EP0673695B1 - Method and control arrangement for interrupting and continuing of the drawing process in double acting presses, in particular hydraulic presses - Google Patents

Description

The invention relates to a method and a double-acting press with a circuit arrangement to interrupt and continue the drawing process, around when converting the press or when retracting tools reduce the number of trial runs and one enable faster production start-up.

According to the patent specification DE 37 35 582 C1, a double-acting press for drawing sheet metal parts with a stroke-driven sheet metal holder tappet and with a separate, likewise stroke-driven drawing tappet is known.
In order to be able to individually adjust the sheet holder force reproducibly and with low inertia and to keep it at the preset value, a servohydraulic sheet holder force control is provided separately for each connecting rod of the sheet holder. A piston / cylinder unit that determines the sheet metal holder force is assigned to each sheet metal connecting rod. The force applied to the associated area of the sheet metal holder can be measured via a pressure transducer connected to it or via a force transducer integrated in the sheet metal holder. The force-determining pressure chamber of the piston / cylinder unit can be individually adjusted to the required pressure via a control path formed by a controller and an electrohydraulically pilot-controlled proportional valve. This controlled system operates with low inertia and largely free of hysteresis, so that the setpoint for the sheet metal holder force can be changed after a repetitive functional sequence even during a deep-drawing cycle. For example, the controlled sheet metal holder force can be individually increased or decreased during the drawing process.
When converting the press to a new sheet metal part, this solution only realizes the adjustability and changeability of the ram and sheet metal holder values after or during the working cycle of a press according to previously known or theoretically determined values.
Better drawing results can be achieved if the die cushion forces can be varied depending on the drawing path. Likewise, a different distribution of the die cushion forces over the drawing edge of the sheet metal part by means of drawing devices with a plurality of separately controllable pressure bolts leads to better control of the drawing process. DE-A-37 44 177 discloses a method and a press for deep-drawing of blanks, in particular deep-drawing sheets for body elements of motor vehicles, by means of hydraulic deep-drawing presses, in which the hydraulic drawing press can be quickly and easily adapted to the manufacture of others and deep-drawn parts to be designed can be reached.
With this method, the blank is firmly clamped to its edge area between a sheet metal holder and a support during the deformation process caused by a stamp of a press ram, and the sheet metal holder pressure is applied by valve-controlled sheet metal holder cylinders, which also determine the movement sequence of the sheet metal holder. The valves upstream of the sheet metal holder cylinder are acted upon by electronic control with switching signals which change the cylinder pressure of the sheet metal holder cylinders several times during the shaping process. The method represents the draw path-dependent regulation of the contact pressure transmitted to the edge of the drawn part, the sheet metal holder being acted upon by four sheet metal holding cylinders which are arranged in a uniformly distributed manner and are preferably of identical construction. However, the prerequisite is knowledge of the required force curve over the drawing path for each individual pressure point. Because of the many influencing factors, its empirical determination is more practical. To do this, it is necessary to interrupt the drawing process at any desired point and continue from there again.
This is where the present invention comes in and has set itself the task of a method and a circuit arrangement, whereby an interruption and continuation of the drawing process while largely maintaining the independence of the individual devices, such as tappet drive, tappet parallelism, die cushion, clamping and possibly synchronous device any place is made possible, during the "intermediate stop" there are no relative movements of the participating facilities, which lead to a change in the drawn part geometry, the "intermediate stop" takes place in clearly definable phases, the participating facilities are essentially given tasks in the individual phases, which they also meet during the "normal" drawing process, ie that no additional devices are required and the reproducibility of the ram position and tilting as well as the pressure pin forces, as is best immediately before the drawing process is interrupted anden, is guaranteed during the "intermediate stop".
According to the invention, the method is implemented by the method steps mentioned in claim 1. Further expedient refinements of the method are presented in claims 2 to 11.

This process causes the tools to be retracted with a small number of test pulls, thus increasing the productivity and economy of the press. By securing the drawing device in position, there is no danger when handling the sheet metal part by the operating personnel during the course of the process.
The method can be carried out particularly advantageously with a pulling device with a plurality of separately controllable pressure bolts.
A press with a circuit arrangement for performing the method is defined in claim 12. The features of claims 13 and 14 relate to further detailed configurations of the press.

Fig. 1:

Zustand zum Zeitpunkt des Signals "Zwischenhalt" bis zum Anhalten des Stößels

Fig. 2:

Lagesicherung des Ziehkissens und Öffnen des Werkzeugs

Fig. 3:

Zustand beim Rückhub des Stößels und Begutachtung des Blechteils

Fig. 4:

Zustand beim Schließen des Werkzeugs

Fig. 5:

Aktivierung des Ziehkissens

Fig. 6:

Zustand bei Fortsetzung des Ziehvorgangs

Fig. 7:

Anordnung der Meßsysteme mit Schaltungsanordnung

Fig. 8:

Zustand nach Fig. 1 unter Verwendung einer Zieheinrichtung mit mehreren separat regelbaren Druckbolzen

The invention is explained in more detail using exemplary embodiments. The accompanying drawings show the basic representation of the plunger, the plunger parallel holding, the tool, the die cushion and the clamping device for the various process steps and the circuit arrangement.

Fig. 1:

State at the time of the "intermediate stop" signal until the plunger stops

Fig. 2:

Secure the die cushion and open the tool

Fig. 3:

Condition of the ram return stroke and inspection of the sheet metal part

Fig. 4:

Condition when the tool is closed

Fig. 5:

Activation of the die cushion

Fig. 6:

Condition when drawing continues

Fig. 7:

Arrangement of the measuring systems with circuit arrangement

Fig. 8:

1 using a pulling device with several separately controllable pressure bolts

Each of the following devices on the presses, such as the ram drive 1, the ram parallel holding device 2, the drawing device 3 in the form of at least one two-point cushion and the safety-related clamping and locking device 4 of the drawing cushion, have their own sensors and their own control for the respectively assigned one To be able to carry out the task independently within one lifting cycle. They correspond to the control for process sequence 30.
A pressure transducer 13 for the pressing force is arranged on each separately controllable pressure chamber 11 of the working cylinders 12 of the tappet drive 1. The signals from the pressure sensor 13 are processed in the control of the ram movement 14 and in the pressing force control 15. To measure the ram travel, at least one displacement measuring system 17 is arranged between the press frame and the ram 8, the signals of which are recorded by the control of the ram movement 14. A pressure transducer 20 is arranged on the pressure chamber 18 of each counter-holding cylinder 10 of the plunger parallel holder 2. The signals of this pressure sensor 20 are processed by the control of the counter-holding system 21. This regulation of the counter-holding system 21 consists of control loops for the load-dependent tilting, the constant tilting and the position control. The ram tilt is calculated by measuring the distance between the ram 8 and the press table 31 using a displacement measuring system 23. The signals of the displacement measuring system 23 are processed by the control of the counter-holding system 21. The pressure chambers 18 and 19 of each counter-holding cylinder 10 are pressurized, the values are set by regulating the counter-holding system 21. Each pressure chamber 18 can also be pressurized with a pressure oil source 32 that can be switched on and off. A pressure sensor 26 is arranged on the pressure chamber 28 of each drawing cylinder 25, the signals of which are processed by the control of the cushion force 27. Furthermore, each pressure chamber 28 can also be pressurized with a pressure oil source 33 that can be switched on and off. Each clamping and locking device 4 on the drawing cylinders 25 is operatively connected to the control for safety clamping 29. If the pulling process is not interrupted, the plunger movement coordinates the interaction of the devices 1, 2, 3 and 4 in that the distance measuring system 17 of the plunger drive 1 detects the plunger movement, the pulling device 3 and possibly the plunger parallelism 2 and the clamping and locking device 4 the current one Ram position transmitted. The tappet parallel position 2 takes up tappet position and tipping with its own position measuring systems. The individual actions are initiated or ended in accordance with the programmed plunger positions.
The pressure values of the counter cylinder 10 and the drawing cylinder 25 of the die cushion are stored and displayed centrally over a cycle (visualization) and are available for further calculations (e.g. load-dependent tilting specification) and for controlling the intermediate stop.
The controls of the individual facilities have the necessary pressure and position control loops and provide them with the setpoints according to the program.
The control of the intermediate stop is superior to the controls of the individual facilities and has the highest priority during the intermediate stop. It receives information about the current tappet position via the tappet drive 1, can access all pressure values and receives feedback about the state of the clamping and locking device 4.The control of the intermediate stop specifies the tappet drive 1 to maintain constant pressure values (preload pressure) and the tappet parallelism 2 the command for maintaining and resuming defined tilts and the position for the position control. In addition, it can e.g. B. affect the biasing pressure in the counter-cylinders 10 during the closing movement of the tool. The die cushion is given time-controlled pressure setpoints when the "original" die cushion forces are to be adjusted again.

The clamping and locking device 4 receives commands for the safety-related locking of the pulling device 3. If a synchronizing device is present, this is brought into the required position by controlling the intermediate stop.
During the "normal" pulling process, the tappet movement and thus the tappet drive 1 have the highest priority. The plunger 8 can be positioned very "sensitively" in the presence of an inking device, but is clearly only influenceable either with regard to its position or its pressing force. However, if both process variables are to be adjustable in a defined manner, a system that can be braced in itself is required. It is formed by the plunger drive 2 counteracting the plunger drive 1.
A special feature of the method is that the “power reserves” which are still available when the ram parallelism 2 is tilted are used to pretension the ram drive 1. All other force effects on the plunger 8, the intermediate stop, z. B. caused by the die cushion or the die, can now be compensated for by a variation of these reserves. The prerequisite here is that the force components to be expected from the die cushion and the die part can be additionally applied by the ram parallel holding device 2. If the tappet parallel position 2 is in the position control position and the pressure in the working cylinders 12 is set to a certain value, the preload pressure in the counter-cylinders 10 automatically increases. If the tappet drive 1 remains in the pressure control (constant setpoint) and additional forces now act, e.g. B. by the die cushion on the plunger, the preload pressure of the plunger parallel position 2 is automatically reduced to the same extent. The position of the plunger 8 remains unchanged, the forces "migrate" under the plunger 8, so to speak, between the pulling device 3 and the plunger parallel position 2 and are by a embodies constant "resultant".

• bei Signalgabe "Zwischenhalt", um den Stößel 8 über einen verkürzten Nachlaufweg mit definierter Kippung zum Stillstand zu bringen,
• bei erneutem Aufsetzen des Stößels 8 auf das Ziehteil und dem Aufbringen der originalen Ziehkissenkräfte und
• bei Fortführung des Ziehvorgangs um eine definierte Richtung der Stößelbewegung zu erzwingen auf.

This described feature occurs in the process

• with signaling "intermediate stop" in order to bring the plunger 8 to a standstill via a shortened overtravel with a defined tilt,
• when placing the plunger 8 again on the drawing part and applying the original drawing cushion forces and
• if the pulling process continues to force a defined direction of the plunger movement.

1. Das Anhalten des Stößels (Fig. 1)
Sobald das Signal "Zwischenhalt" aktiviert ist, erfolgt das kürzestmögliche Abbremsen des Stößels 8. Damit die gewünschte Zwischenhaltposition eingehalten wird, ist die Signalgabe "Zwischenhalt" um den Nachlaufweg des Stößels 8 vorverlegt. Mit dem Signal "Zwischenhalt" erfolgt gleichzeitig die Speicherung der Druckwerte von den Arbeits-, Zieh- und Gegenhaltezylindern 12, 25 und 10 und der Lagewerte des Stößels 8. Die Stößelparallelhaltung 2 geht gleichzeitig in die Lageregelung über und hält den Stößel 8 in seiner ggf. geneigten Lage. In den Ziehzylindern 25 stellt sich ein Druckwert ein, der die Massenkräfte des Ziehkissens und des Blechhalters kompensiert und den Stößel 8 somit nur wenig belastet.

2. Lagesicherung des Ziehkissens und Öffnen des Werkzeugs (Fig. 2)
Mittels einer Klemm- und Verriegelungseinrichtung 4 erfolgt das sicherheitsgerichtete "Einfrieren" der Lage von der Kissenplatte bzw. der Gleichlaufplatte 34 bei einem Ziehkissen mit mehreren geregelten Druckbolzen. Die Gleichlaufplatte 34 wird bis zu einem Wert von 2 mm abgesenkt, um bei extremer Schiefstellung des Stößels 8 und damit der Druckbolzen 24 dafür zu sorgen, daß alle Druckbolzen 24 durch die Gleichlaufplatte 34 in ihrer Lage fixiert sind. Der Druck in den Ziehzylindern 25 wird so weit reduziert, daß eine Gefährdung von Personen ausgeschlossen ist. Wenn alle Signalgeber den programmgemäßen Zustand anzeigen, erfolgt das Öffnen des Werkzeugs 7. Hierbei führt die Stößelparallelhaltung 2 den Stößel 8 über einen vereinbarten Wegbereich (z. B. 10 mm), wobei die Lage des Stößels 8 durch Regelung (konstante Kippung) beibehalten wird. Der Hauptantrieb des Stößelantriebs 1 wird mit einem Gegendruck beaufschlagt, der zum geregelten Rückhub erforderlich ist.

3. Rückhub des Stößels und Begutachtung des Blechteils (Fig. 3)
Wenn das Werkzeug 7 weit genug geöffnet ist, wird der Eilrückhub des Stößels 8 eingeleitet. Nach Erreichen des oberen Totpunktes kann das Blechteil zur Begutachtung dem Werkzeug 7 entnommen werden. Die Teilebegutachtung kann auch automatisiert erfolgen, beispielsweise

• durch Einführen einer Prüfvorrichtung in den Werkzeugraum oder
• durch Entnehmen des Werkstücks und Zuführen zu einer Prüfvorrichtung sowie Rückführung oder
• durch eine im Werkzeug integrierte Prüfvorrichtung.

Wird bei der Teilebegutachtung festgestellt, daß eine Änderung der Kissenkraft (oder der Druckbolzenkräfte) zu einer Verbesserung des Ziehergebnisses führt, können die in der Steuerung gespeicherten Ziehkissenkräfte, die bei Signalabgabe "Zwischenhalt" gemessen wurden, geändert werden.

4. Schließen des Werkzeuges (Fig. 4)
Das Schließen des Werkzeuges 7 erfolgt in umgekehrter Reihenfolge. Nach der Eilsenkbewegung des Stößels 8 und dessen Aufsetzen auf die Kolbenstangen 22 der Stößelparallelhaltung 2 schließt sich das Zusammenführen der beiden Werkzeugteile unter Beibehaltung der Stößelkippung an, die bei dem "Zwischenhalt" ermittelt und gespeichert wurde. Bei Erreichen der Zwischenhaltposition geht die Stößelparallelhaltung 2 in die stationäre Lageregelung über und stoppt somit die Stößelbewegung. Anschließend wird im Hauptantrieb des Stößelantriebs 1 eine der gespeicherten Ziehkissenkraft entsprechende Preßkraft aufgebaut. Dabei erhöht sich in den Gleichlaufzylindern der Druck selbständig so weit, wie es die Beibehaltung des Gleichgewichtszustandes durch Lageregelung erfordert.

5. Aktivierung des Ziehkissens (Fig. 5)
Die Klemm- und Verriegelungseinrichtung 4 wird deaktiviert und die Zieheinrichtung 3 legt sich aufgrund eines geringen "Kraftüberschusses", welcher aus der jetzt aktivierten Auswerferkraft resultiert, an den Stößel 8 an. Im Anschluß daran wird mittels einer externen Druckölquelle 33 der Druck in den Ziehzylindern 25 so lange erhöht, bis die Kissenkraft erreicht ist, die in der Steuerung 27 gespeichert ist. Während dieses Vorgangs wird der Druck in den Arbeitszylindern 12 konstantgehalten und die Stößelparallelhaltung 2 hält den Stößel 8 weiterhin in seiner Lage. Somit kann sich unter dem Stößel 8 ein Belastungswechsel zwischen Zieheinrichtung 3 und Stößelparallelhaltung 2 vollziehen, ohne daß sich die Lage des Stößels 8 ändert. Nach Abschluß dieses Vorgangs hat sich auch automatisch der in den Gegenhaltezylindern 10 eingestellte Druck auf einen minimalen Wert reduziert.

6. Fortsetzung des Ziehvorgangs (Fig. 6)
Der Druck in den Arbeitszylindern 12 wird kontinuierlich erhöht, bis die vor Unterbrechung des Ziehvorgangs ermittelte Summe der Kräfte in den Gegenhaltezylindern 10 erreicht ist. Das Erreichen dieses Anteils der vorher gespeicherten Preßkraft ist gleichzeitig für die Stößelparallelhaltung 2 das notwendige Signal, um die Lageregelung aufzugeben. Damit ist die Fortsetzung des Ziehvorgangs kontrollierbar und erfolgt in die Richtung der Stößelkraft.

The process takes place in the following phases:

1. Stopping the plunger (Fig. 1)
As soon as the "intermediate stop" signal is activated, the ram 8 is braked for as short a time as possible so that the desired intermediate stop position is maintained, the "intermediate stop" signal is advanced by the overtravel of the ram 8. With the signal "intermediate stop", the pressure values of the working, pulling and counter-holding cylinders 12, 25 and 10 and the position values of the tappet 8 are stored at the same time. The tappet parallelism 2 simultaneously changes into the position control and keeps the tappet 8 in its position inclined position. A pressure value is set in the drawing cylinders 25, which compensates for the inertial forces of the drawing cushion and the sheet metal holder and thus only slightly loads the ram 8.

2. Securing the position of the die cushion and opening the tool (Fig. 2)
By means of a clamping and locking device 4, the safety-related "freezing" of the position of the cushion plate or the synchronizing plate 34 takes place in the case of a die cushion with a plurality of regulated pressure bolts. The synchronizing plate 34 is lowered to a value of 2 mm in order to ensure that all the plungers 24 are fixed in their position by the synchronizing plate 34 when the plunger 8 and thus the pressure pin 24 are extremely inclined. The pressure in the drawing cylinders 25 is reduced to such an extent that there is no danger to persons. When all signal transmitters indicate the status according to the program, the tool 7 is opened. In this case, the plunger parallel holding 2 guides the plunger 8 over an agreed travel range (e.g. 10 mm), the position of the plunger 8 being maintained by regulation (constant tilting) . The main drive of the tappet drive 1 is subjected to a back pressure which is required for the controlled return stroke.

3. Return stroke of the ram and inspection of the sheet metal part (Fig. 3)
When the tool 7 is opened far enough, the rapid return stroke of the ram 8 is initiated. After reaching the top dead center, the sheet metal part can be removed from the tool 7 for inspection. The parts inspection can also be automated, for example

• by inserting a test device into the tool room or
• by removing the workpiece and feeding it to a test device and return or
• through a test device integrated in the tool.

If it is found in the parts assessment that a change in the cushion force (or the pressure pin forces) leads to an improvement in the drawing result, the drawing cushion forces stored in the control system, which were measured when the signal "intermediate stop" was emitted, can be changed.

4. Closing the tool (Fig. 4)
The tool 7 is closed in the reverse order. After the rapid lowering movement of the plunger 8 and its placement on the piston rods 22 of the plunger parallel holding 2, the two tool parts are brought together while maintaining the plunger tilt, which was determined and stored during the “intermediate stop”. When the intermediate stop position is reached, the ram parallel position 2 changes to the stationary position control and thus stops the ram movement. Then a pressing force corresponding to the stored die cushion force is built up in the main drive of the tappet drive 1. The pressure in the synchronous cylinders increases automatically as much as it is necessary to maintain the state of equilibrium through position control.

5. Activation of the die cushion (Fig. 5)
The clamping and locking device 4 is deactivated and the pulling device 3 rests on the plunger 8 due to a slight “excess force” which results from the now activated ejector force. Subsequently, the pressure in the drawing cylinders 25 is increased by means of an external pressure oil source 33 until the cushion force which is stored in the controller 27 is reached. During this process, the pressure in the working cylinders 12 is kept constant and the plunger parallelism 2 keeps the plunger 8 in its position. Thus, a load change between the pulling device 3 and plunger parallel position 2 can take place under the plunger 8 without the position of the plunger 8 changing. After completion of this process, the pressure set in the counter-cylinders 10 has also automatically been reduced to a minimum value.

6. Continuation of the drawing process (Fig. 6)
The pressure in the working cylinders 12 is continuously increased until the sum of the forces in the counter-cylinders 10 determined before the drawing process is interrupted. Reaching this portion of the previously stored pressing force is at the same time the necessary signal for the ram parallelism 2 to give up the position control. The continuation of the drawing process is thus controllable and takes place in the direction of the ram force.

The method can, as in Fig. 8 and partially in the shown previous description, also using a pulling device with several separately controllable Operate push pin. It should be noted that to secure the position of the die cushion, the synchronizing plate 34 by one defined path can be lowered.

Reference list

1

- tappet drive

2nd

- Ram parallelism

3rd

- pulling device

4th

- Clamping and locking device

5

- sheet metal part

6

- sheet holder

7

- Tool

8th

- pestle

9

- Piston of the counter cylinder

10th

- counter cylinder

11

- pressure room

12th

- working cylinder

13

- pressure transducer

14

- Control ram movement

15

- Press force control

16

- piston rod

17th

- Path measuring system for the ram path

18th

- pressure room

19th

- pressure room

20th

- pressure transducer

21

- Regulation of the counterhold system

22

- Piston rod of the ram parallel position

23

- measuring system

24th

- pressure bolts

25th

- drawing cylinder

26

- pressure transducer

27

- Control cushion power

28

- pressure room

29

- Regulation of safety clamping

30th

- Process control control

31

- press table

32

- pressure oil source

33

- pressure oil source

34

- synchronization plate

Claims (14)

1. Method of interrupting and resuming the drawing process on a double-action press, particularly on a hydraulic press, which has a ram drive (1), a system (2) for keeping the ram parallel, a drawing arrangement (3) and a clamping and locking arrangement (4) for the drawing cushion that complies with safety requirements, these also being capable of autonomous operation in each case,

   wherein, after the giving of the "intermediate stop" signal at any desired point in time in the forming process, the ram (8) is immediately braked to a standstill

   and, in the process, the instantaneous tilting values of the system (2) for keeping the ram parallel and the values of the ram drive force and of the drawing cushion forces are stored in the operating cycle control system (30) and the following steps run their course in the sequence indicated:

   the drawing cushion forces are changed to the value which compensates for the inertia forces operating from the sheet metal-holder (6) and sheet-metal part (5) without any change in location,

   the clamping and locking arrangement (4) for the drawing arrangement (3) is activated for the defined securing of the drawing arrangement (3) in the intermediate stop position,

   the drawing arrangement (3) is moved into the wattless or low-energy condition,

   the ram return stroke is initiated and is supported, in the process, by the counter-holding cylinders (10) of the system (2) for keeping the ram parallel, during the first millimetres of the ram return stroke, for defined divergence and while maintaining any tilting of the ram that may exist, and also acts upon the working cylinders (12) of the ram drive (1) with a defined counterholding pressure,

   a parts appraisal takes place,

   the stored values of the drawing cushion force or of the thrust bolt forces are changed if necessary,

   the ram stroke for closing the tool (7) begins and, during the last millimetres of the ram stroke travel before the intermediate stop position is reached, the counter-holding cylinders (10) of the system (2) for keeping the ram parallel are activated in such a way that the ram (8) once again assumes any tilting that may have existed at the point in time of the intermediate stop,

   the system (2) for keeping the ram parallel changes over, when the intermediate stop position is reached, into a stationary location-regulating system and, at the same time, the counter-pressure resulting from the stored drawing cushion forces is built up and kept constant in the working cylinders (12) of the ram drive (1),

   the clamping and locking arrangement (4) for the drawing arrangement (3) is released,

   the drawing cushion forces are applied in the drawing cylinders (25) of the drawing arrangement until the stored values are reached and the drawing arrangement (3) is thereby supported on the ram (8), a gradual change of load coming about, under the ram (8), from the drawing arrangement (3) to the system (2) for keeping the ram parallel, and

   after the stored drawing cushion forces have been reached, the ram force is continuously increased until the sum of the forces in the counter-holding cylinders (10) which is ascertained before the interruption of the drawing operation is reached, whereupon a clearing of the system (2) for keeping the ram parallel, and at the same time the resumption of the drawing operation, takes place and, in the process, the system (2) for keeping the ram parallel leaves the location-regulating system and compensates, in a load-dependent manner, for tiltings of the ram which are brought about by the forces involved in the method, which are operating once more.
2. Method of interrupting and resuming the drawing process, in accordance with claim 1,
   in which the braking of the ram (8) to a standstill in the intermediate stop position may additionally take place through the application of a counter-force in the system (2) for keeping the ram parallel.
3. Method of interrupting and resuming the drawing process, in accordance with claim 1,
   in which, during the ram return stroke and in accordance with the range of the defined divergence of the tool halves (7), the ram (8) and the pistons (9) of the counter-holding cylinders (10) can be brought, in rapid motion, into the particular upper end location.
4. Method of interrupting and resuming the drawing process, in accordance with claim 1,
   in which the parts appraisal takes place manually.
5. Method of interrupting and resuming the drawing process, in accordance with claim 1,
   in which the parts appraisal takes place in an automated manner, for example,

   by the introduction of a testing device into the tool space, or

   by the removal of the workpieces and the feeding thereof to a testing device and also the returning thereof, or

   by a testing device integrated into the tool.
6. Method of interrupting and resuming the drawing process, in accordance with claim 1,
   in which the drawing cushion forces ascertained and stored at the point in time of the intermediate stop can be changed in dependence upon the appraisal of the sheet-metal part.
7. Method of interrupting and resuming the drawing process, in accordance with claim 1,
   in which the application of the drawing cushion forces in the drawing cylinders (25) for the purpose of supporting the drawing arrangement (3) on the ram (8) takes place in a time-dependent and controlled manner, as a result of which a gradual rise in force, for example in the form of a ramp function, is achieved.
8. Method of interrupting and resuming the drawing process, in accordance with claim 1,
   in which the acting-upon of the ram drive (1) with energy for the purpose of resuming the drawing process takes place in accordance with a time function.
9. Method of interrupting and resuming the drawing process, in accordance with claim 1,
   in which compensation for tilting is employed instead of the load-dependent regulation of tilting throughout the operating cycle of the method.
10. Method of interrupting and resuming the drawing process, in accordance with claim 1,
    in which tiltings of the drawing arrangement (3) during the intermediate stop are reduced by retraction of the said drawing arrangement (3) by a defined amount.
11. Method of interrupting and resuming the drawing process, in accordance with claim 1,
    in which tiltings of the drawing arrangement (3) are maintained during the intermediate stop by clamping by means of the safety-directed clamping and locking arrangement (4).
12. Double-action press, particularly a hydraulic press with a switching arrangement for interrupting and resuming the drawing process,

    wherein the press has a ram drive (1) with one or more working cylinders (12), a system (2) for keeping the ram parallel having counter-holding cylinders (10), a drawing arrangement (3) as a refinement of at least one two-point drawing cushion and a clamping and locking arrangement (4) for the drawing cushion that complies with safety requirements, these also being capable of autonomous operation in each case, wherein

    there is disposed, on each separately activatable pressure chamber (11) of the working cylinders (12), a pressure pick-up (13) for the pressing force, the signals from which are further processed in the control system for the ram movement (14) and also in the pressing force-regulating system (15), and for the purpose of measuring the ram travel, there is also disposed, between the frame of the press and the ram (8) or the ram drive elements, at least one travel-measuring system (17), the signals from which are picked up by the control system for the ram movement (14), wherein

    there is disposed, on the pressure chamber (18) of each counter-holding cylinder (10) of the system (2) for keeping the ram parallel, a pressure pick-up (20), the signals from which are processed by the regulating system for the counter-holding system (21), consisting of regulating circuits for

    -- load-dependent tilting,

    -- constant tilting and

    -- location regulation

    and, for the purpose of calculating the tilting of the ram, a measurement of the distance between the ram (8) and the table (31) or frame of the press takes place by means of a travel-measuring system (23), the signals from which are processed by the regulating system for the counter-holding system (21), and both pressure chambers (18) and (19) of each counter-holding cylinder (10) are acted upon by pressure, but in a manner regulated by the regulating system for the counter-holding system (21), and each pressure chamber (18) can be additionally acted upon by pressure with the aid of a pressure oil source (32) which can be switched on and off, wherein

    there is disposed, on the pressure chamber (28) of each drawing cylinder (25), a pressure pick-up (26), the signals from which are processed by the regulating system for the cushion force (27), and each pressure chamber (28) can be additionally acted upon by pressure with the aid of a pressure oil source (33) which can be switched on and off, wherein

    each clamping and locking arrangement (4) is in operative connection, for signal-processing purposes, with the control system for safety clamping (29), and wherein

    the pressing force-regulating system (15), the regulating system for the counter-holding system (21), the regulating system for the cushion force (27) and the control system for the safety-clamping system (29) correspond, independently of one another in each case, with the control system (30) for the operating cycle of the method.
13. Double-action press in accordance with claim 12, in which the travel-measuring system (23) is constructed as an incremental travel-measuring system.
14. Double-action press in accordance with claim 12, in which the travel-measuring system (23) is constructed as a contact-less travel-measuring system, preferably a laser-type travel-measuring system, and is disposed in the press table (31) or in the ram (8).

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