VIBRANT INCREMENTAL SHEET METAL FORMING TOOL

Description

1. Technical field

[0001] 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 classically carried out by compressing or pressing the by-product between two predetermined shapes, called the die and the counter-die, respectively.

[0002] Conventional forming, however, is a rigid process, as a given pair of dies can only produce a specific shape. Therefore, if one wishes to change the shape of the formed product, or even marginally modify it, the dies used must be changed. Furthermore, conventional forming is cumbersome to set up, particularly when the sheet metal to be formed is large, and requires significant effort to apply sufficient and uniform pressure to the sheet.

[0003] In order to overcome the shortcomings of conventional forming, incremental forming has been developed over the last two decades (" incremental sheet forming Incremental forming (IFF) involves locally and iteratively deforming a sheet metal to achieve the desired shape. This is done by locally deforming the sheet metal using a tool head (also called a punch), and this process is repeated until the desired shape is reached. The position of the tool head is controlled during incremental forming. It is even possible to control the tool's acceleration to control the force the punch exerts on the sheet metal.

[0004] Incremental forming offers numerous advantages, particularly compared to conventional forming processes. The shape of the formed product results from the numerical control that governs the position of the tool head, and can therefore be easily modified by changing the instructions underlying the numerical control (for example, a 3D model), without having to change the forming tool itself, unlike in conventional forming. Incremental forming can be advantageously implemented by integrating the tool head into the end of a robotic arm or into a CNC machine. The robotic arm or CNC machine then controls the position of the tool head, thereby shaping the sheet metal.

[0005] Incremental forming using a robotic arm is particularly advantageous because it can allow forming on a large sheet while being significantly less expensive than a CNC machine, and even less expensive than traditional forming with a die and counter-die (which are, by definition, the size of the sheet to be formed).

[0006] Thanks to its flexibility and ease of implementation, incremental forming offers many advantages, and finds its place in the manufacture of prototypes or small series. 2. Previous technique

[0007] As explained above, incremental forming relies on the local deformation of a sheet of metal. This deformation results from the local application of a force (or pressure) to the sheet by the punch. This force, in turn, causes an elastic deformation of the robotic arm. If this deformation is too great, it displaces the robotic arm and disrupts the control of the punch's position. Thus, the forces that can be applied to the sheet of metal being formed are limited by the rigidity of the robotic arm used to form the sheet, and are even more limited the thicker the sheet. Similar problems arise when using a CNC machine to perform incremental forming.

[0008] However, it has been discovered that by vibrating the punch at frequencies ranging from kilohertz to low-frequency ultrasound, the sheet metal material is locally softened, making it more ductile and thus reducing the forces required to deform it. The elastic rebound of the sheet metal is also reduced. These vibrations can therefore be used to form thicker sheets and / or to employ smaller robotic arms. The vibrations are typically generated by a piezoelectric actuator.

[0009] More specifically, when vibrations are in a direction normal to the sheet metal (i.e., perpendicular to the sheet at the punch-sheet contact), these longitudinal vibrations would activate a dislocation movement and its propagation within the material, potentially resulting in the observed smoothing. Transverse vibrations (i.e., perpendicular to the normal vibrations) reduce friction between the sheet metal and the punch, which also reduces the effort the robotic arm must exert to perform the 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 finish of the formed sheet metal.

[0010] Normal and transverse vibrations of the sheet metal thus produce complementary effects that greatly reduce the effort required for incremental forming. However, generating these vibrations in all three directions (one normal and two transverse) is not easy.

[0011] We are familiar with the article by 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 located at the center of the forming tool body. Each actuator 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 through the tool body to the drill bit. Thus, this drill bit is capable of vibrating in all three directions.

[0012] We are also familiar with 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 that uses elliptical excitation of a stack of actuators to couple vibrational modes.

[0013] However, these solutions are not satisfactory. First, they require the implementation of several actuators (in the form of stacks), and therefore a multitude of power supplies to control. Furthermore, since there are multiple actuators, each actuator is smaller (to comply with size constraints), 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 individually control the vibration modes to produce the desired vibrations.

[0014] Moreover, these solutions concern machining and drilling, areas quite different from incremental forming.

[0015] We also know from document CN 212 634 025 U an incremental forming tool according to the preamble of claim 1.

[0016] There is a need for a solution to vibrate a punch in a multidirectional manner that does not have the defects of the prior art, particularly in terms of ease of implementation, size and ease of control.

[0017] The invention improves the situation. 3. Description of the invention

[0018] The invention was designed to overcome at least some of the drawbacks of the prior art.

[0019] To this end, the invention proposes an incremental forming tool, comprising: a tool body; a tool head, integral with the body and ending in a punch at its end; a housing provided within the body; an actuator, taking place in the housing and capable of producing a vibration in a first direction which can propagate to the punch; at least one means of transforming the first direction of vibration into a second direction of vibration, during its propagation to the punch.

[0020] Thus, thanks to this transformation mechanism, the tool is able to transform a vibration produced by a unidirectional actuator (along the first direction) into a multidirectional vibration, or at least a vibration along the second direction. For example, the tool is able, starting from an actuator producing an axial vibration (i.e., along a principal axis of the body, also called the axial or longitudinal direction), to produce a transverse vibration, that is, perpendicular to this principal axis, at the punch. The actuator can therefore be a unidirectional actuator without 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 capable of vibrating in the normal and transverse directions to the sheet metal (i.e.along 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 friction between the tool head and the sheet metal is reduced. The springback of the sheet metal on the tool is consequently decreased. As the forces required to form the sheet metal are reduced, the springback of the arm is also reduced.

[0021] 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 effort at the level of the punch by structural resonance effect, particularly when the punch is in contact with a sheet metal.

[0022] This results in a gain on the machine side (i.e. a smaller size of the robotic arm or the CNC machine supporting the tool) but also on the quality of the part formed, since the lower elastic return and the reduction of friction induce a better surface finish of the part after forming.

[0023] According to one aspect, said at least one means of transformation includes at least one portion that is not axisymmetric with respect to a principal axis of said tool.

[0024] Thus, the tool has a generally axisymmetric shape, which improves its vibrational properties, notably by separating the vibrational modes of the punch compared to a tool with a particularly asymmetrical shape. This also allows the transformation method to be integrated into a pre-existing tool, partially modifying its axisymmetric nature.

[0025] According to one aspect, the means for transforming the direction of vibration is arranged so that a unidirectional vibration parallel to a principal axis of said tool and produced by the actuator causes the production of a vibration of the punch in at least one direction orthogonal to the principal axis.

[0026] The ability to vibrate the punch in multiple directions from a unidirectional vibration using the transformation method also allows for the use of a single, unidirectional actuator, and therefore a more powerful one. Consequently, the tool can be smaller for the same actuator power, since the effects of reduced ductility and friction are more pronounced with higher vibration amplitudes.

[0027] To generate a controllable transverse vibration of the punch, the body and / or the head (in particular the non-axisymmetric portion of the processing 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 possible not only to generate a multidirectional vibration of the tool head using a unidirectional actuator, but also to control it precisely.

[0028] This innovative tool enables incremental forming using a robotic arm or a CNC machine (more generally, any machine capable of supporting the tool) smaller than those typically used, since the springback is reduced. This allows for the forming of larger and / or thicker sheets with the same robotic arm. Furthermore, friction is reduced, limiting the stick-slip effect. Consequently, this improves the precision of the tool head's position control and the surface finish of the formed sheet.

[0029] According to one aspect, the housing is offset from a main axis of the body, constituting at least partially said non-axissymmetric portion.

[0030] According to one aspect, the axis of fixing the actuator within the housing is offset from a main axis of said tool, constituting at least partially said non-axissymmetric portion.

[0031] According to one aspect, the actuator is installed within the body in a manner substantially offset from a main axis of the body, constituting at least partially said non-axissymmetric portion.

[0032] Thanks to the offset of the tool housing and / or actuator, the parallel vibration induced by a unidirectional actuator mounted in the housing causes a transverse vibration to the main axis of the tool body. By precisely dimensioning this offset, the transfer function between the vibration in the Z-axis (axial direction) and the vibration in the X, Y, and Z directions (respectively radial transverse, tangential transverse, and axial) can be determined. This can be achieved, for example, using a simulation (e.g., a numerical analysis using the finite element method) based on a tool model obtained through computer-aided design (CAD). With a thorough understanding of this transfer function, it is possible to control the punch vibrations by controlling the frequency of the actuator's power supply (i.e., the unidirectional vibration it produces in the tool housing).

[0033] According to one aspect, the body includes an addition of material that is not axisymmetric with respect to a principal axis of the body, said addition constituting at least partially said non-axisymmetric portion.

[0034] Thus, adding material helps propagate vibration in a non-axial direction. It should be noted that this non-axisymmetric material addition can fulfill a third function, such as acting as a keying feature in tool assembly, allowing the attachment of power supplies or sensors to the tool, etc., thereby saving space by combining functions within this addition.

[0035] According to one aspect, the body includes at least one non-axissymmetric recess with respect to a principal axis of the body, said at least one recess being formed within the body and constituting at least partially said non-axissymmetric portion.

[0036] Here, the recess helps to propagate vibration in a non-axial direction. It should be noted that this non-axisymmetric recess can serve a third function, such as providing access to parts of the tool, acting as a keying device, housing power supplies or sensors, etc., thus saving space by combining functions within this recess.

[0037] According to one aspect, the hollowing includes at least one hole made in the body and forming an access to the housing from outside the body.

[0038] This hole serves a dual purpose: it contributes to the tool's asymmetrical shape and helps power the actuator by providing access to the housing (for routing power cables). This results in a space saving, in addition to generating transverse vibration at the punch.

[0039] It should be noted that the asymmetry induced by material removal or addition, or drilling, is not incompatible with misalignment of the housing and / or actuator. On the contrary, these two characteristics can combine and amplify the asymmetry of the part, thereby enhancing the generation of transverse vibration.

[0040] According to one aspect, the tool further includes one or more shims installed in the housing so as to exert a preload on the actuator (particularly when the tool is in operation and in the absence of actuator vibrations).

[0041] In this example, compressing the actuator using a shim (or a set of shims) induces preload, that is, a force (or pre-load) applied to the actuator in the absence of any specific action (such as powering the actuator). Preload drastically improves the energy transfer between the actuator and the tool body (thus increasing the gain of the transfer function described above). Furthermore, preload increases the transfer of vibration produced by the actuator to the rest of the tool.

[0042] According to one aspect, the punch is capable of vibrating in at least one resonance mode in reaction to a vibration generated by the actuator along a principal 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 in the vibration mode and the amplitude of the vibration of the actuator parallel to the principal axis, said resonance frequency being between 5 kHz and 30 kHz.

[0043] The transfer function exhibits resonance. This resonance advantageously improves energy transfer, thereby increasing the amplitude of the punch vibration. This enhances the smoothness of the sheet metal being formed and reduces the effort required to form it.

[0044] Achieving this resonance results from a carefully chosen tool design, particularly for its non-axisymmetric portion. This can be accomplished through simulation during computer-aided design, or empirically; the important thing is to ultimately obtain a resonant frequency for the desired mode(s).

[0045] According to one aspect, the mode of vibration belongs to the group comprising a vibration parallel to the principal axis, a vibration orthogonal to the principal axis and parallel to a direction of misalignment of the non-axisymmetric portion, a vibration orthogonal to the principal axis and to the direction of misalignment of the non-axisymmetric portion, and a combination of these.

[0046] The direction orthogonal to the principal axis and parallel to a misalignment direction of the non-axisymmetric portion (typically the misalignment of the actuator and / or housing, or the misalignment direction of a hole or material addition) is the radial transverse direction (X-axis). The direction orthogonal to both the principal axis and the misalignment 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) are called coupled mode(s).

[0047] The fact that the punch can vibrate in an axial, radial, tangential, or coupled mode allows for different effects on the sheet metal, which is smoothed by the 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 activate microscopic dislocations in the sheet metal, thus smoothing it. The various coupled modes (notably axial / radial and axial / tangential) combine these effects. Furthermore, coupled modes allow for anticipating variations in the punch's trajectory, and therefore maintaining the transverse direction of the punch's vibration collinear with the tool's feed direction, which further improves the reduction of punch / sheet friction.In other words, the direction of the tool's transverse vibration can be controlled so that it is tangent to the punch's movement. These vibration modes can also be single, double, or triple, meaning they induce a single, double, or triple bending of the tool head, respectively. 4. Brief description of the drawings

[0048] Other features and advantages of the invention will become clearer upon reading the following description of a particular embodiment, given by way of simple illustration and not limitation, and the accompanying drawings, among which: [ Fig. 1 ] represents a perspective view of a tool according to an example of an implementation of the invention; [ Fig. 2 ] represents a perspective view of the underside of the tool figure 1 ; Fig. 3 ] represents a top cross-sectional view of the tool of the figure 1 ; Fig. 4] represents a side cross-sectional view of the tool along plane IV-IV of the figure 3 ; Fig. 5 ] represents a side view of the tool of the figure 1 ; Fig. 6 ] represents a view of detail VI of the figure 4 according to another embodiment of the invention; [ Fig. 7 ] represents an exploded view of the tool of the figure 1 ; Fig. 8 ] represents a tool head variant of the tool of the figure 1 ; Fig. 9 ] represents a frequency response of the tool of the figure 1 in all three directions; and [ Fig. 10 ] represents detail IX of the figure 9 . 5. Detailed description 5.1 General Principle

[0049] 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 incorporating means within the tool to transform the vibrations produced by the actuator. This aspect is clearly illustrated in the following figures, which represent one example of an embodiment. 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 through which the vibration propagates. This vibration can be observed as a vibrating force (for example, at the actuator mounted in the housing) or as a vibratory displacement (or oscillations, for example, observable at the punch when it is free, i.e., not in contact with a sheet metal).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.

[0050] We describe, with reference to figures 1 to 5 A forming tool 1 according to the invention. 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 within the tool body 2 and / or the tool head 3.

[0051] The tool body 2 and the tool head 3 can be made of steel (but not necessarily the same steel). More generally, the tool body 2 and the tool head 3 can be made of any material capable of providing mechanical strength suitable for the forming forces induced by the incremental forming process.

[0052] The tool body is generally axisymmetric about a principal axis 20, which defines a first direction Z called the axial direction. For the remainder of this description, two directions X and Y are defined orthogonally to the Z direction and to each other. The directions X, Y, and Z form an orthonormal coordinate system (X, Y, Z), represented figure 4 .

[0053] The tool head 3 includes a punch 30 at one end. The punch is intended to come into contact with a sheet metal during an incremental forming process.

[0054] 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 the opposite end. The punch 30 forms a tip of the tool head 3. The punch 30 can be hemispherical, allowing the contact angle with the sheet metal to be varied continuously. The tool head 3 further includes a base 32 through which the tool head 3 is fixed to the tool body 2. The base 32 and the punch 30 are connected 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 is substantially axisymmetric in shape. The tool head 3 has a profile that tapers from the base 32 to the punch 30. Here, the section of the tool head 3 has a substantially progressive cross-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 helps to reduce the overall size of the tool, allowing for a wider variety of shapes. The slender design of tool head 3 (i.e., its finger shape) makes it possible to create concave parts with a steeper resulting sheet metal angle and greater depth.

[0055] 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, with screws. Alternatively, the tool head 3 and the tool body 2 can be a single unit, with access to the interior of the tool body 2 from the end opposite the tool head 2.

[0056] The tool body 2 forms an internal housing 22, visible in particular figure 4 The housing 22 is suitable for receiving the actuator 4. The tool head 3 can close one end of the housing 22, also called the proximal end, as can be seen on the figure 4Alternatively, this first end of the housing 22 (near the punch) can be closed by the tool body 2 itself. The tool 1 may include 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 further from the punch) of the housing 22. Alternatively, the tool body 2 itself can close this second end.

[0057] The actuator 4 is installed in the housing 22. The actuator 4 may include an actuator interface 42 for 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 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 that 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.

[0058] Tool 1 includes a means for transforming the direction of vibrations, hereinafter referred to as the transformation means. The transformation means is capable of changing the direction of vibrations during their propagation from the actuator 4 in the housing 22 to the punch 30.

[0059] The transformation means includes a non-axissymmetric portion of the tool with respect to the principal axis 20. In other words, the transformation means includes at least one portion (or element) that is non-axissymmetric with respect to this principal axis 20.

[0060] This non-axisymmetric portion of tool 1 may include the actuator 4 itself, offset from the main axis 20 according to an actuator offset 40, visible figure 4 Here, the housing 22 is axisymmetric with respect to the main axis 20 of the tool body 2, and the actuator 4 is mounted off-center within the housing 22. Alternatively, the housing 22 can be offset with respect to the main axis 20, and the actuator 4 can be centered within the housing 22 (this resulting in the offset 40). It is possible to combine an offset of the housing 22 with an off-center mounting of the actuator 4 within the housing 22.

[0061] The asymmetry induced by the offset of the actuator 4 (whether by placing it offset in a symmetrical housing 22 or by having a housing 22 that is itself offset) allows the axial vibrations of the actuator 4 to be transformed into transverse vibrations at the punch 30. These transverse vibrations occur particularly at specific vibration frequencies corresponding to bending modes of the tool. The actuator can thus be unidirectional while having a punch capable of vibrating both axially and transversely. In the example shown 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 offset from the tool body 2 and the tool head 3.

[0062] Because the actuator can be unidirectional, it can be significantly more powerful than a multidimensional actuator (for example, a stack of small piezoelectric actuators, each layer of which can vibrate in its own direction). The vibration amplitude at punch 30 is therefore much greater, enabling higher-quality incremental forming (surface finish) on thicker sheets, with equivalent tool and machine sizes.

[0063] The tool body 2 may include at least one hole 44; on the figure 1The tool body 2 includes two holes. Hole 44 is formed in a side wall of the tool body 2 and opens into the housing 22. When there are several holes 44, these holes may not be equally distributed on the tool body 2, thus contributing to the asymmetry of the tool body 2, and therefore of the tool 1. In addition to the asymmetry, hole 44 allows access to the inside of the housing 22 to supply the actuator 4. Hole 44 can 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 holes 44 is particularly noticeable. figure 3 where it is clear that they are formed only on one side of the tool. The drilling also allows the heat produced by the operating actuator to dissipate.

[0064] Alternatively or in combination with drilling 44, a material recess can be made in the tool body 2, which also produces an asymmetry of the tool body 2, the cover 24 or the tool head 3. 5.2 Transfer Function

[0065] The various components of tool 1, particularly the tool body 2, the tool head 3, and the actuator 4, can be carefully dimensioned to not only cause a change in the direction of vibration (as it propagates from the actuator 4 housed within the tool body 2 to the punch 30), but also to precisely determine the nature (amplitude, frequency) of the punch 30's vibration as a function of the vibrations of the actuator 4. The frequency of the tool's vibrations corresponds to the actuator's excitation frequency. Exciting the system in a given mode (i.e., at a given frequency) does not cause excitation of the other modes.

[0066] This dimensioning can be the result of a computer simulation, for example, within the framework of computer-aided design. It is also possible to replace or supplement this simulation with empirical measurements on a prototype tool. The inventors, in developing the present invention, were thus able to combine these two methods (simulation and experimentation) to obtain a tool whose vibrational properties are satisfactory, particularly with regard to vibration control.

[0067] The vibration of the punch can be measured using a laser vibrometer. The reflection off the punch allows the measurement of its displacement, and therefore its vibrations. This measurement of the punch vibration can be observed by controlling the actuator and generating a frequency sweep of the actuator's excitation using a sinusoidal control signal, and then observing the punch's vibration response. The frequency of the sweep (i.e., the sinusoidal control signal) can vary linearly or logarithmically.

[0068] For example, we first perform a rapid frequency sweep over the entire spectrum (for example from 1 kHz to 22 kHz), then we retrieve 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 supply voltage of the actuator (image of the force) is maximum, and which therefore has the most chance of being close to a resonance frequency (i.e. a natural mode of the tool).

[0069] A second, finer frequency sweep is then performed around the maximum frequency or frequencies obtained above, which allows us to obtain a precise gain profile in the vicinity of the resonance frequency or frequencies.

[0070] The vibrations of punch 30 can be in several modes of vibration: unidirectional modes (axial Z, radial X or tangential Y), or coupled ((X,Z), (X,Y) or (Y,Z), or (X,Y,Z)).

[0071] For a given mode, it is possible to determine (by experiment and / or simulation) the associated transfer function, that is to say 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).

[0072] The tool 1 as a whole is further dimensioned so that for at least one vibration mode, its associated transfer function includes a resonance frequency 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 maximal (at least locally).

[0073] In some embodiments, tool 1 exhibits several transfer functions, each with its own resonance in the 5-30 kHz range. These resonances can be disjoint, meaning that at a given frequency where resonance occurs in one mode, there is no resonance in another mode. If the excitation frequency does not correspond to the specific frequency of a mode, then the punch's vibrational response is a linear combination of all the natural modes. This allows for precise control over which resonance mode is favored for a given incremental forming process. 5.3 Prestressing

[0074] Reference is now being made to the figure 6 and to the figure 7 .

[0075] In this embodiment, the tool 1 further includes a shim 46 (or a set of shims) housed in the extension of the actuator in the 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 concerning the body 3, the head 2, and the cover 24, which together form the walls of the housing 22.

[0076] The shim 46 can be housed in a recess 48 formed in the cover 24 in line with the housing 22. Here, the shim 46 is housed on the cover 24 side, but the shim 46 could be positioned on the tool head 3 side. In the case of a set of shims 46, part of the shim set 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.

[0077] In the visible example figure 7The shim set includes one thick shim and three thinner shims.

[0078] 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 sufficient, this preload significantly increases the efficiency of energy transfer between the actuator 4 and the rest of the tool 1. The inventors estimated that a preload of approximately 80 to 85 kN is optimal for a developed prototype, in order to maximize energy transfer without damaging the actuator 4. More generally, the preload is on 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 specific torque (for example, with a torque wrench) to ensure control of the tightening force on the screws. 5.4 Non-axisymmetric head

[0079] In one embodiment, head 3 is not axisymmetric with respect to the principal axis 20. In the example shown figure 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.

[0080] The asymmetry of the head 3 results in the head 3 vibrating asymmetrically when the actuator 4 produces vibrations along the principal axis 20. The punch 30 can thus vibrate transversely (radially X or tangentially Y). This advantageously allows the installation of a non-axisymmetric head 3 on a pre-existing axisymmetric tool.

[0081] 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, without edges or angles.

[0082] The asymmetry of head 3 is compatible with the other non-axissymmetric portions described above to constitute the means of transformation. 5.5. Example of a transfer function

[0083] Reference is now being made to the figure 9 , which represents a frequency response of tool 1, or in other words the transfer function of this tool 1.

[0084] There figure 9 includes three graphs, each representing a frequency response of 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 directions (from top to bottom).

[0085] The frequencies studied here range from 4 kHz to 16 kHz, and the x-axis (representing frequency) is on a linear scale. The gain studied (i.e., the ratio of the amplitude of the punch vibration to the amplitude of the vibration generated by the actuator) is measured in dB.

[0086] In this example, the transfer function has: a coupled (X, Y) mode around 4.5 kHz (with a significantly higher gain in the Y direction compared to the X direction), a coupled (X, Y, Z) mode around 14 kHz, and a natural mode along the X direction around 7.5 kHz.

[0087] Looking more closely at the transfer function around 14 kHz, see the 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 has two very similar eigenmodes around 14 kHz: A first coupled mode, 100 kHz, occurs around 14.2 kHz, corresponding to a resonance peak in the three directions X, Y, and Z. A second, 102 kHz resonance mode is also present in the Y direction around 14 kHz. In this second resonance mode, the frequency responses in both X and Z exhibit a very pronounced dip (and therefore the punch does not vibrate; this phenomenon is called anti-resonance and is more pronounced in the Z direction), while the frequency response in Y is close to the resonance peak. The punch's frequency response is thus particularly "pure" in the Y direction (in other words, the punch does not vibrate in the X and Z directions).

[0088] The transfer function also has a third mode 104 very close to the first mode 100.

[0089] Thus, in this example, the tool exhibits simple 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 actuator 4. The vibration produced by the punch 30 can therefore be precisely controlled by adjusting the actuator 4, while maintaining a unidirectional actuator 4.

Claims

1. Incremental sheet forming tool (1), comprising: - a tool body (2); - a tool head (3), integral with the body (1) and comprising a punch (30); - a housing (22) formed inside the body (2); - an actuator (4), located in the housing (22) and capable of producing a vibration in a first direction that can be propagated to the punch (30); the incremental sheet forming tool (1) being characterised in that it comprises: - 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).

2. Sheet forming tool according to claim 1, characterised in that said at least one transforming means comprises at least one portion which is non-axisymmetrical with respect to a main axis (20) of said tool (1).

3. Sheet forming tool according to claim 1, characterised in that the means for transforming the direction of vibration is arranged so that a unidirectional vibration parallel to a main axis (20) of said tool (1) and produced by the actuator (4) causes the punch (30) to vibrate in at least one direction orthogonal to the main axis (20).

4. Sheet forming tool according to claim 2, characterised in that the housing (22) is offset with respect to said main axis (20) of said tool (1), constituting at least partially said non-axisymmetrical portion.

5. Sheet forming tool according to claim 2, characterised in that the axis of fixing the actuator inside the housing (22) is offset with respect to a main axis (20) of said tool (1), constituting at least partially said non-axisymmetrical portion.

6. Sheet forming tool according to claim 2, characterised in that the actuator (4) is installed inside the body in a substantially offset manner with respect to a main axis (20) of the body (2), constituting at least partially said non-axisymmetrical portion.

7. Sheet forming tool according to claim 2, characterised in that the body (2) 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. Sheet forming tool according to claim 2, characterised in that the body (2) comprises at least one recess (44) which is non-axisymmetrical with respect to a main axis of the body (2), said at least one recess (44) being provided inside the body (2) and constituting at least partially said non-axisymmetrical portion.

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

10. Sheet forming tool according to claim 1, characterised in that it also comprises one or more shims (46) installed within the housing (22) so as to exert a mechanical preloading stress on the actuator (4).

11. Sheet forming tool according to claim 1, characterised in that the punch (30) is capable of vibrating in at least one resonance mode in response to a vibration generated by the actuator (4) along a main axis (20), and characterised 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, characterised 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 direction of misalignment (40) of a non-axisymmetric portion of said tool, - a vibration orthogonal to the main axis (20) and the direction of misalignment (40) of a non-axisymmetric portion of said tool (1), and a combination thereof.