DESCRIPTION

Field of the invention

The invention relates to a clamp for a substantially cylindrical tool of a machine tool . The inventive clamp is suitable in particular for precision rotary tools having a small diameter; a preferred field of application is the clamping of electrodes for electric discharge machining (EDM) or micro- EDM.

Prior art

A known technique to clamp a rotary cylindrical tool is to provide a rotary chuck with two or more clamping jaws . For example, a plurality of clamping jaws are arranged radially around a clamping axis, and lock the tool by moving towards each other. The tool is actually clamped between suitable contact areas of the jaws. Said contact areas need to be relatively small, in order to ensure a proper clamping action. For example, the jaws have a tapered end portion, carrying the aforesaid contact areas, and a larger body portion. A disadvantage of this known art is that a tool having a small diameter, compared to the clamping surfaces of the jaws, may escape between the jaws. As soon as the jaws are slightly open, the clearance between the tapered ends of the jaws may be sufficient for the tool to fall down from the chuck. Also during the locking action, i.e. while the jaws are approaching, the tool may fall or remain positioned in a wrong manner. In other words, a clamping system according to known art is not able to retain a tool with a small diameter, until the jaws are very close to the locking position. A wrong contact between the jaws and the tool may also damage the tool, for example due to excessive contact pressure. The above problems may result in machine downtime and/or loss of precision. Moreover, the larger body portions define a m inimum d iameter for the tools that can be clamped. A tool with a diameter less than said minimum cannot be clamped, because the clamps would interfere each other before the tool is actually secured. Hence, in the known art, a very small tool require smaller clamps which, however, may have the disadvantage of less rigidity and reduce the applicable clamping force.

This problem is felt for example in the field of electric discharge machining (EDM), where the tool is a cylindrical electrode that may have a small diameter. For example, EDM makes use of cylindrical electrodes of about 1 mm diameter or smaller; in some cases as small as 0.2 mm diameter.

Summary of the invention

The technical problem underlying the invention is to adapt a clamping device in order to provide accurate centering of a cylindrical tool, even of a small diameter, and avoid the above discussed drawbacks.

The idea underlying the invention is to provide the larger body portions of the clamps with matching patterns of protrusions and recesses, so that the clamps may "intersect" each other while the device is closing.

Accordingly, the above problem is solved with a device for clamping a substantially cylindrical tool, comprising:

- a plurality of clamps arranged radially around a central axis, each clamp being displaceable in a radial direction to/from said axis;

- said clamps comprising a tapered portion with a contact area for clamping said tool, and a body portion;

- the body portions of the clamps being larger than the tapered portions, and the body portions having inner faces towards the central axis, characterized in that:

- said inner faces of the body portions of the clamps have respective patterns of protrusions and recesses, the patterns being arranged in such a way that the protrusions of each clamp can be received in recesses of the other clamps upon displacement of said clamps towards said axis.

Sa id protru s ion s a nd recesses a re prefera bly l yi ng on pl a n es perpendicular to said central axis. The protrusions are arranged on different planes or layers, so that any protrusion of a clamp matches a recess of another clamp, while the clamping device close. Preferably the protrusions are received in the recesses continuously from the open position to the close position.

In a preferred embodiment, said inner surfaces are realized as toothed surfaces. More preferably, a plurality of teeth is distributed over the inner surface of body portions of the clamps, so that the teeth form said protrusions and spaces between consecutive teeth form said recesses; the sets of teeth of the clamps are suitably staggered, so that any tooth of a clamp matches a recess of other clamps, while the device close and the clamps are approaching each other. The toothed surface in other words may freely intersect each other, without interfere, while the clamps are closing against each other.

Said teeth and recesses can be realized by cuts on the inner surfaces of body portions of the clamps. The teeth and recesses may be for example machined on the clamps.

It can be understood that the matching patterns of protrusions and recesses allow the body portions of the clamps to intersect each other, when the device is closing. Hence, the clamps are able to keep the tool in a small area central area around the axis, preventing the tool from falling outside. In more preferred embodiments, the device has three clamping elements arranged at 120° around the central axis.

Preferably, the device comprises a base flange that supports the clamps. More preferably, each clamp is supported by the flange with a parallel structure, adapted to allow the clamp to translate without rotating to/from the clamping position. In this way, a better contact between the clamping area and the tool is assured, as well as more precise matching between the protrusions and recesses. In a preferred embodiment, said parallel structure is formed by two parallel and flexible support elements having two points of deflection, preferably at an upper end close to the clamp, and at a lower end close to the base flange. In a more preferred embodiment, the parallel structure comprises blade-like flexible connections between the body portion of each clamp and said base flange.

A more preferred embodiment provides that said device is realized as a single-piece. For example, the flange, the clamps with the pattern of protrusions and recesses, and said flexible connections are integrally formed in single piece. In one embodiment, a metal piece is machined to form said flange, the clamps and the flexible connections. The piece can be mach ined with conventional techniques or EDM techn iques, or a combination thereof.

According to some embodiments, the body portions of the clamps are larger than the tapered portions perpendicularly to the aforesaid radial directions.

A further aspect of the invention is a self-centering rotary head for a machine tool, the head comprising a device according to any of the above disclosed embodiments. The term of rotary head is used to mean any rotary unit or spindle adapted to carry a tool of a machine tool. A preferred but not exclusive application is to EDM machines. Hence, one of the applications of the invention provides a self-centering head for an EDM machine, with a device for clamping an electrode according to any of the disclosed embodiments. Other applications for exemplificative purpose include conventional machine tools such as a milling machine or a drilling machine.

The advantage of keeping the tool between the clamps is particularly important when the tool is small compared to the clamps. This is the case for example of small electrodes for EDM, but the same advantages are reached with conventional tools. Another advantage is that the clamps can be realized with a massive and rigid body portion, and a smaller tapered portion carrying the contact surface for the tool.

The preferred embodiments involve further advantages. The parallel structure allows the clamps, and in particular the clamping areas thereof, to remain parallel to the central axis while approaching the tool . The clamping force is better distributed on the surface of the tool thus resulting in a safer and more reliable clamping action. The realization in a single piece of the main components such as clamps, flange and flexible connections, allows to obtain a better rigidity and more precise tolerances.

The features and the advantages of the invention shall become clearer from the following detailed description and with the help of the attached figures.

Description of the figures

Fig . 1 is a perspective view tool-carrying head of an EDM machine according to an embodiment of the present invention.

Fig. 2 is a view of a clamping device of the head of Fig. 1 , according to a preferred embodiment of the invention.

Fig. 3 is another view of the device of Fig. 2, showing the clamping elements in open position.

Fig. 4 is a close-up detail of the device of Fig. 2. Fig. 5 is a front view of the clamping elements of the device of Fig. 2.

Fig. 6 is a sequence showing the clamping elements of the device of Fig. 2, while they close around the smallest applicable tool in the particular embodiment of the figure.

Fig. 7 shows an EDM machine comprising the head of Fig. 1 .

Detailed description of preferred embodiments

Fig. 1 shows a tool-carrying rotary head 1 suitable for an EDM machine. The head 1 is suitable to receive a tool such as a cylindrical electrode T (shown in Figs. 5 to 7). In use, the head 1 rotates with the tool T.

The electrode is received at the tip of said head 1 , between clamps 2a, 2b and 2c which are part of a clamping device 3 as shown in Figs. 2 and 3. The clamps 2a, 2b and 2c are radially arranged around a central axis 4 which is also the axis of the electrode, when mounted on the head 1 . The clamps are displaceable in radial directions to/from the axis 4, so that they can firmly clamp or release the electrode on said axis 4.

Each of said clamps 2a, 2b and 2c is connected to a base flange 5 by two flexible connections, made by parallel blades 6. The base flange 5, the clamps 2a to 2c and the blades 6 are preferably machined from a single metal piece, i.e. the device 3 of Figs. 2 and 3 is preferably a single piece. The clamps 2a to 2c (Figs. 3 to 5) have tapered portions 7 with respective contact areas 7a, 7b and 7c for the electrode, and body portions 8. Fig. 3 for example indicates the tapered portion 7 and body portion 8 of the clamp 2a.

Each clamp 2a to 2c has an inner face, directed towards the axis 4, with a respective set of teeth 9a to 9c. The teeth of each set are regularly spaced on the inner surfaces of the respective clamp 2a, 2b or 2c, so that the spaces between consecutive teeth form a plurality of recesses. Hence, a recess 10a can be recognized between two of teeth 9a of the clamp 2a; in the same manner, recesses 10b and 10c can be recognized between teeth 9b and 9c of the other two clamps.

The toothed, inner surfaces of clamps 2a to 2c are arranged to match each other. More in detail, the sets of teeth 9a, 9b and 9c are staggered in the direction of the axis 4, so that the clamps have matching patterns of teeth/recesses, the teeth of each clamp being received in the recesses of th e oth er clamps while the device 3 is closing. Referri ng to the exemplificative embodiment of the figures, it can be seen from Fig. 4 that a tooth 9b and a tooth 9c are received in a recess 10a between two of the teeth 9a; similarly, a couple of teeth 9a and 9c are received in a recess 10b between two teeth 9b, and so on.

Said teeth and recesses can be realized substantially by cuts on the inner surfaces of body portions of the clamps. For example, the clamps are machined to form the recesses 10a to 10c, thus forming also the teeth 9a to 9c.

Referring now to Fig. 5, it can be appreciated how the body portions 8 are larger than tapered portions 6 in directions perpendicular to the clamping directions. For example, Fig. 5 shows the clamping directions 1 1 a, 1 1 b and 1 1 c of the clamps 2a to 2c, which are perpendicular to the axis 4. Perpendicularly to said directions 1 1 a to 1 1 c, the body portions 8 of the clamps are larger than the upper tapered portions 7. In Fig. 5, the inclined faces 12 of tapered portion 7 of clamp 2a are shown, as well as sides 13 of the underlying, larger body portion 8 and trace 14a of the plane of front surfaces of teeth 9a.

The trace 14a appear to intersect with the traces 14b, 14c of planes of the front surfaces of the teeth of the other two clamps, due to the layered arrangement of the teeth 9a to 9c, and due to the fact that the teeth of one clamp are actually accommodated in recesses of other clamps. This effect is shown in the sequence a) to d) of Fig. 6. Looking in greater detail to Fig. 6, the position a) shows the teeth 9a - 9c that start intersect each other. In positions b) and c), the clamping elements are further approaching and the space between the clamps is becoming smaller, thus self-centering the tool T. The arrangement on parallel planes of the teeth 9a - 9c can be appreciated in positions b) and c) showing for example a tooth 9a being above a tooth 9b, and the tooth 9b being above the tooth 9c. Position d) is the clamping position with contact areas 7a to 7c in contact with the tool T.

It should be noted, in particular, that in intermediate positions such as a) or b), the distance between the contact areas 7a - 7c is greater than the diameter of the tool T and hence the tool T would be free to escape between the clamping elements, without the retain ing action of the inventive teeth 9a-9c. Also in Fig. 5 it can be appreciated that the distance f between the edges of teeth of two clamps is smaller than the diameter of the tool T and, hence, the tool cannot escape the substantially triangular area (in the sectional view of Fig. 5) between the clamps.

Fig. 7 shows the smallest tool T compatible with the particular embodiment of clamping device 3. Larger tools T of course can be used with the same clamping device, the invention being not limited to a particular range of tool size.

Turning back to Figs. 1 and 2, the clamping elements 2a - 2c may be operated by a wedge-like coupling between inclined surfaces 20 and corresponding surfaces of a jacket 21 . Operation may be for example pneumatic. In a preferred embod iment, the clamping elements are normally closed under the force of suitable springs mounted inside the head 1 , and are opened by feeding compressed air to a suitable opening piston. These details however are not essential for the invention, and are not described further.

A further feature of the invention can be appreciated from Figs. 2 and 3. The flexible blades 6 between clamping elements 2a - 2c and the base flange operate as a parallel structure, i.e. the contact areas 7a to 7c remain substantially parallel each other, when the clamping elements open. Fig. 3 shows the blades 6 assuming an S-like deformed shape, that keeps the clamps 2a to 2c and contact areas 7a to 7c parallel to axis 4, with no rotation of the clamps. The advantage is that the clamping of tool T is more accurate and contact pressure on the tool is uniformly distributed.

The blades 6 are realized to be flexible. In one embodiment for example the blades 6 are thin enough to be deformed to the S-like shape of Fig. 3; in equivalent embodiments the blades 6 are replaced with supporting elements having selected thin parts, preferably at the upper and lower ends. Deflection is concentrated in said thin parts, which act substantially as "hinges" to allow a deformation similar to that of Fig. 3.

Fig . 7 shows a preferred application of the head 1 as part of an EDM machine 30, comprising a frame 31 and a rotation unit 32 to rotate the tool T acting on a workpiece W. The application to EDM machines is preferred, but shall not be intended as a limitation. The invention is applicable to any kind of substantially cylindrical tools and related machines with the aforesaid advantages.