Punching a workpiece

Abstract

This disclosure relates to methods and apparatuses for punching workpieces. A punching tool is configured to move during a punching stroke along a stroke axis towards a workpiece to be punched. The punching tool is configured to move away from the punched workpiece during a return stroke. The punching tool includes first and second components configured to be coupled hydraulically for concurrent movement along the stroke axis. The punching tool includes a punching drive for moving the first component along the stroke axis. The punching apparatus is configured to move the second component relative to the first component at a first transmission ratio during the punching stroke. The punching apparatus is configured to move the second component relative to the first component at a second transmission ratio in response to a reaction force of the workpiece exceeding a threshold value of the punching drive during the punching stroke.

Claims

1. A punching apparatus comprising: a punching tool configured to move during a punching stroke along a stroke axis (Z) toward a workpiece to be punched, wherein the punching tool is configured to move away from the punched workpiece during a return stroke, and wherein the punching tool comprises a first component and a second component configured to be coupled hydraulically for concurrent movement along the stroke axis (Z); and a punching drive for moving the first component along the stroke axis (Z), wherein the punching apparatus is configured to move the second component relative to the first component at a first transmission ratio during the punching stroke, wherein the punching apparatus is configured to move the second component relative to the first component at a second transmission ratio different than the first transmission ratio in response to a reaction force (F) of the workpiece exceeding a threshold value of the punching drive during the punching stroke, and wherein the punching apparatus is configured to maintain a relative position (P) of the first component with respect to the second component taken up after the workpiece has been punched through with a punching force greater than the threshold value along at least a portion of the return stroke of the punching tool along the stroke axis (Z).

2. The punching apparatus of claim 1, wherein the second component includes a cavity and an end portion of the first component is configured as a piston projecting into the cavity.

3. The punching apparatus of claim 1, further comprising a first hydraulic cylinder, wherein the first component comprises a first piston guided in the first hydraulic cylinder so as to be displaceable in the stroke direction (Z).

4. The punching apparatus of claim 3, wherein the second component comprises a second piston guided in a second hydraulic cylinder so as to be displaceable in the stroke direction (Z).

5. The punching apparatus of claim 4, wherein an effective piston surface of the first component matches an effective piston surface of the second component during operation of the punching apparatus at the first transmission ratio.

6. The punching apparatus of claim 4, wherein the first hydraulic cylinder and the second hydraulic cylinder are configured to act as synchronized cylinders during operation at the first transmission ratio and during operation at the second transmission ratio.

7. The punching apparatus of claim 3, further comprising a positionally fixed piston of the first hydraulic cylinder projecting into a cavity of the first component.

8. The punching apparatus of claim 4, further comprising an auxiliary cylinder in the second hydraulic cylinder and a further piston of the second component projecting into the auxiliary cylinder.

9. The punching apparatus of claim 1, wherein the second component comprises a punching die.

10. The punching apparatus of claim 1, further comprising a ram comprising a piston configured to be guided in a cavity of the first component so as to be displaceable in the stroke direction (Z).

11. The punching apparatus of claim 10, wherein the ram comprises a further piston configured to be guided in the cavity of the second component so as to be displaceable in the stroke direction (Z).

12. The punching apparatus of claim 1, further comprising: at least one hydraulic switching valve for switching between movement of the second component with respect to the first component at the first transmission ratio and movement of the second component with respect to the first component at the second transmission ratio.

13. The punching apparatus of claim 12, wherein the switching valve comprises a control line connected to a pressure-side pressure chamber of the punching tool to switch between the movement of the second component with respect to the first component at the first transmission ratio and the movement of the second component with respect to the first component at the second transmission ratio in the event that the threshold value of the reaction force (F) is exceeded.

14. The punching apparatus of claim 1, further comprising: a resetting device comprising at least one hydraulic reset valve for changing the relative position of the second component with respect to the first component during the return stroke along the stroke axis (Z).

15. The punching apparatus of claim 14, in which the at least one hydraulic reset valve is configured as a control valve.

16. The punching apparatus of claim 14, wherein the reset valve is configured to hydraulically isolate at least one pressure chamber of the second hydraulic cylinder to change the positioning of the second component with respect to the first component.

17. The punching apparatus of claim 12, wherein the switching valve is configured to establish a hydraulic connection between a pressure chamber of a first hydraulic cylinder configured to guide a first piston of the first component in the stroke direction (Z) and further comprising a reservoir for a hydraulic fluid used in the first hydraulic cylinder in the event that the threshold value of the reaction force (F) is exceeded.

18. The punching apparatus of claim 12, wherein the switching valve is configured to establish a hydraulic connection between a first and a second pressure chamber of a first hydraulic cylinder configured to guide a first piston of the first component in the stroke direction (Z) in the event that the threshold value of the reaction force (F) is exceeded.

19. The punching apparatus of claim 18, wherein the switching valve is configured to break a hydraulic connection between a first and a second pressure chamber of a second hydraulic cylinder configured to guide a second piston of the second component in the event that the threshold value of the reaction force (F) is exceeded.

20. The punching apparatus as claimed in claim 19, further comprising a resetting device comprising at least one hydraulic reset valve for changing the relative position of the second component with respect to the first component during the return stroke along the stroke axis (Z), wherein the hydraulic reset valve is configured to establish a hydraulic connection between a first pressure chamber and a second pressure chamber of a cavity formed by the first component to change the positioning of the second component with respect to the first component.

21. The punching apparatus of claim 1, further comprising: a control device for controlling the punching drive and for controlling at least one reset valve of a resetting device comprising at least one hydraulic reset valve for changing the relative position of the second component with respect to the first component during the return stroke along the stroke axis (Z).

22. A method for punching a workpiece, the method comprising: moving a punching tool comprising a first component and a second component configured to be coupled hydraulically in a punching stroke along a stroke axis (Z) toward a workpiece to be punched, wherein moving the punching tool comprises moving the second component with respect to the first component at a first transmission ratio during the punching stroke, and moving the second component with respect to the first component at a second transmission ratio, different than the first transmission ratio, in response to the workpiece transferring a reaction force (F) that exceeds a threshold value to the punching tool during the punching stroke; punching through the workpiece by the punching tool; and moving the punching tool away from the punched workpiece during a return stroke along the stroke axis (Z), wherein the punching apparatus maintains a relative position (P) of the first component with respect to the second component taken up after the workpiece has been punched through with a punching force greater than the threshold value and maintained at least along a portion of the return stroke of the punching tool.

23. The method as claimed in claim 22, further comprising: changing the relative position (P) of the first component with respect to the second component during the return stroke along the stroke axis (Z) to re-establish a prior relative position (P) of the first component with respect to the second component held before the threshold value of the reaction force (F) was exceeded.

Description

DESCRIPTION OF DRAWINGS

(1) FIG. 1 shows a schematic illustration of an exemplary embodiment of a punching apparatus having a punching tool with two components that are movable relative to one another along a stroke axis at the beginning of a punching stroke.

(2) FIG. 2 shows an illustration similar to FIG. 1 with the punching tool at the bottom dead center of a punching stroke in which the two components take up a changed relative position with respect to one another.

(3) FIG. 3 shows an illustration similar to FIG. 1 and FIG. 2 with the punching tool in a resetting position in which the two components have been displaced along the stroke axis during a return stroke while maintaining their relative position.

(4) FIG. 4 shows an illustration of a further example of another embodiment of a punching apparatus in which the two components are guided so as to be displaceable in the stroke direction in two synchronized cylinders.

(5) FIG. 5 shows an illustration of a further example of another embodiment of a punching apparatus having two components, in the cavities of which a piston rod of a ram is guided in a displaceable manner, during operation at a first transmission ratio.

(6) FIG. 6 shows an illustration of the punching apparatus of FIG. 5 during operation at a second transmission ratio different from the first.

DETAILED DESCRIPTION

(7) In the following description, identical reference numbers and letters are used for identical or functionally identical components.

(8) FIG. 1 shows an example of a structure of a punching apparatus 1 for punching a plate-shaped workpiece 2 that is arranged in a supporting plane (XY plane) on a die 3, which is arranged at a predetermined spacing L from a housing 4 of the upper part of the punching apparatus 1. Both the die 3 and the housing 4 are positionally fixed in the example shown here, i.e. they do not move along a stroke axis (Z direction) perpendicular to the supporting plane. This is not the case for a punching tool 5, likewise shown in FIG. 1, which, like all parts of the punching apparatus 1 that are movable along the stroke axis Z, is illustrated without hatching to distinguish it from the positionally fixed components.

(9) The punching tool 5 that is movable or displaceable along the stroke axis Z comprises a first component 6 and a second component 7, the relative position P of which along the stroke axis Z can be changed, as is described in more detail further below. The first component 6 is coupled to a punching drive 8 that is configured as an electric drive for example in the form of a torque motor that acts on a threaded nut 8a that sets a ball screw 9 formed on the first component 6 into rotation in order to displace the first component 6 along the stroke axis Z.

(10) The first component 6 of the punching apparatus 1 has a piston rod 10 on which a first, upper piston 11 is formed, which is guided so as to be displaceable in the stroke direction Z in a first, upper hydraulic cylinder 12. In a corresponding manner, the second component 7 also has a piston 13, which is guided so as to be displaceable in a second, lower hydraulic cylinder 14 formed in the housing 4. The second component 7 includes a cavity 15, into which an end portion, forming a further, lower piston 16, of the piston rod 10 of the first component 6 projects. As is likewise discernible in FIG. 1, a punching die 17 is attached to the lower end, facing the workpiece 2, of the second component 7. The punching die 17 is moved so as to abut the workpiece 2 during the punching operation.

(11) FIG. 1 shows the punching tool 5 at the beginning of the punching operation, i.e. at the top dead center of a reciprocating movement, which the punching tool 5 executes in a punching stroke toward the workpiece 2 and in a return stroke, after the punching-through operation, positioned away from the workpiece 2. In the starting position shown in FIG. 1, the two components 6, 7 take up a relative position P with respect to one another along the stroke axis Z, in which the top side of the lower piston 16 of the first component 6 bears against an axial shoulder 18 of the cavity 15 in the second component 7. This relative position P is (arbitrarily) set as the zero position, i.e., P=0 in the starting position shown in FIG. 1.

(12) Starting from the starting position shown in FIG. 1, the punching tool 5 is moved toward the workpiece 2 in that the punching drive 8 moves the first component 6 downward along the stroke axis Z. A first, upper pressure chamber D1 of the first hydraulic cylinder 12 is hydraulically coupled to a second, lower pressure chamber D2 of the second hydraulic cylinder 14 via a second switching valve UV2 that is located in a first switched position and serves as a reset valve. In a corresponding manner, a second, lower pressure chamber D2 of the first hydraulic cylinder 12 is hydraulically coupled to a first, upper pressure chamber D1 of the second hydraulic cylinder 14 via a first switching valve UV1 that is located in a first switched position. The first pressure chamber D1 of the second hydraulic cylinder 14 is permanently hydraulically connected to a third pressure chamber D3, located in the cavity 15 of the second component 7, of the second hydraulic cylinder 14.

(13) The piston surface B3 on the top side of the first piston 11 of the first component 6 is the same size as the piston surface A3 on the underside of the piston 13 of the second component 7. In a corresponding manner, the piston surface B2 on the underside of the first piston 11 of the first component 6 is also the same size as the piston surface A2 on the top side of the piston 13 of the second component 7. In the starting position shown in FIG. 1, in which the lower piston 16 of the first component 6 bears against the shoulder 18 of the second component 7, the piston surface A3 on the underside of the lower piston 16 of the second component 7 plays no active part in punching. The effective piston surface B3-B2 of the upper piston 11 of the first component 6 and the effective piston surface A3-A2 of the piston 13 of the second component 7 are thus the same size. As a result, the two components 6, 7 are displaced along the stroke axis Z at a transmission ratio of 1:1, i.e. the two components 6, 7 are moved toward the workpiece 2 during the punching stroke, without the relative position P thereof along the stroke axis Z changing.

(14) If a reaction force F, which the workpiece 2 exerts on the punching tool 5, does not rise above a predetermined threshold value, the drive force of the electric punching drive 8 is sufficient to punch through the workpiece 2. In this case, both the punching stroke and the return stroke of the punching tool 5 take place without the relative position of the two components 6, 7 changing, i.e., P=0 is maintained throughout the reciprocating movement.

(15) If, during the punching operation, the reaction force F of the workpiece 2 and thus the pressure in the upper pressure chamber D1 of the second hydraulic cylinder 14 rises to such an extent that a pressurized control line 19 hydraulically connected to the upper pressure chamber D1 switches the first switching valve UV1 from the first switched state shown in FIG. 1 into a second switched state shown in FIG. 2, operation is switched between the first operating state with the first transmission ratio (1:1) between the first component 6 and the second component 7 and a second operating state with a second, greater transmission ratio (for example about 5:1 or more), as is explained in the following text with reference to FIG. 2.

(16) In the second operating state (transmission operation), the first switching valve UV1 connects the second, lower pressure chamber D2 of the first hydraulic cylinder 12 to a reservoir 20 for the hydraulic fluid in the form of a high-pressure tank to which a pressure of for example about 10 bar is applied. The reservoir 20 is connected to the upper pressure chamber D1 of the first hydraulic cylinder 12 and to the upper and the lower pressure chamber D1, D2 of the second hydraulic cylinder 14 via three non-return valves RV1 to RV3. When the first component 6 moves toward the workpiece 2, the hydraulic fluid is delivered from the second pressure chamber D2 of the first hydraulic cylinder 12 into the reservoir 20 in transmission operation. At the same time, hydraulic fluid is delivered from the second pressure chamber D2 of the second hydraulic cylinder 14 into the upper pressure chamber D1 of the first hydraulic cylinder 12, since both are hydraulically connected in the second switched position of the first switching valve UV1. The lower piston 16 of the first component 6 is raised from the shoulder 18 of the second component 7 in transmission operation so as to result in a transmission ratio that is formed from the sum of the piston surface A2 of the piston 13 of the second component 7 and the piston surface A1 in the further pressure chamber D3 hydraulically connected to the upper pressure chamber D1 to the piston surface A1 of the piston 16 in the further pressure chamber D3, i.e., for the transmission ratio: A2/A1.

(17) In the case of a circular piston surface A1 with a diameter of 35 cm and a circular piston surface A2 with a diameter of 100 cm, a transmission ratio of about 8:1 arises in transmission operation. The first component 6 thus travels eight times the distance traveled by the second component 7 along the stroke axis Z, with the result that the pressure that the second component 7 exerts on the workpiece increases in a corresponding manner. The hydraulic fluid that is missing from the upper pressure chamber D1 of the first hydraulic cylinder 12 on account of the different speeds of the two components 6, 7 in transmission operation is fed from the reservoir or from the tank 20 via the first non-return valve RV1.

(18) FIG. 2 shows the punching tool 5 in transmission operation at the bottom dead center of the movement along the stroke axis, at which the workpiece 2 has been fully punched through. On account of the transmission ratio, different than 1:1, of 8:1 in transmission operation, the two components 6, 7 exhibit a relative position P different from zero immediately after punching through the workpiece 2, said relative position P depending on the distance traveled along the stroke axis Z at the second transmission ratio. Unlike the depiction in FIG. 2, the punching tool 5 can be displaced further downward after punching through the workpiece 2, until the bottom dead center of the movement is reached. Since, after the workpiece has been punched through, the further downward movement takes place at the transmission ratio of 1:1, the relative position P of the two components 6, 7 does not change in this case.

(19) As is described below, the relative position P that the two components 6, 7 take up with respect to one another at the end of the downward movement is maintained at least along a section of the return stroke of the punching tool 5 into the starting position shown in FIG. 1, i.e., the relative position P is as it were frozen until a position referred to as the resetting position along the stroke axis Z is reached, at which the relative position P of the two components 6, 7 is transferred into the original relative position P=0.

(20) Since, after the workpiece 2 has been fully punched through, the reaction force F of the workpiece 2 and thus the pressure in the upper pressure chamber D1 of the second hydraulic piston 14 drops abruptly, the first switching valve UV1 is switched from the second switched state into the first switched state via the control line 19. Since the transmission ratio is 1:1 in the first switched position of the first switching valve UV1, during the movement of the first component 6 away from the workpiece 2 by means of the punching drive 8, the second component 7 is entrained without the relative position P changing. Thus, in the punching apparatus 1, it is not necessary to carry out a relative movement between the first component 6 and the second component 7 at the beginning of the return stroke in transmission operation. Such a movement would have the result that, for the movement of the second component 7 upward out of the workpiece 2, a comparatively large stroke movement of the first component 6 and thus a comparatively long duration would be necessary on account of the transmission ratio of 8:1. As a result of the movement of the punching tool 5 at least at the beginning of the return stroke in normal operation, the punching tool 5 and the second component 7 can be withdrawn quickly from the workpiece 2, such that, in the region of the supporting plane, the workpiece 2 can be quickly repositioned for a subsequent punching stroke. Since resetting into the original relative position P also does not take place in transmission operation, the duration that is required overall for the return stroke is also considerably reduced.

(21) In order to re-establish the original relative position P of the two components 6, 7 with respect to one another, the punching tool 5 is moved into a resetting position shown in FIG. 3, which is located between the top dead center position shown in FIG. 1 and the bottom dead center position shown in FIG. 2 of the movement along the stroke axis Z. The resetting position should be selected such that at least that section of the return stroke that is required to withdraw the punching die 17 from the workpiece 2 has already been traveled along during the upward movement. If the reset is achieved, as in the example shown, in that the second component 7 is prevented from moving along the stroke axis Z, it can be favorable for the resetting position to be distant from the top dead center of the movement of the punching tool 5 at least by the amount of the relative position P.

(22) To effect the reset, an electronic control device 21 of the punching apparatus 1 acts on the punching drive 8 to move the first component 6 and thus the punching tool 5 into the desired resetting position along the stroke axis Z. Once the desired resetting position has been reached, the control device 21 acts on the second switching valve UV2 to switch the latter from the first switched state into a second, in which the second switching valve UV2 serves as a reset valve. The control device 21 and the reset valve UV2 together form a resetting device 23 of the punching apparatus 1. The activation of the reset valve UV2 by the control device 21 can take place, for example, by way of a pneumatic control line illustrated by a dashed line. In the second switched position, shown in FIG. 3, of the reset valve UV2, the lower pressure chamber D2 of the second hydraulic cylinder 14 is hydraulically isolated. With the aid of compression springs 22 provided in the upper pressure chamber D1 of the second hydraulic cylinder 14, the second component 7 is fixed or clamped in place in the second hydraulic cylinder 14, such that it can no longer be displaced along the stroke axis Z in the second switched state of the reset valve UV2.

(23) While the second component 7 is clamped in place in the stroke direction Z, the second component 7 is displaced further upward until the two components 6, 7 take up their original relative position P=0 with respect to one another, in which the first and the second component 6, 7 bear against one another at the shoulder 18. During this resetting movement, the reset valve UV2 establishes a hydraulic connection between the upper pressure chamber D1 of the first hydraulic cylinder 12 and the reservoir 20 in order to deliver the hydraulic fluid passed into the reservoir 20 in transmission operation back into the upper pressure chamber D1. After the reset, the reset valve UV2 can be deactivated and the two components 6, 7 can be displaced along the stroke axis Z again at the top dead center (cf. FIG. 1) without any change in the relative position P=0. The resetting position along the stroke axis Z can also be selected such that, after the first component 6 has been displaced so as to reach the original relative position P=0, the starting position, shown in FIG. 1, of the punching tool 5 along the stroke axis Z is taken up.

(24) To suitably control or regulate the punching tool 5 or the punching drive 8 by means of the control device 21, the punching apparatus 1 has, for example, an optical sensor 24 for determining the position of the second component 7 along the stroke axis Z. Further sensors for determining the position of the first component 6 and/or for determining the reaction force F that the workpiece 2 exerts on the punching tool 5 can be provided in the punching apparatus 1.

(25) A further exemplary embodiment of a punching apparatus 1, which is shown in FIG. 4, is based on the basic principle described further above in conjunction with FIG. 1 to FIG. 3. In the punching apparatus 1 shown in FIG. 4, the upper component 6 is moved along the stroke axis Z via an electric punching drive 8 in the form of a linear drive and a hydraulic fluid transmission is realized with the aid of the further component 7. The major difference of the punching apparatus in FIG. 4 with respect to the punching apparatus 1 described further above is that, in the example shown in FIG. 4, both the first hydraulic cylinder 12 and the second hydraulic cylinder 14 are configured as synchronized cylinders, i.e. the piston surfaces that act against one another, or the corresponding surfaces of the pressure chambers, are the same size in each of the two hydraulic cylinders 12, 14, as is described below.

(26) The first, upper hydraulic cylinder 12 has a first, upper pressure chamber D1. Formed in the first component 6 of the punching apparatus 1 in FIG. 4 is a cavity 25 into which a positionally fixed plunger piston 26 of the housing 4 projects and in which a second pressure chamber D2 is formed. The first hydraulic cylinder 12 also has a lower, third pressure chamber D3. The hydraulically effective surfaces of the pressure chambers D1 to D3 and the hydraulically effective surfaces of the piston 11 of the first component 6 are matched to one another such that the upper hydraulic cylinder 12 forms a synchronized cylinder.

(27) The second, lower hydraulic cylinder 14 likewise has a first, upper pressure chamber D1 and a second, lower pressure chamber D2, between which a piston 13 of the second component 6 is guided in a displaceable manner. The second hydraulic cylinder 14 has an auxiliary cylinder 27 into which a further piston 28 of the second component 7 projects in order to reduce the overall height of the second hydraulic cylinder 14. The further piston 28 is rigidly connected to the piston 13, guided in parallel, of the second component 7 via a carrier plate 29. A punching die of the punching apparatus 1 can be attached to the carrier plate 29 in order to punch through a workpiece 2 (not depicted in FIG. 4). Additionally, formed in the auxiliary cylinder 27 is a third pressure chamber D3 that is permanently hydraulically connected to the second, lower pressure chamber D2 of the second hydraulic cylinder 14. The hydraulically effective surfaces of the pressure chambers D1, D2, D3 and the corresponding hydraulically effective surfaces of the piston 16 and of the further piston 28 are matched to one another such that the lower hydraulic cylinder 14 likewise forms a synchronized cylinder.

(28) In normal operation, i.e. in the position, shown in FIG. 4, of the two switching valves UV1, UV2, both the third pressure chamber D3 and the second pressure chamber D2 of the upper hydraulic cylinder 12 are hydraulically connected to the upper pressure chamber D1 of the lower hydraulic cylinder 14. The hydraulically effective surfaces of the two pressure chambers D2, D3 (and the associated piston surfaces) of the upper hydraulic cylinder 12 are the same size as the hydraulically effective surface of the upper pressure chamber D1 of the lower hydraulic cylinder 14. The first, upper pressure chamber D1 of the first hydraulic cylinder 12 is permanently connected to the second, lower pressure chamber D2 (and thus also to the third pressure chamber D3) of the second hydraulic cylinder 14. The hydraulically effective surface of the upper pressure chamber D1 of the upper hydraulic cylinder 12 corresponds to the hydraulically effective surfaces of the second and third pressure chambers D2, D3 of the lower hydraulic cylinder 14. In this way, in normal operation, a transmission ratio of 1:1 is realized, i.e., in normal operation, the two components 6, 7 move in the relative position P=0 shown in FIG. 4, in which the lower piston 16 of the first component 6 bears against a shoulder 18 of the second component 7, along the stroke axis Z.

(29) In transmission operation, i.e. when a threshold value of the reaction force F that the workpiece 2 exerts on the punching tool 5 is exceeded, the pressure in the upper pressure chamber D1 of the lower hydraulic cylinder 14 increases and the first switching valve UV1 is activated via the control line 19 hydraulically connected thereto and switched from the first switched state into the second. In the second switched state, the first switching valve UV1 establishes a hydraulic connection between the first pressure chamber D1 and the third pressure chamber D3 of the upper hydraulic cylinder 12 and breaks the hydraulic connection between the third pressure chamber D3 of the upper hydraulic cylinder 12 and the first pressure chamber D1 of the lower hydraulic cylinder 14. The hydraulically effective surfaces of the first pressure chamber D1 and of the third pressure chamber D3 are opposed, such that the hydraulically effective surface of the second pressure chamber D2, which acts on the hydraulically effective surface of the first pressure chamber D1 of the second hydraulic cylinder 14, arises as the resulting hydraulically effective surface of the first hydraulic cylinder 12. On account of the different sizes of the hydraulically effective surfaces of the second pressure chamber D2 of the first hydraulic cylinder 12 and of the first pressure chamber D1 of the second hydraulic cylinder 14, a transmission ratio of D1/D2, which can be for example about 5:1 or more, results in transmission operation.

(30) After the workpiece 2 has been punched through, the pressure in the upper pressure chamber D1 of the second hydraulic cylinder 14 drops quickly and the first switching valve UV1 switches back into the first switched state. The punching tool 5 is retracted, in normal operation, i.e. at a transmission ratio of 1:1, along the stroke axis Z until a resetting position is reached. In the resetting position, as was described further above in conjunction with FIG. 1 to FIG. 3, the second component 7 is clamped in place in the second hydraulic cylinder 14, to displace the first component 6 relative to the second component 7 by the punching drive 8 and to re-establish the original relative position P=0, shown in FIG. 4, of the two components 6, 7.

(31) To allow this resetting movement, both the first switching valve UV1 and a second switching valve UV2 that serves as a reset valve are switched simultaneously from the first switched state into the second, in that the control device 21 acts on both switching valves UV1, UV2 by a respective pneumatic control line. As a result of the activation of both switching valves UV1, UV2, the upper pressure chamber D1 of the second hydraulic cylinder 14 is hydraulically isolated such that the second component 7 guided therein cannot be displaced further upward. At the same time, the second switching valve UV2 establishes a hydraulic connection between the first pressure chamber D1 and the third pressure chamber D3 of the upper hydraulic cylinder 12, i.e., the upper hydraulic cylinder is short-circuited.

(32) If the two switching valves UV1, UV2 are not switched exactly synchronously, this does not have any negative effects on the punching apparatus 1, i.e., it does not result in stresses. The punching apparatus 1 shown in FIG. 4 additionally has the advantage that no pressure tank or the like for holding hydraulic fluid is required, since the two hydraulic cylinders 12, 14 are configured as synchronized cylinders and the hydraulically effective surfaces of the two hydraulic cylinders 12, 14 are matched to one another such that, even in transmission operation, i.e. at the second transmission ratio, synchronism is ensured. The matching of the hydraulically effective surfaces is realized in FIG. 4 in that, for the pressure chambers D1, D2, D3 of the upper hydraulic cylinder 12 and for the first pressure chamber D1 of the lower hydraulic cylinder 14, D1=D2+D3. Moreover, in the example shown, D2=D3.

(33) As can be seen in FIG. 4, only one reservoir 20 with a very small capacity is required, the reservoir 20 being connected via two non-return valves RV1, RV2 to the third pressure chamber D3 of the upper hydraulic cylinder 12 and to the second pressure chamber D2 of the lower hydraulic cylinder 14 (i.e., to the unpressurized side). The reservoir 20 serves as a compensating volume, i.e., as a temperature compensating volume and as a compression compensating volume. Overall, the punching apparatus 1 shown in FIG. 4 manages with a small number of component parts and can therefore be realized with a compact design. In addition, during surface switching, i.e., during switching between the first transmission ratio and the second transmission ratio, there is no jump in force but rather a continuous transition, such that the (closed) hydraulic circuit and in particular the switching valves UV1, UV2 are not excessively loaded. Additionally, in the embodiment shown in FIG. 4, no force transmission via the stop 18 is necessary during the return stroke, i.e., the shoulder 18 serves merely for safety and is not absolutely necessary for carrying out the punching stroke.

(34) A further embodiment of the punching apparatus 1 is described in the following text with reference to FIG. 5 and FIG. 6. A major difference between the punching apparatus 1 described in FIG. 5 and FIG. 6 and the punching apparatuses 1 described further above is that a ram 30 is additionally provided, which serves to punch the workpiece 2 and that can be displaced relative to the first component 6 and the second component 7 along the stroke axis Z. The ram 30 has a piston rod 33 on which a first piston 31 and a second piston 32 are formed. The first piston 31 of the ram 30 is guided so as to be displaceable in the stroke direction Z in a cavity 25 of the first component 6. The second component 7, too, has a cavity 15 in which the second piston 32 of the ram 30 is guided so as to be displaceable in the stroke direction Z. On its outer side, the second component 7 additionally has a piston 13 that is guided so as to be displaceable in a second or only hydraulic cylinder 14 in the housing 4 of the punching apparatus 1. By contrast, the first component 6 is not guided so as to be displaceable in a hydraulic cylinder, but is driven directly by means of an electric punching drive 8 that can be configured for example as a linear drive, such that the first component 6 acts as a linear actuator.

(35) In the position, shown in FIG. 5, of the two components 6, 7 relative to one another, the piston 16 of the first component 6 bears with its upper side against an axial stop 18 of the second component 7, i.e., the two components 6, 7 take up a relative position P=0 with respect to one another. In normal operation, in which a first switching valve UV1 is in a first switched state, a hydraulic connection is established between an upper pressure chamber D1 and a lower pressure chamber D2 of the hydraulic cylinder 14. The piston 13 of the second component 7 or of the hydraulic cylinder 14 is configured as a synchronized cylinder, i.e. the upper and lower piston surfaces C1, C2 of the piston 13 are the same size. In normal operation, a first, upper pressure chamber D3 of the cavity 15 in the second component 7 is hydraulically separated by the second switching valve UV2 in a first switched state, i.e. the upper piston 31 of the ram 30 is clamped in place such that the ram 30 is displaced along the stroke axis Z in a conjoint or concurrent movement together with the first component 6 and the second component 7.

(36) FIG. 6 shows the punching apparatus 1 in transmission operation, in which the threshold value of the reaction force F of the workpiece 2 has been exceeded, such that the pressure in an upper pressure chamber D3 of the cavity 13 in the lower component 7 has risen to such an extent that the first switching valve UV1 has switched into the second switched position via the control line 19. In the second switched position, the upper pressure chamber D1 and the lower pressure chamber D2 of the hydraulic cylinder 14 are hydraulically separated such that the second component 7 is clamped in place in the hydraulic cylinder 14. The first component 6, acting as a linear actuator, is displaced further downward by means of the punching drive 8 in transmission operation until the workpiece has been fully punched through and the two components 6, 7 take up the position P, shown in FIG. 6, relative to one another along the stroke axis Z. On account of the smaller effective piston surface A1 of the piston 16 of the first component 6 relative to the effective piston surface B1 on the top side of the lower piston 32 of the ram 30, a transmission ratio of B1/A1 is created in transmission operation. In this case, the fact that the lower pressure chamber D4 of the cavity 15 of the second component 15 is permanently hydraulically connected to an upper pressure chamber D3 of the cavity 25 of the upper component 6 is exploited.

(37) During a return stroke, to displace the first component 6 relative to the second component 7 and in the process to create the relative position P=0, shown in FIG. 5, of the two components 6, 7 again, the second switching valve UV2, serving as a reset valve, is switched into the second switched state during the return stroke, i.e. during the movement of the first component 6 along the stroke axis. In the second switched state, the second switching valve UV2 connects the upper pressure chamber D3 to the lower pressure chamber D4 of the cavity 25 of the first component 6.

(38) The second switching valve UV2 is a control valve in which the flow rate in the second switched state can be set or regulated by means of the control device 21 depending on the resetting speed. In this way, during the movement of the first component 6 along the stroke axis Z, the relative movement between the ram 30 and the first component 6 can be directly influenced. Depending on the valve opening or on the throughflow through the second switching valve UV2 that serves as a reset valve, the return stroke or the relative movement between the ram 30 and the first component 6 and between the first component 6 and the second component 7 can be regulated, with the result that the dynamics during the return stroke can be substantially increased. The regulation can take place such that, at the end of the braking movement, the reset is complete.

(39) The difference in volume between the hydraulic fluid, which passes into the respective pressure chambers D1, D2, D3, D4, D3, D4 on account of the different speeds of the ram 30 and of the first component 6 and of the second component 7, can be compensated by two reservoirs (pressure tanks) 20, 20a, into which the corresponding fluid volume is delivered or from which the required fluid volume of hydraulic fluid can be removed. For this purpose, and to compensate leakage losses of the hydraulic fluid, three non-return valves RV1 to RV3 are arranged in the punching apparatus 1.

(40) To reset the ram 30 into the relative position P=0 shown in FIG. 5, a first compression spring 34, which defines the zero position of the first component 6 relative to the first piston 31, is arranged in the upper pressure chamber D3 of the cavity 25 of the first component 6, and a second compression spring 35, which serves for resetting and that exerts on the upper pressure chamber D3 of the cavity 13 a force that increases the pressure in the upper pressure chamber D3, is arranged in the second, lower pressure chamber D4 of the second component 7. The second compression spring 35 thus influences the threshold value of the reaction force F at which switching takes place between the first transmission ratio and the second transmission ratio.

(41) The exemplary embodiments, described further above, of the punching apparatus 1 can also be modified. For example, it is possible to dispense with the provision of a shoulder 18 on the second component 7, or a piston, which interacts with such a shoulder 18, at the lower end of the first component 6 is not absolutely necessary. In this case, the lower end face of the portion, configured as a piston rod, of the first component 6 can serve as a hydraulically effective surface, for example. Also, in the embodiment described in FIG. 1 to FIG. 3, the second component 6 can be clamped in place in the manner described in FIG. 5 and FIG. 6, i.e., in that the two compression chambers D1, D2 of the second hydraulic cylinder 14 are hydraulically isolated such that the piston 13 of the second component 7 is hydraulically clamped in place.

(42) In summary, high dynamics can be achieved during the return stroke in the punching apparatuses 1 described further above, since in particular the beginning of the return stroke, i.e. the beginning of the movement from the bottom dead center, is not carried out at the second, greater transmission ratio, but with the relative position of the two components 6, 7 being maintained, respectively, at the first transmission ratio. In this way, a highly dynamic punching movement with two (or possibly more) force stages can be realized with a closed, energy-efficient hydraulic circuit.

OTHER EMBODIMENTS

(43) A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.