The present invention relates to a workpiece holder for machine tools and machining centers, provided with a system for detecting the position of the claws.

As is known, a workpiece to be machined in a generic machining center can be mounted on a workpiece holder provided with one or more claws which open/close automatically, typically under pneumatic or hydraulic control.

The workpiece holder can be provided with sensor means adapted to detect that the two stroke limit positions (fully open or fully closed) of the claws have been reached.

This information is of interest since the fully open position enables the loading of the workpiece, whereas the fully closed position indicates an anomaly (workpiece not present): in grip conditions, the claws must instead be in a position that is intermediate between the two stroke limit positions.

Two proximity sensors are currently used which are arranged so as to detect respective magnetic locators fixed to one of the claws when said claw is in the two stroke limit positions.

One drawback of this system is that the proximity sensors must be supplied with electric power and this circumstance introduces considerable design and manufacturing complexities, especially when the workpiece holder is supported rotatably during working.

Therefore, the aim of the present invention is to provide a workpiece holder for machine tools and machining centers which is provided with a system for detecting the position of the claws that is reliable and precise and can be easily implemented on any application without requiring an electric power supply.

This aim, as well as these and other objects which will become better apparent from the continuation of the description, are achieved by a workpiece holder having the characteristics described in claim 1, while the dependent claims define other advantageous albeit secondary characteristics of the invention.

The invention is now described in greater detail with reference to some preferred but not exclusive embodiments thereof, illustrated by way of nonlimiting example in the accompanying drawings, wherein:

Figure 1 is a perspective view of a workpiece holder according to the invention;

Figure 2 is a front elevation view of the workpiece holder of Figure 1;

Figure 3 is a sectional view of Figure 2, taken along the line III-III;

Figure 4 is a sectional view of Figure 3, taken along the line IV-IV;

Figure 5 is a view, similar to Figure 4, showing the workpiece holder in a different operating configuration;

Figure 6 is a sectional view of Figure 3, taken along the line VI- VI;

Figure 7 is a view, similar to Figure 6, showing the workpiece holder in a different operating configuration;

Figure 8 is an enlarged- scale view of a detail of Figure 6;

Figures 9a, 9b are two views similar to Figure 3, which show the workpiece holder according to an alternative embodiment of the invention in respective two different operating positions;

Figures 10 and 11 are two sectional views of Figures 9a, 9b, taken along the axes X-X and XI-XI;

Figure 12 is an enlarged-scale view of a detail of Figure 9a;

Figure 13 is a sectional view of Figure 12, taken along the line XIII-

XIII;

Figure 14 is a perspective view of a workpiece holder according to a further alternative embodiment of the invention;

Figure 15 is a sectional view, similar to Figure 3, which refers to the alternative embodiment of Figure 14; Figures 16 is a sectional view of Figure 15, taken along the line XVI-

XVI;

Figure 17 is a view, similar to Figure 16, showing the workpiece holder in a different operating configuration;

Figure 18 is a sectional view of Figure 15, taken along the axis

XVIII-XVIII.

With initial reference to Figures 1-7, a workpiece holder 10 for machine tools and machining centers comprises a body 12 in which a pneumatically sealed cylindrical chamber 14 is formed (Figures 4-7).

One end of the cylindrical chamber 14 is closed by a bottom plate 16.

The base of the body 12 also forms the base for the anchoring of the workpiece holder 10 and for this purpose is provided with centering/fixing pins 17a, 17b.

The opposite end of the cylindrical chamber 14 is delimited by a back wall 18 provided monolithically from the body 12.

The cylindrical chamber 14 slidingly accommodates a piston 19. The piston 19 is provided with a stem 20 which slidingly passes, with a pneumatically hermetic seal, through a hole 22 provided in the back wall 18. A transmission element is fixed at the top of the stem 20, externally to the cylindrical chamber 14, and is constituted by a sleeve 26 which has two mirror-symmetrical oblique guides 28, 30 on respective opposite sides, which converge away from the piston 19.

The workpiece holder 10, in this embodiment, is provided with two claws 32, 34 on which it is possible to fix respective grip elements (not shown) which are contoured as a function of the part to be gripped.

The two claws 32, 34 are supported so that they can translate at right angles to the axis of the piston 19 between two rectilinear guides 36, 38 obtained monolithically from the body 12.

The claws 32, 34 have respective oblique contrast guides 40, 42 which engage slidingly the two oblique guides 28, 30 of the sleeve 26, thus producing the opening and closing of the claws 32, 34 in relation to the axial movements of the piston 19.

The workpiece holder 10 is provided with sensor means adapted to detect the reaching of the stroke limit positions by the claws 32, 34.

The sensor means according to the invention comprise a pneumatic valve 44, which affects a circuit C of pressurized air that is provided in the body 12 and is connected to a pump P (shown only schematically in Figure 8) and is provided with a flow control element 45 which can move between a closed position and a venting position of the circuit C under the control of a contoured surface 48 which can move in correlation with the movement of the claws 32, 34 and actuates for opening/closing the flow control element 45 by cam action; in particular, the contoured surface 48 has such a profile as to determine the switching between the closed position and the venting position when the claws 32, 34 reach the stroke limit positions.

In this embodiment, the contoured surface 48 is provided on the sleeve 26 and has a rectilinear central portion 48a which is parallel to the axis of the piston 19, delimited between two mutually opposite slopes 48b, 48c which are mutually mirror-symmetrical.

With particular reference to Figure 8, the pneumatic valve 44 is accommodated in a valve seat 49 provided on the body 12.

The pneumatic valve 44 comprises a valve body 50 in which there is a cylindrical seat 52 which is delimited axially between a base 54 obtained monolithically from the valve body 50 and a lid 56.

The cylindrical seat 52 is in fluid connection, by means of an opening 53a, with an annular groove 53b which surrounds the valve body 50 and is connected to the circuit C.

The base 54 is provided with a hole 58 in which the flow control element 45 is inserted slidingly.

The flow control element 45, which has the shape of a piston, has an end that extends into a plate 62 which slidingly engages the cylindrical seat 52.

The opposite end of the flow control element 45 acts coaxially on a finger-shaped slider 64. The slider 64 is inserted slidingly in a bushing 66 accommodated in a hole 67 that passes through the body 12 at right angles to the axis of the piston 19. The slider 64 engages the contoured surface 48 with a contact end 64a thereof which has a hemispherical profile and is opposite with respect to the end on which the piston 45 acts.

The piston 45 and the slider 64 are pushed elastically toward the contoured surface 48 by primary elastic means. The primary elastic means comprise a first compression spring 68 which is accommodated coaxially in the cylindrical seat 52 between the plate 62 and the bottom plate 56.

The slider 64 is pushed further toward the contoured surface 48 by auxiliary elastic means. The auxiliary elastic means comprise a second compression spring 70 accommodated in a spring seat 72 of the bushing 66, between the valve body 50 and an annular protrusion 74 of the slider 64.

The plate 62 is provided with a flat region 62a which forms an interspace G with respect to the cylindrical seat 52.

Hermetic sealing means constituted by an annular gasket 76 are interposed between the plate 62 and the base 54.

Inside the piston 45 there is a channel 78 which has a first end that leads into the spring seat 72, which is in fluid communication with the atmosphere, and the opposite end that leads into the cylindrical seat 52 upstream of the annular gasket 76.

In this manner, with the plate 62 in abutment against the annular gasket 76, the cylindrical seat 52 is sealed hermetically. With the plate 62 detached from the annular gasket 76, the cylindrical seat 52 is connected to the atmosphere by means of the air passage formed by the interspace G, by the channel 78, and by the spring seat 72.

In operation, when the claws 32, 34 and the sleeve 26 are in an intermediate position, the slider 64 faces the rectilinear central portion 48a of the seat 48 without contact. The plate 62 is pushed into abutment against the annular gasket 76 by the first compression spring 68 and the cylindrical chamber 52 is therefore pressurized.

With the claws 32, 34 in a fully open or fully closed position, the slider 64 retracts, by cam engagement with the slopes 48b or 48c of the seat 48, and pushes the plate 62 away from the annular gasket 76, connecting the seat 52 to the atmosphere by means of said air passage.

At this point, a pressure sensor PS (shown only schematically in Figure 8) detects a pressure drop and reports it to the control unit CU of the machining center (also shown only schematically in Figure 8). The control unit CU, depending on the open or closed configuration of the claws 32, 34, processes the signal as enabling the loading of the workpiece or as indicating an anomaly.

Figures 9-13 describe an alternative embodiment of the invention, in which the elements that are similar to the preceding embodiment are identified by the same reference numeral increased by 100. The description shall not be repeated for the elements that are similar to the preceding embodiment.

In this embodiment, the piston 119 is pushed downward both pneumatically and elastically by means of a series of springs S which are accommodated in the pneumatic chamber 114 between the bottom plate 116 and the back wall 118.

The space occupation of the springs 119 does not allow to position the pneumatic valve 144 so as to interact with the sleeve 126. Therefore, in this embodiment, the pneumatic valve 144 interacts directly with one of the claws 132, 134 on which the seat 148 is provided.

With particular reference to Figures 12 and 13, the pneumatic valve 144 is again accommodated in a valve seat 149 provided in the body 112 and comprises a valve body 150 which has an axially symmetrical profile, in which there is a cylindrical seat 152 delimited between a base 154 and a bottom plate 156. The back wall 154 is provided with a hole 158 in which a piston 145 is inserted slidingly.

The end of the piston 145 that faces the cylindrical seat 152 is extended with a plate 162 which engages slidingly the cylindrical seat 152.

The opposite end of the piston 145, or contact end 145a, has a hemispherical profile and engages the contoured surface 148 directly. In this case, therefore, there is no intermediate slider.

The piston 158 is pushed elastically toward the contoured seat 148 by a compression spring 168 which is accommodated coaxially in the cylindrical seat 152 between the plate 162 and the bottom plate 156.

A sealing gasket 176 is interposed between the plate 162 and the base

154.

Like the previous embodiment, inside the piston 145 there is a channel 178 which extends from the contact end 145, which is in fluid connection to the atmosphere, to the cylindrical seat 156 upstream of the annular gasket 176.

According to an important characteristic of this embodiment, which is clearly visible in Figure 13, the cylindrical seat 152 is slightly eccentric with respect to the valve body 150. This allows to adjust finely the stroke of the piston by rotating the valve body 150 about its own axis.

This possibility of fine adjustment is particularly advantageous in this embodiment, since the stroke of the claws 132, 134 is much shorter than the stroke of the sleeve 126 owing to the force multiplication ratio between them.

Figures 14-18 describe a further alternative embodiment of the invention, in which the elements similar to the first embodiment are identified by the same reference increased by 200.

In this embodiment, the workpiece holder 210 is provided with three claws 232, 233, 234 which are received slidingly in respective guiding seats 232', 233', 234' provided in the body 212 of the device. In a manner similar to the preceding embodiments, the claws 232, 233, 234 open and close in relation to the axial translation of a sleeve 226, which is actuated by a piston 219 accommodated in a pneumatic chamber 214 formed in the body 212.

In this embodiment also, as in the directly preceding one, the piston 219 is pushed downward both pneumatically and elastically by means of a series of springs S' accommodated in the pneumatic chamber 214 between a bottom plate 216 and a back wall 218.

In a manner similar to the first embodiment, the contoured surface 248 is provided on the sleeve and the piston 245 does not engage the contoured seat 248 directly but by means of a slider 264. The slider 264 is inserted slidingly in a hole 266 provided in the body 212. The pneumatic valve 244 is fully similar to the one described with reference to the first embodiment and therefore its description will not be repeated.

In practice it has been found that according to the intended aim and objects the workpiece holder according to the invention fully achieves the intended aim and objects. In particular, the pneumatic system described above for detecting the position of the claws is reliable and precise and can be implemented easily on any application since it does not require an electric power supply.

Some preferred embodiments of the invention have been described herein, but of course the person skilled in the art may apply various modifications and variations within the scope of the claims.

In particular, the cam system for transmitting the motion from the sleeve or from the claws to the flow control element may be replaced with other known transmission means, for example kinematic systems using levers and linkages, and the like.

Likewise, although in the embodiment described herein the claws open/close with a translational motion, the same principles are applicable to the case in which the claws have an oscillating opening/closure movement, with mere constructive refinements that will be obvious to the person skilled in the art.

Furthermore, although the possibility to finely adjust the stroke of the piston that is achieved by providing its sliding seat so that it is eccentric with respect to the valve body is particularly advantageous if the contoured surface is provided on the claws, the same principle can of course be applied to the case in which the contoured surface is provided on the sleeve.

Moreover, the constructive embodiment of the pneumatic valve 44 can be changed easily, and in particular the flow control element can have different configurations as might be devised by the person skilled in the art. For example, instead of consisting of a sliding piston, the flow control element might be constituted by a flexible membrane arranged so as to open/close a passage for air toward the atmosphere under the control of cam transmission means such as the ones described, or others which are within the scope of the same inventive concept.

The detection system according to the invention is of course likewise applicable both to the case in which grip on the workpiece is performed from the outside inward, as in the examples described herein, and to the opposite case, in which grip on the workplace is performed from the outside inward.

The invention is of course applicable independently of the claw actuation system, which, particularly in the field of fluid dynamics, can be pneumatic, as in the examples described above, or hydraulic.

The disclosures in Italian Patent Application no. 102018000000538, from which this application claims priority, are incorporated herein by reference.

Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.