Spindle equipped with a piezoelectric actuator

Abstract

A spindle (10) for performing non-ultrasonic axial oscillation-assisted machining, comprising a tool-holding shaft (12) and an exciter section for subjecting the shaft to a non-ultrasonic axial oscillation, particularly during rotation thereof, the exciter section comprising a first exciter stage comprising at least one piezoelectric actuator (24) and a second exciter stage comprising at least one piezoelectric actuator (22) having a non-zero axial overlap with the first exciter stage, wherein the actuators of the two stages are arranged to add up their respective actions.

Description

Technical Field

[0001] The present invention relates to a spindle for a machine tool or a robot.

Background Art

[0002] Machine tools, particularly those said to be numerically controlled, are provided with a movable support, especially movable in three directions, for holding an electric spindle, which is a spindle having a motor and a tool holder for rotationally driving it about its axis.

[0003] It has already been proposed to subject a rotationally driven tool to ultrasonic vibrations ("Ultrasonic Machining", i.e., UM) to increase the effectiveness of the tool and enable hard materials, such as glass, to be drilled. The ultrasonic vibrations make it possible to generate shear during cutting, but have no significant effect on the chip thickness and evacuation.

[0004] U.S. Patent Application Publication No. 2011 / 0222975 describes a UM module provided with a transducer configured to receive a tool. This transducer is powered by a rotating electrical contact.

[0005] U.S. Patent No. 3561462 describes another example of a UM spindle provided with an ultrasound exciter operating at a frequency on the order of 16 - 20 kHz, which also requires a rotating contact.

[0006] U.S. Patent No. 8240396 discloses another UM spindle provided with a rotating piezoelectric element, where the excitation frequency ranges from 17 - 60 kHz.

[0007] Furthermore, it is a known practice to use a piezoelectric actuator within the spindle to generate a variable preload in order to compensate for mechanical stresses that occur during operation in the rolling bearings due to expansion caused by the rising temperature during operation.

[0008] Thus, U.S. Patent No. 6,422,757 and European Patent Application Publication No. 1,004,783 disclose spindles with piezoelectric actuators for this purpose. The European Patent Application Publication No. 1,004,783 teaches using the piezoelectric actuator in combination with a cylinder to increase the movement of the axial compensation of the preload.

[0009] Furthermore, it is a known practice to subject the tool to axial oscillations during the drilling operation, the frequency of which is linked to the rotational frequency. Such axial oscillations are at a much lower frequency than in the case of ultrasonic machining (UM) and basically aim to subdivide the chips generated by the tool during rotation of the tool.

[0010] Conventionally, the frequency of these axial oscillations is between 5 Hz and 500 Hz, depending on the speed of rotation.

[0011] In International Publication No. WO 2017 / 087377, it has already been proposed to use a piezoelectric actuator to generate such oscillations. However, this application is brief about its implementation.

[0012] U.S. Patent Application Publication No. 2016 / 0129505 describes a system that enables the feed rate of the tool to be automatically generated as a function of the rotation of the tool, which comprises an electromechanical actuator that can be piezoelectric in order to superimpose axial oscillations on the feed movement. This application remains silent about the actual implementation of the piezoelectric actuator.

[0013] At present, the use of piezoelectric elements to reliably generate axial oscillations to subdivide the chips and to change the instantaneous trajectory of the tool presents many problems that must be solved in a way compatible with the industrial use of the spindle.

[0014] First of all, the piezoelectric element is relatively brittle and has to work under load, and therefore a suitable loading system has to be provided.

[0015] Secondly, the elongation rate of the piezoelectric element when electrically excited is relatively low on the order of 1 / 1000, whereas the amplitude of the axial oscillation necessary to obtain chip subdivision is not negligible. Therefore, it is necessary to find a means to obtain the required amplitude. Furthermore, a piezoelectric actuator of large length is at risk of buckling under high axial strain.

[0016] Furthermore, the thermal energy dissipated by the piezoelectric actuator at high frequencies is not negligible and has to be discharged.

[0017] Finally, the power supply of the piezoelectric element also has to be highly reliable over time and must not present maintenance problems, which is difficult.

[0018] Apart from the problems associated with the use of piezoelectric actuators, the spindle has to remain small enough to be used in numerically controlled machine tools and has to operate over the widest possible range of rotational speeds, preferably up to 15000 or 18000 rpm, so as to cope with the maximum number of uses.

[0019] At present, in the case of chip ejection, the frequency of the axial oscillation is linked to the rotational frequency and increases with it. However, at high frequencies, the inertia of the moving parts causes a significant increase in the power required to generate the axial oscillation.

[0020] As a result, there is a need for a spindle that enables the generation of axial oscillations using a piezoelectric element that can cope with such various constraints.

SUMMARY OF THE INVENTION

PROBLEM TO BE SOLVED BY THE INVENTION

[0021] Accordingly, the present invention aims to improve machining performance, and in particular to improve the discharge of the chips, by using a piezoelectric element to generate non-ultrasonic axial oscillations within a spindle (which is preferably an electric spindle).

MEANS FOR SOLVING THE PROBLEM

[0022] The present invention aims to address this need, and the subject matter according to its first aspect is a spindle for performing machining assisted by non-ultrasonic axial oscillations, comprising a tool holding shaft, and an exciter section for subjecting the shaft to non-ultrasonic axial oscillations, particularly during its rotation, wherein the exciter section comprises a first exciter stage comprising at least one piezoelectric actuator, a second exciter stage comprising at least one piezoelectric actuator having a non-zero axial overlap with the first exciter stage and, wherein the actuators of the two stages are arranged so as to add their respective actions, the spindle being as described above.

[0023] The two stages can be joined by a link member on which the actuators of each stage rest at one end, such that the actuator of one stage has one end resting on the link member fixed relative to the end of the actuator of the other stage, and also by resting on this link member, the elongation of the actuator of the first stage is added to the elongation of the actuator of the second stage.

[0024] In a preferred implementation of the present invention, the spindle is a main body, a front bearing that is at least partially axially movable relative to the main body, a rear bearing that is at least partially axially movable relative to the main body a tool holding shaft that is axially movable relative to the main body, wherein at least partial axial displacement of the plurality of bearings is accompanied by axial displacement of the tool holding shaft. between the intermediate link member and the main body arranged therein, their therein non-rotating operating, the first oscillator stage, among the plurality of bearings and one two and between the intermediate link member arranged therein, their therein non-rotating operating, the second oscillator stage is provided.

[0025] "Stationary" should be understood to mean not rotating relative to the main body, as opposed to the tool rotating relative to the main body. Thus, "stationary" is synonymous with "non-rotating" in this specification.

[0026] "Non-ultrasonic" means that the frequency of the axial rocking is well below the frequency of ultrasound-assisted machining ("Ultrasonic Machining"), and thus well below 16 kHz. The effect of the axial rocking is, in the present invention, to change the instantaneous trajectory of the tool in order to subdivide the chips.

[0027] "The front bearing" refers to the bearing closest to the tool. "Bearing" should be understood to mean one or more parts that ensure the function of guiding the rotation of the shaft. Thus, the bearing can include rolling bearings and supports for these rolling bearings to hold them relative to the body of the spindle.

[0028] "Non-zero axial overlap" should be understood to mean that the piezoelectric actuators occupy at least a group of common abscissas along the longitudinal axis of the spindle.

[0029] The two stages of the actuator can add their respective actions when electrically excited because the elongation of the actuator in one of the stages can be added to the elongation of the actuator in the other stage.

[0030] The actuator of the second stage can be axially interposed between one of the bearings and the intermediate link member. The first actuator can be axially interposed between the intermediate link member and the body.

[0031] The present invention enables a particularly sophisticated and effective solution to many of the problems described above presented by the use of piezoelectric elements within the spindle.

[0032] First of all, the axial overlap between the oscillator stages makes it possible to keep most of the longitudinal direction of the spindle in a state compatible with being used in a numerically controlled machine, while having a length of the piezoelectric actuator sufficient to obtain the amplitude required for the axial oscillation.

[0033] And the presence of the two stages makes it possible to excite only one at a high rotational frequency in order to reduce the number of moving parts and the corresponding inertia, and thus the required energy. Thereby, it becomes possible to generate axial oscillation at a higher frequency for a given power.

[0034] Also, the limited longitudinal bulk of the oscillator part according to the present invention also enables the easy incorporation of an electric motor into the spindle in order to make the spindle an electric spindle.

[0035] The body of the spindle can be formed by one or more parts assembled together.

[0036] Preferably, each stage of the oscillator part comprises several piezoelectric actuators, for example 2 to 4, especially 3, actuators. These actuators are advantageously identical and have a longitudinal axis parallel to each other and to the axis of rotation.

[0037] Preferably, the piezoelectric actuators of one stage alternate with the piezoelectric actuators of the other stage in a manner that is angularly equally distributed about the longitudinal axis of the spindle. Thereby, good dispersion of the load is enabled.

[0038] Preferably, the spindle comprises an axial preloading system for applying stress to the tool holding shaft with an axial displacement that is the reverse of the axial displacement caused by the excitation of the actuator. Thereby, it becomes possible to keep the piezoelectric element compressed.

[0039] This preloading system can be axially interposed between the body and one of the bearings, preferably the front bearing. As a variant, it is interposed between the body and the rear bearing.

[0040] The preloading system can comprise at least one elastic return member that acts by compression, for example, it can comprise several sets of elastic washers arranged in a recess of a part fixed relative to the body of the spindle. Each set can, at one end, rest on the bottom of the recess. The preloading system can comprise at least one tie rod that is connected, at one end, to one of the bearings, preferably the front bearing, and that, at the other end, rests on a set of washers on the side of the opening of the said recess. Thus, the sets of elastic washers act by compression to apply a stress to the bearing in the direction of compression of the piezoelectric actuator.

[0041] In a variant, the preloading system comprises springs that act by tension. Those springs are, for example, attached, at one end, at a point fixed relative to the body and, at the other end, to one of the bearings, preferably the front bearing, to apply a stress to the bearing so as to cause it to bear on the piezoelectric actuator and thus subject the latter to an axial compressive force.

[0042] The intermediate link member preferably comprises a plurality of recesses that open alternately towards one axial end of the spindle and towards the opposite end, where the plurality of piezoelectric actuators are received in the plurality of recesses. This link member can be a single piece or can be formed by the assembly of several parts. In each recess, the corresponding piezoelectric actuator can be axially placed on a part provided with a centering O-ring seal arranged at the bottom of the recess.

[0043] Preferably, at least one of the bearings, more preferably each bearing, is provided with at least one rolling bearing held by a flexible support that can bend under the action of the force generated by the piezoelectric actuator to allow axial displacement of the shaft, while the radial position of the rolling bearing is kept relatively invariant relative to the body.

[0044] The use of such a support is advantageous because it reduces the inertia of the parts that are axially moved to generate the axial rocking and limits the use of rolling bearings or other guiding means.

[0045] The flexible support preferably has a transverse, in particular direction transverse, perpendicular to the oriented, plate in the shape of structure. These plates are preferably introduced by cutting sheet metal. Their thickness is, for example, in the range of 0.1 to 1 mm, and their number is 10 to 50 per support.

[0046] The support can be provided in various forms, especially to increase flexibility and / or to allow the passage of tie rods, cables, or the flow of cooling air. Also, holes can be made in the support to provide higher flexibility.

[0047] If necessary, centering screws can be provided on the support to refine the centering of the support.

[0048] Each support can be fixed to the body at a fixed point on one side and, on the other side, fixed to the rolling bearing housing at a location angularly offset from the fixed point about the rotational axis of the shaft.

[0049] For example, the support generally has a triangular shape when viewed from the front, allowing the body of the spindle to be fixed at the apex of the triangle and the rolling bearing housing of the bearing to be fixed in the middle along each side of the triangle. In a variant, the support has a corrugated form in the circumferential direction.

[0050] Preferably, each bearing comprises two flexible supports arranged on both sides of the one or more rolling bearings.

[0051] Each bearing preferably comprises two ball bearings arranged side by side.

[0052] Each stage of the piezoelectric actuator preferably comprises 2 to 4 actuators, more preferably 3 actuators.

[0053] Each piezoelectric actuator can transmit its thrust via a flexible transmission part which, at at least one of its axial ends, has, for example, a T-shaped form with a widened part in cross-section, where the piezoelectric actuator bears on this flexible part-piece, in particular on the widened part of this part-piece. The elongated part of the part-piece allows pivoting due to its flexibility. As a variant, a transmission system with a spherical reach is used. The advantage of the T-shaped transmission part is its simplicity of machining compared to a spherical reach. The two ends of the actuator can be received, as mentioned above, in centering part-pieces provided with O-ring seals which press on the outer surface of the actuator.

[0054] Preferably, the piezoelectric actuator is cooled by the circulation of compressed air.

[0055] The spindle can comprise a spacer-forming sleeve surrounding the piezoelectric actuator, where the front bearing comes to rest axially on the sleeve. This simplifies the mounting of the bearing. This sleeve can have longitudinal grooves on its radially outer surface, in which the power supply cables connected to the actuator and / or the sensors engage. When the sleeve is made of metal, it can provide additional shielding against the electromagnetic radiation generated by the piezoelectric actuator, in particular against the cables connected to the sensors, for example, through the said grooves.

[0056] The spindle can be provided with a release system for acting on the clamping of the tool holder, where this release system is preferably arranged between the motor and the piezoelectric actuator. The release system can be pneumatic and can comprise several successive pressure chambers, for example three chambers, in order to obtain a greater operating force for a given pressure. It makes it possible to reduce the radial bulk of the release system and facilitates its installation between the oscillator stage and the motor. The arrangement of the motor behind such a release system simplifies the manufacture of the motor in which the shaft can be rigid except for the cutting oil intake channel, and this intake channel can also be manufactured more easily.

[0057] Another main subject of the invention according to another aspect of the invention is a drilling or machining method in which a tool is rotationally driven using the spindle according to the invention, where the tool is subjected to non-ultrasonic axial oscillations simultaneously with its rotation by periodically exciting the piezoelectric actuator.

[0058] The two stages of the actuator can be excited simultaneously. It makes it possible to have the maximum amplitude of the axial oscillations and the elongation of each actuator is concentrated.

[0059] Preferably, the actuator is excited by a sinusoidal voltage. The frequency of the axial oscillations can be between 0 and 350 Hz.

[0060] Each stage of the actuator can be powered by its own power stage.

[0061] Also, it is possible to excite a single stage of the actuator, in particular the one operating between the front bearing and the link member. It makes it possible to operate at a higher excitation frequency for a given power by reducing the inertia of the part that is axially moved during such oscillations.

[0062] In this case, the non-oscillating actuator stage can be powered off or powered at a fixed voltage selected to provide a more refined compensation of the rotor displacement.

[0063] The spindle can be mounted on a numerically controlled machine or effector.

[0064] And the spindle is customarily axially displaced during the drilling or the machining of the shape, where the feed per revolution of the spindle and the peak-to-peak amplitude of the axial oscillation are of the same order of magnitude (i.e., at most 10 times between them).

[0065] The method can be a method for drilling assisted by non-vibrating vibrations of a metal, in particular aluminum or titanium or other metals or materials, where the vibration parameters can be adjusted (amplitude and frequency).

[0066] The method can also be a machining of a shape, in particular a countersinking or a boring. In this case, the axial oscillation can be stopped before the end of the feed of the tool.

[0067] Also, for example, it is also possible to generate an axial oscillation without rotation in order to perform an amplitude measurement on the tool or to assist in its removal.

[0068] The present invention will be better understood by reading the following description of non-limiting examples of its implementation and considering the accompanying drawings.

Brief Description of the Drawings

[0069]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Mode for Carrying Out the Invention

[0070] Next, an example of the spindle 10 according to the present invention will be described with reference to FIG. 1.

[0071] Since the spindle 10 incorporates an electric motor 11, for example a synchronous motor having permanent magnets, in this specification, it is an electric spindle.

[0072] This motor 11 rotationally drives the shaft 12 by means of a link 14, which allows a certain axial clearance sufficient for the shaft to oscillate axially with an amplitude required along its longitudinal axis between the shaft and it.

[0073] The shaft 12 drives a tool holder 13 at the end opposite to the motor 11, and the tool holder 13 can receive any type of tool, for example, a drill bit or any other tool for machining of shapes, schematically represented in FIG. 1.

[0074] The interface between the tool holder and the tool can be of the clamping type and can be manufactured according to various standardized or self-developed standards, for example, ER or D type in particular.

[0075] The tool can also be held in the tool holder by means of hydraulic clamping, thermal shrinkage, mechanical shrinkage or transmission by obstacles, especially of the Weldon or Wissonnotch type.

[0076] The interface between the tool holder and the spindle can also be determined by self-developed standards or specifications, for example, of the HSK, BIG, CAPTO, BT or ISO (SK40) type, or a Morse taper can be used, where this listing is non-limiting.

[0077] The interface between the spindle and the numerically controlled machine can be made by clamping or any other suitable means.

[0078] The front bearing 15 and the rear bearing 16 guide the rotating shaft 12 while allowing axial movement.

[0079] These bearings 15 and 16 are axially movable at least on the shaft 12 relative to the body 19 of the spindle 10.

[0080] The latter includes a first oscillator stage 21 having several piezoelectric actuators 22 operating in parallel between the body 19 and a link member 18 that is movable relative to the body 19, and a second oscillator stage 23 having several piezoelectric actuators 24 operating between the link member 18 and one of the bearings, in this particular case the front bearing 15 considered in the example.

[0081] An elastic preloading system 30 is axially interposed between the body 19 and one of the bearings, in this particular case the front bearing 15, to maintain the piezoelectric actuators 22 and 24 with the necessary prestress.

[0082] As can be seen in FIG. 1, the link member 18 is arranged to allow a certain axial overlap of the piezoelectric actuators along the longitudinal axis X of the spindle 10.

[0083] The actuators 22 and 24 do not rotate relative to the body 19, which simplifies their power supply and makes it possible to avoid the use of rotating slip rings.

[0084] They are connected to a control system, which delivers a variable voltage to oscillate them in conjunction with the rotation of the shaft 12 so that the tool has a sufficiently accurate trajectory according to the required application.

[0085] The actuators 22 and 24 are powered, for example, by two respective amplifiers.

[0086] At low rotational frequencies, all of the actuators 22 and 24 are powered and their elongations are aggregated, which makes it possible to obtain the maximum amplitude of axial oscillation of the tool. The actuators 22 and 24 are preferably excited by sinusoidal voltage of frequencies and phases selected as a function of the rotational frequency of the tool and its angular phase.

[0087] At high rotational frequencies, only the second stage 23 closest to the front bearing is excited to reduce the inertia of the movable part, and the link member 18 becomes stationary in the axial direction.

[0088] The spindle 10 according to the present invention preferably satisfies the following relationships. The frequency of the axial rocking generated by the piezoelectric actuator: 0 to 500 Hz, more preferably 0 to 350 Hz, The rotational frequency ω of the tool rotation : 0 to 18000 rpm, The peak-to-peak amplitude of the axial rocking: 0 to 0.25 mm, more preferably 0.02 to 0.25 mm, Frequency ω axial oscillation / ω rotation The ratio of is preferably non-integer, for example 1 / 2, 3 / 2, 5 / 2, or 7 / 2. When f represents the feed per revolution, i.e., the axial displacement of the entire spindle with respect to a 360° rotation of the tool, preferably 1 / 10 f < a < 10 f, where a refers to the peak-to-peak amplitude of the axial rocking. f is preferably 0.01 to 0.5 mm.

[0089] The control system of the piezoelectric actuator can be configured to enable the start of the axial rocking over the entire machining phase or, as a variant, only over a part thereof, according to the method which is the subject of the applicant's European Patent No. 2790860, and the rocking is stopped at the end of the machining.

[0090] In the figure shown in FIG. 1, the bearings 15 and 16 are axially movable relative to the body 19. The bearings 15 and 16 can be manufactured to have a deformable part that allows axial displacement of the shaft while accurately guiding the rotating shaft as will be described later.

[0091] FIGS. 2 to 6 represent examples of spindles 10 manufactured according to the present invention.

[0092] This spindle 10 is an electric spindle, and its motor 11 is housed in a housing provided with separate fluid connections (liquid and / or gas) 70 and electrical connections 71, which ensure the cooling of the spindle, the power supply to the motor and the piezoelectric actuator, the control of the tool holder, and the exchange of signals with various sensors. In particular, in the example under consideration, the spindle is provided with an engine cooling water intake and outlet connection and a compressed air intake for cooling for the piezoelectric actuator.

[0093] In this example, the front bearing 15 comprises two ball bearings 80 and 81, which axially abut against the shoulder 82 of the shaft 12 at one end and axially abut against the housing 83 at the other end. The latter is fixed by screws 86 to two supports 84 arranged on both sides of the housing 83.

[0094] Each support 84 is formed by the superposition of metal sheets having a substantially triangular shape as given, for example, in FIG. 7, presenting great rigidity in the thickness direction and a certain flexibility in a direction perpendicular to the plane of the metal sheet. The screw 86 passes through an opening 90 present in the middle along each side of the support 84.

[0095] Also, the support 84 is also fixed by a screw 88 to an element 92 forming part of the body 19 of the spindle 10 through an opening 93 present at its vertex as visible in FIG. 7.

[0096] The flexibility of the support 84 allows the axial clearance of the housing 83 necessary to enable the axial rocking of the shaft 12, while keeping the housing 83 axially centered relative to the body 19 of the spindle 10.

[0097] The rear bearing 16 has a similar structure, with ball bearings 95 and 96 resting on the shoulder of the shaft 12 at one end and on the housing 97 at the opposite end.

[0098] Two supports 84 identical to the support of the front bearing are fixed to this housing 97 through the opening 90. The support 84 of the rear bearing is immobilized on an element 101 of the body of the spindle by a screw 102 engaged in the opening 93 of the support 84.

[0099] For the front bearing, the flexibility of the metal sheet of the support 84 allows the axial clearance of the rear bearing in the rolling bearings 95 and 96, while ensuring its centering.

[0100] The piezoelectric actuator 24 is adapted to rest via a transmission part 107 having a T-shaped axial cross-section, allowing the distribution of the load over the entire front face of the actuator and the compensation in case of misalignment of the front bearing with respect to the housing 83.

[0101] The actuator 24 of one of the stages is adapted to rest, at the rear, on the link member 18 via a part 110. The part 110 can, as shown, comprise a seal 111 for centering the actuator.

[0102] As can be seen especially in FIG. 5, the mounting of the actuator 22 of the other stage is similar, where the latter is adapted to rest, at the rear, on a bearing part 113 forming part of the body 19 of the spindle by the transmission part 107 and, at the front, on the link member 18. The bearing part 113 is hollow and houses, in the example under consideration, a preload system 30 consisting of a stack of Belleville washers through which respective tie rods 120 pass.

[0103] Each stack of elastic washers 230 is compressed, as can be seen especially in FIG. 4, between a nut 121 engaged with the tie rod 120 and the bottom 122 of a recess of the bearing part 113 receiving it.

[0104] Each tie rod 120 is adapted to rest on the front face of the housing 83 via a nut 123, thereby exerting a rearward tension on it.

[0105] Each tie rod freely passes through the support 84 through an opening made in the support between openings that serve as passages for the fixing screws mentioned above.

[0106] Thus, the elastic washer 230 ensures the necessary preload of the piezoelectric actuator.

[0107] A metal part 130 forming a spacer in the form of a sleeve is disposed around the link member 18 and is adapted to rest axially on the bearing part 113 at one end and on the fixing element 92 at the other end.

[0108] As shown in FIG. 8, this spacer forming part 130 can have longitudinal grooves 133 on its radially outer surface for passing the power supply cable of the piezoelectric actuator and / or a cable connected to a sensor.

[0109] These grooves 133 can also promote the circulation of cooling air within the spindle 10.

[0110] The shaft 12 is hollow, and the operating rod 140 of the tool holder mounted on the spindle 10 passes longitudinally therethrough. The rod 140 is also hollow so that cutting fluid can be conveyed to the tool.

[0111] The rod 140 is maintained in the locked position by a stack of elastic washers 142 and is adapted to rest axially on a shoulder 143 of the shaft 12.

[0112] When unlocking the tool (not shown), a forward thrust is exerted on the rod 140 by a pneumatic unlocking system 150, which comprises three chambers 151, 152, and 153 that aggregate their actions on a transmission part 154.

[0113] In the illustrated example, this unlocking system 150 is axially positioned between the exciter unit located in front of the spindle equipped with the piezoelectric actuator and the motor 11.

[0114] The latter includes a rotor 141 having a permanent magnet and a stator 142, and is cooled by a liquid.

[0115] The shaft of the rotor 141 is connected to the shaft 12 of the spindle 12 by an elastic coupling system 155, enabling torque transmission and an axial gap between the two shafts. This system 155 includes two metal parts having teeth that fit into each other in a keyed manner, with an elastomeric cushion between the teeth.

[0116] The motor 11 is equipped with a coder 156 at the rear, which enables accurate knowledge of the angular position of the shaft. The piezoelectric actuator is preferably driven in response to the information supplied by this coder.

[0117] The spindle 10 includes a cylindrical housing 160 divided into several parts, which is closed at the front by a front flange 161 and at the rear by a rear flange 157 that houses the coder 156.

[0118] Obviously, numerous changes can be made to the spindle 10 without departing from the framework of the present invention.

[0119] A spindle 10 of a variant in which the method of ensuring the preload of the piezoelectric actuator is different from that described above will next be described with reference to FIGS. 9 to 12.

[0120] In this example, the preload system 30 includes a coil spring 180, which acts by tension between a ring 181 that is axially displaced together with the rolling bearings 80, 81 of the front bearing and a tie rod 183 that is fixed relative to the body of the spindle 10.

[0121] The actuator 24 is seated on the front bearing through transmission with the spherical reach 184 at the front and on the link member 18 at the rear.

[0122] The latter is shown separately in FIG. 11.

[0123] In this specification, it takes the form of a single-piece part having a recess 200 opening towards the rear for receiving the piezoelectric actuator 22 and a recess 202 opening towards the front for receiving the actuator 24. A groove 203 is formed on the outside of the part for receiving the spring 180.

[0124] The front bearing and the rear bearing have a support 84 whose shape shown in FIG. 12 is different from that of the support 84 in the example described above.

[0125] The support 84 has a waveform shape and has an opening 210 through which a screw for axially immobilizing on the main body of the spindle 10 can pass, and an opening 211 through which a fixing screw for the rolling bearing housing passes.

[0126] The depression formed between the opening 210 and the opening 211 allows an electric cable to pass through if necessary.

[0127] In this example, the motor 11 (not shown) is located immediately behind the rear bearing, and the tool holder release system is located immediately behind the spindle 10, behind the motor 11.

[0128] Other modifications can be made to the spindle without departing from the framework of the present invention.

[0129] For example, the spindle is not provided with a motor and is driven by a pulley and a motor remote from the machine.

[0130] The spindle can be used on a robotic arm rather than a numerically controlled machine.

[0131] The ball bearing can be replaced by a roller bearing.

Claims

1. A spindle (10) for performing non-ultrasonic axial oscillation assisted machining, comprising: A tool holder shaft (12), and an exciter section for subjecting the tool holder shaft (12) to a non-ultrasonic axial oscillation; It is equipped with The exciter section is a first exciter stage comprising at least one piezoelectric actuator (22); a second exciter stage comprising at least one piezoelectric actuator (24) having a non-zero axial overlap with the first exciter stage; It is equipped with The spindle, wherein the piezoelectric actuator (22) of the first exciter stage and the piezoelectric actuator (24) of the second exciter stage are arranged to add up their respective effects.

2. The spindle, A main body (19), a front bearing (15) at least partially axially movable relative to said body (19); a rear bearing (16) at least partially axially movable relative to said body (19); the tool holder shaft (12) being axially movable relative to the body (19), where at least partial axial displacement of the bearings is accompanied by axial displacement of the tool holder shaft (12). the first exciter stage being disposed between an intermediate link member (18) and the body (19) and moving non-rotatably therebetween, wherein the intermediate link member (18) and the body (19) are in abutment with the first exciter stage; the second exciter stage disposed between one of the plurality of bearings and the intermediate link member (18) and non-rotatably moving therebetween, wherein the one of the plurality of bearings and the intermediate link member (18) are in abutment with the second exciter stage; The spindle of claim 1 , comprising:

3. 3. The spindle according to claim 1 or 2, wherein the piezoelectric actuators of one of the first and second exciter stages alternate with the piezoelectric actuators of the other of the first and second exciter stages in an angularly equidistant manner about a longitudinal axis (X) of the spindle.

4. The spindle of any one of claims 1 to 3, comprising an axial preload system (30) for stressing the tool holder shaft (12) with an axial displacement that is the inverse of the axial displacement caused by excitation of the piezoelectric actuators (24, 22).

5. The spindle of claim 4 , wherein the axial preload system operates in compression.

6. A spindle as claimed in claim 2 or any one of claims 3 to 5 which rely on claim 2, wherein the intermediate link member (18) has a plurality of recesses which open alternately towards one axial end of the spindle and towards the opposite axial end, and wherein the piezoelectric actuators (24, 22) are received in the plurality of recesses.

7. A spindle according to claim 2 or any one of claims 3 to 5 which depend on claim 2, wherein at least one of the or each bearing comprises a rolling bearing carried by a flexible support which is capable of yielding under the action of forces generated by the piezoelectric actuators (24, 22) to allow an axial displacement of the tool holder shaft (12), while the radial position of the rolling bearing remains unchanged relative to the body (19).

8. 8. A spindle according to claim 7, wherein the flexible support has a sheet-like overlapping structure, the plane of each sheet being oriented perpendicular to the axis of rotation of the tool-holding shaft (12).

9. 9. A spindle according to claim 7 or 8, wherein each of the flexible supports is fixed on the one hand to the body (19) at a fixed point and on the other hand to a rolling bearing housing at a location angularly offset from said fixed point around the rotation axis of the tool holder shaft (12).

10. Spindle according to any one of claims 7 to 9, wherein each bearing comprises two of said flexible supports arranged on either side of said rolling bearing or rolling bearings.

11. The spindle of any one of claims 1 to 10, wherein each of the first and second exciter stages comprises between 2 and 4 piezoelectric actuators.

12. Each of the piezoelectric actuators (24, 22) transmits its thrust through a flexible transmission part at at least one of its axial ends, where each of the piezoelectric actuators (24, 22) abuts against the front bearing (15) and the body respectively on the flexible transmission part. The spindle according to any one of claims 1 to 11.

13. The spindle according to any one of claims 1 to 12, wherein the piezoelectric actuator (24, 22) is cooled by the circulation of compressed air.

14. A spacer forming sleeve (130) surrounding the piezoelectric actuator (24, 22) is provided, where the front bearing (15) is axially mounted on the spacer forming sleeve. The spindle according to claim 2 or any one of claims 3 to 5 citing claim 2.

15. The spacer forming sleeve (130) has longitudinal grooves (133) on its radially outer surface, and a power supply cable for the piezoelectric actuator (24, 22) and / or connected to a sensor is engaged in the longitudinal grooves. The spindle according to claim 14.

16. The spindle includes a motor for driving the tool holding shaft (12), a tool holder driven by the tool holding shaft (12), and a release system for acting on the clamping of the tool holder, where the release system is disposed between the motor and the piezoelectric actuator (24, 22). The spindle according to any one of claims 1 to 15.

17. The spindle according to claim 16, wherein the release system is pneumatic and includes several consecutive pressure chambers (151, 152, 153).

18. A method of drilling or machining a shape, in which a tool is rotationally driven using the spindle (10) according to any one of claims 1 to 17, where the tool is subjected to non-ultrasonic axial oscillation simultaneously with its rotation by periodically exciting the piezoelectric actuator (24, 22). The method.

19. The method according to claim 18, wherein the first oscillator stage and the second oscillator stage are excited simultaneously.

20. The method according to claim 18, wherein only one of the first oscillator stage and the second oscillator stage is excited.

21. The method according to any one of claims 18 to 20, wherein the spindle is mounted on a numerically controlled machine or an effector. **Claim 22** The method according to any one of claims 18 to 21, wherein the frequency of the axial oscillation is 0 to 350 Hz. **Claim 23** The method according to any one of claims 18 to 22, wherein the spindle is displaced axially during the drilling or the contour machining, wherein the feed per revolution of the spindle and the peak-to-peak amplitude of the axial oscillation are of the same order of magnitude (mm).

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