Vibratory incremental sheet forming tool

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

However, conventional forming is a rigid process, since a given pair of dies can only produce one given form. Thus, if you wish to change the shape of the product shaped by forming, even just marginally modify it, it is necessary to change the dies used. Conventional forming is also cumbersome to set up, particularly when the sheet to be shaped has large dimensions and requires very high forces to apply sufficient and uniform pressure to the sheet to be formed.

To address the shortcomings of conventional forming, incremental sheet forming (ISF) has been developed over the last two decades. Incremental sheet forming consists of iterative local deformation of a sheet to give it the desired shape. To achieve this, the sheet is locally deformed using a tool head (also referred to as a punch), and this process is iterated until the desired shape is achieved. To control incremental sheet forming, the position of the tool head is controlled. It is even possible to control the acceleration of the tool, so as to control the force that the punch exerts on the sheet.

Incremental sheet forming has a number advantages over conventional forming processes. The shape of the formed product results from the numerical control that controls the position of the tool head, and can thus be easily modified by changing the instructions at the origin of the numerical control (for example a 3D model), without having to change the sheet forming tool itself, unlike with conventional forming. Incremental sheet 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 computer numerical control machine then controls the position of the tool head, thereby shaping the sheet metal.

Incremental sheet forming using a robotic arm is particularly advantageous in that it can be used to shape a large sheet at much lower cost than a CNC machine, and is even less costly than traditional forming with a die and a counter die (which are, by definition, the same size as the sheet to be formed).

Due to its flexibility and ease of use, incremental sheet forming has many advantages, and is ideal for the manufacture of prototypes or small production runs.

As explained above, incremental sheet forming is based on the local deformation of a metal sheet. This deformation is a result of the local application of a force (or pressure) to said sheet by the punch. This force thus produces in turn the elastic deformation of the robotic arm. If this deformation is too great, it moves the robotic arm and disrupts the control of the position of the punch. Thus, the forces that can be applied to the sheet to be formed are limited by the rigidity of the robotic arm with which the sheet is formed, and even more limited the thicker the sheet. Similar problems arise when a CNC machine is used for incremental sheet forming.

However, it has been discovered that vibrating the punch at frequencies in the kilohertz to low-frequency ultrasonic range, locally softens the sheet material, making it more ductile, thereby reducing the forces required to deform it. The springback of the sheet is thus reduced. These vibrations can be used to shape thicker sheets and/or allow the use of a robotic arm with smaller dimensions. The vibrations are typically generated by a piezoelectric actuator.

More precisely, when the vibrations are in a direction normal to the sheet (i.e. perpendicular to the sheet at the point of punch-sheet contact), these longitudinal vibrations would activate a dislocation movement and their propagation within the material, from which the observed softening potentially occurs. Transverse vibrations (i.e. perpendicular to normal vibrations) reduce the friction between the sheet and the punch, which also reduces the forces that the robotic arm needs to produce to carry out the incremental sheet forming. These transverse vibrations also limit the ‘stick-slip’ phenomenon, which also improves the quality of tool movement control and the surface finish of the sheet metal formed.

Vibrations normal and transverse to the sheet thus produce complementary effects which greatly reduce the effort required to perform incremental sheet forming. However, generating these vibrations in three directions (one normal and two transverse) is not easy.

The article 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 comprising a stack of piezoelectric actuators placed in the centre of the sheet forming tool body. Each of the actuators in the stack can vibrate in one of the three directions, as a function of 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 can vibrate in all three directions.

The article by Gao, J., & Altintas, Y. is also known (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 describes a machining tool that uses elliptical excitation of a stack actuators to couple vibration modes.

However, these solutions are not satisfactory. Firstly, it requires the use of several actuators (in the form of stacks), and therefore a multitude of power supplies to be controlled. In addition, since there are several actuators, each actuator is smaller (to respect the space limitations), and can therefore product less powerful vibrations. Finally, in practice these solutions are difficult to control, as multiple actuators interfere with the tool and it is not easy to control the vibration modes individually to produce the desired vibrations.

Furthermore, these solutions relate to machining and drilling, which are very different from incremental sheet forming.

There is a need to find a solution for vibrating a punch in a multidirectional manner which does not have the shortcomings of the prior art, in particular in terms of simplicity of implementation, size and ease of control.

The invention improves the situation.

The invention has been designed to overcome at least some of the disadvantages of the prior art.

To this end, the invention proposes an incremental sheet forming tool, characterised in that it comprises:

Thus, due 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 referred to as the axial direction or longitudinal direction), to produce a transverse vibration, i.e. orthogonal to this main axis, at site of the punch. The actuator can thus be a unidirectional actuator without this preventing the tool from vibrating in several directions (including one transverse and one axial direction). The actuator is for example a piezoelectric actuator. During an incremental sheet forming process, the punch is able to vibrate in directions normal and transverse to the metal sheet (i.e. in the main axis of the tool body and in one or more directions orthogonal to this main axis, respectively). The sheet is thus locally smoothed and friction between the tool head and the sheet is reduced. The springback of the sheet to the tool is therefore reduced. As the forces required to form the sheet are reduced, the springback of the arm is also reduced.

The actuator can excite tool head and cause it to vibrate by imposing a vibratory force on it to obtain, at certain frequencies, a large vibratory force at the punch by the structural resonance effect, particularly when the punch is in contact with a metal sheet.

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 formed part, since the lower springback and reduction in friction result in a better surface finish of the part after sheet forming.

According to one aspect, said at least one transformation means comprises at least one non-axisymmetrical portion with respect to a main axis of said tool.

Thus, the tool has a generally axisymmetrical shape, which improves its vibratory properties, in particular by separating the vibratory nodes of the punch in relation to a tool whose shape would be particularly dissymmetric. This also makes it possible to integrate the transformation means into a pre-existing tool, by partially modifying its axisymmetrical nature.

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

The fact of being able to make the punch vibrate in several directions from a unidirectional vibration due to the transformation means also makes it possible to use a single unidirectional actuator, and therefore a more powerful actuator. The tool can therefore be smaller for equivalent actuator power, since the effects of reduced ductility and reduced friction are all the greater the higher the amplitude of the vibration.

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, such 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.

This new type of tool thus allows incremental sheet forming using a robotic arm or a CNC machine (more generally any machine that supports the tool) of smaller dimensions than those usually used, since the springback is lower. Alternatively, larger and/or thicker sheets can be formed using the same robotic arm. Finally, there is less friction, which limits the stick-slip effect. This improves the quality of the tool head position control and the surface finish of the sheet once it has been formed.

According to one aspect, the housing is off-center with respect to a main axis of the body, constituting at least partially said non-axisymmetrical portion.

According to one aspect, the axis of attachment of the actuator within the housing is off-centre with respect to a main axis of said tool, constituting at least partially said non-axisymmetrical portion.

According to one aspect, the actuator is installed within the body in a substantially off-centre manner with respect to a main axis of the body, constituting at least partially said non-axisymmetrical portion.

Due to the offset 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 offset, it is then possible to determine the transfer function between the vibration in the Z axis (axial direction) and the vibration in X, Y and Z directions (transverse radial, transverse tangential and axial direction respectively). This can be achieved for example by means of a simulation (for example, numerical analysis using the finite element method) based on a model of the tool obtained by computer-aided design (CAD). With good knowledge of this transfer function, it is possible to control the vibrations of the punch from the frequency control of the power supply to the actuator (i.e. the unidirectional vibration it produces in the housing).

According to one aspect, the body comprises an addition of material which is non-axisymmetrical with respect to a main axis of the body, said addition constituting at least partially said non-axisymmetrical portion.

Thus, an addition of material makes it possible to contribute to the propagation of the vibration in a non-axial direction. It should be noted that this non-axisymmetrical addition of material can perform a third function, such as acting as a behavior-shaping constraint in the assembly of the tool, making it possible to attach power supply means or sensors to the tool, etc. which saves pace by pooling the function within this addition.

According to one aspect, the body comprises at least one recess that is not axisymmetrical with respect to a main axis of the body, said at least one recess being provided within the body and constituting at least partially said non-axisymmetrical portion.

Here, the recess makes it possible to contribute to the propagation of the vibration in a non-axial direction. It should be noted that this non-axisymmetrical recess can perform a third function, such as providing access to parts of the tool, acting as a behavior-shaping constraint, housing power supply means or sensors etc., which makes it possible to save space by sharing the functions within this recess.

According to one aspect, the recess comprises at least one hole made in the body and forming an access to the housing from the outside of the body.

This hole has a dual function: it contributes to the non-axisymmetry of the tool and helps to power the actuator by allowing access to the housing (to run power cables). This saves space, as well as generating a transverse vibration in the punch.

It should be noted that the dissymmetry caused by the removal or addition of material or drilling is not incompatible with an offset of the housing and/or actuator. On the contrary, these two particular features can be combined to amplify the dissymmetry of the part, thereby improving the generation of a transverse vibration.

According to one aspect, the tool also comprises 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 vibrations of the actuator).

According to this example, the compression of the actuator with a shim (or a set of shims) induces a preload, i.e. a force (or preload) applied to the actuator in the absence of any specific action (such as powering the actuator). The preload drastically improves the energy transfer between the actuator and the tool body (hence the gain in the transfer function described above). The preload also increases the transfer of the vibration produced by the actuator to the rest of the tool.

According to one aspect, the punch is also able to vibrate according to at least one resonance mode in reaction to a vibration generated by the actuator in a main axis, and the tool has at least one resonance frequency for said vibration mode for the transfer function defined by the relationship 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.

The transfer function has a resonance. This resonance advantageously makes it possible to improve the energy transfer, and therefore increase the amplitude of the vibration of the punch. The smoothness of a sheet to be formed is increased in this way and reduces the forces required to form the sheet.

This resonance is achieved by carefully selecting the dimensions of the tool, in particular its non-axisymmetrical portion, selected judiciously. This can be achieved using simulation means during computer-aided design, or empirically, the important factor being that in the end a resonance frequency is obtained for the desired mode or modes.

According to one aspect, the vibration mode belongs to the group comprising a vibration parallel to the main axis, a vibration orthogonal to the main axis and parallel to a direction of misalignment of the non-axisymmetric portion, a vibration orthogonal to the main axis and to the direction of misalignment of the non-axisymmetric portion, and a combination thereof.

The direction orthogonal to the main axis and parallel to a direction of misalignment of the non-axisymmetric portion (typically the offset of the actuator and/or the housing, or the direction of misalignment of a hole or an addition of material) is the radial transverse direction (X axis). The direction orthogonal to the main axis and to the direction of misalignment 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 referred to as the coupled mode(s).

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 are softened by vibrations. In particular, forces generated by transverse vibrations reduce friction at the point of contact between the sheet and the punch during sheet forming, as axial vibrations would activate dislocations in the sheet at the microscopic level, thus softening the sheet. The various coupled modes (in particular axial/radial and axial/tangential), for their part, would make it possible to combine these effects. Coupled modes also make it possible to anticipate variations in the trajectory of the punch, and therefore to maintain the transverse direction of the vibration of the punch co-linear with the feed direction of the tool, which in turn improves the reduction in punch/sheet friction. In other words, the direction of the transverse vibration of the tool can be controlled so that it is tangential to the movement of the punch. These modes of vibration can also be single, double or triple, i.e. inducing single, double or triple bending of the tool head, respectively.

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

With reference to, a sheet forming toolaccording to the invention is described. The toolcomprises a tool body, a tool headand an actuator. The tool bodyand the tool headare fixed to one another. The actuatoris housed in the tool bodyand/or the tool head.

The tool bodyand the tool headcan be made of steel (but not necessarily the same steel). The tool bodyand the tool headcan more generally be made from any material capable of withstanding the sheet forming forces induced by the incremental sheet forming process.

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