EP1782897B1 - Press with cutting shocks damping. - Google Patents

Description

The invention relates to a press, which is particularly adapted for cutting thick and / or high-strength sheets, and a method for operating such a press.

When punching or cutting high-strength sheets occur between the ram and punching tool very temporally very fluctuating forces, which can change in particular abruptly. As long as the material of the workpiece resists the punch, a very high force is present, whereby parts of the press are elastically deformed. this concerns the press table, the punching tool, the press stand, the press head piece and, to some extent, the push rod together with the connecting rod and eccentric shaft. If the workpiece gives way under the action of the punch, the elastically stored in the said elements energy is released relatively uncontrolled.

To better control this process, suggests the DE 102 52 625 A1 a system for cutting impact reduction, in which a number of hydraulic cylinders are provided in the tool. These can be arranged below, above or to the side of the workpiece. Sensors, such as ultrasonic sensors, or sensors that measure the flow velocity of the hydraulic fluid flowing out of the hydraulic cylinders, close a valve through which hydraulic fluid could hitherto flow out of the hydraulic cylinders. The hydraulic cylinders are connected to pressure accumulators, which are under relatively high pressure. Therefore, they now generate a high drag. The force previously exerted by the punch on the workpiece is thus transferred to the moment in the hydraulic cylinder, in which the stamp begin to break through the workpiece.

Similar presses and methods with flow-dependent switching valves are also from the non-prepublished documents WO 2005/120741 A2 and EP 1 602 419 A1 and also from the DE 28 04 185 A1 known.

This way to cut impact damping has basically preserved. However, the adjustment of the sensors to detect the workpiece breakthrough is critical. Also, with the arrangement of the hydraulic cylinder next to the tool still a certain cutting blow exists, which should be further reduced.

The US 5,673,601 describes a damping device for a press to reduce noise and vibration. The press angle is measured and a noise measurement is carried out at certain press angle values. The damping device is then activated as a function of the noise measured values.

On this basis, it is an object of the invention to improve the said prior art.

• Die Presse weist nach Anspruch 1 eine Abstützeinrichtung auf, die zwischen Stößel und Pressentisch wirksam ist und eine zwischen diesen wirksame Kraft erzeugt. Z.B. ist die Abstützeinrichtung Teil der Blechhaltereinrichtung, die das Werkzeug während des Umformvorgangs gegen das Unterwerkzeug presst. Das Unterwerkzeug ist beispielsweise ein Stanzwerkzeug während das Oberwerkzeug beispielsweise ein Stempel ist. Die als Abstützeinrichtung ausgebildete Blechhalteeinrichtung ist in der Lage, unterschiedliche Kräfte auszuüben. Eine der Abstützeinrichtung zugeordnete Steuereinrichtung kann die von der Abstützeinrichtung ausgeübte Kraft beeinflusen.
• Die Steuereinrichtung beeinflusst die von der Abstützeinrichtung ausgeübte Kraft erfindungsgemäß anhand einer Große, die mit der Stößelposition in eindeutigem Zusammenhang steht. Diese Größe ist die Stößelposition selbst oder auch, wie es bevorzugt wird, der Pressenwinkel. Wenn der Pressenwinkel zugrunde gelegt wird, wird von einer Presse ausgegangen, die Von einer drehenden Welle angetrieben wird, wie es beispielsweise bei Exzenterpressen, Kniehebelpressen oder dergleichen der Fall ist. Die Drehposition der Antriebswelle, insbesondere der Exzenterwelle wird dabei als "Pressenwinkel" bezeichnet. Der Pressenwinkel (oder die sonstige, die Stößelposition eindeutig kennzeichnende Größe), bei dem vorzugsweise eine sprungartige Erhöhung der von der Abstützeinrichtung aufgebrachten Gegenkraft erfolgen soll, wird beispielsweise vom Werkzeughersteller vorgegeben und voreingestellt. Es kann vorgesehen werden, diesen voreingestellten Wert des Pressenwinkels, bei dem die Kraftumschaltung erfolgt, beim Einarbeiten der Presse zu korrigieren, d.h. probeweise zu veränkern, um eine möglichst ruhige Arbeitsweise der Presse im Sinne einer maximalen Schnittschlagdampfung zu erreichen. Der so eingestellte Wert wird vorzugsweise abgespeichert und dann für den nachfolgenden Betrieb der Presse zugrunde gelegt.

This object is achieved with the press according to claim 1 and the method according to claim 13:

• The press according to claim 1, a support means which is effective between plunger and press table and generates an effective force between them. For example, the support device is part of the sheet holder device, which presses the tool against the lower tool during the forming process. The lower tool is for example a punching tool while the upper tool is a stamp, for example. Trained as a support sheet holding device is able to exert different forces. A control device associated with the support device can influence the force exerted by the support device.
• According to the invention, the control device influences the force exerted by the support device on the basis of a variable that is unambiguously associated with the plunger position. This size is the plunger position itself or, as it is preferred, the press angle. If the press angle is taken as a basis, it is assumed that a press, which is driven by a rotating shaft, as is the case for eccentric presses, toggle presses or the like. The rotational position of the drive shaft, in particular the eccentric shaft is referred to as "press angle". The press angle (or the other, the plunger position clearly characterizing size), in which preferably a sudden increase in the applied force from the support device counterforce should be made, for example, specified by the tool manufacturer and preset. It can be provided to correct this preset value of the press angle at which the power is switched when incorporating the press, ie to try to change, in order to achieve the quietest possible operation of the press in terms of a maximum Schnittschlagdampfung. The value thus set is preferably stored and then used as the basis for the subsequent operation of the press.

The set value for the press angle can be determined workpiece and tool-specific. Such values may be retrievably stored in a table so as to have new setting values for the press angle at which the force changeover of the support device takes place when the press is converted.

By switching in the simplest case exclusively based on the press angle switching the support force, the press can be easily adjusted to different conditions, in particular with regard to the stroke rate or the cutting force.

To the sheet holding device usually includes a hold-down plate, which is supported directly on the workpiece. The hold-down plate extends into the immediate vicinity of the punches (punches) and thus close up to the section to be produced. So that the sheet in the immediate Schnittnahe is firmly clamped between the hold-down plate and the lower tool (punching tool) to achieve a high quality cut. In the press according to the invention preferably takes the sheet holding device or other support means after the breakthrough of the punch by the workpiece, the force applied by the plunger during this passes through its bottom dead center and stores the energy thereby emitted by the plunger. During the return stroke of the plunger, this energy is given back to the plunger and thus to the press drive. By thus avoiding the uncontrolled release of the elastically stored energy in the press, the press drive is relieved as a whole, i. Energy is saved. In addition, the mechanical load on the press is prevented by avoiding too large sudden force changes. Furthermore, by transferring the force exerted on the stamps up to the workpiece breakthrough onto the sheet-metal holding device, a particularly firm clamping of the workpiece takes place during the breakthrough, resulting in particularly high cutting qualities. In addition, the force on the sheet holding device can be introduced over a particularly large area and thus gently in the workpiece, so that the same undesirable deformations, such as bruising and the like, can be avoided.

It is also possible to detect and monitor the progression of the ram force over time or the press angle. If this leaves a tolerance range-dependent predetermined tolerance range, the switching time (switching press angle) for the opposing force applied by the support device can be moved forward or backward. This allows in particular the achievement of high stroke rates.

Preferably, the sheet holding device or the support means to a hydraulic cylinder which is connected to a first and a second hydraulic pressure accumulator. Both accumulators have e.g. a slidably mounted piston with damped end stop on. Alternatively, membrane storage devices or storage devices may be provided in which a gas pressure pad is directly in communication with the hydraulic fluid. Both accumulators preferably have different rest pressures. The path leading from the hydraulic cylinder to the lower pressure accumulator is preferably regulated by an electrically controlled valve controlled by the controller.

The switching of the supporting force, preferably from a smaller value to a larger value, at a given press angle makes it possible to start the switching operation of the hydraulic valve just before the press angle, at which the counterforce applied by the workpiece collapses, for example because the punch by the Pierces the material of the workpiece. The press angle difference by which the hydraulic valve is opened prematurely may be referred to as "lead angle". This can be time delays that arise due to the switching of the hydraulic valve and by the response delay of other components, effectively compensate. In contrast, systems that use large forces to switch the hydraulic valve can force collapse Identify the workpiece, only in hindsight, ie react with a time delay. The proposal according to the invention thus makes possible an effective compensation of the otherwise occurring cutting stroke, especially at a faster operating speed of the press (large number of strokes).

Further details of advantageous embodiments of the invention are the subject of subclaims, the drawing or the description of the figures.

Figur 1

die erfindungsgemäße Presse in schematisierter Übersichtsdarstellung,

Figur 2

das Werkzeug der Presse nach Figur 1 in einer schematisierten Vertikalschnittdarstellung und

Figur 3

ein Diagramm zur Veranschaulichung der Abhängigkeit des Stößelhubs und der Stößelkraft vom Pressenwinkel.

In the figure description an embodiment of the invention is illustrated. Show it:

FIG. 1

the press according to the invention in a schematic overview,

FIG. 2

the tool after the press FIG. 1 in a schematic vertical sectional view and

FIG. 3

a diagram illustrating the dependence of the ram stroke and the ram force of the press angle.

In FIG. 1 is in an extremely schematic representation of a press 1 illustrated, which has a press frame with press stands 2, 3, a press table 4 and a head piece 5. On the head piece 5, a drive 6, for example, held in the form of an electric motor which drives a plunger 9 back and forth via a schematically illustrated and shown in dashed lines eccentric 7 and a connecting rod 8 also shown in dashed lines. Between the plunger 9 and the press table 4, a tool 10 with an upper tool 11 and a lower tool 12 is provided. The lower tool 12 is formed as a punching tool. On the upper tool 11 stamp 13, 14, 15 are held, which, like the other details of the tool 10 in particular FIG. 2 can be seen. The tool 10 is used for punching a workpiece 16, the in FIG. 2 is illustrated as a flat workpiece. Of course, however, non-planar workpieces can be subjected to a punching operation in a corresponding manner. In this case, the lower tool 12 then has a contour corresponding to the non-planar workpiece.

To the upper tool 11 includes a sheet holder plate 17 which is held on not further illustrated means on a base body 18 of the upper tool 11. The main body 18 connected to the plunger 9 carries the punches 13 to 15, which are thereby rigidly connected to the plunger 9. In addition, the base body 18 includes one or more hydraulic cylinders 19, 20, which together with the sheet holder plate 17 form a sheet holding device 21. To the sheet holding device 21 also includes pressure pins 22 to 27, which are approximately or exactly parallel to the punches 13 to 15 are arranged and supported with its lower end face on the sheet holder plate 17. The rest, by the way essentially cylindrical pins are supported by their upper front end on floating plates 28, 29, which are thus on top of the pressure pins 22 to 27. The hydraulic cylinders 19, 20 include pistons 30, 31, which delimit in the hydraulic cylinders 19, 20 corresponding with hydraulic fluid filled working spaces 32, 33 and sealed and slidably mounted in these. Piston rods 34, 35 of the pistons 30, 31 press from above onto the floating plates 28, 29 and thus the sheet metal holder plate 17 against the workpiece 16.

The hydraulic cylinders 19, 20 are connected via an in FIG. 2 not and in FIG. 1 only schematically shown fluid line 36 connected to a hydraulic system 37, which serves to generate a blank holder force and at the same time to take over the force exerted by the plunger 9 force during and after breakthrough of the workpiece 16. This power transfer should be as stepless as possible, ie without a sudden force change. The sheet holding device thus forms a support device which generates a controlled force between the plunger 9 and the press table 4.

To the hydraulic system 37 includes a first pressure accumulator 38 and a second pressure accumulator 39, which are both formed in the embodiment as accumulator cylinder 40, 41 with sealingly and slidably mounted therein pistons 42, 43. Both pistons 42, 43 divide in the accumulator cylinders 40, 41 each from two working chambers, the upper, each filled with a gas cushion. The pressure accumulator 38 is, for example, under a pressure of about 200 bar while the pressure accumulator 39, for example, under a pressure of 400 bar, for example.

The pistons 42, 43 have at their lower, the respective end pieces 44, 45 side facing preferably a profiling, which is complementary to a profiling of the respective end piece 44, 45 is formed. The profiling is achieved by straight or curved, e.g. formed annular concentric strips or webs, wherein the strips or webs of each piston 42, 43 in correspondingly shaped recesses of each end piece 44, 45 fit. The profiles serve as cushioning, so that the pistons 42, 43, when they run against the fittings 44, 45, are gently braked.

Both pressure accumulators 38, 39 are connected to the fluid line 36. Preferably, the pressure accumulator 39 is connected via a check valve 46 and a throttle device 47 to the fluid line 36. The check valve 46 is oriented so that the hydraulic fluid from the hydraulic line 36 can flow unhindered into the pressure accumulator 40, while it is forced on its way back through the throttle device 47.

The pressure accumulator 38 is connected via a valve device 48 to the fluid line 36 and thus the hydraulic cylinders 19, 20. The valve device 48 includes, for example, a directional control valve 49 which is switchable between two states. In a first state, the flow of fluid into and out of the pressure accumulator 38 is unrestricted and unthrottled (or even throttled in an alternative embodiment), while in its other state it blocks this flow of fluid. It is thus designed as open / close valve. The valve device 48 may be connected to an electric actuator 50, which is connected to a control device 53. Preferably, the Control device 53 designed as a microprocessor control or as another suitable electronic control. It may be formed as an independent controller for the tool 10 or may be part of the other press control. Preferably, the control device is provided or connected to a suitable input / output means 52, such as a screen, a keyboard and the like. Operating parameters for the control of the hydraulic system 37 can be input via these input / output means 52. Such an operating parameter is, for example, the press angle α, at which the valve device 48 is to switch over.

Among other input signals, the controller 53 receives at least one position signal, e.g. marks the press angle. The position signal can originate, for example, from an encoder 54, which detects the position of the plunger 9, in particular in the vicinity of its bottom dead center, as a displacement sensor. Additionally or alternatively, an encoder 55 may be provided which controls the angular position of the eccentric shaft, i. detects the press angle at least in a rotation angle range in which the plunger 9 is in the vicinity of its bottom dead center.

In a preferred embodiment, a sensor 56 is also provided in the form of a force sensor which detects the force applied to the workpiece. For this purpose, the sensor 56 is provided, for example, at the point of articulation of the connecting rod 8 on the plunger 9. If a plurality of connecting rods are provided, sensors may be provided at each of the articulation points which, like the sensor 56, are then respectively connected to the control device 53. With the sensor 56, the force exerted by the plunger 9 is detected. This force is the sum of the force exerted on the workpiece and the force exerted by the workpiece Sheet holding device 21 is received. Alternatively, it is possible to accommodate corresponding force sensors at other locations, for example as a deformation sensor in the press table 4 or as force sensors in the upper tool 11 and / or the lower tool 12.

The control device 53 is adapted to switch the directional control valve 49 at a certain predetermined press angle so that it locks. This press angle is just before the bottom dead center at the point at which it is expected that a punching operation to be performed on the workpiece leads to a material separation. The position of the eccentric at which this occurs is called the breakthrough angle. The breakthrough angle is just before the bottom dead center of the plunger 9. After passing through the bottom dead center, the controller 53, the valve device 48 open again.

• Zur Funktionsveranschaulichung wird ein einziger Stanzhub beschrieben. Zur Durchführung desselben wird zunächst das Werkstück 16 auf das Unterwerkzeug 12 gelegt, wonach sich der Stößel 9 senkt. Die Blechhalterplatte 17 ist dabei in ihrer untersten Position, in der sie mit ihrer Unterseite zumindest etwas unterhalb der Stirnflächen der Stempel 13, 14, 15 steht. Bevor die Blechhalterplatte 17 auf dem Werkstück 16 aufsetzt, sind die Kolben 30, 31 in den Hydraulikzylinder 19, 20 in Ruhe. Das Hydraulikfluid steht in dem Hydrauliksystem 37 unter einem Ruhedruck.

The press 1 described so far operates in a first embodiment as follows:

• To illustrate the function, a single punching stroke is described. To carry out the same, the workpiece 16 is first placed on the lower tool 12, after which the plunger 9 lowers. The blank holder plate 17 is in its lowest position, in which it is at least slightly below the end faces of the punches 13, 14, 15 with its underside. Before the blank holder plate 17 rests on the workpiece 16, the pistons 30, 31 in the hydraulic cylinder 19, 20 at rest. The hydraulic fluid is in the hydraulic system 37 under a static pressure.

As soon as the blank holder plate 17 touches the workpiece 16, it presses the workpiece 16 against the lower tool 12. The blank holder plate 17 thus stops while the plunger 9 moves toward the workpiece 16. Also remain standing the pressure pins 22 to 27, the floating plates 28, 29 and the pistons 30, 31. As a result of the further downward movement of the plunger 9, the volume of the working chambers 32, 33 is reduced and hydraulic fluid via the fluid line 36 and the open directional control valve 49th the valve device 48 is driven into the pressure accumulator 38, which has a lower static pressure than the pressure accumulator 39. Thus, the piston 43 in. FIG. 1 moved upwards against the force of the upper gas cushion. Because of the pressure prevailing in the accumulator 39 higher pressure piston 42 remains in the lowest position.

Then put the end faces of the punches 13, 14, 15 on the workpiece 16. The workpiece 16 opposes the penetration of the punches 13, 14, 15 a considerable resistance, so that the movement of the punches 13, 14, 15 stops first. The drive power of the drive device 6 is now briefly used to somewhat elastically deform the drive train and the press frame including press table 4 and lower tool 12, ie to tension. Thus, an increasing force is built up, until finally the punches 13, 14, 15 abut through the workpiece 16. The press angle (breakthrough angle) at which this occurs is known to the control device as an operating parameter. Therefore, it monitors constantly, or at least in the vicinity of the bottom dead center, the press angle switches on reaching the breakthrough angle, the valve means 48. The leading to the pressure accumulator 38 hydraulic line is shut off. Switching the Valve device 48 may alternatively be made shortly before reaching the breakthrough angle, dH with an angular or temporal overfeed. The necessary Vorhaltewinkel is eg via the input / output device 52 can be entered.

If now the directional control valve 49 is closed, no further hydraulic fluid can flow into the pressure accumulator 38. It must therefore escape into the pressure accumulator 39, which is under considerably higher pressure. Thus, the hydraulic cylinders 19, 20 now generate a considerable back pressure, which is supported on the one hand on the sheet holder plate 17 and on the other hand counteracts the plunger 9. Thus, the force previously absorbed by the punches 13, 14, 15 commutes to the sheet holder 21, so that the tensioned press can not relax. Against the great force of the sheet holder device, the plunger now passes through its bottom dead center, the sheet holder device then pushes the plunger 9 on the first portion of the upstroke with great force upwards. In this phase, the stored in the press 1 elastic energy is given to the plunger 9 and thus to the drive means 6 back.

The return switching of the valve device 48 can take place at a predetermined, after bottom dead center press angle and done by the controller 53 based on the monitored press angle. Alternatively, it is also possible to provide the pressure accumulator 39 with a sensor device, for example a proximity switch, which recognizes when the piston 42 approaches the end piece 44. As soon as the piston reaches the end piece 44 or at least almost reaches it, the valve 48 can be opened again in order to reactivate the pressure accumulator 38. Of the corresponding sensor is connected to the control device 53.

Alternatively, the fluid line 36 may be connected to a pressure sensor connected to the controller 53. This then switches the valve device 48 back to the open state when the fluid pressure prevailing in the fluid line 36 drops below a given limit, which is approximately on the order of magnitude of the pressure of the pressure accumulator 38.

Furthermore, in a more refined embodiment, it is possible to monitor the force exerted by the plunger 9 by means of the sensor 56. The result is the in FIG. 3 as an example illustrated time course I of the force F, which, like FIG. 3 shows, in the vicinity of the bottom dead center of the time course of the movement X of the plunger has a maximum. It is possible to monitor whether this expected course I adheres to or leaves a tolerance band. The tolerance band can be specified by two further time profiles II and III, which is obtained by corresponding X and α offset of the expected force curve I. If the course I of the actually occurring ram force leaves the tolerance band, the control device 53 switches over the valve device 48. This is especially true if the course I intersects the course II from top to bottom. The switching of the valve device 48 from the open state to the closed state causes at this moment an increase in the counterforce on the plunger, so that a sudden, ie sudden force drop is avoided.

When breaching the course I by the upper limit III can be reacted in reverse. It is possible the To limit the function of the monitoring of the course I on leaving the tolerance band to an angle window α1, α2, in which the break-through angle is expected.

Alternatively, the time profile of the force measured by the sensor 56 can be determined. If the drop in force, ie the negative slope of the curve I too steep, this can be detected by the control device 53 as a breakthrough of the workpiece 16. Based on this, the control device 53 can close the valve device 48 immediately. Alternatively or additionally, the stored breakdown angle can be corrected to the now detected value. In this way, an adaptive, ie self-learning control device 53 is created, which adapts itself during operation in terms of the breakdown angle itself. The optimization goal lies in that in the course I according to FIG. 3 To minimize between the angles α1 and α2 occurring force drop. The controller 53 may be configured by doing so automatically by adjusting the advance angle. However, it is also possible to specify the angle, as mentioned above, manually.

It is also possible to adjust the break-through angle dynamically by first roughly setting, monitoring the force curve and thus determining the actual current break-through angle. The press angle from the previous punching stroke or an average value from previous punching strokes is then used as the press angle of the punching breakthrough. It is also possible to accommodate in the press frame, the press table or other parts of the press, force sensors which respond to a deformation of the respective press element or directly to the force acting in the press. Such can be, for example, force sensors be in the tool 10. The signals emitted by these sensors can be fed to the control device 53 and serve to define the breakthrough angle.

The system according to the invention allows a significant increase in the hold-down force, in particular during the performance of the punching operation, ie while the punches 13, 14, 15 penetrate through the material of the workpiece. The actual cutting force can be reduced to one sixth of the theoretical thrust. The sheet holding device 21 causes a particularly firm clamping of the workpiece 16 and thus causes an improvement of the cut as well as a cutting impact damping. The press 1 is biased to equalize or compensate the games. This leads to a reduction in the total compressive force of the system compared to classical cutting impact damping systems. However, this also means that older presses can continue to be used even for difficult separation operations. The force exerted on the blank holder plate force is preferably designed approximately to 40% of the pressing force. The separation process can be monitored, evaluated and controlled by the use of a fast evaluation and control device, such as the control device 53. The system can be largely self-sufficient, ie independently formed and used by the press 1. For example, it can be part of the tool, and thus be used in principle in different presses. When changing the press data, press-specific parameters can be changed via program or system-specific flash cards.

The pressures in the hydraulic cylinders 19, 20 can be permanently monitored angularly or distance-dependent. The resulting envelopes allow for permanent process monitoring. The control of the bypass valve 52 takes place crank angle or path-dependent on the same system. The process data and faults can be stored via data storage systems and traced back in case of damage. In addition, systems for detecting overload cases can be provided.

To increase the quality of cut and improve the operation of a press, especially when punching high-strength martensitic materials or when punching thick sheets, a sheet holding device is provided which firmly clamps the workpiece in question during the punching process. The clamping force is increased up to 40% or more percent of the ram force. In particular, the force exerted by the sheet holding device during the workpiece breakthrough can be increased again. The increase of the clamping force is preferably controlled in dependence on the press angle. On the one hand improves the cut quality while on the other hand results in an efficient cutting impact reduction or prevention on the press. A cutting stroke is significantly weakened or does not occur.

Claims (21)

1. Press (1) for punching metal sheets,
   with a press frame, which includes a press bed (4) to receive a lower die (12) and in which a slide (9), which connects to a drive device (6) and carries an upper die (11), is is movably disposed,
   with a controllable support device (21), which is fitted to generate a variable force active between the slide and the press bed,
   with a control device (53), which is associated with the support device (21) and by means of a sensor device (54, 55) detects as magnitude (α, x), which stands in a clear relationship with the slide position, a pressing angle (α) or a slide position (x), characterised in that in the case of a predetermined pressing angle (α) or in the case of a predetermined slide position (x) the force exerted by the support device (21) and active between the slide (9) and the workpiece (16) switches over from a lower value to a higher value.
2. Press according to claim 1, characterised in that the magnitude is the pressing angle (α) and the sensor device is a rotation position sensor (55).
3. Press according to claim 2, characterised in that the rotation position sensor (55) is fitted to detect the pressing angle in increments.
4. Press according to claim 1, characterised in that the magnitude is the slide position (x) and the sensor device is a position measuring device (54).
5. Press according to claim 1, characterised in that the support device (21) belongs to the die (10) and presses the workpiece (16) against the lower die (12) during the deformation process and to do this rests against the slide (9) at one end and on the workpiece (16) at its other end.
6. Press according to claim 1, characterised in that the support device (21) has at least one hydraulic cylinder (19), which is connected to a hydraulic system (37), which applies a pressurised hydraulic fluid to the hydraulic cylinder (19).
7. Press according to claim 1, characterised in that the hydraulic system (37) has at least one first hydraulic accumulator (38) and at least one second hydraulic accumulator (39).
8. Press according to claim 2, characterised in that the control device (53) has an electrically controlled changeover valve (48) for controlling the hydraulic flow out of the hydraulic cylinder (19, 20).
9. Press according to claim 1, characterised in that the sensor device includes at least one force sensor (56), which detects at least a portion of the force exerted on the workpiece.
10. Press according to claim 9, characterised in that the control device (53) is fitted to monitor the time curve (I) of the detected force and abruptly increase the force exerted by the support device (21) when the curve of the monitored force deviates by more than a predetermined dimension from a time-dependently predetermined value (II, III).
11. Press according to claim 10, characterised in that the control device (53) determines the rate of change of the time curve (I) and abruptly increases the force exerted by the support device (21) when the rate of change exceeds a limit dimension.
12. Press according to claim 11, characterised in that the control device (53) determines the rate of change of the time curve (I) only for a limited section of the slide path.
13. Process for punching metal sheets with a press, which has
    a press frame, which includes a press bed (4) to receive a lower die (12) and in which a slide (9), which connects to a drive device (6) and carries an upper die (11), is movably disposed,
    a controllable support device (21), which at least intermittently presses against the slide (9) against the slide working direction, and
    a control device (51, 53), which is associated with the support device (21) and by means of a sensor device (54, 55) detects as magnitude (α, x), which stands in a clear relationship with the slide position, a pressing angle (α) or a slide position (x),
    characterised in that in the process the force exerted by the support device (21) switches over from a lower value to a higher value in the case of a predetermined pressing angle (α) or in the case of a predetermined slide position (x).
14. Process according to claim 13, characterise in that a sheet holding device (21) is used as support device (21).
15. Process according to claim 13, characterised in that the support device (21) has at least one hydraulic cylinder (19, 20), which is connected to a hydraulic system (37), by means of which a pressurised hydraulic fluid is applied to the hydraulic cylinder (19, 20).
16. Process according to claim 15, characterised in that the hydraulic system (37) has at least one first hydraulic accumulator (38) and at least one second hydraulic accumulator (39).
17. Process according to claim 16, characterised in that the control device (53) controls a valve device (48), by means of which the pressure of the first accumulator (38) or the pressure of the second accumulator is alternately active in the support device (21).
18. Process according to claim 13, characterised in that the control device (53) determines the current slide position by means of a sensing device (54).
19. Process according to claim 13, characterised in that the control device (53) detects the pressing angle by means of a sensing device (55).
20. Process according to claim 13, characterised in that the control device (53) detects the slide force by means of a sensor device (56).
21. Process according to claim 20, characterised in that the control device (53) changes the force of the support device (21), when the curve (I) of the detected force leaves a tolerance range (II, III).

Images