Vibratory incremental sheet forming tool

Abstract

An incremental sheet forming tool. The tool includes: a tool body; a tool head, rigidly attached to the body and having a punch; a housing provided in the body; an actuator, located in the housing and capable of producing a vibration in a first direction which can propagate to the punch); and at least one element for transforming the first direction of the vibration into a second direction of the vibration, as it is propagating to the punch.

Claims

1. An incremental sheet forming tool, which comprises: a tool body; a tool head, integral with the body and comprising a punch; a housing formed inside the body; an actuator, located in the housing and capable of producing a vibration in a first direction that can be propagated to the punch; and at least one element configured to transform the first direction of vibration into a second direction of vibration, during propagation of the vibration to the punch.

2. The incremental sheet forming tool according to claim 1, wherein said at least one element comprises at least one portion which is non-axisymmetrical with respect to a main axis of said tool.

3. The incremental sheet forming tool according to claim 1, the at least one element is arranged so that a unidirectional vibration parallel to a main axis of said tool and produced by the actuator causes the punch to vibrate in at least one direction orthogonal to the main axis.

4. The incremental sheet forming tool according to claim 2, wherein the housing is offset with respect to said main axis of said tool, constituting at least partially said non-axisymmetrical portion.

5. The incremental sheet forming tool according to claim 2, wherein an axis of fixing the actuator inside the housing is offset with respect to the main axis of said tool, constituting at least partially said non-axisymmetrical portion.

6. The incremental sheet forming tool according to claim 2, wherein the actuator is installed inside the body in a substantially offset manner with respect to a main axis of the body, constituting at least partially said non-axisymmetrical portion.

7. The incremental sheet forming tool according to claim 2, wherein the body comprises an addition of material non-axisymmetrical with respect to a main axis of the body, said addition constituting at least partially said non-axisymmetrical portion.

8. The incremental sheet forming tool according to claim 2, wherein the body comprises at least one recess-which is non-axisymmetrical with respect to a main axis of the body, said at least one recess being provided inside the body and constituting at least partially said non-axisymmetrical portion.

9. The incremental sheet forming tool according to claim 8, wherein the at least one recess comprises at least one hole-formed within the body and forming access to the housing from an outside of the body.

10. The incremental sheet forming tool according to claim 1, wherein the tool also comprises one or more shims-installed within the housing so as to exert a mechanical preloading stress on the actuator.

11. The incremental sheet forming tool according to claim 1, wherein the punch is capable of vibrating in at least one resonance mode in response to a vibration generated by the actuator along a main axis, and wherein the tool has at least one resonance frequency for a vibration mode for a transfer function defined by a ratio between an amplitude of the vibration of the punch according to the vibration mode and an amplitude of the vibration of the actuator parallel to the main axis, said resonance frequency being between 5 kHz and 30 kHz.

12. The incremental sheet forming according to claim 11, wherein the vibration mode comprises at least one of: a vibration parallel to the main axis, a vibration orthogonal to the main axis and parallel to a direction of misalignment of a non-axisymmetric portion of said tool, a vibration orthogonal to the main axis and the direction of misalignment of a non-axisymmetric portion of said tool, or a combination thereof.

Description

4. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Other features and advantages of the invention will become clearer on reading the following description of a particular exemplary embodiment, given merely as an illustrative and non-limiting example, and of the accompanying drawings, in which:

[0053] FIG. 1 shows a perspective view of a tool according to an exemplary embodiment of the invention;

[0054] FIG. 2 shows a perspective view from below of the tool of FIG. 1;

[0055] FIG. 3 shows a cross-sectional view of the top of the tool in FIG. 1;

[0056] FIG. 4 shows a cross-sectional view of the side of the tool in plane IV-IV of FIG. 3;

[0057] FIG. 5 shows a side view of the tool of FIG. 1;

[0058] FIG. 6 shows a detailed view VI of FIG. 4 according to another embodiment of the invention;

[0059] FIG. 7 shows an exploded view of the tool of FIG. 1;

[0060] FIG. 8 shows a variant of the tool head of the tool of FIG. 1;

[0061] FIG. 9 shows a frequency response of the tool of FIG. 1 in all three directions; and

[0062] FIG. 10 shows detail IX of FIG. 9.

5. DETAILED DESCRIPTION

5.1. General Principle

[0063] As explained above, the general principle of the disclosure is to produce vibrations in the desired direction by modifying the structure of the sheet forming tool and in particular by providing, within the tool, means for transforming the vibrations produced by the actuator. This aspect is clear from the description of the following figures, which represent one of the exemplary embodiments. The vibrations are produced at one or more given frequencies. In this application, vibration is defined as a mechanical wave at a certain frequency (or a plurality of superimposed frequencies) within the medium in which the vibration propagates. This vibration can be observed in the form of a vibratory force (for example at the actuator mounted in the housing) or in the form of a vibratory movement (or oscillations, for example observable at the punch when the latter is free, i.e. not in contact with a metal sheet). This vibration is also manifested physically in the form of a vibratory energy, i.e. the combination of a vibratory force and a vibratory movement.

[0064] With reference to FIGS. 1 to 5, a sheet forming tool 1 according to the invention is described. The tool 1 comprises a tool body 2, a tool head 3 and an actuator 4. The tool body 2 and the tool head 3 are fixed to one another. The actuator 4 is housed in the tool body 2 and/or the tool head 3.

[0065] The tool body 2 and the tool head 3 can be made of steel (but not necessarily the same steel). The tool body 2 and the tool head 3 can more generally be made from any material capable of withstanding the sheet forming forces induced by the incremental sheet forming process.

[0066] The tool body is generally axisymmetrical around a main axis 20, which defines a first direction Z referred to as the axial direction. For the remainder of this description, two directions X and Y are defined as orthogonal to direction Z and to each other. The directions X, Y and Z form an orthonormal reference framework (X, Y, Z), shown in FIG. 4.

[0067] The tool head 3 comprises a punch 30 at one of its ends. The punch is intended to come into contact with a metal sheet during an incremental sheet forming process.

[0068] In the example described here, the tool head 3 forms the punch 30 at one end, and is connected (fixed) to the tool body 2 at an opposite end. The punch 30 forms a tip of the tool head 3. The punch 30 may be hemispherical, which allows the angle of contact with the sheet metal to be varied continuously. The tool head 3 also comprises a base 32 via which the tool head 3 is secured to the tool body 2. The base 32 and the punch 30 are connected to one another by a section 34. The punch 30 and the section 34 can thus form a finger projecting from the base 32. The tool head 3 has a substantially axisymmetrical form. The tool head 3 has a profile which tapers from the base 32 to the punch 30. Here, the section of the tool head 3 has a substantially progressive cross-section, without any sharp angles, between the base 32 and the punch 30, so as to improve the mechanical properties of the part formed by the tool head 3. This also reduces the overall size of the tool, making it possible to obtain more varied shapes. In particular, the slender nature of the tool head 3 (i.e. its finger shape) makes it possible to form concave parts with a steeper resulting sheet angle and greater depth.

[0069] In the examples described here, the tool head 3 and the tool body 2 are described as separate parts, and are fixed together using screws, for example. Alternatively, the tool head 3 and tool body 2 may be in one piece, with access to an interior of the tool body 2 from the end opposite the tool head 2.

[0070] The interior of the tool body 2 forms a housing 22, shown in particular in FIG. 4. The housing 22 is suitable for receiving the actuator 4. The tool head 3 can close a first end of the housing 22, also referred to as the proximal end, as shown in FIG. 4. Alternatively, this first end of the housing 22 (close to the punch) can be closed by the tool body 2 itself. The tool 1 may comprise a cover 24, fixed to the tool body 2, and closing a second end (also referred to as the distal end of the housing 22, as it is remote from the punch) of the housing 22. Alternatively, the tool body 2 itself may close this second end.

[0071] The actuator 4 is installed in the housing 22. The actuator 4 may comprise an actuator interface 42 capable of controlling the actuator. The actuator 4 is capable of generating vibrations. The vibrations produced by actuator 4 are propagated throughout tool 1, and in particular to punch 30. The actuator 4 is able to vibrate in Z direction, i.e. parallel to the main axis 20. When the actuator 4 vibrates, it generates a vibration which can propagate to the rest of the tool 1. The actuator 4 may be unidirectional, i.e. capable of vibrating specifically in Z direction.

[0072] The tool 1 comprises means for transforming the direction of vibration, hereinafter referred to as transformation means. The transformation means is capable of modifying the direction of the vibrations as they propagate from the actuator 4 in the housing 22 to the punch 30.

[0073] The transformation means comprises a non-axisymmetric portion of the tool with respect to the main axis 20. In other words, the transformation means comprises at least one non-axisymmetrical portion (or element) with respect to this main axis 20.

[0074] This non-axisymmetrical portion of the tool 1 may comprise the actuator 4 itself, offset relative to the main axis 20 by an actuator offset 40, shown in FIG. 4. Here, the housing 22 is axisymmetrical relative to the main axis 20 of the tool body 2 and the actuator 4 is mounted off-centre in the housing 22. Alternatively, the housing 22 may be offset with respect to the main axis 20 and the actuator 4 housed centred in the housing 22 (this causes the offset 40). It is possible to combine an offset of the housing 22 and an off-center mounting of the actuator 4 in the housing 22.

[0075] The dissymmetry induced by the offset of the actuator 4 (whether by placing it off-center in a symmetrical housing 22 or by having a housing 22 itself off-center) enables axial vibrations of the actuator 4 to be transformed into transverse vibrations at the punch 30. These transverse vibrations occur in particular for particular frequencies of vibrations corresponding to bending modes of the tool. The actuator can thus be unidirectional while having a punch capable of vibrating axially and transversely. In the example shown in FIGS. 3 and 4, the actuator 4 is fixed to the tool head 3 by a hole 46 formed in the tool head 3, in line with the axis of the actuator 4 but offset from the tool body 2 and the tool head 3.

[0076] As the actuator can be unidirectional, this actuator can be significantly more powerful than a multidimensional actuator (for example a stack of small piezoelectric actuators, each layer of the stack being able to vibrate in its own direction). The amplitude of the vibrations at the level of the punch 30 is then much greater, which allows incremental sheet forming of better quality (surface finish) and on thicker metal sheets, for the same size of tool and machine supporting the tool.

[0077] The tool body 2 may comprise at least one hole 44; in FIG. 1, the tool body 2 comprises two holes. Hole 44 is formed in a side wall of the tool body 2 and opens into housing 22. When there are several holes 44, these holes 44 may not be equally spaced on the tool body 2, so as to contribute to the asymmetry of the tool body 2, and therefore of the tool 1. In addition to the asymmetry, the hole 44 allows access to the inside of the housing 22 so that the actuator 4 can be powered. the hole 44 can also be made in the tool head 3 or in the cover 24 to allow access to the housing 22. The asymmetrical appearance of the holes 44 is particularly apparent in FIG. 3, where it is clear that they are only formed on one side of the tool. The hole also enables the heat produced by the actuator during operation to be dissipated.

[0078] Alternatively, or in combination with the hole 44, a material recess can be made in the tool body 2, also producing a dissymmetry of the tool body 2, the cover 24 or the tool head 3.

5.2 Transfer Function

[0079] The different parts of the tool 1, in particular the tool body 2, the tool head 3 and the actuator 4, can be carefully dimensioned so as not only to cause the transformation of the direction of vibration (as it propagates from the actuator 4 housed in the tool body 2 to the punch 30), but also so as to know precisely the nature (amplitude, frequency) of the vibration of the punch 30 as function of the vibrations of the actuator 4. The vibration frequency of the tool corresponds to the excitation frequency of the actuator. The excitation of the system in a given mode (i.e. at a given frequency) does not cause the excitation of other modes.

[0080] This dimensioning can be the result of a computer simulation, for example as part of computer-aided design. It is also possible to replace or complete this simulation with empirical measurements on a tool prototype. In developing the present invention, the inventors were thus able to combine these two methods (simulation and experiment) to obtain a tool with satisfactory vibration properties, particularly in terms of vibration control.

[0081] The vibration of the punch can be measured using a laser vibrometer, the reflection of which on the punch makes it possible to determine the displacement of the punch, and therefore its vibrations. This measurement of the vibration of the punch can be observed by controlling the actuator and generating a frequency sweep of the excitation of the actuator by a sinusoidal command and observing the vibratory response of the punch. The frequency of the sweep (i.e. of the sinusoidal command) can evolve linearly or logarithmically.

[0082] For example, firstly a rapid frequency sweep is carried out over the whole spectrum (for example from 1 kHz to 22 kHz), then the frequency for which the gain between the displacement measured at the end of the tool by a laser vibrometer (or acceleration by an accelerometer) and the supply voltage to the actuator (image of the force) is maximum is recovered, and which is therefore most likely to be close to a resonance frequency (i.e. an own mode of the tool).

[0083] A second, finer frequency sweep is then carried out around the maximum frequency or frequencies obtained above, which makes it possible to obtain a precise gain profile in the vicinity of the resonant frequency or frequencies.

[0084] The punch 30 can vibrate in several vibration modes: unidirectional modes (axial Z, radial X or tangential Y), or coupled modes ((X,Z), (X,Y) or (Y,Z), or (X,Y,Z)).

[0085] For a given mode, it is possible to determine (by experiment and/or simulation) the associated transfer function, i.e. the function taking as input the vibration of the actuator 4 in Z direction (amplitude, frequency) and as output the vibration of the punch 30 in this given mode (amplitude, frequency).

[0086] The tool 1 as a whole is also dimensioned so that for at least one vibration mode, its associated transfer function comprises a resonance frequency of between 5 and 30 kHz. There may be several resonance frequencies for a transfer function of a given mode, i.e. several peaks in which the energy transfer from the actuator 4 to the punch 30 is at a maximum (at least locally).

[0087] In some embodiments, the tool 1 has several transfer functions, each with its own resonance in the 5-30 kHz range. These resonances can be disjointed, i.e. when a given frequency is selected for which there is resonance in a given mode, there is no resonance in another mode. If the excitation frequency does not correspond to the specific frequency of a mode, then the vibratory response of the punch is a linear combination of all the own modes. This allows for the precise control of which resonance mode is preferred for a given incremental sheet forming process.

5.3 Preload

[0088] Reference is now made to FIG. 6 and FIG. 7.

[0089] In this embodiment, the tool 1 also comprises a shim 46 (or a set of shims) housed in the extension of the actuator in Z direction. The shim 46 exerts a preload on the actuator 4 when the housing 22 is closed. The shim 46 can be dimensioned by a suitable chain of dimensions, for example a chain of dimensions relating to the body 3, the head 2 and the cover 24 together forming the walls of the housing 22.

[0090] The shim 46 can be housed in a recess 48 formed in the cover 24 in extension of the housing 22. Here, the shim 46 is housed on the cover side 24, but the shim 46 could be arranged on the tool head 3 side. In the case of a set of shims 46, one portion of the set of shims 46 can be on the cover 24 side and another portion on the tool head 3 side. Here, the shim 46 on the cover 24 side sandwiches the interface 42 with the actuator 4.

[0091] In the example shown in FIG. 7, the set of shims comprises one shim of greater thickness and three shims of smaller thickness.

[0092] The shim 46 precisely reduces the length of the housing 22 in which the actuator 4 is housed. Thus, the shim 46 exerts a preload on the actuator 4. When this preload is sufficient, it considerably increases the efficiency of the energy transfer between the actuator 4 and the rest of the tool 1. The inventors have estimated that a preload of in the order of 80 to 85 kN is optimal for a prototype that has been developed, in order to maximise the energy transfer without damaging the actuator 4. More generally, the preload is in the order of several tens of kN to a few hundreds of kN depending on the dimensions of the prototype. This preload can also be adjusted by tightening the screws 32 to a certain torque (for example with a torque spanner) to ensure control of the screw tightening force.

5.4 Non-Axisymmetrical Head

[0093] In one embodiment, the head 3 is non-axisymmetrical with respect to the main axis 20. In the example shown in FIG. 8, the punch 30 is offset from the main axis 20. The base 32 of the head 3 is substantially axisymmetrical with respect to the main axis 20.

[0094] The dissymmetry of the head 3 means that the head 3 vibrates non-axisymmetrically when the actuator 4 produces vibrations along the main axis 20. The punch 30 can therefore vibrate transversely (radially X or tangentially Y). This makes it advantageous to install a non-axisymmetrical head 3 on a pre-existing axisymmetric tool.

[0095] The punch 30 is connected to the base 32 by a section 34 whose cross-section becomes thinner the closer it is to the punch 30. The section 34 has a smooth profile, with no edges or corners.

[0096] The asymmetry of the head 3 is compatible with the other non-axisymmetrical portions described above to constitute the transformation means.

5.5. Example of Transfer Function

[0097] Reference is now made to FIG. 9, which represents a frequency response of the tool 1, or in other words the transfer function of this tool 1.

[0098] FIG. 9 comprises three graphs, each representing a frequency response of the punch 30 (i.e. the amplitude of the vibration produced at the punch) according to the excitation frequency of the actuator, in the X, Y and Z directions respectively (from top to bottom).

[0099] The frequencies studied here vary between 4 kHz to 16 kHz, and the x-axis (which represents the frequency) is on a linear scale. The gain studied (i.e. the ratio of the amplitude of the vibration of the punch to the amplitude of the vibration generated by the actuator) is measured in dB.

[0100] In this example, the transfer function has: [0101] a coupled mode (X, Y) around 4.5 kHz (with a substantially higher gain in the Y direction compared to the X direction), [0102] a coupled mode (X, Y, Z) around 14 kHz, and [0103] an own mode in X direction around 7.5 kHz.

[0104] With specific reference to the transfer function around 14 kHz, cf. FIG. 10 which shows this transfer function along the X, Y and Z axes around 14 kHz, it can be seen in reality that the transfer function has two very similar own modes around 14 kHz: [0105] a first coupled mode 100 at around 14.2 kHz which corresponds to a resonance peak in the three directions X, Y and Z, and a second own mode 102 in Y direction at around 14 kHz. In this second resonance mode 102 the frequency responses in X and Z both show a very marked dip (and so that the punch does not vibrate; this phenomenon being known as anti-resonance and being more marked in Z direction), whereas the frequency response in Y is close to the resonance peak. The frequency response of the punch is therefore particularly pure in the Y direction (in other words, the punch does not vibrate in X and Z directions).

[0106] The transfer function also has a third mode 104 which is very close to the first mode 100.

[0107] Thus, in this example, the tool has simple modes of vibration (e.g. at 14 kHz in the Y direction or at 7.5 kHz in the X direction) and coupled (in X, Y and in X, Y, Z). As the transfer function is linear, it is possible to excite each of these modes independently, by superposing frequencies in the excitation of the actuator 4. In this way, the vibration produced by the punch 30 can be finely controlled by adjusting the control of the actuator 4, while maintaining a unidirectional actuator 4.