Vibratory Incremental Sheet Forming Tool

Abstract

An incremental sheet forming tool (1) is disclosed. According to the present disclosure, such a tool comprises a tool body (2), a tool head (3) firmly attached to the body (1) and provided with a punch (30), a housing (22) provided in the body (2), an actuator (4) disposed within the housing (22) capable of generating a first vibration direction that propagates to the punch (30), and at least one means (4, 22, 3) for converting the first vibration direction that propagates to the punch (30) into a second vibration direction.

Description

Detailed Description of the Invention

[0001] [1. Technical Field] The present invention relates to the field of sheet forming, and more particularly to incremental sheet forming. Forming is a manufacturing process that consists of forming a by-product, typically a plate or sheet. Conventionally, forming has been performed by compressing or pressurizing the by-product between two predetermined molds, respectively called a die and a counter die.

[0002] However, conventional forming has been a strict process because a set of dies can produce only one predetermined shape. Therefore, if it is desired to change the shape of the formed product even slightly, it is necessary to change the mold used. In particular, when the size of the sheet to be formed is large, a very large force is required to apply sufficient and uniform pressure to the formed sheet.

[0003] To address the drawbacks of conventional forming, incremental sheet forming (ISF) has been developed over the past 20 years. Incremental sheet forming repeatedly performs local deformation of a sheet to obtain a desired shape. To achieve this, the sheet is locally deformed using a tool head (also called a punch), and this process is repeated until the desired shape is obtained. To control incremental sheet forming, the position of the tool head is controlled. It is also possible to control the acceleration of the tool to control the force exerted by the punch on the sheet.

[0004] Incremental sheet forming has a number of advantages not found in conventional forming processes. Since the shape of the formed product is obtained from numerical control that controls the position of the tool head, unlike conventional forming, it can be easily modified by changing the command (e.g., 3D model) at the origin of the numerical control without changing the sheet forming tool itself. Incremental sheet forming can be advantageously implemented by integrating the tool head at the tip of a robotic arm or incorporating it into a numerically controlled machine. The robotic arm or computer numerical control machine forms the sheet metal by controlling the position of the tool head.

[0005] Incremental sheet forming using a robotic arm is particularly advantageous in that it can form large sheets at a much lower cost than a CNC machine and is even less costly than conventional forming using a die and a counter die (which are by definition the same size as the sheet being formed).

[0006] Due to its flexibility and ease of use, incremental sheet forming has many advantages and is optimal for prototypes and small batch production. [2. Prior Art]

[0007] As described above, incremental sheet forming is based on local deformation of a metal sheet. This deformation is the result of local force (or pressure) being applied to the sheet by a punch. This force successively causes elastic deformation of the robotic arm. If this deformation is too large, it will move the robotic arm and disrupt the position control of the punch. Thus, the force applied to the sheet being formed is limited by the rigidity of the robotic arm on which the sheet is formed, and is further limited the thicker the sheet is. The same problem also occurs when using a CNC machine for incremental sheet forming.

[0008] However, it has been discovered that vibrating the punch in the ultrasonic region from kilohertz frequencies down to low frequencies causes the sheet material to locally soften and increase in ductility, thereby reducing the force required for deformation. The springback of the sheet is reduced. These vibrations can be used for forming thicker sheets and / or for robotic arms of smaller dimensions. The vibrations are typically generated by a piezoelectric actuator.

[0009] More precisely, when the vibration is in the normal direction of the sheet (i.e., the direction perpendicular to the sheet at the point where the punch and the sheet are in contact), these longitudinal vibrations activate the translational displacement, activate the propagation of dislocations within the material, and the softening observed therefrom may occur. The transverse vibration (i.e., the vibration in the direction perpendicular to the normal vibration) reduces the friction between the sheet and the punch. As a result, the force required for the robotic arm to perform incremental sheet forming is also reduced. Such transverse vibration also limits the "stick-slip" phenomenon and improves the quality of the operation control of the tool and the surface finish of the formed sheet metal.

[0010] Thus, the vibrations in the normal and transverse directions of the sheet bring about complementary effects and significantly reduce the labor required for incremental sheet forming. However, it is not easy to generate these vibrations in three directions (the normal direction and two transverse directions).

[0011] The paper by Kurniawan, R., Ali, S., Park, K. M., Li, C. P., & Ko, T. J. is known (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 with a stack of piezoelectric actuators arranged at the center of the sheet forming tool body. Each actuator within the stack can vibrate in one of the three directions as a function of the electrical command received by each actuator. The vibration generated by the actuator is propagated to the drill bit by the tool body. Therefore, this drill bit can vibrate in all three directions.

[0012] The paper by Gao, J., & Altintas, Y. (2019), also known as "Development of a three-degree-of-freedom ultrasonic vibration tool holder for milling and drilling", IEEE / ASME Transactions on Mechatronics, 24(3), 1238-1247, describes an electric tool that uses elliptical vibration of a stack actuator to combine vibration modes.

[0013] However, these solutions are not satisfactory. First, multiple actuators (in the form of stacks) need to be used, so a large number of power supplies are controlled. Further, due to the presence of multiple actuators, each actuator becomes small (considering space limitations), and as a result, it is not possible to generate stronger vibrations. Finally, these solutions are actually difficult to control because multiple actuators interfere with the tool and it is not easy to individually control the vibration modes to generate the desired vibrations.

[0014] Furthermore, these solutions relate to machining or drilling, which is very different from incremental sheet forming. In particular, it is necessary to find a solution for vibrating the punch in multiple directions that does not have the drawbacks of the prior art in terms of simplicity of implementation, size, and ease of control. The present invention improves this situation. [3. Disclosure of the Invention]

[0015] The present invention is designed to overcome at least some of the drawbacks of the prior art. For this purpose, the present invention proposes an incremental sheet forming tool. The incremental sheet forming tool is characterized by comprising a tool body, a tool head integrated with the body and attached to a punch at its tip, a housing disposed within the body, an actuator (4) disposed within the housing and capable of generating vibrations in a first direction propagable to the punch (30), and at least one means for converting the first vibration direction into a second vibration direction during propagation to the punch.

[0016] Therefore, by means of this conversion means, the tool can convert vibrations generated by a unidirectional actuator (first direction) into multi-directional vibrations or at least into a second vibration direction. For example, the tool can generate lateral vibrations orthogonal to this main axis at the site of the punch from an actuator that generates axial vibrations (i.e., vibrations in the main axis direction of the body, also called the axial or longitudinal direction). The actuator can be a unidirectional actuator without preventing the tool from vibrating in multiple directions (including the lateral direction and one axial direction). The actuator is, for example, a piezoelectric actuator. During the incremental sheet forming process, the punch can vibrate in the normal direction and the lateral direction of the metal sheet (i.e., the main axis of the tool body and one or more directions orthogonal to this main axis). Thus, the sheet is locally smoothed and the friction between the tool head and the sheet is reduced. Therefore, the springback of the sheet with respect to the tool is reduced. When the force required for sheet forming is reduced, the springback of the arm is also reduced.

[0017] The actuator can vibrate and excite the tool head by applying a vibration force to the tool head in order to obtain a large vibration force at a specific frequency at the punch by means of a structural resonance effect, especially when the punch is in contact with the metal sheet.

[0018] This results in gains not only on the machine side (i.e., miniaturization of the CNC machine supporting the robotic arm or tool), but also in terms of reduced springback and friction, which improves the surface finish of the parts after sheet forming, thus resulting in gains in the quality of the formed parts. According to one aspect, the at least one conversion means comprises at least one non-axisymmetric portion with respect to the spindle of the tool.

[0019] In this way, the tool normally has an axisymmetric shape, and in particular, by separating the vibration nodes of the punch with respect to a tool whose shape is particularly asymmetric, its vibration characteristics are improved. This also makes it possible to integrate the conversion means into an existing tool by partially changing the axisymmetry of the tool.

[0020] According to one aspect, the means for converting the vibration direction is arranged parallel to the spindle of the tool and is such that the unidirectional vibration generated by the actuator causes the punch to vibrate in at least one direction orthogonal to the spindle.

[0021] Also, the fact that the conversion means can vibrate the punch in multiple directions from a single unidirectional vibration enables the use of a single unidirectional actuator, so a more powerful actuator can be used. The effects of reduced ductility and reduced friction increase with the amplitude of the vibration, so the tool can be miniaturized with the same actuator output.

[0022] To generate controllable lateral vibrations of the punch, the body and / or the head (in particular the non-axisymmetric portion of the conversion means) can be carefully dimensioned such that a given unidirectional vibration of the actuator at a given frequency results in a given lateral vibration of the tool head. Thus, not only can a unidirectional actuator be used to generate multi-directional vibrations of the tool head, but the tool head can also be accurately controlled.

[0023] This new tool type enables incremental sheet forming using a robotic arm or CNC machine (more generally, any machine that supports the tool) with smaller dimensions than those normally used, due to its low springback. Also, the same robotic arm can be used to form larger and / or thicker sheets. Finally, by reducing friction, the stick-slip effect is limited, improving the quality of the position control of the tool head and the surface finish of the sheet after forming.

[0024] According to one aspect, the housing is offset from the spindle of the body and at least partially constitutes the non-axisymmetric part. According to one aspect, the attachment scale of the actuator within the housing is offset from the center with respect to the spindle of the tool and at least partially constitutes the non-axisymmetric part. According to one aspect, the actuator is installed within the body in a manner substantially offset from the center with respect to the spindle of the body and at least partially constitutes the non-axisymmetric part.

[0025] Due to the offset of the housing and / or the actuator, the parallel vibration induced by a unidirectional actuator attached to the housing causes lateral vibration with respect to the spindle of the tool body. By accurately sizing this offset, it becomes possible to determine the transfer function between the vibration in the Z-axis direction (axial direction) and the vibrations in the X, Y, and Z directions (radial direction of the horizontal axis, tangential direction of the horizontal axis, and axial direction, respectively). This can be achieved, for example, by simulation (e.g., numerical analysis by the finite element method) based on the model of the tool obtained by computer-aided design (CAD). By being familiar with this transfer function, it becomes possible to control the vibration of the punch from the frequency control of the power supply to the actuator (i.e., the unidirectional vibration generated by the actuator in the housing).

[0026] According to one aspect, the body includes an addition of a material that is non-axisymmetric with respect to the main axis of the body, and the addition at least partially constitutes the non-axisymmetric portion.

[0027] In this way, the addition of the material can contribute to the propagation of non-axisymmetric vibrations. It should be noted that this non-axisymmetric addition of the material can perform a third function that acts as a constraint for forming behavior in the assembly of the tool that enables attaching a power supply means or a sensor to the tool. Thereby, it is possible to save pace by pooling functions within this addition.

[0028] According to one aspect, the body includes at least one recess that is not axisymmetric with respect to the main axis of the body, and the at least one recess is provided within the body and at least partially constitutes the non-axisymmetric portion.

[0029] Here, the recess can contribute to the propagation of vibrations in the non-axis direction. It should be noted that this non-axisymmetric recess can perform a third function such as providing access to components of the tool, acting as a constraint for forming behavior, and accommodating a power supply means or a sensor. It is possible to save space by sharing functions within this recess.

[0030] According to one aspect, the recess is formed in the body and includes at least one hole that forms access from the outside of the body to the housing. This hole has two functions: contributing to the non-axisymmetry of the tool and enabling access to the housing (routing of the power cable), which helps to supply power to the actuator. This not only generates lateral vibrations of the punch but also saves space.

[0031] It should be noted that the asymmetry caused by material removal or addition, or by drilling, is not incompatible with the offset of the housing and / or the actuator. On the contrary, by combining these two special features and amplifying the asymmetry of the part, the generation of lateral vibrations can be improved. According to one aspect, the tool also comprises one or more shims installed in the housing so as to apply a preload to the actuator (in particular, when the tool is in operation and there is no vibration of the actuator).

[0032] According to this embodiment, by compressing the actuator using a shim (or a set of shims), a preload, that is, a force (or preload) applied to the actuator in the absence of a specific operation (such as power supply to the actuator), is induced. The preload dramatically improves the energy transfer between the actuator and the tool body (therefore, the gain of the transfer function described above is obtained). The preload also increases the vibration transfer generated by the actuator to the body to the rest of the tool.

[0033] According to one aspect, the punch can also vibrate according to at least one resonance mode in reaction to the vibration generated on the spindle by the actuator. Also, the tool has at least one resonance frequency for the vibration mode with respect to a transfer function defined by the relationship between the amplitude of the vibration of the punch according to the vibration mode and the amplitude of the vibration of the actuator parallel to the spindle, and the resonance frequency is between 5 kHz and 30 kHz.

[0034] The transfer function has a resonance. This resonance advantageously enables improving the energy transfer and thus increasing the amplitude of the vibration of the punch. In this way, the smoothness of the sheet to be formed is improved and the force required for sheet forming is reduced. This resonance is achieved by carefully selecting the dimensions of the tool, in particular the non-axisymmetric part. This can also be achieved using simulation means during computer-aided design or empirically. The important element is to finally obtain the resonance frequency for the desired mode or modes.

[0035] According to one aspect, the vibration modes belong to the group comprising vibrations parallel to the main axis, vibrations orthogonal to the main axis and parallel to the displacement direction of the non-axisymmetric part, vibrations orthogonal to the main axis and orthogonal to the displacement direction of the non-axisymmetric part, and combinations thereof. The direction orthogonal to the main axis and parallel to the displacement direction of the non-axisymmetric part (typically, the offset of the actuator and / or the housing, or the displacement direction of the hole or the material addition direction) is the radial transverse direction (X-axis). The direction orthogonal to the main axis and orthogonal to the displacement direction of the non-axisymmetric part is the tangential transverse direction (Y-axis). The possible combinations of these three directions X, Y, Z are called combined modes.

[0036] The fact that the punch can vibrate in an axial mode, a radial mode, a tangential mode, or a combined mode allows different effects to be obtained on the sheet metal softened by vibration. In particular, the force generated by the lateral vibration reduces the friction at the contact point between the sheet and the punch during sheet forming, since the axial vibration activates the dislocations in the sheet at the micro level. The plurality of combined modes (in particular axial / radial, axial / normal) allow these effects to be combined. The combined mode also allows the variation of the punch's trajectory to be predicted, and as a result, it is possible to keep the lateral direction of the punch's vibration in line with the feed direction of the tool. In other words, the direction of the lateral vibration of the tool can be controlled to be tangential to the movement of the punch. These vibration modes can be single, double, or triple, each inducing a single, double, or triple bending of the tool head respectively. [4. Brief Description of the Drawings]

[0037] Other features and advantages of the present invention will become apparent from the following description of specific exemplary embodiments given by way of illustration and not limitation, and from the accompanying drawings: [FIG. 1] is a perspective view of a tool according to an exemplary embodiment of the present invention; [FIG. 2] is a perspective view of the tool of FIG. 1 seen from below; [FIG. 3] is a cross-sectional view of the upper part of the tool of FIG. 1; [FIG. 4] is a cross-sectional view of the side of the tool in the plane IV-IV of FIG. 3; [FIG. 5] is a side view of the tool of FIG. 1; [FIG. 6] shows a detailed view VI of FIG. 4 according to another embodiment of the present invention; [FIG. 7] shows an exploded view of the tool of FIG. 1; [FIG. 8] shows a modified example of the tool head of the tool of FIG. 1; [FIG. 9] shows the frequency characteristics of the tool of FIG. 1 in all three directions. [FIG. 10] shows a detailed view IX of FIG. 9. [5. Detailed Description] [5.1. General Principles]

[0038] As described above, the general principle of the present disclosure is to generate vibrations in a desired direction by changing the structure of the sheet forming tool, in particular, by providing means for converting vibrations generated by an actuator within the tool. This aspect will be apparent from the following description of the figures, which represent one exemplary embodiment. The vibrations occur at one or more predetermined frequencies. In this specification, vibrations are defined as mechanical waves at a certain frequency (or multiple superimposed frequencies) within the medium through which the vibrations propagate. These vibrations can be observed in the form of a vibration force (e.g., an actuator attached to the housing) or a vibratory motion (or vibration, e.g., observable with a punch when the latter is in a free state, i.e., not in contact with the metal sheet). These vibrations are also physically represented in the form of vibrational energy, i.e., a combination of a vibration force and a vibratory motion.

[0039] Referring to FIGS. 1 to 5, the sheet forming tool 1 according to the present invention will be described. The tool 1 includes 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.

[0040] The tool body 2 and the tool head 3 can be made of steel (however, they do not have to be the same steel). More generally, the tool body 2 and the tool head 3 can be made of any material capable of withstanding the sheet forming forces induced by an incremental sheet forming process. The tool body is generally axially symmetric about a spindle 20 that defines a first direction Z called the axial direction. In the remainder of this description, two directions X and direction Y are defined as being orthogonal to direction Z and orthogonal to each other. Directions X, Y, and Z form a right-handed reference frame (X, Y, Z) as shown in FIG. 4.

[0041] The tool head 3 is provided with a punch 30 at one of its ends. This punch is intended to come into contact with the metal sheet during the incremental sheet forming process. 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 the tip of the tool head 3. The punch 30 may be hemispherical, whereby the contact angle with the sheet metal can be continuously varied. The tool head 3 also includes a base 32 to 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. Thus, the punch 30 and the section 34 can form a finger protruding from the base 32. The tool head 3 has a substantially axisymmetric shape. The tool head 3 has a tapered shape from the base 32 towards the punch 30. Here, the cross-section of the tool head 3 has a substantially continuous cross-section without acute angles between the base 32 and the punch 30, improving the mechanical properties of the parts formed by the tool head 3. Thereby, the overall dimensions of the tool are also reduced, making it possible to obtain more diverse shapes. In particular, the elongated shape (i.e., finger shape) of the tool head 3 enables the formation of a recess having a container resulting from the sheet angle and having a greater depth.

[0042] In the example described here, the tool head 3 and the tool body 2 are described as separate parts and are fixed together using, for example, screws. Alternatively, the tool head 3 and the tool body 2 may be integral, and access can be made to the inside of the tool body 2 from the end opposite to the tool head 2.

[0043] The interior of the tool body 2 forms a housing 22, particularly as shown in FIG. 4. The housing 22 is suitable for receiving the actuator 4. The tool head 3 can close the first end of the housing 22, also called 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 be fixed to the tool body 2 and include a cover 24 that closes the second end of the housing 22 (which is also called the distal end of the housing 22 because it is away from the punch). Alternatively, the tool body 2 itself may close this second end.

[0044] The actuator 4 is installed within the housing 22. The actuator 4 may include an actuator interface 42 that can control the actuator. The actuator 4 can generate vibrations. The vibrations generated by the actuator 4 are transmitted throughout the tool 1, particularly to the punch 30. The actuator 4 can vibrate in the Z direction, i.e., the direction parallel to the spindle 20. When the actuator 4 vibrates, vibrations are generated that can be transmitted to other parts of the tool 1. The actuator 4 may specifically be unidirectional and vibrate only in the Z direction. The tool 1 includes means (hereinafter referred to as conversion means) for converting the direction of vibration. The conversion means can change the direction of the vibrations transmitted from the actuator 4 within the housing 22 to the punch 30.

[0045] The conversion means includes an axially asymmetric portion of the tool with respect to the spindle 20. In other words, the conversion means includes at least one axially asymmetric portion (or element) with respect to this spindle 20.

[0046] This non-axisymmetric portion of tool 1 can include actuator 4 itself offset with respect to spindle 20 by actuator offset 40 shown in FIG. 4. Here, housing 22 is axisymmetric with respect to spindle 20 of tool body 2, and actuator 4 is mounted offset from the center of housing 22. Alternatively, housing 22 may be offset with respect to spindle 20 and actuator 4 may be housed in the center of housing 22 (resulting in offset 40). It is also possible to combine an offset of housing 22 with a mounting offset from the center of actuator 4 within housing 22.

[0047] The asymmetry induced by the offset of actuator 4 (whether the actuator 4 is arranged offset from the center within the symmetric housing 22 or the housing 22 itself is offset from the center) enables the axial vibration of actuator 4 to be converted into lateral vibration by punch 30. These lateral vibrations occur at specific frequencies corresponding in particular to the bending modes of the tool. Thus, the actuator can vibrate in one direction while having a punch that can vibrate axially and laterally. In the example shown in FIGS. 3 and 4, actuator 4 is fixed to tool head 3 by hole 46 formed in tool head 3, and coincides with the axis of actuator 4, but is offset from tool body 2 and tool head 3.

[0048] Since the actuator is in one direction, this actuator is much more powerful than a multi-dimensional actuator (e.g., a stack of small piezoelectric actuators where each stacked layer can vibrate in its respective direction). The amplitude of the vibration at the level of punch 30 becomes much larger. This enables incremental sheet forming of better quality (surface finish) and thicker metal sheets in the case of tools and machines supporting tools of the same dimensions.

[0049] The tool body 2 may be provided with at least one hole 44. In FIG. 1, the tool body 2 is provided with two holes. The holes 44 are formed in the side wall of the tool body 2 and open to the housing 22. When there are a plurality of holes 44, these holes 44 may not be arranged at equal intervals on the tool body 2, which contributes to the asymmetry of the tool body 2 and thus the asymmetry of the tool 1. In addition to the asymmetry, the holes 44 enable access to the inside of the housing 22 so as to supply power to the actuator 4. The holes 44 can also be provided in the tool head 3 or the cover 24 to enable access to the housing 22. The asymmetric appearance of the holes 44 is particularly obvious in FIG. 3, and it is clear that the holes 44 are formed only on one side of the tool. The holes also enable the dissipation of heat generated by the actuator during operation.

[0050] Alternatively, in combination with the holes 44, the material recess formed in the tool body 2 can also create the asymmetry of the tool body 2, the cover 24 or the tool head 3. [5.2 Transfer function]

[0051] Different parts of the tool 1, in particular the tool body 2, the tool head 3 and the actuator 4, can not only cause a change in the direction of vibration (so as to propagate from the actuator 4 housed in the tool body 2 to the punch 30), but also be carefully dimensioned so that the nature (amplitude, frequency) of the vibration of the punch 30 as a function of the vibration of the actuator 4 can be accurately known. 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., a given frequency) does not cause the excitation of other modes.

[0052] This dimensional determination can be carried out, for example, as part of computer-aided design, as a result of computer simulation. Also, it is possible to replace or complete this simulation with experimental measurements on a tool prototype. In developing the present invention, the inventors were able to obtain a tool having satisfactory vibration characteristics, particularly in terms of vibration control, by combining these two methods (simulation and experiment).

[0053] The vibration of the punch can be measured using a laser vibrometer. By reflecting the laser vibrometer on the punch, the displacement of the punch and, as a result, the vibration of the punch can be determined. The measurement of the vibration of this punch can be carried out by controlling the actuator, generating a frequency sweep of the excitation of the actuator by a sine wave command, and observing the vibration response of the punch. The frequency of the sweep (i.e., the frequency of the sine wave command) changes linearly or logarithmically.

[0054] For example, first, a high-frequency sweep is performed over the entire spectrum (e.g., from 1 kHz to 22 kHz), and then the frequency at which the gain between the displacement measured at the end of the tool by the laser vibrometer (or the acceleration by the accelerometer) and the supply voltage to the actuator (image of the force) is maximized is recovered. Therefore, this is the frequency that is most likely to be close to the resonance frequency (i.e., the natural mode of the tool).

[0055] Next, a second, finer frequency sweep is performed around the maximum frequency or group of frequencies obtained above. This makes it possible to obtain an accurate gain profile in the vicinity of the resonance frequency or group of frequencies.

[0056] The punch 30 can vibrate in a plurality of vibration modes, namely, a unidirectional mode (axial direction Z, radial direction X or tangential direction Y), or a combined mode ((X,Z), (X,Y) or (Y,Z), or (X,Y,Z)). For a given mode, it is possible to determine (by experiment and / or simulation) the associated transfer function, that is, a function that takes the vibration (amplitude, frequency) of the actuator 4 in the Z-axis direction as the input and outputs the vibration (amplitude, frequency) of the punch 30 in this given mode.

[0057] The tool 1 as a whole is dimensioned such that for at least one vibration mode, its associated transfer function has a resonance frequency in the range of 5 to 30 kHz. There may be multiple resonance frequencies for the transfer function of a given mode, that is, there may be multiple peaks at which the energy transfer from the actuator 4 to the punch 30 is (at least locally) maximized. In multiple embodiments, the tool 1 has a plurality of transfer functions, each having its own resonance in the range of 5 to 30 kHz. These resonances can be separated, that is, when a specific frequency at which resonance occurs in a given mode is selected, there is no resonance in another mode. If the excitation frequency does not correspond to a specific frequency of a certain mode, the vibration response of the punch will be a linear combination of all the eigenmodes. Thereby, in a given incremental sheet forming process, it is possible to accurately control which resonance mode is preferred. [5.3 Preload]

[0058] Now refer to FIGS. 6 and 7. In this embodiment, the tool 1 also includes a shim 46 (or a set of shims) housed in the Z-direction extension of the actuator. The shim 46 applies a preload to the actuator 4 when the housing 22 is closed. The shim 46 can be dimensioned by a chain of appropriate dimensions, for example, a chain of dimensions related to the body 3, the head 2 and the cover 24 that together form the wall of the housing 22.

[0059] The shim 46 can be accommodated in a recess 48 formed in the cover 24 at an extension of the housing 22. Here, the shim 46 is accommodated on the cover 24 side, but the shim 46 can also be arranged on the tool head 3 side. In the case of a set of shims 46, a part of the set of shims 46 can be arranged 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 respect to the actuator 4. In the example shown in FIG. 7, the set of shims includes one thick shim and three thin shims.

[0060] The shim 46 accurately reduces the length of the housing 22 in which the actuator 4 is accommodated. Therefore, the shim 46 applies a preload to the actuator 4. If this preload is sufficient, the efficiency of energy transmission between the actuator 4 and the other parts of the tool 1 is significantly improved. The inventors estimate that in the developed prototype, a preload of approximately 80 to 85 kN is optimal in order to maximize energy transmission without damaging the actuator 4. More generally, the preload is approximately several tens of kN to several hundreds of kN depending on the dimensions of the prototype. This preload can also be adjusted by tightening the screw 32 to a certain torque (e.g., with a torque wrench) to ensure control of the screw tightening force. [5.4 Non-axisymmetric head]

[0061] In one embodiment, the head 3 is non-axisymmetric with respect to the main shaft 20. In the example shown in FIG. 8, the punch 30 is offset from the main shaft 20. The base 32 of the head 3 is substantially axisymmetric with respect to the main shaft 20. The non-symmetry of the head 3 means that when the actuator 4 generates vibrations along the main shaft 20, the head 3 vibrates non-axisymmetrically. Therefore, the punch 30 can vibrate in the lateral direction (radial direction X or tangential direction Y). This is advantageous for attaching the non-axisymmetric head 3 to an existing axisymmetric tool.

[0062] The punch 30 is connected to the base 32 by a section 34 whose cross-section becomes thinner as it approaches the punch 30. The section 34 has no edges or corners and has a smooth shape. The asymmetry of the head 3 is compatible with the other non-axisymmetric parts described above that make up the conversion means. [5.5 Examples of Transfer Functions]

[0063] Next, refer to FIG. 9. FIG. 9 represents the frequency response of the tool 1, in other words, the transfer function of this tool 1. FIG. 9 comprises three graphs respectively showing the frequency characteristics of the punch 30 (i.e., the amplitude of the vibration generated in the punch) according to the excitation frequency of the actuator in each of the X, Y, and Z directions (from top to bottom).

[0064] The frequencies investigated here vary between 4 kHz and 16 kHz, and the X-axis (representing the frequency) is on a linear scale. The gain investigated (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.

[0065] In this example, the transfer function is as follows: A coupling mode (X, Y) of approximately 4.5 kHz (the gain in the Y direction is significantly higher than that in the X direction), and A coupling mode (X, Y, Z) of approximately 14 kHz, and A natural mode in the X direction near 7.5 kHz.

[0066] Specifically referring to the transfer function near 14 kHz (see FIG. 10), FIG. 10 shows this transfer function along the X-axis, Y-axis, and Z-axis near 14 kHz, and it can be realistically understood that the transfer function has two very similar natural modes near 14 kHz: A first coupling mode 100 of approximately 14.2 kHz corresponding to the resonance peaks in the three directions of X, Y, and Z, and and a second eigenmode 102 in the Y direction at about 14 kHz. In this second resonant mode 102, the frequency response in both the X and Z directions shows a very significant tilt (so that the punch does not vibrate; this phenomenon is known as anti-resonance and is more pronounced in the Z direction), whereas the frequency response in the Y direction is closer to a resonant peak. Thus, the frequency response of the punch is particularly "pure" in the Y direction (in other words, the punch does not vibrate in the X and Z directions).

[0067] The transfer function also has a third mode 104 which is very close to the first mode 100. Thus, in this example, the tool has simple vibration modes (e.g. 14 kHz in the Y direction or 7.5 kHz in the X direction) and coupled modes (X, Y and X, Y, Z). Because the transfer function is linear, it is possible to excite each of these modes independently by superimposing frequencies in the excitation of the actuator 4. In this way, by adjusting the control of the actuator 4, the vibration generated by the punch 30 can be finely controlled while maintaining the actuator 4 in one direction. [Brief description of the drawings]

[0068]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Claims

1. A tool body (2), a tool head (3) integrated with the body (1) and provided with a punch (30), a housing (22) formed inside the body (2), an actuator (4) disposed within the housing (22) capable of generating vibrations in a first direction that can be propagated to the punch (30), and at least one means (4, 22, 3) for converting the first vibration direction into a second vibration direction while the vibrations are being propagated to the punch (30). An incremental sheet forming tool (1) characterized by comprising the above.

2. The sheet forming tool according to claim 1, wherein the at least one conversion means comprises at least one portion that is non-axisymmetric with respect to the spindle (20) of the tool (1).

3. The sheet forming tool according to claim 1, wherein the means for converting the vibration direction is parallel to the spindle (20) of the tool (1), and the vibrations in one direction generated by the actuator (4) are arranged to vibrate the punch (30) in at least one direction orthogonal to the spindle (20).

4. The sheet forming tool according to claim 2, wherein the housing (22) is offset with respect to the spindle (20) of the tool (1) and at least partially constitutes the non-axisymmetric portion.

5. The sheet forming tool according to claim 2, wherein the fixed axis of the actuator inside the housing (22) is offset with respect to the spindle (20) of the tool (1) and at least partially constitutes the non-axisymmetric portion.

6. The sheet forming tool according to claim 2, wherein the actuator (4) is installed substantially offset with respect to the spindle (20) of the body (2) within the body and at least partially constitutes the non-axisymmetric portion.

7. The body (2) includes an addition of a non-axisymmetric material with respect to the spindle of the body, and the addition at least partially constitutes the non-axisymmetric portion. The sheet forming tool according to claim 2, characterized by this.

8. The body (2) is provided with at least one recess (44) that is non-axisymmetric with respect to the spindle of the body (2), The at least one recess (44) is provided inside the body (2) and at least partially constitutes the non-axisymmetric part, characterized in that the sheet forming tool according to claim 2.

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

10. The sheet forming tool according to claim 1, further comprising one or more shims (46) installed in the housing (22) so as to apply a mechanical preload stress to the actuator (4).

11. The punch (30) is capable of vibrating in at least one resonance mode in response to vibrations generated by the actuator (4) along the spindle (20). The tool (2) has at least one resonance frequency for the vibration mode with respect to 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 spindle (20). The resonance frequency is between 5 kHz and 30 kHz, characterized in that the sheet forming tool according to claim 1.

12. The vibration mode is vibration parallel to the spindle (20), vibration perpendicular to the spindle (20) and parallel to the deviation direction (40) of the non-axisymmetric part of the tool, vibration perpendicular to the spindle (20) and the deviation direction (40) of the non-axisymmetric part of the tool (1), and combinations thereof, belonging to the group consisting of, characterized in that the tool according to claim 11.

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