DEVICE FOR LOCKING TOOLS FOR A PRESS BRAKE

DESCRIPTION

The present invention concerns a device for locking tools in a press brake, in particular in a press for bending sheet metal or the like.

In the machine tool sector, press brakes, configured to give certain bending angles to metal sheets, plates or the like, through cold deformation, are well know.

Press brakes are generally composed of a bed defining a fixed table on which to place the metal sheet and carrying a bending die according to the profile of which said metal sheet is bent.

An upper beam is supported so that it can move towards and away from the bed, along a substantially vertical plane. The beam carries integral therewith a bending tool adapted to cooperate with the die to give the sheet the desired shape.

Movement of the upper beam is generated by hydraulic, mechanical or electrical actuator means, which provide the necessary force for the deformation operation.

Generally, the tool has a lower end structured to cooperate with the upper surface of the die and an upper portion defining a shank for attaching to the upper beam.

Depending on the type of machining operation to be carried out, said tool can be single, i.e. with a monolithic body that extends for the whole, or for a part, of the length of the crossbeam, or divided into parts, i.e., formed of multiples placed side by side in the direction of extension of the crossbeam.

For this purpose, the upper beam can be provided with one or more locking devices (known in the field as “intermediate devices”) which, by clamping the shanks of the tools make them integral with it to allow press bending.

Among the various shapes and types of shank, some of the more widely used have become standardized shapes to allow tools to be installed on presses made by different manufacturers.

According to a very widely used variant, the shank has on at least one side a seat, with a grooved profile with a rectangular section, in which a pawl of the locking device engages to prevent the tool from slipping out and falling when the locking device is loosened.

Prior art locking devices are generally structured as a clamp and comprise a fixed jaw integral with the upper beam and a movable jaw moved by a closing device adapted to move the movable jaw towards or away from the fixed jaw to clamp or release the tool.

Some examples of prior art locking devices are described, for example, in EP 0682995 Al, EP 1762310 Al and WO 2011/092558 Al.

A common problem that occurs in the use of these devices regards the tool change operations.

The methods for inserting and removing the tools in prior art locking devices are essentially two, as a function of their configuration: sliding the tool horizontally (parallel to the upper beam) or from below, through a combined translation and rotation movement.

In the first case, the tool is inserted at the sides of the press and is moved along the length of the upper beam until it is in the predetermined operating position. This method has some drawbacks, the most important of which is the difficulty of the operation and the time it takes to carry out, causing prolonged machine downtime.

Moreover, if the tool mounted on the machine comprises multiple segmented elements placed side by side, in order to remove or replace an intermediate element it is necessary to first remove all those preceding it, with a further increase in the times required for replacement.

For these reasons, locking devices in which the tool can be inserted from the bottom upwards are preferable as, besides requiring much less time to install or remove a single tool, they also allow the replacement of an intermediate tool without having to remove or move those close to it already installed. WO 2011 141774 describes a device of this kind for locking tools provided with a movable jaw provided with a pawl adapted to engage the shank of the tool to hold it suspended and prevent it from falling when the movable jaw.

Moreover, said pawl, guided along a clamping path of the jaw, also allows the tool to be taken to abut against the fixed jaw to hold it in position during pressing.

However, this prior art locking device has some drawbacks and, in particular, is not always capable of guaranteeing conditions of safety during tool change operations.

More in detail, the drawback consists of the fact that when multiple segmented tools having a smaller width than the width of the locking device are installed in the same jaw, the operation for removal and insertion of another tool can cause those supported by the pawl of the jaw to fall.

This happens because the pawl obtained on the movable jaw is integral therewith mobile and extends for the whole of its width; therefore, during insertion or removal of a tool, when the movable jaw is moved away from the fixed jaw, the pawl can disengage from the shank of the other segmented tools that are installed on the locking device.

These problems are partially solved by EP 1244528 B 1 which describes a locking device in which the movable jaw is provided with a lower pawl, facing the fixed jaw, which rests on a plurality of springs fixed on a protrusion of the same number of safety hooks, on which it exerts, through said springs, the force required to clamp the tool and move it to abut against the fixed jaw.

The movable jaw is also provided with an upper pawl, facing upwards, which defines a cavity for insertion of corresponding spikes, projecting from the top surface of the safety hooks, which enables said safety hooks to remain suspended during clamping or release of the jaws.

Insertion and extraction of the tool takes place with a combined translation and rotation movement of the tool on the vertical plane from bottom to top and vice versa, respectively. In this case, the presence of a plurality of safety hooks, having a smaller width than the width of the jaw, also allows segmented tools of smaller size to be held suspended independently.

However, this prior art locking device also has some drawbacks, among which it must be mentioned that particularly laborious and costly precision machining is required to produce the pawls on the jaw and the corresponding spikes and protrusions on the safety hook.

Another drawback of this device consists in the fragility of the safety hooks which, having a thin and elongated shape and being subject to continuous impacts with the shank of the tool during operations for insertion and removal of the tool, through time can become deformed or break.

Another drawback of the device described above consists of the use of springs to transmit the clamping force from the lower pawl of the movable jaw to the safety hook . In particular, the use of flat springs fixed to the top surface of the hook requires the safety hooks to have a minimum width which, in some cases, can still allow tools to become unhooked and to fall during replacement operations (in particular of segmented tools of very small dimensions are used).

EP 1884298 Bl also describes a locking device comprising a first jaw and a second jaw, movable relative to each other, and safety hooks interposed between the first and the second jaw, provided with a pawl adapted to be inserted into a cavity provided in the shank of the tool. The second jaw is provided with a seat in which said safety hooks are movably housed, said seat being closed, on the opposite side relative to said first jaw, by an arch-shaped wall that coacts, through elastic deformation, with a portion of the safety hook during clamping and release of the second jaw. The seat of the second jaw also houses a crossbar that engages a claw-like portion of the safety hook to support it inside said seat.

This device also has a highly complex construction, both due to the shape of the safety hook and to the large number of parts to be assembled. Moreover, with this device the clamping force is transferred to the tool through an elastic element that can be inclined to break after a large number of clamping and release cycles of the device. Moreover, this elastic element must be designed and calibrated to operate correctly both with small and lightweight tools as well as with very heavy tools.

WO 2015/036981 Al, by the same applicant, 141774 describes a locking device comprising a first jaw and a second jaw, movable towards and away from each other, and at least one safety hook interposed between said first and said second jaw provided with a pawl adapted to be inserted into a groove in the shank of the tool, in which an inner side of said second jaw is provided with at least one thrust surface adapted to coact in sliding support with at least a corresponding support surface obtained on an outer side of the safety hook. Said thrust surface and said support surface lie parallel along a thrust plane inclined in the opposite direction to the first jaw. Due to fact that the safety hook and the jaw coact with each other directly, this device is less complex and costly to produce.

However, a further drawback common to all the above-described prior art devices regards mounting of segmented tools of limited width, in particular less than 20 mm, on the locking device.

More in detail, the problem regards alignment of adjacent segmented tools clamped in the same locking device.

In fact, although the dimensional precision of the tools has reached very satisfactory levels, these objects still have minimum tolerances in the coupling area, or shank, for example in the thickness thereof or in the distance between the grooved profile that is engaged by the pawl and a stop surface adapted to abut against a corresponding surface of the fixed jaw.

Due to these dimensional differences, when two or more segmented tools are gripped by a same pawl of the movable jaw, one or more of these could remain slightly slack, with the possibility of small oscillations that give rise to imperfect alignment. This problem occurs both with the locking devices provided with a monolithic movable jaw with a single pawl, and with devices equipped with several safety hooks, such as those cited above, when two tools are clamped by the same safety hook.

A rough method currently used to overcome this problem is that of interposing a strip of elastically deformable material, rubber or the like, between the jaw and the shank, which allows any slack between the jaw or the safety hook and the shank of the tool to be eliminated, so as to transfer to all the segmented tools a certain clamping force.

However, this operation is somewhat impractical for the operator who installs the tools in the locking device. In some cases, a deformable insert is provided, fitted directly on the inner side of the movable jaw. However, this insert is not removable and, as it is used continuously, is subject to considerable wear.

Moreover, an additional machining operation is generally required on the inner side of the movable jaw to obtain the housing for the insert, which increases the costs of the device.

In devices provided with a plurality of safety hooks, it would be desirable for the latter to have a very limited width, for example lower than the minimum width of the segmented tools, i.e., around 10 mm. In this way it would be difficult for two segmented tools to be clamped by the same safety hook.

However, this is not practicable with prior art locking devices, such as those described above, as the safety hooks must accommodate or coact with guide and/or thrust means, which must have a minimum lateral footprint in order to be housed. Moreover, the size of the locking device being equal, if the safety hooks have a limited size a larger number of hooks would be required, increasing the costs and complexity of construction of the locking device.

Another limit of prior art locking devices, when used with segmented tools of limited width, is that said tools must be arranged as close as possible to the centreline of the locking device to avoid misalignments of the jaw and imprecise clamping. In this context, the object of the present invention is to propose a device for locking tools, in particular punches, in a press brake, which overcomes the problems of the prior art.

More in detail, the object of the present invention is to provide a locking device that allows two or more segmented tools of limited width to be maintained in perfect alignment.

Another object of the present invention is to produce a locking device that allows practical and rapid mounting of the tools.

A further object of the present invention is to provide a locking device that is simple and inexpensive to produce.

These and other advantages are achieved by a device for locking tools, in particular punches, in a press brake, in conformity with claim 1.

Within the context of the present invention, the terms horizontal, vertical, front, rear, height, width, etc., referring to the locking device, must be understood in relation to the working position of the locking device mounted in a press brake.

In detail, the locking device comprises a first fixed jaw, a second jaw, mounted movable towards and away from the first jaw, and actuator means acting on said second jaw to move it relative to the first jaw.

The second jaw, on an inner side thereof, i.e. facing the first jaw, is provided with a pawl adapted to engage the groove defined in the shank of a tool.

When the jaws are clamped, the pawl engages said groove in the shank of the tool. Following clamping, the second jaw draws the tool to abut against the first jaw.

On the contrary, when they are released, the pawl remains engaged in the groove of the shank preventing the tool from falling.

According to the invention, the second jaw comprises a laminated body formed by a plurality of plates arranged parallel, adhering to and substantially aligned with one another. More in detail, said plates are aligned along a direction parallel to the profile of the tool to be gripped, i.e., parallel to the bend line. Moreover, preferably, but not necessarily, the plates all have the same profile.

Said plates generally have a thickness of less than 10 mm, typically less than 8 mm, preferably between 3 mm and 6 mm.

Moreover, according to the invention the actuator means are configured to exert a thrust action on each plate of the second jaw to move it towards or away from the first jaw.

In this way, when segmented tools (punches) of limited width are mounted on the locking device, the coupling part (shank) is engaged by one or more plates of the second jaw. Due to the limited thickness of the plates, clamping of two segmented tools by a single plate, with all the drawbacks described above that characterize prior art locking devices, is avoided.

On the contrary, the thrust action exerted independently on each plate ensures that if two segmented tools are arranged side by side and adjacent, each one can in any case be clamped against the first jaw independently by respective portions of the second jaw, i.e., by respective plates.

This configuration of the locking device allows any differences in dimension of the coupling parts of the tools to be offset.

Moreover, due to the structure of the second jaw, it is possible to mount a segmented tool in the locking device in any position, not necessarily in the centreline, as the thrust provided by the actuator means is transferred only to the plates aligned with and superimposed on the shank of the tool. This condition limits the “warp” effect of the second jaw that afflicts prior art locking devices.

The locking device thus conceived will be even less expensive to produce relative to prior art devices, In fact, the second movable jaw is formed by a pack of shaped plates, which are typically made of steel; therefore, the jaw has a considerably lower production cost relative to an equivalent monolithic jaw obtained from machining a solid block.

Moreover, machining of the plates, typically performed by laser cutting, makes it possible to produce more complex shapes, which would require special machining operations if made through material removal on a solid block.

Finally, if some of the plates are worn in the gripping area, i.e., the part that grips the shank of the tool, it is possible to replace only these plates and not the whole jaw.

According to an aspect of the invention, said actuator means comprise a guide rod, arranged with its axis of extension along a direction parallel to the width of the tool, i.e., parallel to the bend line, and an actuator, connected to said guide rod, configured to move it along a substantially horizontal direction perpendicular to the axis of extension of the latter.

Said guide rod can therefore coact with each plate to push the second jaw towards or away from the first jaw.

According to a preferred variant of embodiment, the guide rod is housed in a seat obtained in the second jaw, which extends parallel to the bending direction. Said seat is formed by a plurality of identical holes obtained in each plate.

Said holes are also substantially aligned along a direction parallel to the bending direction.

The guide rod, engaged in each hole, draws said plates, and consequently the second jaw, towards or away from the first jaw.

According to an aspect of the invention, the holes obtained in the plates of the second jaw are slotted holes. Said slotted holes are oriented with the axis of the slot inclined relative to a horizontal direction.

The guide rod is housed sliding in a seat formed by the slotted holes along a direction perpendicular to its axis of extension.

Following the substantially horizontal movement of the guide rod, it slides on the edge of the slotted holes, obliging the second jaw to carry out a combined movement towards the first jaw and a lifting movement.

This combined movement allows, during locking of the tool, slight lifting of the tool so that a stop surface of the tool is moved to abut against a corresponding stop surface of the first jaw. According to a preferred variant, the angle a comprised between the axis of the slotted hole and a horizontal plane has an amplitude comprised between 35° and 55°, more preferably between 40° and 50°.

According to another aspect of the invention, the ends of the guide rod protrude laterally from the second jaw, i.e., from the seat obtained in the latter. Said ends are each engaged in respective seats obtained in a pair of slides mounted sliding on the first jaw.

Said slides are connected to the actuator so as to be able to impart to the guide rod the movement that allows clamping and release of the second jaw.

According to a preferred embodiment, said slides comprise two thin plates arranged adjacent to the lateral faces of the first jaw, in each of which the seat for housing one end of the guide rod and a slot adapted to house a pin protruding from the lateral face of the first jaw are obtained.

Said thin plates are also fixed to a cross member, part of the actuator, arranged on the rear side of the first jaw, movable along a substantially horizontal direction.

According to a preferred variant, the slot obtained in each thin plate is oriented with its axis inclined relative to a horizontal plane, and to the direction of movement of the actuator, by an angle 6 comprised between 0.5° and 5°, preferably between 1° and 3°.

This arrangement allows a slight lifting of the thin plates during movement towards the locking position. This upward movement of the thin plates also determines a slight lifting of the guide rod and, hence, a further lifting of the plates of the second jaw. This allows complete clamping of the shank of the tool along the vertical direction to be achieved at all times, i.e., to move the upper face of the shank abutting against the seat obtained in the first jaw, offsetting slight dimensional tolerances of the shank.

According to another aspect of the invention, the locking device further comprises a stabilizer bar, housed in a seat obtained in the second jaw, formed by a plurality of aligned holes obtained in each plate. The stabilizer bar is also fixed, at its ends, to the slides that guide the guide rod.

The function of the stabilizer bar is to prevent, or in any case to limit, rotation of each plate around the guide rod during clamping of the tool, and hence transfer the entire clamping force to the latter.

According to an embodiment of the invention, the holes in which the stabilizer bar are housed have the shape of a slot arranged with its axis inclined relative to a horizontal plane.

The angle P comprised between the axis of said holes and a horizontal plane has an amplitude comprised between 50° and 70°, preferably between 55° and 65°.

According to a preferred variant, one end of the slot, in which the stabilizer bar is housed, has a widening. This widening facilitates rotation of the second jaw when the locking device is in the release position, during the operation to remove the tool, as will be described in more detail below.

According to another aspect of the invention, the outer side of the first jaw has a stop face adapted to receive abutting against it a stop edge provided on the inner side of the second jaw, when the locking device is in locking position.

Said stop edge is formed by all the portions of stop edge obtained on each plate.

An elastically deformable insert is affixed to said stop face, so that at least a portion of the stop edge of the second jaw, i.e., a section of each portion of stop edge of all the plates, presses against said insert in the locking position.

Therefore, in the locking position, the insert exerts an elastic force directed outward on each plate.

More in detail, the insert is affixed to the first jaw at a greater height or distance relative to that of the guide rod and of the stabilizer bar, if present.

This positioning of the insert means that the force exerted on the plates tends to rotate these latter in a vertical plane perpendicular to the bend line.

More in detail, the lower part of the second jaw, i.e., of each plate, namely the part that pushes the shank of the tool against the first jaw, receives a thrust towards the first jaw.

If segmented tools are mounted in the locking device, any slack between the shank and the jaws, due to the dimensional tolerances of the tool, are offset by the elastic thrust provided by the insert.

In fact, each plate always receives a thrust that, in turn, is transferred to the shank of the tool engaged by said plate.

According to a preferred embodiment, the insert comprises a flattened strip housed in a groove obtained on the stop face of the first jaw. Preferably, the insert and the groove extend for the whole of the width of the first jaw.

The strip has a greater thickness relative to the depth of the groove so as to protrude beyond said stop face.

In this way, when the stop edge of the second jaw is moved towards the first jaw, it comes into contact first with the insert, deforming it, and subsequently it abuts against the stop face of the first jaw.

Typically, the outer face of the insert protrudes relative to the stop face by a distance comprised between 0.5 mm and 3 mm.

According to a preferred variant, said insert is made of polymer material with a hardness comprised between 40 Shore and 90 Shore. More preferably, the insert is made of polyurethane.

Further features and details will be better understood from the description below, provided by way of non-limiting example, and from the accompanying drawings, wherein:

- Fig. 1 is a front view of the locking device according to the invention;

- Fig. 2 is a lateral view of the locking device of Fig. 1;

- Fig. 3 is an exploded perspective view of the locking device of Fig. 1;

- Fig. 4 is a lateral sectional view of the locking device of Fig. 1, in the release position;

- Fig. 5 is a lateral sectional view of the locking device of Fig. 1, in the release position with a tool mounted;

- Fig. 6 is a lateral sectional view of the locking device of Fig. 1, in locking position with a tool mounted;

- Fig. 7 is a sectional view of the locking device of Fig. 1, during removal of the tool.

With reference to the accompanying Figs. 1 to 3, the number 1 indicates as a whole a device for locking tools, in particular punches, in a press brake.

In detail, the locking device 1 comprises a first fixed jaw 10, a second jaw 20, mounted movable towards and away from the first jaw, and actuator means 30 that act on said second jaw 20 to move it relative to the first jaw 10 along a mainly horizontal locking and release direction.

In the example of the figures, the first jaw 10 is formed of two portions joined integrally to each other. A lower portion 11 coacts with the second jaw to grip the shank of the tool and a second upper portion 12 is adapted to be connected to gripping means of the upper cross member of the press brake.

However, the first jaw 10 can also be formed by a single monolithic body, just as by more than two portions joined to one another.

The actuator means 30, represented schematically in the accompanying figures, comprise a crossbar 31 placed on the rear side of the first jaw, i.e., on the opposite front face facing the second movable jaw. Said crossbar 31 is subject to the action of actuators, preferably pneumatic, which, during locking of the tool, push said crossbar away from the first jaw 10. Said actuators are generally obtained directly in the body of the first jaw 10 and of the crossbar 31, or interposed between said two parts.

According to the invention, the second jaw 20 comprises a laminated body 21 formed by a plurality of plates 200 having the same profile and arranged parallel, adhering to and substantially aligned with one another along a transverse direction X, substantially parallel to the profile of the tool to be gripped, i.e., to the bend line. The plates 200 generally have a thickness comprised between 3 mm and 6 mm.

Each plate 200, on an inner side 201 thereof, i.e., facing the first jaw 10, is provided with a portion of pawl 202 adapted to engage the groove defined in the shank of a tool.

Each plate 200 has a first slotted hole 210 and a second slotted hole 220, both passing through the thickness of the plate. The second slotted hole 220 is preferably arranged at a greater height relative to the first slotted hole 210.

Both the slotted holes are arranged with the respective axes Al, A2 inclined relative to a horizontal plane X-Y.

More precisely, said axes Al, A2 are oriented upwards and towards the outside of the locking device relative to the front side thereof.

The angle a comprised between the axis Al of the first slotted hole 210 and the horizontal plane X-Y has an amplitude preferably comprised between 40° and 50°.

The angle P comprised between the axis A2 of the second slotted hole 220 and the horizontal plane X-Y has an amplitude comprised between 55° and 65°.

The plates 200, forming as a whole the second jaw 20, are maintained grouped together and in contact with one another by two thin plates 24, respectively placed substantially adjacent to the two plates 200 placed at the lateral ends of the second jaw 20.

Said thin plates 24 are connected to the actuator means 30, specifically to the crossbar 31, for example by screws 33. Said actuator means 30 are configured to move the thin plates 24 along a substantially horizontal direction Y. The movement of the thin plates 24 between the locking position and the release position is guided by a pair of elements 13, for example pins, protruding on both sides from the first jaw 10. Said protruding elements 13 are housed in respective slots 27 obtained in each thin plate 24.

According to a preferred embodiment, the axis of the slots 26 is inclined relative to the horizontal plane X-Y by an angle comprised between 1° and 4° in the direction towards the rear part of the locking device. As explained previously, this inclination of the slot ensures that during locking, when the thin plates 24 are moved by the actuator means 30 towards the first jaw, said thin plates 24 are also lifted slightly.

A first hole 25 and a second hole 26 are also obtained in each thin plate 24.

The ends of a guide rod 40 are housed in a fixed manner in the first holes 25 of the thin plates 24. The central part of the guide rod 40, i.e., the part comprised between said thin plates 24, is inserted into the first slotted holes 210 of each plate 200, so as to be able to slide along the axis Al of the slot. The guide rod 40 preferably has a circular cross section.

Starting from a release position, as illustrated in Fig. 4, when the actuator means 30 are operated they push the two thin plates 24 towards the first jaw 10 (i.e., towards the rear part of the locking device) and these latter draw the guide rod 40. Said guide rod 40 slides inside the first slotted hole 210 and draws the plates 200, i.e., the second jaw 20, towards the first jaw 10. Due to the inclined arrangement of said slotted hole 210, the movement carried out by the second jaw 20 has both a horizontal component and a vertical component in upward direction.

This movement allows the tool T, in particular the shank C, to be taken to abut against the first jaw.

Fig. 5 illustrates the locking device, still in the release position but with the tool T mounted. In this operating position, the portions of the pawl 202 of the plates 200 that are superimposed on the tool T (i.e., the plates 200 that cover the width of said tool) are engaged in the groove S of the shank C of the tool T, supporting it.

Fig. 6 illustrates the locking device in the locking position of the tool T. As shown in the figure, when the actuator means 30 are operated, the movement of the crossbar 31 away from the first jaw 10 causes movement of the thin plates 24 and, as explained previously, also translation of the plates 200 towards the first jaw 10 and upwards so that the abutment faces Cfl, Cf2 of the shank C are abutting against respective stop faces 16,17 of the first jaw

10. According to the invention, the ends of a stabilizer bar 50 are housed in a fixed manner in the second holes 26 of each thin plate 24. The central part of the stabilizer bar 50, i.e., the part comprised between said thin plates 24, is inserted into the second slotted holes 220 of each plate 200, so as to slide along the axis A2 of the slot. Preferably, also the stabilizer bar 50 has a circular cross section.

The stabilizer bar 50, just as the guide rod 40, provides a contribution to the movement of the second jaw 20 towards the first jaw 10 during locking of the tool T. In particular, the stabilizer bar 50 also limits rotation of the second jaw 20 around the guide rod 40 so that the latter, i.e., the plates 200 that are superimposed on the tool T, adhere well to the shank C thereof.

Cooperation between the stabilizer bar 50 and the second slotted holes 220 during locking is substantially the same as described above in relation to the guide rod 40.

According to a preferred variant, the lower end of the second slotted hole 220 is widened, i.e., substantially wider relative to the diameter of the stabilizer bar 50. This widened area allows the second jaw to rotate about the axis of the guide rod 40 when the device is in the release position to facilitate removal of the tool T between the two jaws.

With reference to Fig. 7, this illustrates the locking device 1 during a point in an operation to remove the tool T. As can be appreciated from the figure, to disengage the portions of the pawls 202 from the groove S of the shank C, the tool T must be rotated so as to push the plates 200 of the second jaw 20 that engage the tool T outwards. This thrust, exerted by the shank C on the inner side 201 of the plates 200, causes these latter to rotate around the guide rod 40.

The widened area in the second slotted hole 220 allows a wider travel of the rotation of the plates 200, facilitating disengagement of the portions of pawls 202 from the groove S of the shank C and hence removal of the tool T.

In the example of embodiment illustrated in the figures, a groove 14 is present on the front side 101 of the first jaw 10, arranged horizontally and which extends for the whole of the width of the first jaw 10. The groove 14 has a typically quadrangular, generally rectangular, section.

The height h of the groove 14 is preferably comprised between 5 mm and 10 mm, while the depth p is preferably comprised between 2 mm and 3 mm.

Said groove 14 is produced on a stop face 102 against which a stop edge 205, provided on the inner side 201 of each plate 200 of the second jaw 20, abuts when the locking device is in locking position.

An insert 15, in the form of a strip made of elastically deformable polymer, for example, polyurethane, with a hardness comprised between 40 Shore and 90 Shore, is inserted in the groove 14.

The width of the insert 15 is preferably the equal to the width of the groove 14. The thickness of the insert 15 is instead greater relative to the depth of the groove by a value of around 0.5 mm to 1.5 mm, so as to protrude beyond the stop face 102.

This protrusion of the insert ensures that, in the locking position, at least a section of the stop edge 205 of the plates 200 is superimposed on, and in contact with, the insert 15, pressing it and deforming it elastically, as can be seen in Fig. 6.

The present invention, as described and illustrated, may be subject to various modifications and variants, all of which fall within the scope of the invention; moreover, all the details may be replaced with other technically equivalent elements.