresponds approximately to the cutting gap defined between the edge  18  and the area  13  of the cutting knife  11 . 
         [0070]    While the sheet metal holder  17  rest on the workpiece  3  the upper tool  7  moves further down. Then, the protrusion  20  touches the workpiece  3  and bends it during its further downward movement. Due to this bending a high bending stress results, in particular directly next to the sheet metal holder  17 , i.e., in the area of the cutting contour to be created. With further downward movement of the cutting knife  11  the tensile stress increases, however, without reaching the flow limit of the material. Then, as illustrated in  FIG. 7 , the cutting edge  12  touches the upper side of the workpiece  3  and begins to apply a shearing stress to it. This results together with the already initiated tensile stress in a comparative tension that leads to the separation of the material. This is illustrated in  FIG. 8 . With the beginning of the separation process the so far bent section of the sheet metal relaxes again. 
         [0071]    The cutting process described hereinbefore coincides with weak forces at the cutting edge  12  and with little wear and tear of the tool. High qualities of separation or cutting surfaces are achieved. The resulting separation plane  21  can cut through individual grains or crystallites  22  of the workpiece  3 , as shown in  FIG. 9 . This is possible because of the loading with very high tensile stresses, indicated in  FIG. 9  through arrows  23 ,  24 , which are preferably higher than the introduced shearing stresses (arrows  25 ,  26 ). This leads to very smooth cutting or fractured surfaces. 
         [0072]      FIGS. 5 to 8  illustrate a more detailed embodiment, wherein the cutting knife  11  and the bending apparatus  19  are configured to be independent of each other, or separate. The bending apparatus  19  is formed by a separate biasing element  27 , which is arranged in vicinity of the cutting knife  11  and at a distance to its cutting edge  12 . The bending element  27  can, for example, be connected with a biasing device with the upper tool  7 . For example, the biasing device can be a hydraulic cylinder under a predetermined pressure that biases the bending element  27  with a defined load to bend, i.e., to preload, the workpiece  3  in a defined manner. The workflow is then as follows: 
         [0073]    After the placing the sheet metal holder  17  and its tensioning towards the workpiece  3 , the pre-bending element  27  touches the workpiece  3 , as illustrated in  FIG. 5 . While the upper tool  7 , and with it the cutting knife  11 , moves further towards the workpiece  3  the biasing device assigned to the bending element  27  builds up a force. This phase is illustrated in  FIG. 5 , in which the workpiece  3  is not yet substantially bent. 
         [0074]    At the latest, when the cutting edge  12  touches the workpiece  3 , as illustrated in  FIG. 7 , the bending element  27  provides the necessary biasing force to bend or bias the workpiece  3  right before the flow limit of the material. The shearing off, as illustrated in  FIG. 8 , occurs then with an even lower shearing force in that the cutting knife  11  penetrates the material of the workpiece  3  at the cutting line  3  and introduces a shearing stress. 
         [0075]    As in the embodiment described in the foregoing, here, the majority of the tension required for separating the material is provided by the bending device  19 , while the lesser share of the material tension is introduced by the cutting knife  11 . The contribution of the bending device  19   
         [0076]    If the tensile stress exceeds the flow limit it can disadvantageously result in micro cracks in the material. As a precaution, one does not set the break limit, but the flow limit. 
         [0077]    to the comparison tension σν is thereby greater than the contribution of the cutting knife  11 . It is: 
         [0000]      σν=√σ 2 +3τ 2    
         [0000]    wherein σν is the comparison tension, σ the tensile stress, τ the shearing stress, and wherein 3τ 2 &lt;σ 2 . 
         [0078]    As an alternative to the embodiment described with reference to  FIGS. 5 to 8 , it is also possible to modulate the force provided by the bending element  27  during the cutting process, i.e., to influence it in a controlled manner depending on a set function of time, or a set position of the cutting knife  11 . Thereby, the quality of the cut can be further influenced. For example, it is possible to substantially reduce the bending force after the separation process has been introduced. It is further possible to use specific increasing and decreasing forces, or to vary the bending force along the desired cutting contour  8 . For example, the bending force can be set to be greater in concave areas of the cutting line than in convex areas of the cutting line, or other profiles may be applied. 
         [0079]    A method according to the invention for cutting high-tensile, in particular non ductile sheet metals, in particular sheet steels, is a combination of a breaking process and a cutting process. In this process, the material to be separated is biased nearly up to its flow limit, and is, then, subjected to a cutting process. A substantially increased lifetime of the tool results together with a substantially improved surface quality of the separation surface compared to pure breaking or pure cutting processes. 
         [0080]      FIG. 12  describes an apparatus for cutting high-tensile sheet metals, also referred to as bending cutting. 
         [0081]    State of the art: Sheet metals are cut in die plates by means of cutting apparatuses and presses having cutting dies. With high-tensile, in particular martensite hardened sheet metals, an extreme wear and tear results at the cutting edges when using conventional cutting structures. 
         [0082]    The disadvantages of the existing technique are solved in that the cutting die is provided with a chamfer whose task is, before the cutting edge sets down for shearing, to pre-bend the sheet metal up to the break limit under tensile stress, and then only to shear by means of providing a little push. The cutting structure by means of the push has the task to exactly cut the sheet metal precisely at the predetermined line. The shear fracture as such is virtually assisted by a predetermined bending pre-stress. From the hypotheses of the change of structure, the comparative tension is the root of the bending stress squared, added with three times the shearing stress squared. That means that the shear stress is significantly improved by the bending shearing. Experiments have shown that this system allows handling of even complex cuts using lasting knives.  FIG. 12  shows the cutting structure for controlled bending cutting.
         1 . Cutting die, cutting knife,     2 . Cutting matrix     3 . Holding-down device     4 . Sheet metal     5 . Bending edge     6 . Cut     7 . Die plate     8 . Bending angle     9 . Bending stress     10 . Equation of the hypotheses of the change of structure     11 . Break by push     12 . Remaining tension with remaining push     13 . Shearing stress, remaining break       
 
         [0096]    Device or method for cutting of high-tensile, nearly non-ductile sheet metals with a reduced required cutting force, characterized in that the sheet metal is biased, prior to the shearing, by a pre-bending stress provided by applying a bending edge, wherein the cutting die or the cutting knife are provided with a chamfer for pre-bending, wherein the pre-bending is provided by a separate pre-bending rail, wherein the die is slightly negatively dressed to size, wherein a contact edge  6  is maintained at a defined distance, wherein the contact edge is concave with respect to the cutting edge. 
         [0097]    In  FIG. 10 , in a highly schematic illustration, a press  1  is depicted, which has a press structure with press supports  2 ,  3 , a press table  4  and a head piece  5 . To the head piece  5 , a drive  6 , for example, in form of an electric motor, is mounted, which drives a back and forth moving push-rod via a schematically and in dashed lines depicted eccentric  7  and a crankshaft  8 , as well shown in dashed lines. Between the push-rod  9  and the press table a tool  10  is provided having an upper tool  11  and a lower tool  12 . The lower tool  12  is configured as a punching tool. The upper tool  11  holds dies  13 ,  14 ,  15 , which are shown in particular in  FIG. 2 , just like the other details of the tool  10 . The tool  10  is for punching a workpiece  16 , which is shown in  FIG. 2  as a flat workpiece. Of course, non-flat workpieces can in a corresponding manner be subjected to a punching process as well. In this case, then, the lower tool  12  has a contour that corresponds to the non-flat workpiece. 
         [0098]    A sheet metal holding plate  17  belongs to the upper tool  11  that is held by a basic body  18  of the upper tool  11  by means not shown in detail. The basic body  18  connected to the push-rod  9  carries the dies  13  to  15 , so that they are rigidly connected with the push-rod  9 . The basic body  18  has further one or more hydraulic cylinders  19 ,  20 , which together with the sheet metal holding plate  17  form a sheet metal holding apparatus  21 . To the sheet metal holding apparatus  21  belong further pressure pins  22  to  27 , which are arranged substantially or precisely parallel to the dies  13  to  15  and rest with their lower frontal end on the sheet metal holding plate  17 . The remaining substantially cylindrical pins, at their upper frontal ends, are supported by suspended intermediate plates  28 ,  29 , which therefore rest on top of the pressure pins  22  to  27 . The hydraulic cylinders  19 ,  20  have pistons  30 ,  31 , which limit and seal in the hydraulic cylinders  19 ,  20  working volumes  32 ,  33  filled with hydraulic fluid, and which are movably mounted therein. Piston rods  34 ,  35  of the pistons  30 ,  31  press from above against the suspended intermediate plates  28 ,  29 , and thereby the sheet metal holding plate  17  against the workpiece  16 . 
         [0099]    The hydraulic cylinders  19 ,  20  are via a fluid line  36 , which is not shown  FIG. 11 , but schematically illustrated in  FIG. 10   a, b , connected to a hydraulic system  37 , which is for generating a force of the sheet metal holder and taking over the force exerted by the push-rod  9  during and after the breakthrough of the workpiece  16 . This transfer of a force has to occur, if possible, in a steady manner, i.e., without stepwise changes of force. 
         [0100]    To the hydraulic system  37  belong a first pressure accumulator  38  and a second pressure accumulator  39 , which are both configured in one embodiment as pressure accumulator cylinders  40 ,  41  with pistons  42 ,  43  sealed and movable mounted therein. Both pistons  42 ,  43  divide in the pressure accumulator cylinders  40 ,  41  two working chambers each, whose upper ones are each filled with gas. The pressure accumulator  38  is, for example, under a pressure of about 200 bar while the pressure accumulator  39  is, for example, under a pressure of 400 bar. 
         [0101]    The pistons  42 ,  43  have on their bottom side, respectively facing the connecting pieces  44 ,  45 , preferably a profile structure that is configured to be complementary to a profile structure of the respective connecting piece  44 ,  45 . 
         [0102]    The profile structure is formed by straight or curved, for example, ring shaped concentric ledges or rails, wherein the ledges or rails of each piston  42 ,  43  fit into correspondingly shaped recesses of each connecting piece  44 ,  45 . The profile structures serve as end point dampening so that the pistons  42 ,  43  are softly slowed down when approaching the connecting pieces  44 ,  45 . 
         [0103]    Both pressure accumulators  38 ,  39  are connected with the fluid line  36 . The pressure accumulator  39  is preferably connected with the fluid line  36  via a check valve  46  and a throttle device  47 . The check valve  46  is thereby positioned so that the hydraulic fluid can flow undisturbed from the hydraulic line  36  into the pressure accumulator  40 , while being forced through the throttle device  47  on its way back. 
         [0104]    The pressure accumulator  38  is connected with the fluid line  36  via a valve device  48 , and, hence, with the hydraulic cylinders  19 ,  20 . The valve device  48  contains, e.g., a directional control valve  49  that can be switched between two states. In a first state, it passes the fluid flow in and out of the pressure accumulator unlimited or throttled, while it blocks this fluid flow in its other state. The valve device  48  can be connected with a sensor device  50 , which monitors, for example, the mass flow m′ in the fluid line  36 , and interrupts it as soon as the hydraulic flow into the pressure accumulator  38  exceeds a threshold value m th , and keeps it closed until the pressure in the fluid line  36  falls below a threshold value. 
         [0105]    The sensor device  50  forms therefore at the same time a control unit  51  for controlling the valve device  48  depending on the speed of the relative movement between the pistons  28 ,  29  of the hydraulic cylinders  19 ,  20  and the push-rod  9 . 
         [0106]    A bypass valve  52 , which bypasses the valve device  48  and, hence, provides for an alternative path from the hydraulic cylinders  19 ,  20  to the pressure accumulator  38 , may, as needed, belong to the valve device  48 . The bypass valve  52  is, for example, an open/close valve that can be controlled in an electric-pneumatic manner, or else. It is therefore preferably connected to a control apparatus  53 , which can be preferably configured as microcontroller or other suitable electronic controller. Besides other input signals, the control apparatus  53  receives at least one position signal. This may be caused, for example, by a sensor  54 , which detects, as a displacement sensor, the position of the push-rod  9 , in particular in proximity of its lowest end point. In addition, or in the alternative, a sensor  55  can be provided that measures the angle position of the eccentric shaft at least in one angle of rotation range, in which the push-rod  9  is in proximity of its lower end point. 
         [0107]      FIG. 13  illustrates one embodiment of the valve device  48 , which is preferred because of its fast reaction time. It has a base  56  having at least one input  57  and one output  58 . Between them, a channel  59  is formed that extends longitudinally through the base  56 , and that is transverse to the channels of the input  57  and the output  58 . Between the input  57  and the output  58  a valve seat  60  is formed, to which a valve end member in form of a disc  61  is assigned. The latter one sits on a pin and is biased in an opening direction away from the valve seat  60  by means of a spring. The bias can be adjusted, if needed, by means of a handle accessible from the outside, for example, an adjustment screw  62 . 
         [0108]    The press  1  described so far operates in a first simple embodiment that functions generally without the control apparatus  53 , as follows: 
         [0109]    For the illustration of the function a single punch stroke is described. For its performance, the workpiece  16  is first placed on the lower tool  12 , and, then, the push-rod  9  is lowered. The sheet metal holder plate  17  is thereby in its lowest position, in which its lower side is at least a bit below the front surfaces of the dies  13 ,  14 ,  15 . Before the sheet metal holder plate  17  touches the workpiece  16  the pistons  30 ,  31  in the hydraulic cylinders  19 ,  20  do not move. The hydraulic fluid in the hydraulic system  37  is under a static pressure. 
         [0110]    As soon as the sheet metal holder plate  17  touches the workpiece  16  it presses the workpiece  16  against the lower tool  12 . The sheet metal holder plate  17  comes to a rest while the push-rod  9  moves further towards the workpiece  16 . Further, the pressure pins  22  to  27 , the suspended intermediate plates  28 ,  29  and the pistons  30 ,  31  come to a rest. Following the further downward movement of the push-rod  9  the volume of the working chambers  32 ,  33  is reduced, and the hydraulic fluid is forced via the fluid line  36  and the open directional control valve  49  of the valve device  48  into the pressure accumulator  38 , which has a lower static pressure than the pressure accumulator  39 . Hence, the piston  43  in  FIG. 1  is moved upwards against the force of the upper gas cushion. The resulting fluid flow m′ is below a threshold value so that the sensor device  50  is not activated. 
         [0111]    Then, the front surfaces of the dies  13 ,  14 ,  15  touch the workpiece  16 . The workpiece  16  puts up a substantial resistance against the penetration of the dies  13 ,  14 ,  15 , so that the movement of the dies  13 ,  14 ,  15  stops at first. The drive power of the drive device  6  is then temporarily used to elastically deform, i.e., tension, the power train and the press structure together with the press table  4  and the lower tool  12 . Thereby, more and more an increasing force is built up until the dies  13 ,  14 ,  15  finally break through the workpiece  16 . At the time of the break through a very fast relative movement between the base  18 , and, hence, the hydraulic cylinders  19 ,  20 , and the sheet metal holder plate  17  results. This leads to a short-term, very strong increase of the mass flow m′ of the hydraulic fluid from the hydraulic cylinders  19 ,  20  into the pressure accumulator  38 . The increase is so steep that the sensor apparatus  50  detects it and closes the directional control valve  49 . In the embodiment according to  FIG. 3 , this means that the fluid flow, which flows from the input  57  to the output  58 , takes the disc, i.e., the valve closure element  61 , with it and presses it against the spring force against the valve seat  60 . The directional control valve, therefore, closes suddenly, whereby the closed state is maintained until a decreasing system pressure allows the valve closure element  61  to return to its open state, i.e., static position. 
         [0112]    Is the directional control valve  49  closed no additional hydraulic fluid can flow into the pressure accumulator  38 . Therefore, it needs to give way into the pressure accumulator  39 , which is at a substantially higher pressure. Hence, the hydraulic cylinders  19 ,  20  cause a substantial counter pressure that acts, on one hand, upon the sheet metal holder plate  17 , and, on the other hand, counters the push-rod  9 . Thus, the force up to now absorbed by the dies  13 ,  14 ,  15  is transferred to the sheet metal holding device  21  so that the tensioned press cannot relax. Against the strong force of the sheet metal holding device  21  the push-rod passes through its lower dead point, wherein the sheet metal holding device pushes the push-rod  9  with great force upwards during the first section of the upward stroke. In this phase, the elastic energy stored in the press  1  is transferred to the push-rod  9 , and, hence, returned to the drive apparatus  6 . 
         [0113]    A more elaborate embodiment uses the control apparatus  53  for controlling the sheet metal holder device or an alternative support device, for example, in form of hydraulic cylinders between the push-rod  9  and the press table  4 , or the upper tool  11  and the lower tool  12 . The control apparatus  53  monitors the position X of the push-rod  9  or the rotational angle φ of the drive apparatus  6 , i.e., the eccentric  7 . The relationships are illustrated in  FIG. 4 . Thereby, a press with a high stroke number is assumed. A first graph I illustrates the path X of the push-rod  9  over the rotational angle φ of the eccentric shaft. An approximately sinusoidal relationship is assumed. With a press angle (PO, the sheet metal holding plate  17  touches the workpiece. 
         [0114]    The graph II illustrates the mass flow of the hydraulic fluid displaced from the hydraulic cylinders  19 ,  20 . As can be seen, it increases stepwise to a relatively high value when the sheet metal holding plate  17  touches the workpiece  16 . When the push-rod  9  approaches its lower dead center the mass flow m′ continues to decrease further. This is because the speed of the push-rod  9  decreases when approaching the lower dead center. As a consequence of the resistance the material of the workpiece  16  puts up against the punching process, the push-rod  9  is further slowed down, whereby the mass flow strongly decreases according to graph II. 
         [0115]    At a press angle φ 1 , which exists with certainty after the sheet metal holding plate  17  touches the workpiece  16 , and with certainty before the break through of the dies  13 ,  14 ,  15  through the workpiece  16 , the control device  53  closes the bypass valve  52  activating the sensor device  50 . Instead of the crank handle or press angle (PI the push-rod  9  passing through the point x 1  can be used as a criterion for releasing the sensor device  50  and the valve device  48 . However, monitoring the press angle is preferred because it provides a better resolution. 
         [0116]    If, after activation or release of the sensor device  50 , or after passing through (PI, the break through of the workpiece  16  occurs, the fluid flow m′ exceeds a threshold value m′ th . This is illustrated in  FIG. 4  by means of the tip III in graph II. An exceeding of the threshold value of the fluid flow is detected that leads to the closure of the valve device  48 , as described above, and, hence, to the support of the push-rod  9  on the sheet metal holding device  21 . 
         [0117]    As can be seen in  FIG. 14 , by defining the activation window between the press angles φ 1  and φ 2  it can be achieved that flow peeks are detectable that are lower than the flow immediately after the sheet metal holder plate  17  touching the workpiece  16 . This plays a roll with very fast presses (high stroke numbers), large punch strokes, and, in particular, very stiff press structures, in which a very high tension force, but only a low tension change occurs in the whole press structure. Due to the limitation of the monitoring of the fluid flow to an angle window φ 1 , φ 2  of the press drive, within which the break through is to be expected, the activation threshold m′ th  for the valve device  48  can be set very low so that the otherwise noticeable punch is limited to a barely noticeable minimum. 
         [0118]    Instead of the sensor device  50 , which monitors the fluid flow from the hydraulic cylinders  19 ,  20 , other sensor device can be used as well. Further, it is possible to define the press angles φ 1 , φ 2  variably. For example, they may be entered by means of a suitable input device. It is further possible to adjust these press angles φ 1 , φ 2  dynamically. This can be done, for example, by setting φ 1  to an angle distance before the breakthrough that is predetermined or can be input, and φ 2  to an angle distance after the breakthrough that is fixed or adjustable. As press angle of the break through the respective press angle of the previous punch stroke, or an average of previous punch strokes is used. It is also possible to provide the press structure, the press table or other parts of the press with force sensors, which react in response to a deformation of the respective press element, or directly to the force acting upon the press. For example, these may be force sensors in the tool  10 . The signals generated by these sensors can be fed to the control device  53  and can be used to define the press angles φ 1 , φ 2 . Is, for example, the force acting upon the dies  13 ,  14 ,  15  detected, the sensor device  50  can be released at a moment, i.e., then released, when a significant increase of force is detected at the dies  13 ,  14 ,  15 . At this moment, no erroneous release of the valve device  48  is to be worried about because the relative movement between the sheet metal holding plate  17  and the dies  13 ,  14 ,  15  is close to zero. 
         [0119]    The system according to the invention provides for a substantial increase of the holding-down force, in particular during the performance of the punching, i.e., while the dies  13 ,  14 ,  15  penetrate through the material of the workpiece. The actual cutting force can thereby be reduced up to a sixth of the theoretical shear force. The sheet metal holding device  21  causes a particular strong clamping of the workpiece  16 , and, hence, causes an improvement of the cut and a dampening of the cutting punch as well. The press  1  is biased so that plays are balanced or compensated. With respect to conventional systems for dampening the cutting punch, this leads to a reduction of the total press force of the system. This means further that older presses can continue to be used even for difficult separation tasks. 
         [0120]    The force exerted on to the sheet metal holding plate is preferably set to 40% of the press force. The separating process can be monitored, evaluated and controlled by means of the use of a fast evaluation and control device, such as the control device  53 . The system can essentially be configured and used to be autarkic, i.e., independent of the press  1 . For example, it can be part of the tool, and, hence, used in principle with different presses. When changing the press data, press specific parameters can be changed by means of a program or system specific flash cards. 
         [0121]    The pressures in the hydraulic cylinders  19 ,  20  can be permanently monitored depending on the encoder or path. The resulting envelopes allow a permanent monitoring of the process. The control of the bypass valve  52  occurs shaft-angle dependent or path dependent via the same system. The process data and failures can be stored by means of data storage systems and tracked back in case of damage. Further, systems for detecting overloads can be provided. 
         [0122]    To improve the cut quality, in particular when punching high tensile, austenitic materials or thick sheet metals, a sheet metal holding device  21  is provided that rigidly clamps the respective workpiece  16  during the punching process. The tension force is increased up to 40% or more of the force of the push-rod. In particular, the force exerted by the sheet metal holding device can be increased during the breakthrough of the workpiece. On one side, the cut quality is thereby improved while on the other side an efficient dampening of the cutting punch at the press results. 
         [0123]    The apparatus is in particular for avoiding the cutting punch during the breakthrough through the workpiece. 
         [0124]    When cutting sheet metals the press deflects. That is, with hard sheet metals, due to the release of the cutting energy the total deflection is transformed into oscillation energy. This leads in particular with hard sheet metals or sheet metals of higher strength, as used in the future in particular in vehicle construction, to an early destruction of the components of the installation. The braced frame and the components connected therewith: eccentric shaft, flywheel mount, crankshaft, push-rod, table, tools, guides, tie-rod, base, threading dies, die plate are extremely in danger due to the rapid relaxation. 
         [0125]    State of the art: With cutting punch cylinders oil is sprayed over throttles or pressure limiting valves after the setting down or during the breakthrough. The energy released thereby is completely transformed into heat. It is a disadvantage that energy is thereby wasted. In addition, a holding down device is required that presses the sheet metal against the die plate so that it does not bend towards the threading dies. This leads to an additional increase of the press force. 
         [0126]    An advantage of the new invention is that the holding down device acts at the same time as an absorber of the cutting punch. The deflection energy is only stored temporarily in the high pressure accumulator. The holding down cylinders serve at the same time for maintaining the pressing force constant. The deflection energy is controlled so that it does not lead to an oscillation of the press. The characteristic curve of the deflection and unloading is manipulated from one of a cutting press to one of an embossing press. It is an advantage thereof that the deflection energy of mechanical presses is fed back to the flywheel. With hard sheet metals about 50% of the cutting force is required for the holding down force. With these cutting methods about 60% of the ancillary forces can be avoided. The conventional dampening of the cutting punch requires about 60% of the press force as counter force; 50% of the cutting force as holding down force. If the cutting die broke through the workpiece the high pressure counters via the nitrogen biasing. That means that the maintenance of a constant pressing force acts against the pressing force only when the workpiece already broke through. This does not act as an increase of the pressing force. Assuming that because of high tensile or ultra-high tensile sheet metals—strength about 1200-1900 N/mm2—the pressing force will in the future increase 4-5 fold, it will be necessary to household with the force. 
       DESCRIPTION OF THE INVENTION 
     FIG.  15   
       [0127]    A press having a flywheel ( 18 ) is driven by one or more motors ( 19 ). Via a clutch and eccentric drive the push-rod is, via one or more con-rods ( 13 ), moved towards the table ( 11 ) in nearly parallel manner. At the push-rod ( 2 ), preferably the cutting tool ( 4 ) with a holding-down device is provided. The holding-down device ( 3 ) provides that the sheet metal ( 6 ) does not deflect during the cutting process. The cylinder piston acting upon the holding-down device ( 3 ) holds the sheet metal ( 6 ). The holding down device/piston is biased by means of the low pressure accumulator ( 10 ), or the oil exerts via the nitrogen compression via the oil a predetermined holding down force on the holding-down device. The adjustment occurs via the nitrogen loading pressure at the store ( 10 ). This one can also be biased by the spring force/piston unit. If the sheet metal breaks through the flow valve ( 16 ) closes. This valve ( 30 ) is preferably provided with a low-mass punch plate ( 31 ). For maintaining the pressing force constant the break through is controlled. The controlled break avoids the relaxing of the base and the elements connected therewith, such as the deflection of the table ( 11 ), push-rod deflection, con-rod deflection ( 13 ) . . . . After passing through the lower dead center, the compressed energy of the high pressure accumulator ( 9 ) is fed back to the flywheel via the holding down cylinder. The Jetcon valve ( 16 ) serves as fast-switching valve with flow limitation. 
         [0128]    Apparatus for the cutting of workpieces with a low cutting punch, wherein one or more holding down cylinders act at the same time as counter holders, wherein the deflection energy is fed from the stores back into the drive, wherein the Jetcon valve is actively pre-activated. The punch plate of the Jetcon valve is made of a light-weight, high-tensile material. The holding down cylinders are used for the parallel holding. In  FIG. 17 , the holding down devices  30 , which are also shown in  FIG. 11 , act also as ejectors by means of the oil overflow channels  300 . The low pressure accumulator is used for controlling the off-mean pressing force. The feeding back is monitored via angular acceleration sensor in the flywheel. 
         [0129]      FIG. 16  schematically illustrates a die  11 , which has a so-called variable cutting gap (u 1  . . . u 2 ) between the die  11  and die plate  3 . The cutting gap varies between u 1  and u 2  between 2% to 10% of the sheet metal thickness, but preferably from 2% to 6%. At u 1  the cutting gap is about 2% of the sheet metal thickness to be cut, and at u 2 —at the widest distance—the cutting gap is about 6% of the sheet metal thickness to be cut. As already mentioned in the Handbook of Deformation Technology (Springer Publishing, 1996, page 281) “a large cutting gap generally reduces the required force and effort.” Therefore, the press or cutting force depends also from the cutting gap. In the manner shown in  FIG. 16 , by means of using the variable cutting gap, the press or cutting force can be reduced about one half. This variable cutting gap technique can be used for any geometry in that, as shown in the lower illustration of  FIG. 16 , the u 1  . . . u 2  sections are arranged in segments around the circumference of the die  11 . During cutting, using the variable cutting gap. the sheet metal to be cut is first cut around the u 1  zones, wherein thereby caused cracks spread towards the u 2  zones, which then leads to a—slightly delayed—complete cut. According to the above-mentioned Handbook, page 275, “a reduction of the required cutting force is possible, if instead of a die with even working surface a beveled, shearing cutting die is used.” On page 276, a reduction of the cutting force by at least 30% compared to dies (die plates) with even surface is mentioned. In an alternative embodiment, the variable cutting gap is combined with the technique of the beveled die to achieve a further reduced cutting force. In particular, it is foreseen to bevel the areas starting from the u 1  edges in x-direction to the u 2  areas, which can be seen in the sideview of the die  11  in  FIG. 16 . In a further preferred embodiment, it is foreseen to clamp the sheet metal to be cut by means of the holding-down device  3  shown in  FIG. 12  so that a notch is caused at the place of clamping. In this manner, the required cutting force is reduced by a factor of about 1.2 to 2.2. The so-called bending cutting or hard cutting shown in  FIG. 3 , wherein the lever  1  corresponds to the three to eightfold of the sheet metal thickness to be cut, preferably the six fold of the sheet metal thickness to be cut, is suitable for reducing the cutting force to about one sixth of previously known values. In a preferred embodiment it is foreseen to combine the previously described bending cutting or hard cutting with the equally above described technique of the variable cutting gap to reduce in this manner the cutting force to about one ninth of the known values. In a further embodiment according to the invention, it is foreseen to combine the variable cutting gap, the bending cutting or hard cutting and the pulling cut (slanted die) to reduce the cutting force to about one twelfth of the usual values “because the cutting force may not exceed the nominal pressing force mentioned on the power label of the press within the nominal operational path since this may cause the machine to be overloaded.” (Handbook on page 274.) The initially set objective is solved by means of the above improvements. Q.e.d.