Vibration-assisted roll machining

Abstract

The invention relates to a method for roll-machining a gear-shaped workpiece (23), in particular by means of a continuous generating grinding process, in which the gear-shaped workpiece is rotated about a workpiece axis (C) while meshing with a roll machining tool. The gear-shaped workpiece is subjected to an oscillating movement during the machining process, said oscillating movement being superimposed with the rotation of the gear-shaped workpiece. The invention likewise relates to a clamping device (22) which is designed to carry out the method, to a system which comprises such a clamping device and a controller (50), and to a roll machining machine which comprises a clamping device of the aforementioned type.

Description

[0001] TITLE

[0002] VIBRATION-ASSISTED HOB MACHINING

[0003] TECHNICAL FIELD

[0004] The present invention relates to a method for machining a gear-shaped workpiece by a generating machining method, in particular by continuous generating grinding, a clamping device designed to carry out the method, a system for generating oscillating movements of a gear-shaped workpiece during machining by a generating machining method, and a generating grinding machine on which the clamping device is used.

[0005] STATE OF THE ART

[0006] The final hard finishing step in gear manufacturing is one of the most important steps in determining the quality of the gear teeth. This is where the geometry that will later mesh with the teeth is created. Continuous generating grinding is a process frequently used for hard finishing. In continuous generating grinding, a gear-shaped workpiece is machined in rolling engagement with a grinding tool in the form of a helically profiled grinding wheel (grinding worm). Generating grinding is a highly sophisticated, generative machining process based on a multitude of synchronized, precise individual movements and influenced by numerous boundary conditions. Information on the fundamentals of continuous generating grinding can be found, for example, in Ref. [1].

[0007] Fig. 1 illustrates, by way of example, the engagement between a generating tool in the form of a grinding worm 16 and a helical gear 23 during continuous generating grinding. The grinding worm 16 rotates about a tool axis B, while the gear 23 rotates about a workpiece axis C. The grinding worm 16 and the gear 23 mesh with each other like a helical gear (so-called rolling coupling). During a grinding stroke, the grinding worm 16 moves axially relative to the gear 23 along a feed axis Z, which runs parallel to the workpiece axis C. At the same time, the grinding worm 16 is continuously moved translationally along a Y-direction, which runs parallel to the tool axis B, in order to continuously bring fresh worm material into engagement with the gear 23 (so-called shift movement).The grinding worm 16 is fed along a feed axis X in a radial direction relative to the gear 23.

[0008] As a result of the complex cutting motion between grinding worm 16 and gear 23, regular, wave-like structures form on the tooth flanks along the tooth line (so-called grinding grooves). These regular structures can lead to undesirable noise during gear operation.

[0009] Various approaches have been proposed in the state of the art for the subsequent removal of such regular structures. For example, it has been suggested that continuous generating grinding be followed by a polishing or honing process. However, such an additional process step is often associated with an undesirable increase in costs.

[0010] The state of the art also proposes measures to prevent the formation of these regular structures from the outset. For example, in so-called Low Noise Shifting (LNS), the rotation angles of the grinding worm and dressing wheel are coupled during dressing. A specifically coordinated shifting movement during grinding allows periodic fluctuations in the grinding pattern to be positioned in a targeted manner to achieve a more favorable noise behavior of the gear (see Ref. [2]). Due to the increasing process complexity, this process does not fully cover all noise-critical applications.

[0011] In Ref.

[0030] it is proposed to generate a targeted waviness during continuous generating grinding by exploiting a targeted imbalance of the tool.

[0012] Other gear-cutting processes, such as power skiving, can also produce undesirable, regular structures. SUMMARY OF THE INVENTION

[0013] In a first aspect, an object of the present invention is to provide a method for generating gear-shaped workpieces, in particular for continuous generating grinding, with which undesirable regular structures on the flanks of the machined workpieces can be reduced.

[0014] This object is achieved by a method according to claim 1. Further embodiments are specified in the dependent claims.

[0015] The invention proposes a method for machining a gear-shaped workpiece, in particular its tooth flanks, by a generating machining process, in particular by continuous generating grinding. During machining, the gear-shaped workpiece rotates about a workpiece axis while being in rolling engagement with a generating machining tool, in particular a helically profiled grinding tool, which rotates about a tool axis. The method is characterized in that the gear-shaped workpiece executes an oscillating movement during machining, which is superimposed on the rotation of the gear-shaped workpiece. As a result, the engagement between workpiece and tool is modified such that the surface structure of the workpieces thus manufactured can be improved. In particular, regular structures such as grinding marks can be avoided or interrupted.

[0016] The oscillating movement of the gear-shaped workpiece preferably has a component along at least one of the following movement directions: axially along the workpiece axis; radially to the workpiece axis; and torsional around the workpiece axis.

[0017] The oscillating movement can in particular correspond to a superposition of components along these directions of movement.

[0018] In some embodiments, the oscillating movement has a fundamental frequency above 15 kHz. In other words, in such embodiments, the oscillating movement is an oscillation in the ultrasonic range. As described in more detail below, such movements can be generated particularly efficiently by resonant excitation of a clamping device on which the workpiece is mounted. In other embodiments, the fundamental frequency is below 15 kHz. "Fundamental frequency" refers to the spectral component of the oscillation that has the lowest frequency.

[0019] The oscillating movement can be generated in particular as follows: During the generating grinding process, the gear-shaped workpiece is clamped on a clamping device, which in turn is arranged on a workpiece spindle. The workpiece spindle drives the gear-shaped workpiece clamped on the clamping device to rotate around the workpiece axis. The clamping device has a clamping area that is in direct contact with the workpiece clamped on the clamping device. The oscillating movement of the gear-shaped workpiece is then preferably generated by exciting the clamping area of ​​the rotating clamping device to oscillate. This excitation preferably occurs resonantly, so that the oscillation is a eigenmode of the clamping device with the gear-shaped workpiece clamped thereon. This allows controlled excitation with a high amplitude to be achieved.The corresponding natural frequencies (resonance frequencies) are typically in the ultrasonic range. To reduce unwanted transmission of the vibration to the workpiece spindle, the natural mode is preferably selected such that a vibration node of the natural mode is located in a mounting area of ​​the clamping device that is in contact with the workpiece spindle.

[0020] To excite the vibration, the clamping device can have a vibration generator integrated into the clamping device. The vibration generator converts an electrical excitation signal in the form of an alternating voltage with a suitable frequency into a vibration of the clamping device with the gear-shaped workpiece clamped thereon. To monitor the vibration caused by the excitation signal, an electrical sensor signal can be determined that characterizes the vibration. The excitation signal can then be controlled based on the sensor signal such that the vibration is excited resonantly, as explained above. In particular, the sensor signal can characterize the amplitude of the generated vibration and / or the phase position of the generated vibration relative to the excitation signal.Using such a sensor signal, the excitation signal (especially its frequency) can then be controlled in such a way that the oscillating system consisting of the clamping device and the gear-shaped workpiece clamped thereon is excited resonantly as desired. Suitable control algorithms for controlling the excitation of an oscillating system so that the system oscillates resonantly are known in principle.

[0021] Preferably, the excitation signal and the sensor signal are transmitted contactlessly between the rotating clamping device and a stationary control device. The control device can, in particular, comprise a frequency generator for generating the excitation signal and a controller for the above-described control of the excitation signal. Suitable devices for the contactless transmission of energy and signals are known per se. For example, the transmission can be effected inductively by two coils arranged concentrically around the workpiece axis, with one of the coils being arranged on the clamping device and the other coil on the stationary machine element.

[0022] In a second aspect, the invention provides a clamping device for clamping a gear-shaped workpiece on a workpiece spindle of a gear-cutting machine, in particular a generating grinding machine, which is designed to carry out the method described above according to the first aspect of the invention. The clamping device is characterized in that it has a vibration generator for generating vibration of the clamping device with the gear-shaped workpiece clamped thereon.

[0023] The vibration generator can in particular be designed to generate vibration excitation along at least one of the following directions of movement: axially along the workpiece axis; radially to the workpiece axis; and torsional around the workpiece axis.

[0024] In preferred embodiments, the vibration generator comprises a piezo actuator or is designed as a piezo actuator. In particular, the vibration generator can have at least one piezoelectric actuator element that is disc-shaped or ring-segment-shaped, with a ring axis of the piezoelectric actuator element corresponding to the workpiece axis.

[0025] The vibration generator can be arranged in the clamping device as follows: The clamping device defines a proximal end and a distal end, wherein the proximal end is designed for connection to the workpiece spindle. The clamping device has a clamping area designed to make direct contact with the gear-shaped workpiece. The clamping area can be designed, for example, in a conventional manner as a hydraulically actuated expansion sleeve or as a mechanically actuated, segmented clamping bush. The vibration generator can then be arranged between the proximal end and the clamping area, or it can be arranged distally from the clamping area.

[0026] The clamping device can also have a vibration sensor for determining at least one sensor signal, wherein the sensor signal characterizes the vibration of the clamping device with the gear-shaped workpiece clamped thereon.

[0027] The vibration sensor can, in particular, comprise a piezoelectric sensor or be designed as a piezoelectric sensor. It can be constructed analogously to the vibration generator and have at least one piezoelectric sensor element that is disc-shaped or ring-segment-shaped, with a ring axis of the piezoelectric sensor element corresponding to the workpiece axis. The vibration generator and the vibration sensor can jointly form a vibration transducer comprising a stack of disc-shaped or ring-segment-shaped piezoelectric elements, disc-shaped electrodes, and disc-shaped insulation discs.

[0028] In a third aspect, the present invention provides a system for generating an oscillating movement of a gear-shaped workpiece during machining by a gear machining method, in particular by continuous generating grinding. The system comprises, on the one hand, a clamping device according to the second aspect of the invention. On the other hand, the system comprises a frequency generator for generating an excitation signal for the vibration generator in order to cause the vibration of the clamping device with the gear-shaped workpiece clamped thereon. The system can have a controller configured to receive the sensor signal and, based on the sensor signal, to regulate the excitation signal such that the vibration is excited resonantly.

[0029] The system may also comprise a transmission device for contactless transmission of the excitation signal and the sensor signal between the rotating clamping device and the stationary control device, as already explained above in the context of the method.

[0030] In a fourth aspect, the invention provides a generating machine, in particular a generating grinding machine, which is designed to carry out the method according to the first aspect of the invention. The generating machine comprises: a tool spindle for driving a generating tool, in particular a helically profiled grinding tool, to rotate about a tool axis; a workpiece spindle for driving the gear-shaped workpiece to rotate about the workpiece axis; and a machine controller designed to control the tool spindle and the workpiece spindle in such a way that a rolling coupling is established between the rotation of the tool generated by the tool spindle and the rotation of the workpiece generated by the workpiece spindle.

[0031] The gear-forming machine is characterized in that a clamping device according to the second aspect of the invention is mounted on the workpiece spindle. Of course, the gear-forming machine can also comprise a frequency generator, a controller, and / or a transmission device as specified in the third aspect of the invention.

[0032] BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Preferred embodiments of the invention are described below with reference to the drawings, which are for illustrative purposes only and are not to be construed as limiting. In the drawings:

[0034] Fig. 1 is a sketch illustrating the kinematics of continuous generating grinding; Fig. 2 is a schematic view of a generating grinding machine;

[0035] Fig. 3 is a schematic sketch showing possible vibration types and shapes of a clamping device with a gear wheel clamped thereon;

[0036] Fig. 4 is a schematic view of a clamping device with a gearwheel clamped thereon, with a vibration transducer arranged above the gearwheel for generating and measuring torsional movements of the gearwheel, together with an associated control device and a possible vibration form;

[0037] Fig. 5 is a schematic exploded view of a vibration transducer which forms a vibration generator on the one hand and a vibration sensor on the other hand;

[0038] Fig. 6 is a schematic view of a clamping device with a gear wheel clamped thereon, with a vibration transducer arranged below the gear wheel for generating and measuring oscillating torsional movements of the gear wheel, as well as a possible vibration form;

[0039] Fig. 7 is a schematic view of a clamping device with a gear wheel clamped thereon, with a vibration transducer arranged above the gear wheel for generating and measuring oscillating longitudinal movements of the gear wheel, as well as a possible vibration form;

[0040] Fig. 8 is a schematic view of a clamping device with a gear wheel clamped thereon, with a vibration transducer arranged below the gear wheel for generating and measuring oscillating longitudinal movements of the gear wheel, as well as a possible vibration form;

[0041] Fig. 9 is a schematic view of a clamping device with a gear wheel clamped thereon, with a vibration transducer arranged above the gear wheel for generating and measuring oscillating radial movements of the gear wheel, as well as a possible vibration form; and

[0042] Fig. 10 is a schematic view of a clamping device with a gear wheel clamped thereon, with a vibration transducer arranged below the gear wheel for generating and measuring oscillating radial movements of the gear wheel, as well as a possible vibration form.

[0043] DESCRIPTION OF PREFERRED EMBODIMENTS

[0044] Example structure of a generating grinding machine

[0045] Fig. 2 shows an example of a generating grinding machine 1, as is known per se from the prior art. The machine has a machine bed 11 on which a tool carrier 12 is guided so as to be displaceable along a feed direction X. The tool carrier 12 carries an axial slide 13 which is guided so as to be displaceable relative to the tool carrier 12 along an axial direction Z. A grinding head 14 is mounted on the axial slide 13 and can be pivoted about a pivot axis running parallel to the X-axis (the so-called A-axis) in order to adapt to the helix angle of the gear to be machined. The grinding head 14, in turn, carries a shift slide on which a tool spindle 15 can be displaced relative to the grinding head 14 along a shift axis Y. A grinding worm 16 is clamped on the tool spindle 15. The grinding worm 16 is driven by the tool spindle 15 to rotate about a tool axis B.

[0046] The machine bed 11 further supports a pivotable workpiece carrier 20 in the form of a turret, which can be pivoted about an axis C3 between at least two positions. Two identical workpiece spindles 21 are mounted diametrically opposite one another on the workpiece carrier 20, only one of which is visible in Fig. 1. A clamping device 22 for clamping a workpiece 23 is mounted on each of the workpiece spindles. The workpiece spindle visible in Fig. 1 is in a machining position in which the workpiece 23 clamped on the clamping device 22 can be machined with the grinding wheel 16. The other workpiece spindle, offset by 180° and not visible in Fig. 1, is in a workpiece change position in which a finished workpiece can be removed from the clamping device on this spindle and a new blank can be clamped.Each workpiece spindle 21 drives the clamping device 22 mounted thereon with the workpiece 23 clamped thereon to rotate about a workpiece axis C.

[0047] All driven axes of the generating grinding machine 1 are digitally controlled by a machine controller 30. The machine controller 30 receives sensor signals from a plurality of sensors in the generating grinding machine 1 and, depending on these sensor signals, outputs control signals to the actuators of the generating grinding machine 1. The machine controller 30 comprises, in particular, a plurality of axis modules (NC modules) 32, which provide control signals for a respective machine axis (i.e., for at least one actuator used to drive the respective machine axis, such as a servo motor) at their outputs. It further comprises an operator panel 33 and a control computer 31, which interacts with the operator panel 33 and the axis modules 32. The control computer 31 receives operator commands from the operator panel 33 and sensor signals and calculates control commands for the axis modules 32 therefrom.It also outputs operating parameters based on the sensor signals to the control panel 33 for display. Processing of a workpiece batch.

[0048] To machine a workpiece that has not yet been machined (blank), the workpiece is clamped by an automatic workpiece changer onto the clamping device of the workpiece spindle that is in the workpiece change position. The workpiece change takes place in parallel with the machining of another workpiece on the other workpiece spindle that is in the machining position. Once the new workpiece to be machined is clamped and machining of the other workpiece is complete, the workpiece carrier 20 is pivoted 180° about the C3 axis so that the spindle with the new workpiece to be machined reaches the machining position. Before and / or during the pivoting process, a centering operation is performed using a centering probe. For this purpose, the workpiece spindle 21 is rotated, and the position of the tooth gaps of the workpiece 23 is measured using the centering probe. The pitch angle is determined on this basis.

[0049] When the workpiece spindle, which carries the workpiece 23 to be machined, has reached the machining position, the workpiece 23 is brought into collision-free engagement with the grinding wheel 16 by shifting the tool carrier 12 along the X-axis. The workpiece 23 is then machined by the grinding wheel 16 in rolling engagement. Meanwhile, the tool spindle 15 is slowly and continuously shifted along the shift axis Y in order to continuously utilize unused areas of the grinding wheel 16 during machining (so-called shift movement).

[0050] At the same time as the workpiece is being machined, the finished workpiece is removed from the other workpiece spindle and another blank is clamped onto this spindle.

[0051] Introduction of vibrations during continuous generating grinding

[0052] In the proposed method, a separately excited oscillating motion is introduced into the generating grinding process. By appropriately selecting the amplitude, frequency, and phase position of the oscillating motion, positive effects on the structure of the workpiece surface can be achieved.

[0053] Ultrasonic vibrations have been used in various cylindrical and surface grinding processes to reduce grinding forces, minimize tool wear, and optimize the workpiece surface in terms of structure and roughness parameters (see documents [3]-

[0024] ). With the superposition of ultrasonic vibrations, the continuous contact between tool and workpiece becomes discontinuous, and / or the trajectory of the abrasive grains is altered. This can result in advantages in terms of productivity and quality. However, due to the completely different kinematics and contact conditions in continuous generating grinding, findings obtained in surface or cylindrical grinding cannot be transferred to continuous generating grinding.

[0054] In the method proposed here, the process kinematics of generating grinding are suitably superimposed with oscillating movements in the ultrasonic range (above 15 kHz) or in the low-frequency range (below 15 kHz). This is intended to reduce the waviness of the ground tooth flank surface and prevent or interrupt regular grinding marks. Further objectives include reducing process forces and tool wear. This makes it possible to subject grinding processes to higher intensity in the future (e.g., shortening the grinding time by increasing the axial feed), since the individual grain is subjected to less stress and the grinding forces are reduced. The superposition of oscillating movements with frequencies below the ultrasonic range has the potential to suppress chatter frequencies.

[0055] Within the framework of the approach proposed here, the oscillating movements are generated on the workpiece. For this purpose, the workpiece clamping device (hereinafter also referred to as the clamping set) can be equipped with an actuator to generate vibrations.

[0056] Vibration directions

[0057] Three main directions are possible for aligning the oscillating movements relative to the conventional cutting direction during continuous generating grinding. Table 1 shows these three main directions for the example of machining a spur gear. Table 1: Assignment of the three main directions of superimposed oscillating movements during grinding of a spur gear

[0058] Combinations of these three main directions are also possible and sensible. In real systems, such combinations are even unavoidable to a certain extent. This is especially true for helical gears.

[0059] Vibration frequencies

[0060] Regarding the frequency range, a distinction can be made between low-frequency vibrations (below 15 kHz) and high-frequency vibrations in the ultrasonic range (above 15 kHz). The influences on the process can be very similar and pursue the same objectives. However, with regard to vibration generation, different practical implementation variants arise for the different frequency ranges.

[0061] To generate ultrasonic vibrations, the natural structural dynamics of the gear and clamping assembly to be excited are proposed. Due to the mass and stiffness distribution of such structures, usable eigenmodes typically lie in the ultrasound frequency range and can also be optimized and adapted through design measures. Advantages of resonance excitation include, for example, the possibility of spatially separating the actuators for excitation and the active point of the process, maintaining a relatively high structural stiffness, and high efficiency for generating the vibration amplitudes at the active point of the process. Examples of integrating a vibration transducer into the clamping assembly to achieve these goals are explained in more detail below.

[0062] For the generation of low-frequency vibrations, the possibility of resonant excitation is generally not available. The structural dynamics of the clamping assembly and gear usually do not allow resonant excitation in the low-frequency range, or their stiffness would have to be reduced to such an extent that it would no longer be usable for the process. Therefore, non-resonant excitation is proposed for the generation of low-frequency vibrations. By means of a suitable arrangement of actuators and connecting kinematics for translating and transmitting the deflections, the gear can also be set into low-frequency vibrations without utilizing resonance.

[0063] Frequency modes of ultrasonic vibrations

[0064] In order to specifically couple ultrasonic vibrations into the active site of the process, some designs provide for the excitation of natural eigenmodes at natural resonance, thus creating standing waves (resonance excitation). An eigenmode is characterized by regions of so-called vibration antinodes with maximum deflection and simultaneously minimal strain. Opposite regions with minimal mechanical deflection and maximum mechanical strain are referred to as vibration nodes.

[0065] Preferably, the workpiece (gear) and the workpiece clamping device (clamping set) are excited as a common oscillating structure in suitable eigenmodes. Preferably, eigenmodes are used that result in largely uniform deflections across the entire circumference of the gear. Otherwise, and in the case of vibration modes that randomly orient themselves, as is the case, for example, with bending vibrations relative to the C axis of the workpiece and the clamping set, the vibration generated at the point of action would be subject to randomness, and process reliability could not be guaranteed.

[0066] In Fig. 3, three possible types of vibration for achieving the differently directed oscillating movements of a gear 23 in the three main directions described above from Table 1 are idealized and sketched for the case of a straight-toothed gear 23.

[0067] For the superposition in the cutting direction (diagram a in Fig. 3), the clamping assembly 22 including the gear 23 is excited in a eigenmode with longitudinal oscillation (longitudinal oscillation) in the direction of the gear axis (workpiece axis C). The order of the mode is selected and the structure is designed such that an antinode of the longitudinal oscillation develops in the area of ​​the gear 23. Thus, maximum amplitudes in the axial direction of the gear 23 act at the effective point of the process and thus in the cutting direction.

[0068] In geometrically confined structures, a longitudinal vibration is always accompanied by a transverse vibration (thickness vibration) due to transverse contraction. These vibrations are referred to as "quasi-longitudinal" vibrations. Maximum transverse deflections occur at the points of maximum longitudinal strain. This means that a vibration node of a real longitudinal vibration exhibits no longitudinal displacements, but displacements perpendicular to them. This behavior can be used in the case of generating grinding for superposition in the cutting plane transverse to the cutting direction. This form of vibration excitation is sketched in Fig. 3 in diagram b and is characterized by a longitudinal vibration node in the area of ​​gear 23.

[0069] For superposition in the direction of the cutting depth, the rotationally symmetrical structure consisting of gear 23 and clamping set 22 can be excited in a torsional mode around the workpiece axis C, with a torsional vibration antinode located in the area of ​​gear 23. This form of vibration is sketched in diagram c of Fig. 3.

[0070] The various types of vibration superposition and combinations thereof can be implemented through the type of vibration generator (thickness oscillator or shear oscillator), their positioning, the geometry of the end mass, and the selection of the eigenmode (via excitation frequency). So-called operating deflection shapes, as they occur in real structures, always exhibit a certain degree of combinations of different directional components of the idealized vibration directions. Integration of the vibration generator into the clamping device

[0071] To generate the vibrations, a vibration generator is preferably integrated into the clamping set. The vibration generator preferably comprises one or more piezoelectric actuators, which convert an electrical signal corresponding to the operating frequency into mechanical vibrations. The placement and orientation of the vibration generator determines the vibration parameters achieved on the tooth flank surface to be machined and can be designed according to the intended type of vibration superposition.

[0072] For effective vibration excitation, the actuators are preferably positioned close to a vibration node for the eigenmode to be excited. The mechanical direction of action of the actuators lies in the direction of the vibration mode's stretching at the actuator position. Preferably, several thin actuators are used as an actuator stack with alternatingly polarized electrodes between them for power supply. This setup achieves the required electric field strengths with relatively low electrical voltages.

[0073] To protect the machine components connected to the clamping set and to prevent unwanted vibration transmission to the remaining structures of the generating grinding machine, it is advisable to decouple the vibration of the clamping set with gear from the rest of the machine. To achieve this, a vibration mode is preferably selected that has a vibration node in the area where the clamping set is connected to the workpiece spindle. Vibration transmission can be further reduced by giving the clamping set a high mass in the mounting area and / or by providing geometries that reduce vibration transmission.

[0074] A frequency generator can be used to supply the actuators with an excitation signal in the form of an alternating electrical voltage at the desired excitation frequency. The generated excitation frequency corresponds to the resonance frequency of the eigenmode to be excited. The resonance frequency depends within certain ranges on influences such as temperature and the applied process force. To ensure excitation always within a favorable operating range and with high efficiency, the frequency of the frequency generator should preferably be controlled with a controller. The frequency generator and controller can be integrated into a control unit. This allows the output excitation frequency to track the variable resonance frequency of the system. Furthermore, the ultrasonic power is preferably controlled so that desired amplitudes can be introduced into the process regardless of the load.

[0075] To control the vibration amplitude and track the excitation frequency, the actual vibration of the gear can be recorded in terms of frequency, phase position, and / or amplitude. Since the oscillating movement of the tooth flanks cannot easily be recorded directly during the process, an indirect measurement can be performed, allowing conclusions to be drawn about the vibration on the gear. A single sensor can, assuming the correct excited eigenmode is present, provide the actual values ​​for the vibration on the tooth flanks. A statement as to whether the desired mode is actually being excited is only possible to a limited extent with a single sensor. By arranging several sensors at characteristic points, adjusting the phase position of the sensor signals, and comparing the measured amplitudes, the excited three-dimensional vibration shape can also be deduced.This ensures vibration excitation in the desired eigenmode and increases process stability during controlled vibration operation. Piezoelectric elements, from which a voltage signal can be tapped, can also be used as sensors.

[0076] Both the electrical supply for the actuators and the sensor signals are transmitted between the rotating clamping set with gear and the stationary control device. For this purpose, a contactless rotary transformer with inductive power transmission can be provided, for example.

[0077] Example 1: Generation of torsional vibrations with vibration generator above the gear

[0078] An example of the implementation of the principles explained above in practice is now explained using Fig. 4.

[0079] Fig. 4 shows a highly schematic illustration of a clamping assembly 22 for rigidly connecting to the rotating shaft of a workpiece spindle. The end of the clamping assembly intended for connection to the spindle shaft is referred to below as the proximal end 221, and the opposite end 222 as the distal end. In Fig. 4, the proximal end is at the bottom and the distal end is at the top. The clamping assembly 22 has a clamping region 223 in the form of a radially expandable expansion sleeve or a segmented clamping bush, on which a gear-shaped workpiece 23 is clamped. A vibration transducer 40 is integrated into the clamping assembly above the workpiece 23 (axially between the clamping region 223 and the distal end 222). Above the vibration transducer 40 (distal from the vibration transducer 40), the clamping set 22 has an exchangeable end piece 224 in the form of a preload nut.With this end piece 224, the vibration transducer 40 is fixed to the clamping set 22 and simultaneously compressed in the axial direction along the workpiece axis C.

[0080] As will be explained in more detail below, the vibration converter 40 comprises, on the one hand, a vibration generator 410 which is supplied with a high-frequency excitation signal V A is controlled to excite torsional vibrations 41 of the workpiece 23. On the other hand, the vibration transducer 40 comprises a vibration sensor 420 to measure characteristics of the generated vibrations. The vibration sensor 420 outputs one or more sensor signals V s out of.

[0081] A control device 50 serves to read the vibration sensor 420 and, based thereon, to control the vibration generator 410. The control device 50 comprises, on the one hand, a frequency generator 51 to generate the excitation signal V Afor the actuator 410. On the other hand, the control device 50 comprises a controller 52 which processes the sensor signals V s receives and on the basis of these sensor signals the frequency f and amplitude A of the excitation signal V A The electrical output and input signals of the vibration converter 40 are transmitted through electrical lines inside the clamping set 22. An inductive transmission device 53, shown only schematically, serves for the contactless transmission of the excitation signal V A from the control device 50 to the clamping set 22. A further transmission device 54 is used to transmit the sensor signals V sfrom the clamping set 22 to the control device 50. For example, the transmission can be effected inductively by two coils arranged concentrically around the workpiece axis, with one of the coils being arranged on the clamping device and the other coil on the stationary machine element. In particular, arrangements such as those known from references

[0031] or

[0032] in a different context can be used.

[0082] In the present example, the vibration generator 410 generates a torsional vibration about the workpiece axis C. The frequency of this torsional vibration is controlled by the controller 52 such that the unit consisting of the clamping set 22 and the workpiece 23 is resonantly excited. In the simplest case, the controller 52 controls the excitation frequency f such that the amplitude A measured by the vibration sensor 420 is maximized.

[0083] A standing wave is created with vibration nodes (i.e., locations where the amplitude of the torsional vibration is minimal) and vibration antinodes (i.e., locations where the amplitude of the torsional vibration has a local maximum) along the workpiece axis C. The amplitude distribution of this standing wave along the workpiece axis is illustrated in the left-hand part of Fig. 4 as amplitude distribution 61 on a radius R not equal to zero. The clamping set 22 is designed such that the standing wave has a vibration antinode in the region of the workpiece 23, while the vibration generator 410 is arranged close to a vibration node. In the present example, the vibration generator 410 is arranged close to the first vibration node above the gear 23, which contributes to a particularly large amplitude of the resulting torsional vibration at the vibration antinodes.In order to minimize the transmission of torsional vibrations to the workpiece spindle, the clamping set 22 is designed such that a vibration node is located at its proximal end 221, i.e. where the clamping set 22 is connected to the spindle shaft.

[0084] The geometric design of the clamping set 22 and the positioning of the vibration generator 410 such that resonant excitation can occur, with the vibration antinodes and nodes located at the desired locations, can be easily achieved using known vibration behavior simulation methods, in particular FEM simulations. Since the workpiece 23 forms part of the resonantly vibrating structure, the design of the clamping set 22 and the positioning of the vibration generator 410 are essentially workpiece-specific. However, a clamping set 22, once designed, can certainly be used for different workpieces 23 within certain limits by adjusting the excitation frequency f depending on the workpiece so that resonant excitation occurs.

[0085] Structure of a vibration converter

[0086] The structure of a suitable vibration transducer 40 is illustrated by way of example in Fig. 5. The vibration transducer 40 comprises a stack of flat, disc-shaped elements. The stack has two regions. A first, lower region forms a vibration generator 410, and a second, upper region forms a vibration sensor 420.

[0087] In the present example, the vibration generator 410 comprises two axially stacked, annular piezoelectric shear actuators 411, between which a central annular electrode 412 is arranged. An outer annular electrode 412 is arranged above the upper shear actuator and below the lower shear actuator at each of the two axial ends of the vibration generator 410. The two outer electrodes are electrically connected to each other. The shear actuators are both identically constructed but are arranged in a mirror image with respect to a horizontal plane passing through the central electrode 412.

[0088] Shear actuators are known from the state of the art. They exploit the fact that in many piezo materials the piezoelectric shear deformation coefficient d 15is not equal to zero. In the present case, the shear actuators 411 are designed to generate a shear deformation in the circumferential direction under the influence of an electric field running along the workpiece axis C. The direction of action of the actuators around their axis C can be achieved by a special polarization process in which the closed ring is polarized segment-by-segment over the circumference. Actuators that generate a shear deformation in the circumferential direction are described, for example, in references

[0033] -

[0036] .

[0089] When a voltage is applied between the outer electrodes and the central electrode, both shear actuators 411 generate a co-directional shear force along the circumferential direction. Overall, this creates a torque between the upper and lower ends of the oscillation generator 410. By driving the shear actuators with an alternating voltage, a torsional oscillation can be generated.

[0090] The vibration sensor 420 is fundamentally very similar in design. In this example, it has only a single annular piezoelectric element that generates an output voltage under the influence of a shear deformation between its bottom and top surfaces, acting in the circumferential direction.

[0091] The vibration generator 410 and the vibration sensor 420 are electrically insulated from each other by insulating discs 430. The end piece 224, which is designed as a preload nut, allows the stack of disc-shaped elements to be compressed along the axial direction with a compression force F c be applied.

[0092] Alternative vibration directions and arrangements

[0093] Figure 6 shows an alternative arrangement for exciting torsional vibrations. The vibration transducer 40 is located axially between the proximal end 221 of the clamping assembly 22 and the clamping area 223 for the workpiece 23. As in the previous example, the vibration transducer 40 is arranged in the region of a vibration node. In this embodiment, too, an axial preload can be generated on the vibration transducer 40 by means of a suitable preload device.

[0094] The arrangement in Fig. 7 is structurally similar to the arrangement in Fig. 4, and reference is made to the above explanations in this regard. Unlike in Fig. 4, however, the vibration transducer 40 is designed to primarily generate a longitudinal vibration 42 of the workpiece 23. Again, a resonant excitation occurs. On the left side of Fig. 7, the resulting amplitude distribution of the longitudinal vibration components 62 in the direction of the C-axis of the workpiece and the clamping set is sketched. Analogous to the embodiments in Figs. 4 and 6, the vibration transducer 40 is arranged in the region of a (here longitudinal) vibration node. In this embodiment, annular thickness actuators can be used as piezo actuators utilizing the piezoelectric longitudinal deformation coefficient d 33 be used.

[0095] In the embodiment of Fig. 8, a longitudinal vibration of the workpiece 23 is also excited. In this embodiment, the vibration transducer 40 is located in the region of a longitudinal vibration node below the workpiece 23, between the proximal end of the clamping set 22 and the clamping area for the workpiece.

[0096] In the embodiment of Fig. 9, a transverse (radial) oscillation 63 of the gear 23 is excited, wherein the amplitude of the oscillation is the same at all times over the entire circumference of the gear. Since a longitudinal oscillation is always accompanied by a transverse contraction, a longitudinally acting oscillation converter can be used, which causes the radial oscillation 63 in the region of the workpiece 23, as already explained above in the context of Fig. 3. On the left-hand side of Fig. 9, the longitudinal amplitude distribution 62 and the transverse amplitude distribution 63 are schematically sketched on a radius R not equal to zero. The oscillation converter 40 is arranged in the region of a longitudinal node. The workpiece 23 is arranged in the region of the adjacent longitudinal node and thus in the region of a transverse oscillation antinode.

[0097] In the embodiment of Fig. 10, a radial vibration 63 of the gear 23 is also excited by means of a longitudinally acting vibration converter 40. In this embodiment, the vibration converter 40 is arranged near a longitudinal vibration node between the proximal end of the clamping set 22 and the clamping point for the workpiece 23.

[0098] Variations

[0099] While the invention has been explained above using examples, the invention is not limited to these examples, and various modifications are possible without departing from the scope of the invention.

[0100] For example, other types of vibration generators than those explained above can be used. More than one vibration generator can also be used per clamping set. Likewise, a vibration generator can comprise one of two different numbers of actuators. For example, multiple vibration generators can be arranged at different axial locations to specifically generate specific vibration modes. The generated vibration modes can be more complex than in the examples explained above and, for example, include superpositions of longitudinal, radial, and torsional vibrations.

[0101] The same applies to vibration sensors. As already explained, multiple vibration sensors can be installed at different axial locations to more accurately characterize the type of vibration and its amplitude distribution.

[0102] If the vibration generators and / or vibration sensors comprise flat, disc-shaped piezoelectric elements, as in the above examples, several such elements can be stacked on top of one another with electrodes arranged between them. Instead of disc-shaped piezoelectric elements, piezoelectric elements of other shapes can also be used. In particular, ring-segment-shaped elements can be used, which are arranged as a whole to form a ring, or elements of any other shape can be used, which are arranged evenly distributed around the circumference of the clamping set.

[0103] literature

[0104] [1] H. Schriefer et al., “Continuous Generating Gear Grinding”, self-published by Reishauer AG, Wallisellen 2010, ISBN 978-3-033-02535-6, Chapter 2.3 (“Basic Methods of Generating Grinding”), pages 119 to 127

[0105] [2] US6379217B1

[0106] [3] DE885525C

[0107] [4] DE102007047891 A1

[0108] [5] DE10335376A1

[0109] [6] T. Tawakoli, B. Azarhoushang und M. Rabiey, „Ultrasonic assisted dry grinding of 42CrMo4“, Int J Adv Manuf Technol, Jg. 42, 9-10, S. 883-891 , 2009, doi: 10.1007 / S00170-008- 1646-7.

[0110] [7] H. Chen und J. Tang, „Influence of ultrasonic assisted grinding on Abbott-Firestone curve“, Int J Adv Manuf Technol, Jg. 86, 9-12, S. 2753-2757, 2016, doi: 10.1007 / S00170-016-8370-5.

[0111] [8] H. Chen, J. Tang, X. Lang, Y. Huang und Y. He, „Influences of dressing lead on surface roughness of ultrasonic-assisted grinding“, Int J Adv Manuf Technol, Jg. 71 , 9-12, S. 2011-2015, 2014, doi: 10.1007 / s00170-014-5636-7.

[0112] [9] H. Chen, J. Tang, W. Shao und B. Zhao, „An investigation on surface functional parameters in ultrasonic-assisted grinding of soft steel“, Int J Adv Manuf Technol, Jg. 97, 5-8, S. 2697-2702, 2018, doi: 10.1007 / s00170-018-2164-x.

[0113]

[0010] D.-H. Xiang, Z.-Y. Liu, Z.-K. Zhou und Y.-l. Yao, „Experimental study of single cubic boron nitride grains in the ultrasound-assisted high-speed grinding“, Advances in Mechanical Engineering, Jg. 9, Nr. 2, 168781401769354, 2017, doi: 10.1177 / 1687814017693543.

[0114]

[0011] S. Sun, J. Tang, W. Shao, C. Chen und Y. Liu, „Research on the matching relationship between ultrasonic-assisted grinding parameters and workpiece surface roughness“, Int J Adv Manuf Technol, Jg. 102, 1-4, S. 487-496, 2019, doi: 10.1007 / s00170-018- 3195-z.

[0115]

[0012] H. Chen und J. Tang, „A model for prediction of surface roughness in ultrasonic- assisted grinding“, Int J Adv Manuf Technol, Jg. 77, 1-4, S. 643-651, 2015, doi: 10.1007 / S00170-014-6482-3.

[0116]

[0013] W. Zhou, J. Tang, H. Chen und W. Shao, „A comprehensive investigation of surface generation and material removal characteristics in ultrasonic vibration assisted grinding“, International Journal of Mechanical Sciences, Jg. 156, S. 14-30, 2019, doi: 10.1016 / j.ijmecsci.2019.03.026.

[0117]

[0014] US5540614A

[0118]

[0015] H. Kitzig-Frank, T. Tawakoli und B. Azarhoushang, „Material removal mechanism in ultrasonic-assisted grinding of AI2O3 by single-grain scratch test“, I nt J Adv Manuf Technol, Jg. 91 , 9-12, S. 2949-2962, 2017, doi: 10.1007 / s00170-016-9967-4.

[0119]

[0016] DE102009017248A1

[0120]

[0017] US5165205A.

[0121]

[0018] JPS63278751A

[0122]

[0019] C. Brecher, R. Schug, A. Weber, C. Wenzel and S. Hannig, “New systematic and time-saving procedure to design cup grinding wheels for the application of ultrasonic-assisted grinding”, Int J Adv Manuf Technol, Vol. 47, 1-4, pp. 153-159, 2010, doi: 10.1007 / S00170-009-2204-7.

[0123]

[0020] T. Tawakoli and B. Azarhoushang, “Dry grinding of soft steel with use of ultrasonic vibrations”. The 6th International Conference on Manufacturing Research (ICMR08), ​​Brunel University, UK, 2008.

[0124]

[0021] T. Taghi and A. Bahman, "Developments in ultrasonic-assisted grinding for ceramic and metal II materials: processes and machines" in Yearbook Grinding, Honing, Lapping and Polishing: Processes and Machines, H.-W. Hoffmeister and B. Denkena, eds., 66th ed. Essen: Vulkan-Verl., 2013.

[0125]

[0022] DE102008054309A1

[0126]

[0023] G. Sun, F. Shi und Z. Ma, „Effects of axial ultrasonic vibration on grinding quality in peripheral grinding and end grinding of ULE“, Int J Adv Manuf Technol, Jg. 109, 7-8, S. 2285-2298, 2020, doi: 10.1007 / s00170-020-05761 -5.

[0127]

[0024] DE10060696 A1.

[0128]

[0025] H. Kitzig, T. Tawakoli und B. Azarhoushang, „A novel ultrasonic-assisted dressing method of electroplated grinding wheels via stationary diamond dresser“, I nt J Adv Manuf Technol, Jg. 86, 1-4, S. 487-494, 2016, doi: 10.1007 / s00170-015-8156-1.

[0129]

[0026] JP2002144150A

[0130]

[0027] JPH02124221A

[0131]

[0028] CN108746875A

[0132]

[0029] CN208913295U

[0030] US2013280990A1

[0133]

[0031] US7641537B2

[0134]

[0032] US2016217364A1

[0135]

[0033] Research report "High-performance machining of difficult-to-machine materials by hybrid ultrasonic-assisted machining (ULTRASPAN)", 2018.

[0136]

[0034] T. Morita et al., "A smooth impact rotation motor using a multi-layered torsional piezoelectric actuator", IEEE Trans Ultrason Ferroelectr Freq Control, 46(6), pages 1439-1445, 1999, doi: 10.1109 / 58.808867.

[0137]

[0035] AE Glazounov et al., "Torsional actuator based on mechanically amplified shear piezoelectric response", Sensors and Actuators 79, pages 22-30, 2000, doi:

[0138] 10.1016 / S0924-4247(99)00250-2.

[0139]

[0036] T. Harada et al., "Droplet generation using a torsional Langevin-type transducer and a micropore plate", Sensors and Actuators A: Physical, Volume 155, Issue 1, pages 168-174, 2009, doi: 10.1016 / j.sna.2009.08.007

Claims

PATENT CLAIMS 1. Method for machining a gear-shaped workpiece (23) by a gear-making process, in particular by continuous gear grinding, in which the gear-shaped workpiece (23) rotates about a workpiece axis (C) while in a rolling engagement with a gear-making tool (16), in particular with a helically profiled grinding tool, characterized in that the gear-shaped workpiece (23) is deliberately excited to an oscillating movement during machining, which is superimposed on the rotation of the gear-shaped workpiece (23).

2. The method of claim 1, wherein the oscillating motion has a component along at least one of the following directions of motion: axially along the workpiece axis (C); radially to the workpiece axis (C); and torsional about the workpiece axis (C).

3. Method according to claim 1 or 2, wherein the oscillating motion has a fundamental frequency above 15 kHz.

4. A method according to any of the preceding claims, wherein the gear-shaped workpiece (23) is clamped on a clamping device (22) during the gearing process, wherein the clamping device (22) is arranged on a workpiece spindle (21), wherein the clamping device (22) has a clamping area (223) at which the clamping device (22) is in contact with the gear-shaped workpiece (23), wherein the workpiece spindle (21) drives the gear-shaped workpiece (23) clamped on the clamping device (22) to rotation about the workpiece axis (C), and wherein the oscillating motion of the gear-shaped workpiece (23) is generated by exciting the clamping area (223) with the gear-shaped workpiece (23) in contact therewith to vibrate.

5. Method according to claim 4, wherein the excitation of the vibration is resonant, such that the vibration is a natural mode of the clamping device (22) with the gear-shaped workpiece (23) clamped thereon, wherein the natural mode is preferably has a vibration node in an area where the clamping device (22) is connected to the workpiece spindle (21).

6. Method according to claim 4 or 5, wherein the clamping device (22) has a vibration generator (410) for exciting the vibration.

7. Method according to claim 6, wherein the vibration generator (410) is supplied with an electrical excitation signal (V) A ) is supplied, wherein at least one electrical sensor signal (V s ) is determined, that the excitation signal (V) A ) caused oscillation, and where the excitation signal (V A ) based on the sensor signal (V s) is regulated in such a way that the excitation of the oscillation occurs resonantly.

8. Method according to claim 7, wherein the excitation signal (V A ) and the sensor signal (V s ) are transmitted without contact between the rotating clamping device (22) and a stationary control unit (50).

9. Clamping device (22) for clamping a gear-shaped workpiece (23) onto a workpiece spindle (21) of a gear-cutting machine, in particular a gear grinding machine, in order to drive the gear-shaped workpiece (23) to a rotation about a workpiece axis (C), wherein the clamping device (22) has a clamping area (223) in which the clamping device (22) is in contact with the gear-shaped workpiece (23), characterized in that the clamping device has a vibration generator (410) for generating a vibration of the clamping area (223) with the gear-shaped workpiece (23) in contact therewith.

10. Clamping device (22) according to claim 9, wherein the vibration generator (410) is configured to generate a vibration excitation along at least one of the following directions of movement: axially along the workpiece axis (C); radially to the workpiece axis (C); and torsional about the workpiece axis (C).

11. Clamping device (22) according to claim 9 or 10, wherein the vibration generator (410) comprises at least one piezo actuator.

12. Clamping device (22) according to claim 11, wherein the vibration generator (410) has at least one piezoelectric actuator element (411) which is disc-ring shaped or ring-segment shaped, wherein a ring axis of the piezoelectric actuator element (411) corresponds to the workpiece axis (C).

13. Clamping device (22) according to one of claims 9 to 12, wherein the clamping device (22) defines a proximal end and a distal end, wherein the proximal end is designed for connection with the workpiece spindle (21), and wherein the vibration generator (410) is arranged between the proximal end and the clamping area (223) or distal to the clamping area (223).

14. Clamping device (22) according to one of claims 9 to 13, further comprising a vibration sensor (420) for determining at least one sensor signal (7) S ), which characterizes the vibration of the clamping device (22) with the gear-shaped workpiece (23) clamped on it.

15. Clamping device (22) according to claim 14, wherein the vibration sensor (420) comprises a piezo sensor.

16. Clamping device (22) according to claim 15, wherein the vibration sensor (420) has at least one piezoelectric sensor element (421) which is disc-ring shaped or ring-segment shaped, wherein a ring axis of the piezoelectric sensor element (421) corresponds to the workpiece axis (C).

17. System for generating an oscillating motion of a gear-shaped workpiece (23) during machining by continuous gear grinding, comprising: a clamping device (22) according to one of claims 9 to 16; a frequency generator (51) for generating an excitation signal (V A ) for the vibration generator (410) to cause the vibration of the clamping device (22) with the gear-shaped workpiece (23) clamped thereon; and optionally a controller (52) configured to receive a sensor signal (V s ) to receive and use the sensor signal (V s ) the excitation signal (V A ) to regulate in this way, that the excitation of the oscillation occurs resonantly and with a predetermined amplitude.

18. System according to claim 17, further comprising: a transmission device (53, 54) for contactless transmission of the excitation signal (V A ) and optionally the sensor signal (V s ) between the rotating clamping device (22) and the control device (50).

19. A gear-cutting machine, in particular a gear grinding machine, comprising: a tool spindle (15) for driving a gear-cutting tool (16), in particular a helical-profile grinding tool, to a rotation about a tool axis (B); a workpiece spindle (21) for driving the gear-shaped workpiece (23) to a rotation about the workpiece axis (C); and a machine control configured to control the tool spindle (15) and the workpiece spindle (21) in such a way that a rolling coupling is established between the rotation of the gear-cutting tool (16) generated by the tool spindle and the rotation of the workpiece (23) generated by the workpiece spindle, characterized in that a clamping device (22) according to one of claims 9 to 18 is mounted on the workpiece spindle (21).

Images