Vibratory incremental sheet forming tool

Abstract

An incremental sheet forming tool (1) is disclosed. According to the disclosure, such a tool comprises: - a tool body (2); - a tool head (3) rigidly attached to the body (1) and comprising a punch (30); - a housing (22) provided in the body (2); - an actuator (4) located in the housing (22) and capable of producing a vibration in a first direction which can propagate to the punch (30); - at least one means (4, 22, 3) for transforming the first direction of the vibration into a second direction of the vibration as it is propagating to the punch (30).

Description

[0001] DESCRIPTION

[0002] VIBRATORY INCREMENTAL FORMING TOOL

[0003] 1. Technical field

[0004] The invention relates to the field of forming, and more particularly to incremental forming. Forming is a manufacturing process consisting of shaping a by-product, typically a plate or sheet. Forming is conventionally carried out by compressing or pressing the by-product between two predetermined shapes, called respectively a die and a counter-die.

[0005] Conventional forming is, however, a rigid process, because a given pair of dies can only produce a given shape. Thus, when one wishes to change the shape of the product resulting from the forming, or even to modify this shape marginally, it is necessary to change the dies used. Conventional forming is also cumbersome to set up, particularly when the sheet to be formed is large, and requires very significant efforts to apply sufficient and uniform pressure on the sheet to be formed.

[0006] To overcome the shortcomings of conventional forming, incremental sheet forming (ISF) has been developed over the last two decades. Incremental forming involves locally and iteratively deforming a sheet of metal to give it the desired shape. To do this, the sheet is locally deformed using a tool head (also called a punch), and this process is iterated until the desired shape is achieved. To control incremental forming, the tool head is positioned. It is even possible to control the tool's acceleration, so as to control the force that the punch exerts on the sheet.

[0007] Incremental forming has many advantages, particularly compared to conventional forming processes. The shape of the forming product results from the numerical control controlling the position of the tool head, and can therefore be easily modified by changing the instructions at the origin of the numerical control (for example a 3D model), without having to change the forming tool itself, unlike conventional forming. Incremental forming can advantageously be implemented by integrating the tool head at the end of a robotic arm or in a numerically controlled machine. The robotic arm or the numerically controlled machine then controls the position of the tool head, whereby the sheet is shaped.

[0008] Incremental forming using a robotic arm is particularly advantageous in that it can be used to form a large sheet metal while being significantly less expensive than a numerically controlled machine, and even less expensive than traditional forming with a die and counter-die (which are, by definition, the size of the sheet metal to be formed).

[0009] Thanks to its flexibility and ease of implementation, incremental forming has many advantages and finds its place in the manufacture of prototypes or small series.

[0010] 2. Prior art

[0011] As explained above, incremental forming relies on the local deformation of a sheet metal. This deformation results from the local application of a force (or pressure) on this sheet metal by the punch. This force therefore causes in return an elastic deformation of the robotic arm. If this deformation is too great, it moves the robotic arm and disrupts the control of the punch position. Thus, the forces that can be applied to the sheet metal to be formed are limited by the rigidity of the robotic arm with which the sheet metal is formed, and even more limited the thicker the sheet metal. Similar problems arise when using a numerically controlled machine to carry out incremental forming.

[0012] However, it has been discovered that by vibrating the punch at frequencies in the kilohertz range up to the low-frequency ultrasound range, the sheet material is locally softened, making it more ductile, thereby reducing the forces required to deform it. The elastic return of the sheet is then also reduced. These vibrations can therefore be used to form thicker sheets and / or to use a smaller robotic arm. The vibrations are conventionally generated by a piezoelectric actuator.

[0013] More specifically, when the vibrations are in a direction normal to the sheet metal (i.e. perpendicular to the sheet metal at the punch-sheet contact), these longitudinal vibrations would activate a movement of dislocations and their propagation within the material, hence, potentially, the softening observed. Transverse vibrations (i.e. perpendicular to normal vibrations) reduce friction between the sheet metal and the punch, which also reduces the forces that the robotic arm must produce to carry out incremental forming. These transverse vibrations also limit the "stick-slip" phenomenon, which also improves the quality of the tool movement control, as well as the surface condition of the sheet metal thus formed.

[0014] The vibrations normal and transverse to the sheet thus produce complementary effects which greatly reduce the efforts required to carry out incremental forming. However, generating these vibrations in all three directions (one normal and two transverse) is not easy.

[0015] We know the article Kurniawan, R., Ali, S., Park, KM, Li, CP, & Ko, TJ (2019). Development of a three-dimensional ultrasonic elliptical vibration transducer (3D-UEVT) based on sandwiched piezoelectric actuator for micro-grooving. International Journal of Precision Engineering and Manufacturing, 20(7), 1229-1240, which describes a drilling tool comprising a stack of piezoelectric actuators placed in the center of the body of the forming tool. Each of the actuators in this stack can vibrate in one of three directions, depending on the electrical command it receives. The vibrations produced by the actuators are propagated by the tool body to the drill bit. Thus, this drill bit is able to vibrate in all three directions.

[0016] Also known is the article Gao, J., & Altintas, Y. (2019). Development of a three-degree-of-freedom ultrasonic vibration tool holder for milling and drilling. IEEE / ASME Transactions on Mechatronics, 24(3), 1238-1247. This second article describes a machining tool playing on an elliptical excitation of a stack of actuators to couple vibration modes.

[0017] However, these solutions are not satisfactory. Firstly, this requires the implementation of several actuators (in the form of stacks), therefore a multitude of power supplies to control. In addition, since there are several actuators, each actuator is smaller (to respect a certain space limit), and can therefore produce less powerful vibrations. Finally, in practice these solutions are difficult to control, because the multiple actuators interfere with the tool and it is not easy to control the vibration modes individually to produce the desired vibrations.

[0018] Furthermore, these solutions concern machining and drilling, areas very different from incremental forming.

[0019] There is a need for a solution for vibrating a punch in a multidirectional manner which does not have the defects of the prior art, particularly in terms of simplicity of implementation, size and ease of control.

[0020] The invention improves the situation.

[0021] 3. Statement of the invention

[0022] The invention was designed to overcome at least some of the drawbacks of the prior art.

[0023] To this end, the invention proposes an incremental forming tool, characterized in that it comprises: a tool body; a tool head, integral with the body and terminated by a punch at its end; a housing provided within the body; an actuator, placed in the housing and capable of producing a vibration in a first direction which can propagate to the punch; at least one means for transforming the first direction of the vibration into a second direction of the vibration, during its propagation to the punch. Thus, thanks to this transformation means, the tool is able to transform a vibration produced by a unidirectional actuator (in the first direction) into a multidirectional vibration or at least in the second direction. For example, the tool is able, from an actuator producing an axial vibration (i.e.in a main axis of the body, also called axial direction or longitudinal direction), to produce a transverse vibration, i.e. orthogonal to this main axis, at the punch. The actuator can thus be a unidirectional actuator without this preventing the tool from vibrating in several directions (including a transverse and an axial one). The actuator is for example a piezoelectric actuator. During an incremental forming process, the punch is able to vibrate in the directions normal and transverse to the sheet metal (i.e. in the main axis of the tool body and in one or more directions orthogonal to this main axis, respectively). The sheet metal is thus locally softened, and the friction between the tool head and the sheet metal is reduced. The elastic return of the sheet metal on the tool is consequently reduced. As the forces to form the sheet metal are reduced, the elastic return of the arm is also reduced.

[0024] The actuator can excite the tool head and make it vibrate by imposing a vibratory force on it to obtain, at certain frequencies, a significant vibratory force at the punch level by structural resonance effect, in particular when the punch is in contact with a sheet metal.

[0025] This results in a gain on the machine side (i.e. a smaller size of the robotic arm or the numerically controlled machine supporting the tool) but also in the quality of the formed part, since the reduced elastic return and the reduction in friction lead to a better surface condition of the part after forming.

[0026] According to one aspect, said at least one transformation means comprises at least one non-axisymmetric portion relative to a main axis of said tool.

[0027] Thus, the tool has a generally axisymmetric shape, which improves its vibration properties, in particular by separating the vibration modes of the punch compared to a tool whose shape would be particularly asymmetrical. This also makes it possible to integrate the transformation means into a pre-existing tool, by partially modifying its axisymmetric character.

[0028] According to one aspect, the vibration direction transforming means is arranged such 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.

[0029] The fact that the punch can be vibrated in several directions from a unidirectional vibration using the transformation means also makes it possible to implement a single, unidirectional actuator, and therefore a more powerful actuator. The tool can therefore be smaller with equivalent actuator power, since the effects of lower ductility and lower friction are all the more significant when the vibration is of high amplitude.

[0030] In order to generate a controllable transverse vibration of the punch, the body and / or the head (in particular the non-axisymmetric portion of the transformation means) can be judiciously dimensioned, so that a given unidirectional vibration of the actuator at a given frequency delivers a given transverse vibration of the tool head. It is thus not only possible to generate a multidirectional vibration of the tool head using a unidirectional actuator, but also to control it precisely.

[0031] This new type of tool thus allows incremental forming using a robotic arm or a numerically controlled machine (more generally any machine supporting the tool) of smaller dimensions than those usually used, since the elastic return is less. This alternatively allows larger and / or thicker sheets to be formed with the same robotic arm. Finally, friction is less, which limits the stick-slip effect. This consequently improves the quality of the servo-control in the position of the tool head, and the surface condition of the sheet once formed.

[0032] According to one aspect, the housing is offset relative to a main axis of the body, at least partially constituting said non-axisymmetric portion.

[0033] According to one aspect, the axis of attachment of the actuator within the housing is offset relative to a main axis of said tool, at least partially constituting said non-axisymmetric portion.

[0034] According to one aspect, the actuator is installed within the body in a substantially offset manner relative to a main axis of the body, at least partially constituting said non-axisymmetric portion.

[0035] Thanks to the misalignment of the housing and / or the actuator, the parallel vibration induced by a unidirectional actuator mounted in the housing causes a vibration transverse to the main axis of the tool body. By accurately dimensioning this misalignment, it is then possible to determine the transfer function between the vibration in the Z axis (axial direction) and the vibration in the X, Y and Z directions (respectively transverse radial, transverse tangential and axial). This can be done, for example, using a simulation (for example, a numerical analysis using the finite element method) from a model of the tool obtained by computer-aided design (CAD). By knowing this transfer function well, it is possible to control the vibrations of the punch from the frequency control of the actuator power supply (i.e. the unidirectional vibration it produces in the housing).In one aspect, the body comprises an addition of material that is non-axisymmetric relative to a main axis of the body, said addition at least partially constituting said non-axisymmetric portion.

[0036] Thus, an addition of material helps to propagate the vibration in a non-axial direction. It should be noted that this addition of non-axisymmetric material can fulfill a third function, such as acting as a key in the assembly of the tool, allowing the attachment of power supply means or sensors to the tool, etc., which saves space by pooling the functions within this addition.

[0037] According to one aspect, the body comprises at least one recess which is non-axisymmetric with respect to a main axis of the body, said at least one recess being provided within the body and at least partially constituting said non-axisymmetric portion.

[0038] Here, the recess helps to propagate the vibration in a non-axial direction. It should be noted that this non-axisymmetric recess can fulfill a third function, such as allowing access to parts of the tool, acting as a keying device, housing power supply means or sensors, etc., which saves space by pooling the functions within this recess.

[0039] According to one aspect, the recess comprises at least one bore provided in the body and forming an access to the housing from the exterior of the body.

[0040] This drilling fulfills a dual role: that of contributing to the non-axisymmetry of the tool and of helping to power the actuator by allowing access to the housing (to pass power supply cables through it). This leads to a saving of space, in addition to the generation of a transverse vibration at the level of the punch.

[0041] It should be noted that the asymmetry induced by the removal or addition of material or drilling is not incompatible with an offset of the housing and / or the actuator. On the contrary, these two particular characteristics can combine and amplify the asymmetry of the part, thereby improving the generation of transverse vibration.

[0042] According to one aspect, the tool further comprises one or more shims installed in the housing so as to exert a preload on the actuator (in particular when the tool is in operation and in the absence of vibrations of the actuator).

[0043] In this example, compressing the actuator using a shim (or a set of shims) induces a pre-stress, i.e. a force (or pre-load) applied to the actuator in the absence of a specific action (such as powering the actuator). The pre-stress drastically improves the energy transfer between the actuator and the tool body (hence the gain of the transfer function described above). The pre-stress also increases the transfer of the vibration produced by the actuator to the rest of the tool.

[0044] According to one aspect, the punch is capable of vibrating according to at least one resonance mode in reaction to a vibration generated by the actuator along a main axis, and the tool has at least one resonance frequency for said vibration mode for the transfer function defined by the ratio between the amplitude of a vibration of the punch according to the vibration mode and the amplitude of the vibration of the actuator parallel to the main axis, said resonance frequency being between 5 kHz and 30 kHz.

[0045] The transfer function exhibits resonance. This resonance advantageously improves energy transfer, and therefore increases the amplitude of the punch vibration. This increases the softening of a sheet to be formed, and reduces the effort required to form this sheet.

[0046] Obtaining this resonance results from a judiciously chosen dimensioning of the tool, in particular of its non-axisymmetric portion. This can be achieved using a simulation during a computer-aided design, or empirically, the important thing being that a resonance frequency is obtained at the end for the desired mode(s).

[0047] In one aspect, the vibration mode belongs to the group consisting of vibration parallel to the main axis, vibration orthogonal to the main axis and parallel to an offset direction of the non-axisymmetric portion, vibration orthogonal to the main axis and to the offset direction of the non-axisymmetric portion, and a combination thereof.

[0048] The direction orthogonal to the main axis and parallel to an offset direction of the non-axisymmetric portion (typically the offset of the actuator and / or the housing, or the offset direction of a drilling or an addition of material) is the radial transverse direction (X axis). The direction orthogonal to the main axis and to the offset direction of the non-axisymmetric portion is the tangential transverse direction (Y axis). The possible combination(s) of these three directions X, Y and Z is called coupled mode(s).

[0049] The fact that the punch can vibrate in an axial mode, a radial mode, a tangential mode or a coupled mode makes it possible to obtain different effects on the sheet metal that is softened using vibrations. In particular, forces generated by transverse vibrations reduce friction at the contact between the sheet metal and the punch during forming, because axial vibrations would activate dislocations of the sheet metal at the microscopic level, which would soften the sheet metal. The different coupled modes (in particular axial / radial and axial / tangential), for their part, would make it possible to combine these effects. Furthermore, the coupled modes make it possible to anticipate the variation of the trajectory of the punch, and therefore to maintain the transverse direction of the vibration of the punch collinear with the direction of advance of the tool, which further improves the reduction of punch / sheet metal friction.In other words, the direction of the transverse vibration of the tool can be controlled so that it is tangent to the displacement of the punch. These vibration modes can also be single, double or triple, i.e. induce a single, double or triple bending of the tool head, respectively.

[0050] 4. Brief description of the drawings

[0051] Other characteristics and advantages of the invention will appear more clearly on reading the following description of a particular exemplary embodiment, given as a simple illustrative and non-limiting example, and the appended drawings, among which:

[0052] [Fig. 1] represents a perspective view of a tool according to an exemplary implementation of the invention;

[0053] [Fig. 2] represents a perspective view from below of the tool of Figure 1;

[0054] [Fig. 3] represents a top sectional view of the tool of Figure 1;

[0055] [Fig- 4] represents a side sectional view of the tool according to plane IV-IV of figure 3;

[0056] [Fig. 5] represents a side view of the tool of Figure 1;

[0057] [Fig. 6] represents a view of detail VI of figure 4 according to another exemplary embodiment of the invention;

[0058] [Fig. 7] represents an exploded view of the tool of figure 1;

[0059] [Fig. 8] represents a tool head variant of the tool of Fig. 1;

[0060] [Fig. 9] represents a Sequential response of the tool of Figure 1 in the three directions; and

[0061] [Fig. 10] represents detail IX of figure 9.

[0062] 5. Detailed description

[0063] 5.1 General principle

[0064] As explained previously, the general principle of the disclosure consists of producing vibrations in the desired direction by modifying the structure of the forming tool and in particular by having, within the tool, means for transforming the vibrations produced by the actuator. This aspect appears clearly from the description of the following figures which represent one of the exemplary embodiments. The vibrations are produced at one or more given frequencies. By vibration, in the present application is meant a mechanical wave according to 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 vibrating force (for example at the actuator mounted in the housing) or in the form of a vibratory displacement (or oscillations, for example observable at the punch when the latter is free, that is to say without being in contact with a sheet).This vibration also manifests itself physically in the form of vibratory energy, that is to say the combination of a vibratory effort and a vibratory displacement.

[0065] A forming tool 1 according to the invention is described with reference to Figures 1 to 5. 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 each other. The actuator 4 is housed in the tool body 2 and / or the tool head 3.

[0066] The tool body 2 and the tool head 3 may be made of steel (but not necessarily the same steel). The tool body 2 and the tool head 3 may more generally be made of any material capable of exhibiting mechanical strength suitable for the forming forces induced by the incremental forming process.

[0067] The tool body is generally axisymmetrical in shape around a main axis 20, which defines a first direction Z called the axial direction. For the remainder of this description, two orthogonal directions X and Y are defined to the Z direction and to each other. The directions X, Y and Z form an orthonormal reference frame (X, Y, Z), shown in Figure 4.

[0068] The tool head 3 comprises a punch 30 at one of its ends. The punch is intended to come into contact against a sheet metal during an incremental forming process.

[0069] 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 in shape, which allows the contact angle with the sheet metal to be varied continuously. The tool head 3 further comprises a base 32 via which the tool head 3 is fixed to the tool body 2. The base 32 and the punch 30 are connected to each other by a section 34. The punch 30 and the section 34 may thus form a finger projecting from the base 32. The tool head 3 here has a substantially axisymmetrical shape. 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 section, without 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 makes it possible to limit the overall size of the tool, which allows more varied shapes to be obtained. The slender nature of the tool head 3 (i.e. its finger shape) makes it possible in particular to form concave parts with a more pronounced resulting sheet metal angle and greater depth.

[0070] In the examples described here, the tool head 3 and the tool body 2 are described as separate parts, and are fixed together for example using screws. Alternatively, the tool head 3 and the tool body 2 may be a single piece, with access to an interior of the tool body 2 being via the end opposite the tool head 2. The tool body 2 forms a housing 22 inside it, visible in particular in FIG. 4. The housing 22 is capable of receiving the actuator 4. The tool head 3 may close a first end of the housing 22, also called the proximal end, as can be seen in FIG. 4. Alternatively, this first end of the housing 22 (close to the punch) may 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 called the distal end of the housing 22, because 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 it. The actuator 4 is capable of generating vibrations. The vibrations produced by the actuator 4 propagate throughout the tool 1, and in particular up to the punch 30. The actuator 4 is capable of vibrating in the 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 the Z direction.

[0072] The tool 1 comprises a means for transforming the direction of the vibrations, hereinafter called transformation means. The transformation means is capable of modifying the direction of the vibrations during their propagation 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-axisymmetric portion (or element) with respect to this main axis 20.

[0074] This non-axisymmetric portion of the tool 1 may comprise the actuator 4 itself, offset relative to the main axis 20 according to an actuator offset 40, visible in FIG. 4. Here, the housing 22 is axisymmetric relative to the main axis 20 of the tool body 2 and the actuator 4 is mounted off-center in the housing 22. Alternatively, the housing 22 may be offset relative to the main axis 20 and the actuator 4 housed centered in the housing 22 (this inducing 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 asymmetry induced by the misalignment of the actuator 4 (whether by placing it in an off-axis manner in a symmetrical housing 22 or by having a housing 22 itself off-axis) makes it possible to transform axial vibrations of the actuator 4 into transverse vibrations at the punch 30. These transverse vibrations occur in particular for particular vibration frequencies 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 Figures 3 and 4, the actuator 4 is fixed to the tool head 3 by a bore 46 formed in the tool head 3, in the axis of the actuator 4 but off-axis relative to the tool body 2 and to the tool head 3.

[0076] Since 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 punch 30 is then much greater, which allows for higher quality incremental forming (surface finish) and on thicker sheets, with equivalent tool and machine sizes supporting the tool.

[0077] The tool body 2 may comprise at least one bore 44; in Figure 1, the tool body 2 comprises two bores. The bore 44 is formed in a side wall of the tool body 2, and opens into the housing 22. When there are several bores 44, these bores 44 may not be equally distributed over the tool body 2, so as to contribute to the asymmetry of the tool body 2, and therefore of the tool 1. The bore 44 allows, in addition to the asymmetry, access to the interior of the housing 22 so as to be able to supply the actuator 4. The bore 44 may also be formed in the tool head 3 or in the cover 24 to allow access to the housing 22. The asymmetrical appearance of the bores 44 is particularly visible in Figure 3, where it is clear that they are formed only on one side of the tool. The drilling also helps dissipate the heat produced by the actuator in operation.

[0078] Alternatively or in combination with the drilling 44, a material recess can be provided in the tool body 2, which also produces an asymmetry of the tool body 2, the cover 24 or the tool head 3.

[0079] 5.2 Transfer function

[0080] The various 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 the vibration (when 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 a function of the vibrations of the actuator 4. The frequency of the vibrations of the tool corresponds to the excitation frequency of the actuator. Excitation of the system in a given mode (i.e. at a given frequency) does not cause excitation of the other modes.

[0081] This dimensioning can be the result of a computer simulation, for example in the context of computer-aided design. It is also possible to replace or supplement this simulation with empirical measurements on a tool prototype. The inventors, in the context of the development of the present invention, were thus able to combine these two methods (simulation and experiment) to obtain a tool whose vibration properties are satisfactory, particularly in terms of vibration control.

[0082] The measurement of the punch vibration can be carried out using a laser vibrometer whose reflection on the punch makes it possible to determine the displacement of said punch, therefore its vibrations. This measurement of the punch vibration can be observed by controlling the actuator and generating a frequency sweep of the actuator excitation by a sinusoidal command and observing the vibrating response of the punch. The frequency of the sweep (i.e. of the sinusoidal command) can evolve linearly or logarithmically).

[0083] For example, we first perform a rapid frequency sweep over the entire spectrum (for example from 1 kHz to 22 kHz), then we recover the frequency for which the gain between the displacement measured at the end of the tool by a laser vibrometer (or the acceleration by an accelerometer) and the actuator supply voltage (image of the force) is maximum, and which therefore has the greatest chance of being close to a resonance frequency (i.e. a natural mode of the tool).

[0084] A second, finer frequency sweep is then carried out around the maximum frequency(ies) obtained above, which makes it possible to obtain a precise profile of the gain in the vicinity of the resonance frequency(ies).

[0085] The vibrations of the punch 30 can be in several vibration modes: unidirectional modes (axial Z, radial X or tangential Y), or coupled ((X,Z), (X,Y) or (Y, Z), or (X,Y,Z)).

[0086] 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 the Z direction (amplitude, frequency) and as output the vibration of the punch 30 in this given mode (amplitude, frequency).

[0087] The tool 1 as a whole is further 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 maximum (at least locally).

[0088] In some embodiments, the tool 1 has several transfer functions, each having its own resonance in the 5-30 kHz range. These resonances can be disjoint, i.e., when one is at a given frequency 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 vibration response of the punch is a linear combination of all the natural modes. This offers the possibility of precisely controlling which resonance mode to favor for a given incremental forming process.

[0089] 5.3 Prestressing

[0090] Reference is now made to Figure 6 and Figure 7.

[0091] In this embodiment, the tool 1 further comprises a shim 46 (or a set of shims) housed in the extension of the actuator in the Z direction. The shim 46 exerts a prestress on the actuator 4 when the housing 22 is closed. The shim 46 can be dimensioned by a chain of adapted 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.

[0092] The shim 46 can be housed in a recess 48 formed in the cover 24 in the extension of the housing 22. Here, the shim 46 is housed on the cover 24 side, but the shim 46 could be arranged on the tool head 3 side. In the case of a set of shims 46, part of the set of shims 46 can be on the cover 24 side and another part on the tool head 3 side. Here, the shim 46 on the cover 24 side sandwiches the interface 42 with the actuator 4.

[0093] In the example shown in Figure 7, the set of shims includes one thick shim and three thinner shims.

[0094] 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. This preload, when sufficient, 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 the order of 80 to 85 kN is optimal for a developed prototype, in order to maximize the energy transfer without damaging the actuator 4. More generally, the preload is of the order of a few tens of kN to a few hundred 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 wrench) to ensure control of the tightening force of the screws.

[0095] 5.4 Non-axisymmetric head

[0096] In an exemplary embodiment, the head 3 is non-axisymmetric with respect to the main axis 20. In the example shown in FIG. 8, the punch 30 is offset with respect to the main axis 20. The base 32 of the head 3 is substantially axisymmetric with respect to the main axis 20.

[0097] The asymmetry of the head 3 causes the head 3 to vibrate in a non-axisymmetric manner when the actuator 4 produces vibrations along the main axis 20. The punch 30 can thus vibrate transversely (radial X or tangential Y). This advantageously makes it possible to install a non-axisymmetric head 3 on a pre-existing and axisymmetric tool. The punch 30 is connected to the base 32 by a section 34 whose section becomes thinner the closer it is to the punch 30. The section 34 has a smooth profile, without edges or angles.

[0098] The asymmetry of the head 3 is compatible with the other non-axisymmetric portions described above to constitute the transformation means.

[0099] 5.5. Example of transfer function

[0100] Reference is now made to Figure 9, which represents a frequency response of tool 1, or in other words the transfer function of this tool 1.

[0101] Figure 9 includes 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, respectively in the X, Y and Z direction (from top to bottom).

[0102] The frequencies studied here vary between 4 kHz and 16 kHz, and the abscissa (which represents the frequency) is in 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.

[0103] In this example, the transfer function presents: a coupled mode (X, Y) around 4.5 kHz (with a significantly higher gain in the Y direction compared to the X direction), a coupled mode (X, Y, Z) around 14 kHz, and a natural mode along the X direction around 7.5 kHz.

[0104] Looking more closely at the transfer function around 14 kHz, see Figure 10 which shows this transfer function along the X, Y and Z axes around 14 kHz, we observe that in reality, the transfer function presents two very similar eigenmodes around 14 kHz: a first coupled mode 100 around 14.2 kHz which corresponds to a resonance peak in the three directions X, Y and Z, and a second eigenmode 102 in the Y direction around 14 kHz. In this second resonance mode 102, the frequency responses in X and Z both present a very marked dip (and therefore the punch does not vibrate; this phenomenon is called anti-resonance and is more marked in the Z direction), while the frequency response in Y is close to the resonance peak. The frequency response of the punch is therefore particularly "pure" here in the Y direction (in other words, the punch does not vibrate in the X and Z directions).

[0105] The transfer function also has a third mode 104 very close to the first mode 100.

[0106] Thus, in this example, the tool has single vibration modes (e.g. at 14 kHz in the Y direction or at 7.5 kHz in the X direction) and coupled modes (in X, Y and in X, Y, Z). Since the transfer function is linear, it is possible to excite each of these modes independently, thanks to a superposition of frequencies in the excitation of the actuator 4. It is thus possible to finely control the vibration produced by the punch 30 by adjusting the control of the actuator 4, while retaining a unidirectional actuator 4.

Claims

Claims: An incremental forming tool (1), characterized in that it comprises: a tool body (2); a tool head (3), integral with the body (1) and comprising a punch (30); a housing (22) formed within the body (2); an actuator (4), located in the housing (22) and capable of producing a vibration in a first direction that can propagate to the punch (30); at least one means (4, 22, 3) for transforming the first direction of vibration into a second direction of vibration during its propagation to the punch (30). A forming tool according to claim 1, characterized in that said at least one transformation means comprises at least one non-axissymmetrical portion with respect to a principal axis (20) of said tool (1).A forming tool according to claim 1, characterized in that the means for transforming the direction of vibration is arranged such that a unidirectional vibration parallel to a principal axis (20) of said tool (1) and produced by the actuator (4) causes a vibration of the punch (30) in at least one direction orthogonal to the principal axis (20). A forming tool according to claim 2, characterized in that the housing (22) is offset with respect to said principal axis (20) of said tool (1), constituting at least partially said non-axissymmetric portion. A forming tool according to claim 2, characterized in that the axis for fixing the actuator within the housing (22) is offset with respect to a principal axis (20) of said tool. (1), constituting at least partially said non-axissymmetric portion. Forming tool according to claim 2, characterized in that the actuator (4) is installed within the body in a substantially offset manner with respect to a principal axis (20) of the body (2), constituting at least partially said non-axissymmetric portion.

7. Forming tool according to claim 2, characterized in that the body (2) comprises an addition of material not axisymmetric with respect to a principal axis of the body, said addition constituting at least partially said non-axisymmetric portion.

8. Forming tool according to claim 2, characterized in that the body (2) comprises at least one recess (44) not axisymmetric with respect to a principal axis of the body (2), said at least one recess (44) being formed within the body (2) and constituting at least partially said non-axisymmetric portion.

9. Tool according to claim 8, characterized in that the recess (44) comprises at least one hole (44) provided within the body (2) and forming an access to the housing (22) from outside the body (2).

10. Forming tool according to claim 1, characterized in that it further comprises one or more shims (46) installed within the housing (22) so as to exert a mechanical preload on the actuator (4).

11. Forming tool according to claim 1, characterized in that the punch (30) is capable of vibrating in at least one resonance mode in reaction to a vibration generated by the actuator (4) along a main axis (20), and characterized in that the tool (2) has at least one resonance frequency for said vibration mode for the transfer function defined by the ratio between the amplitude of the vibration of the punch (30) according to the vibration mode and the amplitude of the vibration of the actuator (4) parallel to the main axis (20), said resonance frequency being between 5 kHz and 30 kHz.

12. Tool according to claim 11, characterized in that the vibration mode belongs to the group comprising: a vibration parallel to the main axis (20), a vibration orthogonal to the main axis (20) and parallel to a misalignment direction (40) of a non-axisymmetric portion of said tool, a vibration orthogonal to the main axis (20) and to the misalignment direction (40) of a non-axisymmetric portion of said tool (1), and a combination of these.

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