The invention relates to dies which can be mounted on hydraulic or mechanical presses for machining sheet metals, such as drawing, shearing, bending. In particular, the invention relates to a hydraulic driving system which can be coupled to a die for a press for controllably driving operating elements of said die, intended to interact directly with the workpiece (e.g. for carrying out drawing, bending, cutting, etc.) or for driving sheet-press elements or sheet-press cushions.

The known dies for machining sheet metal typically comprise two half-dies, the lower half die (die) of which is generally mounted on a fixed base of the press, while the upper half die (counter-die) is mounted on the movable part of the press provided with vertical movement, for closing and opening the die itself.

At least one of the two half-dies is provided with one or more operating elements intended to interact with the workpiece to lock it or remove it on/from the die (sheet-press elements) or to perform operations thereon (for example drawing, bending, punching, etc.) or associated with and driving sheet-press cushions.

These operating elements, in particular when they act as sheet-press elements, are moved by respective pneumatic or hydraulic cylinders supplied with fluid (air, nitrogen or oil) at a predefined and settable pressure by means of driving systems which comprise a hydraulic reservoir or accumulator of the pressurized fluid and an appropriate supply circuit. The hydraulic accumulator typically comprises therein a pressurized gas bag (generally nitrogen) which acts as an elastic element to maintain the oil pressure in the supply circuit substantially constant.

The pneumatic or hydraulic cylinders and the relative driving systems therefore guarantee an elastic behaviour of the sheet-press elements which are completely compressed in a die closing step, while extending elastically and automatically with the opening of the die to allow the extraction of the workpiece.

In some machinings, however, this automatic movement of the sheet-press elements at the opening of the die is not acceptable because it can lead to damaging the workpiece, the extraction of which must be carried out only after the complete opening of the die, with the two half-dies suitably spaced apart.

For this purpose, driving systems, in particular hydraulic ones, are known, provided with valves adapted to control the movement of the rods of the hydraulic cylinders of the sheet- press elements. In such systems, a rear or thrust chamber of the hydraulic cylinder intended to receive pressurized oil acting on the piston to extend the sheet-press cushion may be isolated from the hydraulic accumulator by closing a corresponding valve. In this way, the hydraulic cylinder can be locked in the position where the sheet-press element is fully compressed in the die closing position and remain in that compressed position even after the opening of the die.

However, due to the high oil pressure inside the cylinder thrust chamber, and the elasticity of the seals and supply circuit ducts, a minimum movement of the cylinder rod, i.e. the sheet-press cushion, may occur when the die is opened, i.e. when the sheet-press element is free to move. This uncontrolled, albeit minor, movement of the sheet-press element is nevertheless undesirable in some applications because it can lead to damaging the workpiece.

An object of the present invention is to improve the known hydraulic driving systems applicable to a press die for machining sheet metals, to drive in particular sheet-press elements or cushions of said die, in particular when opening the latter after machining the workpiece.

Another object is to provide a hydraulic driving system which allows keeping operating elements of the die, in particular sheet-press elements, locked in the retracted position with the opening of the die so as not to damage the piece.

A further object is to realize a hydraulic driving system for press dies which is simple and inexpensive to realize, reliable and safe in operation and which does not require any restoration after each operating stroke of the press.

These objects and others are achieved by a hydraulic driving system according to one of the claims set forth below.

The invention can be better understood and implemented with reference to the attached drawings which illustrate some exemplifying and non-limiting embodiments thereof, in which:

figure 1 is a diagram of the hydraulic driving system of the invention associated with a press die for machining sheet metals, adapted to drive at least one operating element, for example a sheet-press element of said die, and arranged in an initial step in which said sheet-press cushion is in a maximum extension position;

figure 2 is the diagram of figure 1 illustrating the hydraulic driving system and the sheet-press element in a first operating step; figure 3 is the diagram of figure 1 illustrating the hydraulic driving system and the sheet-press element in a second operating step of compression, with the closing of the die;

figure 4 is the diagram of figure 1 illustrating the hydraulic driving system and the sheet-press element in a third operating step at the end of compression;

figure 5 is the diagram of figure 1 illustrating the hydraulic driving system and the sheet-press element in a fourth operating step with the die opening;

figure 6 is the diagram of figure 1 illustrating the hydraulic driving system and the sheet-press element in a fifth operating step restoring the sheet-press element to the maximum extension position for extracting the workpiece from the die;

figure 7 is a diagram of a first variant of the hydraulic driving system of the invention and the die in the second operating configuration of compression;

figure 8 is a diagram of a second variant of the hydraulic driving system of the invention and the die in the second operating configuration of compression;

figure 9 is a diagram of a third variant of the hydraulic driving system of the invention and the die in the second operating configuration of compression;

figure 10 is a perspective view of a die for presses provided with five hydraulic cylinders for driving respective sheet-press elements (not illustrated) and to which the hydraulic driving system of the invention is associated.

Referring to figures 1 to 6, the hydraulic driving system 1 of the invention is illustrated, associable with a die 50 for a mechanical or hydraulic press for machining sheet metals and arranged to drive in a controlled manner at least one operating element 2 of the die 50 arranged to interact directly with a workpiece 100 housed in the die 50 itself or associated with and driving a sheet-press cushion.

The die 50 of known type and schematically illustrated in figure 10 comprises, for example, two half-dies or half-shells, a lower half-die 50b of which is mountable on a fixed base of a known type of press, while an upper half-die 50a is mounted on a movable part of the press provided with vertical movement, for closing and opening the die 50 itself.

The hydraulic driving system 1 comprises at least one hydraulic cylinder 20, a first hydraulic accumulator 3, a second hydraulic accumulator 4, an auxiliary hydraulic cylinder 8 and a supply circuit 10 formed by a plurality of ducts/pipes adapted to connect to each other the hydraulic cylinder 20, the first hydraulic accumulator 3, the second hydraulic accumulator 4 and the auxiliary hydraulic cylinder 8. The hydraulic cylinder 20 is provided with a respective first piston 21 which forms a first thrust chamber 22 inside the aforementioned hydraulic cylinder 20 and is associated with the operating element 2 to move the latter along a respective operating axis X. More precisely, in the illustrated embodiment, the operating element 2 is a sheet-press element and comprises an end portion of a first rod 23 of the first piston 21 protruding from the hydraulic cylinder 20, said end portion 2 being intended to abut and press the workpiece 100 (sheet metal) inserted into the die 50 moved to the closing position C. In this closing position C, the two half-dies 50a, 50b are abutted on both sides of the workpiece 100 to carry out machining, such as bending, drawing, shearing, etc., thereon.

The first hydraulic accumulator 3 is connected to the first thrust chamber 22 of the hydraulic cylinder 20 and arranged to send a pressurized fluid, for example oil, into the first thrust chamber 22, so as to extend, i.e. move outwardly of the hydraulic cylinder 20 itself, the first piston 21 and position the latter and the sheet-press element 2 in a maximum extension position El, or to receive the pressurized fluid from the first thrust chamber 22 and allow the first piston 21 and the element 2 to be moved to a retracted position R, pushed by the workpiece 100 in the die 50 moved to a closing position C, to machine the aforementioned workpiece 100.

The second hydraulic accumulator 4 is connected to the first thrust chamber 22 and arranged to receive the pressurized fluid from the latter when the first piston 21 and the sheet-press element 2 are in the retracted position R so as to reduce a pressure of the fluid in the first thrust chamber 22 and prevent movements of the first piston 21 and the sheet-press element 2 along the operating axis X when the die 50 is moved to an opening position A, when processed, and therefore not compress the workpiece 100 against said sheet-press element 2 (figure 5). The pressure inside the second hydraulic accumulator 4 is significantly lower than that of the fluid present in the first thrust chamber 22.

The hydraulic driving system 1 further comprises a first valve 5 which is interposed between the first hydraulic accumulator 3 and the first thrust chamber 22 of the hydraulic cylinder 20 and is selectively operable or switchable to open or close a connection between the first hydraulic accumulator 3 and the first thrust chamber 22. More precisely, the first valve 5 is selectively switchable between a first open position D1 for connecting the first hydraulic accumulator 3 and the first thrust chamber 22, so as to move the first piston 21 and the sheet- press element 2 to the maximum extension position El, and a first closing position D2 for blocking a connection between the first hydraulic accumulator 3 and the first thrust chamber 22.

A first one-way valve 6 is interposed between the first hydraulic accumulator 3 and the first thrust chamber 22, positioned parallel to the first valve 5 to allow the fluid to flow at least from the first thrust chamber 22 to the first hydraulic accumulator 3, when the sheet-press element 2 and the first piston 21 are moved to the retracted position R by the workpiece 100 housed in the die 50 moved to the closing position C.

The first valve 5 and the first one-way valve 6 are inserted into a first portion 10a of the supply circuit 10.

A second valve 7 is interposed between the first thrust chamber 22 of the hydraulic cylinder 20 and the second hydraulic accumulator 4 and is selectively operable or switchable to open or close a connection between the first thrust chamber 22 and the second hydraulic accumulator 4. More precisely, the second valve 7 is selectively switchable between a second open position G1 for connecting the first thrust chamber 22 to the second hydraulic accumulator 4 and allowing the latter to receive the pressurized fluid from the first thrust chamber 22 when the first piston 21 and the sheet-press element 2 are in the retracted position R, and a second closing position G2 for blocking the connection between the first thrust chamber 22 and the second hydraulic accumulator 4, in particular during normal operation of the die 50 and the press.

The second valve 7 and the second hydraulic accumulator 4 are inserted into a second portion 10b of the supply circuit 10.

The second hydraulic accumulator 4 comprises an internal separator 41, which forms inside the second hydraulic accumulator 4 an accumulation chamber 42 arranged to receive the fluid coming from the first thrust chamber 22, and a first elastic element 44 inserted inside the second hydraulic accumulator 4 and acting on the internal separator 41 to move and/or maintain the latter in an internal position in which a volume of said accumulation chamber 42 is minimum. More precisely, the first elastic element 44 is inserted into a chamber formed by the internal separator 41 inside the second hydraulic accumulator 4 and opposite the accumulation chamber 42. The first elastic element 44 is selected to exert a limited thrust on the internal separator 41 so that the fluid coming from the first thrust chamber 22 by means of its pressure can easily move said internal separator 41 and penetrate inside the accumulation chamber 42 by filling the latter and emptying the first thrust chamber 22.

The auxiliary hydraulic cylinder 8 is interposed between the second hydraulic accumulator 4 and the first hydraulic accumulator 3 and arranged to receive the fluid from the second hydraulic accumulator 4, so as to empty it, when the second valve 7 is switched to the second closing position G2, the die 50 arranged in the opening position A. The auxiliary hydraulic cylinder 8 is also arranged to send the fluid, received from the second hydraulic accumulator 4, to the first hydraulic accumulator 3 when the die 50 is moved to the closing position C (figure 3). The auxiliary hydraulic cylinder 8 is inserted in a third portion 10c of the supply circuit 10.

The auxiliary hydraulic cylinder 8 comprises, in fact, a second piston 81 which forms inside said auxiliary hydraulic cylinder 8 a second thrust chamber 82 arranged to receive the fluid coming from the second hydraulic accumulator 4 and a second elastic element 84 inserted in the second thrust chamber 82 and acting on the second piston 81 to move and/or maintain the latter in a respective maximum extension position E2 in which the volume of the second thrust chamber 82 is maximum, where the pressure is limited so as to easily allow its filling by the fluid from the second hydraulic accumulator 4. It should be noted that the maximum volume of the accumulation chamber 42 of the second hydraulic accumulator 4 and the maximum volume of the second thrust chamber 82 of the auxiliary hydraulic cylinder 8 are substantially equal.

The second piston 81 comprises a second rod 83 adapted to be abutted and moved by the workpiece 100 when the die 50 is moved to the closing position C so as to send the fluid contained in the second thrust chamber 82 into the first hydraulic accumulator 3. In this way, the second thrust chamber 82 of the auxiliary hydraulic cylinder 8 is emptied and capable of receiving fluid from the second hydraulic accumulator 4 in a subsequent die 50 and press machining cycle.

A second one-way valve 16 is interposed between the second hydraulic accumulator 4 and the auxiliary hydraulic cylinder 8 to allow the fluid to flow only from the second hydraulic accumulator 4 to the auxiliary hydraulic cylinder 8. A third one-way valve 26 is interposed between the auxiliary hydraulic cylinder 8 and the first hydraulic accumulator 3 to allow the fluid to flow only from the auxiliary hydraulic cylinder 8 to the first hydraulic accumulator 3. It should be noted that the second one-way valve 16 opens at a pressure lower than that of the fluid contained in the second hydraulic accumulator 4 and at the pressure required to open the third one-way valve 26 to prevent the fluid exiting the second hydraulic accumulator 4 from flowing directly into the first hydraulic accumulator 3.

In the embodiment illustrated in figures 1 to 6, the selector valves 5, 7 comprise respective mechanically operated valves. More precisely, the first valve 5 is mechanically operated by a cam to switch between the two open and closing positions during the closing movement of the die 50. Similarly, the second valve 7 is mechanically operated by the workpiece 100 housed in the die 50 moved in closing.

The hydraulic driving system 1 of the invention further comprises a safety valve 9 interposed between the first valve 5 and the hydraulic cylinder 10 in the first portion 10a of the supply circuit 10 and arranged to discharge the latter in case of excessive and abnormal fluid pressure.

A pressure gauge 11 is provided to monitor the pressure of the fluid leaving the first hydraulic accumulator 3.

The operation of the hydraulic driving system 1 of the invention associated with a die 50 mountable on a press for carrying out sheet metal machining and arranged to controllably drive at least one operating element interacting with the workpiece, in particular a sheet- press element 2, of said die 50, provides an initial step in which the die 50 is arranged in the opening position A to receive a workpiece 100 and the first valve 5 is arranged in the first opening position D1 to allow the pressurized fluid coming from the first hydraulic accumulator 3 to fill the first thrust chamber 22 of the hydraulic cylinder 20 which moves the sheet-press element 2 (figure 1). More precisely, the pressurized fluid introduced into the first thrust chamber 22 moves the first piston 21 and the sheet-press element 2 along the operating axis X to the maximum extension position El so as to interact with the workpiece 100, in particular when the die 50 is moved to the closing position C. The second valve 7 is arranged in the second closing position G2 to prevent the pressurized fluid from escaping in this step from the first thrust chamber 22 of the hydraulic cylinder 20.

In a subsequent first operating step, the first valve 5 is switched, for example mechanically, and arranged in the first closing condition D2 to close the connection between the first hydraulic accumulator 3 and the hydraulic cylinder 20 and substantially block the supply to the latter (figure 2).

In a second operating step (compression step) in which the die 50 is moved in closing position C to perform the requested machining (drawing, bending, punching, etc.) on the workpiece 100, the latter abuts the sheet-press element 2 and pushes it together with the first piston 21 of the hydraulic cylinder 20 towards the retracted position R. In this second step, the first piston 21, moved from the maximum extension position El towards the retracted position R, pushes the fluid out of the first thrust chamber 22 towards the first hydraulic accumulator 3 via the first one-way valve 6 through the first portion 10a of the supply circuit 10.

At the same time, the workpiece 100 also acts on the second piston 81 of the auxiliary hydraulic cylinder 8 which is moved so as to empty the second thrust chamber 82 (filled with fluid from the second hydraulic accumulator 4 in a previous machining cycle) of the fluid and send it to the first hydraulic accumulator 3 via the third one-way valve 26 through the first portion 10a and the third portion 10c of the supply circuit 10 (figure 3).

At the end of the compression stroke of the die 50 in which the sheet-press element 2 and the first piston 21 of the hydraulic cylinder 20 are in the retracted position R, in a third operating step (figure 4) the second valve 7 is activated, for example mechanically, and arranged in the second opening position G1 so as to connect the first cylinder 20 to the second hydraulic accumulator 4 and allow the pressurized fluid coming from the first thrust chamber 22 to flow into the second hydraulic accumulator 4. This is made possible by the fact that the pressure inside the second hydraulic accumulator 4 is significantly lower than that of the fluid present in the first thrust chamber 22. In this way, the first thrust chamber 22 substantially empties, that is, it reduces or virtually cancels the pressure of the fluid in said first thrust chamber 22 so as to prevent movements, even minimal, of the first piston 21 and the sheet-press element 2 along the operating axis X in the subsequent fourth operating step of opening or rising in which the die 50 is moved to the opening position A after carrying out the machining and does not compress the workpiece 100 abutting the sheet-press element 2 (figure 5).

In such fourth operating step of opening, the auxiliary hydraulic cylinder 8 has the second piston 81 arranged by the second elastic element 84 in the respective maximum extension position E2 in which the volume of the second thrust chamber 82 is maximum. In the latter, the pressure is limited so as to allow its easy filling by the fluid coming from the second hydraulic accumulator 4. In particular, the first elastic element 44 in the second hydraulic accumulator 4 acts on the internal separator 41 to move the latter to the internal position F, in which a volume of said accumulation chamber 42 is minimum, so as to push the fluid out of the accumulation chamber 42.

In the next fifth operating step of restoring the hydraulic cylinder 20, the first valve 5 is repositioned in the first opening position D1 to allow the pressurized fluid coming from the first hydraulic accumulator 3 to fill the first thrust chamber 22 of the hydraulic cylinder 20 which moves the sheet-press element 2 (figure 6). More precisely, the pressurized fluid introduced into the first thrust chamber 22 moves the first piston 21 and the sheet-press element 2 along the operating axis X to the maximum extension position El so as to push and extract the workpiece 100 from the die 50, the latter already arranged in the opening position A.

Once the processed workpiece 100 has been extracted, another workpiece 100 can be inserted into the die 50 and subjected to a machining cycle according to the operating steps described above.

Thanks to the hydraulic driving system 1 of the invention applicable to a die 50 for a sheet metal machining press, it is therefore possible to controllably drive the sheet-press element 2 of said die 50, in particular when opening the die 50 after machining the workpiece. More precisely, the hydraulic driving system 1 of the invention allows, thanks to the use of the second hydraulic accumulator 4 and the auxiliary hydraulic cylinder 8, to maintain the sheet-press element 2 locked in a retracted position with the opening of the die 50 so as not to damage the workpiece, that is, it allows to delay the reset movement of the sheet-press element for a desired and set time and with it the extraction of the workpiece 100 from the die 50. The first thrust chamber 22 is in fact almost emptied in the retracted position R of the first piston 21 of the hydraulic cylinder 20 at the end of the third operating step of compression so that even minimal movements of the first piston 21 and the sheet-press element 2 cannot occur when the die 50 is opened.

Restoring the hydraulic cylinder 20 for the extraction of the workpiece 100 by positioning the sheet-press element 2 and the first piston 21 in the maximum extension position E2 is easily accomplished in the fifth operating step simply by switching the first valve 5 to the first opening position D1 to allow the pressurized fluid coming from the first hydraulic accumulator 3 to fill the first thrust chamber 22 of the hydraulic cylinder 20 which moves the sheet-press element 2 (figure 6).

The hydraulic driving system 1 for press dies is therefore simple and inexpensive to realize, reliable and safe in operation and does not require any restoration after each operating stroke of the press.

With reference to figure 7, a first variant of the hydraulic driving system 1 of the invention is illustrated which differs from the embodiment described above and illustrated in figures 1 to 6 in that the selector valves 15, 17 comprise respective electro-mechanically operated valves, i.e. controlled in opening and closing by a special management and control unit, for example the same which controls the operation of the press, in particular the opening and closing of the die 50. The operation of this first variant of the hydraulic driving system 1 is substantially the same as that of the embodiment already described.

Referring to figure 8, a second variant of the hydraulic driving system 1 of the invention is illustrated which differs from the embodiments described above in that the first valve 15 comprises an electro-mechanically operated valve, i.e., controlled in opening and closing by a special management and control unit, while the second valve 7 comprises a mechanically operated valve, driven by the workpiece 100 housed in the die 50 moved in closing. The operation of this second variant of the hydraulic driving system 1 is substantially the same as the embodiments described above.

Referring to figure 9, a third variant of the hydraulic driving system 1 of the invention is illustrated which differs from the embodiments described above in that the first valve 5 comprises a mechanically operated valve, in particular mechanically operated by a press cam to switch between the two open and closing positions during the closing movement of the die 50. Differently, the second valve 17 comprises an electro-mechanically operated valve, i.e. controlled in opening and closing by a special management and control unit. The operation of this third variant of the hydraulic driving system 1 is substantially the same as the embodiments described above.

A further variant of the hydraulic driving system 1 of the invention is also provided, not illustrated in the figures, which involves a plurality of hydraulic cylinders 20 arranged to drive respective sheet-press elements 2 of the die 50, each hydraulic cylinder 20 being associated and connected to a respective second valve 7, a respective second hydraulic accumulator 4, a respective auxiliary hydraulic cylinder 8. The plurality of hydraulic cylinders 10 is supplied by a first hydraulic accumulator 3 of adequate size via the first valve 5 also of adequate size to supply the various hydraulic cylinders 10.

Another further variant of the hydraulic driving system 1 of the invention, not illustrated in the figures, is provided, which involves a plurality of hydraulic cylinders 20 arranged to drive respective operating elements associated and driving a sheet-press cushion. Also in this case each hydraulic cylinder 20 is associated with and connected to a respective second valve 7, a respective second hydraulic accumulator 4, a respective auxiliary hydraulic cylinder 8.

The operation of such variants of the hydraulic operating system 1 is substantially similar to the embodiments described above.

With reference to figure 10, schematically illustrated is a die 50 of the invention mountable on a press, hydraulic or mechanical, for machining sheet metal and comprising the hydraulic driving system 1 described above, adapted to controllably drive at least one sheet-press element 2 of the die 50.

More precisely, the die 50 comprises two half-dies or half-shells, a lower half-die 50b of which is mountable on a fixed base of a known type of press, while an upper half-die 50a is mounted on a movable part of the press provided with vertical movement, for closing and opening the die 50 itself. The upper half-die 50a comprises a plurality of hydraulic cylinders 20, e.g., five, supplied and controlled by the hydraulic driving system 1 of the invention and arranged to drive respective operating elements 2, not illustrated, comprising sheet-press elements.