METHOD FOR PRODUCING A ROTATIONAL SURFACE OF A WATCH COMPONENT

Description

[0001] The invention relates to a method for creating a surface of revolution for a watch component, more particularly a ceramic watch component. The invention also relates to a watch component obtained by such a manufacturing method. The invention further relates to a watch movement comprising such a watch component. Finally, the invention relates to a timepiece comprising such a watch component or such a watch movement.

[0002] Machining technical ceramic watch components such as those described for example in patent application EP3258325 is a delicate operation which requires mastery of the interactions between the tool and the material, in order not to induce stresses in the ceramic, nor new defects, particularly on the surface, which would prove to be crack initiations detrimental to the shock resistance of the components.

[0003] Manufacturing watch arbors with a diameter of less than 2 mm is particularly challenging, notably due to the presence of very small diameter pivots (< 200 microns, or even < 100 microns) at the ends of the arbor. These pivots must have a perfectly circular geometry and meticulously adhere to their dimensions. They are designed to cooperate with a bearing, and any geometric defect, such as out-of-roundness or an incorrect diameter, will result in a loss of chronometric performance for the movement in which the arbor is integrated. This is especially true for the balance staff. The same applies if the pivot surface is not perfectly polished.

[0004] This is why, according to the state of the art, the axles are machined by turning in steels, then the pivots are polished with shaped grinding wheels to obtain both the final roughness and shape.

[0005] However, this technique is difficult to apply to ceramic pieces: machining using cutting tools or grinding wheels is very time-consuming, and the tools wear out quickly.

[0006] An alternative to grinding ceramics is to machine them using femtosecond laser ablation. The lasers used have ultra-short pulses (femtosecond lasers) so as not to thermally affect the material being machined.

[0007] The equipment currently available, as used by experts in the field, for example for manufacturing hard metal electrodes or other small, generally metallic components (such as micro-tweezers), uses a laser emitting in the infrared (IR) range (wavelength of 1030 nm). It has not been possible to obtain satisfactory results with this type of equipment: the surface roughness obtained after machining is significant, requiring a very lengthy and specific tribofinishing process to achieve the desired roughness, and despite the geometric conformity of the parts, the flexural strength performance does not meet expectations.

[0008] The bending resistance of pivots with axes of identical geometry, produced by grinding or by laser-femtosecond machining according to the state of the art, was measured by applying a force using a knife at a given distance from the end of a pivot, the part being held in support at the level of a bearing surface making the interface between said pivot and the main body of the axis.

[0009] The bending resistance of pivots on different axes is represented by the figure 9 which indicates the stress levels at failure of the pivots of each of these axes. The measured stresses are correlated to the cross-sectional area of ​​each pivot at the point where the maximum force initiating failure is applied.

[0010] As illustrated on the figure 9 , the 3% yttria zirconia (ZRO 2 Y 2 O 3 3%) ground parts are thus more resistant than the 3% yttria zirconia (ZRO 2 Y 2 O 3 3%) laser machined parts according to the known state-of-the-art femtosecond laser machining process, whether they have subsequently been polished or polished and then tribofinished.

[0011] It therefore appears that watch axes made of ceramic machined by femtosecond laser may exhibit unsatisfactory behavior in use, even if they are dimensionally compliant.

[0012] The object of the invention is to provide a method for manufacturing a surface of revolution for a ceramic watch component, improving upon known prior art methods. In particular, the invention proposes a method for reproducibly manufacturing a reliable component, notably a balance staff, an escapement arbor, a seconds arbor, or any other arbor that is part of a finishing chain in a watch movement. The present invention also aims to accelerate and improve the reliability of ceramic machining to obtain arbor-type components with the required dimensions (diameter < 2 mm, tolerance to the micron) while ensuring adequate shock resistance.

[0013] According to the invention, a method of implementation is defined by claim 1.

[0014] Different modes of execution of the process are defined by claims 2 to 11. The accompanying figures illustrate, by way of example, one embodiment of a timepiece according to the invention. figure 1 is a schematic view of the embodiment of a timepiece, including an embodiment of a timepiece component. figure 2 is a graph representing the Ra roughness values ​​of surfaces obtained by different laser machining processes on various ceramics. figure 3 is a graph representing the evolution of the Ra roughness values ​​of surfaces obtained by different laser machining processes of 3% yttria-treated zirconia. figures 4a, 4b et 4c These are photographs of a 3% yttria-stabilized zirconia balance shaft pivot, respectively after IR laser machining (Ref. 1030), after 10 hours of tribofinishing following said machining, and after 20 hours of tribofinishing following said machining. figure 5 This is a graph representing the stress-at-failure values ​​of different axes of 3% yttria-stabilized zirconia balance wheels obtained by different types of laser machining. figures 6a et 6b These are photographs of a 3% yttria-stabilized zirconia balance shaft pivot, respectively after green laser machining (Ref. 515) and after 10 hours of tribofinishing following said machining. figures 7a et 7b These are photographs of a 3% yttria-stabilized zirconia balance shaft pivot, respectively after UV laser machining (Ref. 434) and after 10 hours of tribofinishing following said machining. figures 8a et 8b These are photographs of a pivot of a balance shaft made of 3% yttria-stabilized zirconia, respectively after grinding and after 10 hours of tribofinishing following said grinding. figure 9 is a graph representing the breaking stress values ​​of different axes of 3% yttria-stabilized zirconia balance wheels obtained by different manufacturing processes. figure 10 is a view of another embodiment of a watch component.

[0015] An embodiment of a timepiece 200 according to the invention is described below with reference to the figure 1 .

[0016] A timepiece number 200 is, for example, a wristwatch. A timepiece preferably includes a watch case and a watch movement number 100.

[0017] The watch movement can be a mechanical movement, including an automatic movement.

[0018] The watch movement includes an embodiment of a watch component 1.

[0019] The watch component is, for example, an axle, specifically a balance staff of a balance-spring type watch oscillator, an escapement axle, or a seconds axle. The diameter of the component, in particular its maximum diameter, is, for example, less than or equal to 2 mm.

[0020] The watch component preferably includes a pivot 10. By "pivot," we mean a portion of a component, particularly a shaft, designed to cooperate, especially through contact, with a bearing, particularly a pivot jewel. This pivot may have at least one portion of cylindrical, conical, or frustoconical geometry. The pivot is preferably located at one end of the shaft. The pivot may, for example, be formed as a continuation of a portion of the shaft comprising a curved generatrix surface.

[0021] The watch movement also includes a bearing 12, or even several bearings mounted on an element of the movement, in particular on a movement frame.

[0022] The watch component is, for example, designed to cooperate, particularly through contact, with the bearing 12. In particular, the watch component cooperates with the bearing 12 at the pivot 10. More specifically, the watch component cooperates with the bearing 12 at a surface 20 of revolution located at the pivot 10. The diameter of a cross-section of the surface 20 of revolution is, for example, less than or equal to 200 µm, or even less than or equal to 100 µm. Optionally, the pivot can be axially delimited by a counter-pivot 13 at an end 10a of the pivot 10. The elements 12 and 13 can be arranged within a single element in the form of a shock absorber body. In this particular case, the axis is preferably a balance staff or an escapement axle.

[0023] The bearing includes a bore. The bore and the surface of revolution are dimensioned and / or arranged to present a limited functional clearance, for example on the order of 7 to 17 µm.

[0024] Advantageously, the watch component includes two pivots 10 to cooperate with two bearings 12 so as to guide the watch component relative to the bearings and more generally relative to the watch movement element on which the bearings are mounted.

[0025] According to the invention, the watch component or a part thereof is made of ceramic. The surface of revolution 20 is made of ceramic; that is, the surface of revolution is formed or is located on a ceramic part of the component. Preferably, the ceramic is zirconia, in particular yttria-stabilized zirconia, especially 3% yttria-stabilized zirconia, monocrystalline alumina, or an alumina-zirconia (ATZ) combination.

[0026] Preferably, all or part of the watch component is made of ceramic and includes a surface of revolution 20 with a roughness Ra of 15 nm or less. Even more preferably, the watch component includes a pivot comprising said surface of revolution. Advantageously, the shear strength of the pivot of the component is greater than 200 N.mm⁻², or even greater than 250 N.mm⁻².

[0027] An execution method of a process for producing the surface 20 of revolution of the ceramic watch component 1 is described below.

[0028] The implementation process includes: a machining step with a femtosecond laser beam of a first surface of the watch component so as to obtain a second surface, in particular so as to obtain a second surface whose roughness Ra is less than 100 nm, or even less than 70 nm, then a tribofinishing step applied to the second surface so as to obtain said surface of revolution.

[0029] This process makes it possible to obtain watch components with a ceramic surface of revolution that is geometrically compliant and exhibits adequate roughness after a tribofinishing process of reasonable duration, typically around 10 hours. This tribofinishing step must be parameterized so as not to alter the geometry of the surface, in particular to avoid excessively rounding the end of the pivot(s) comprising said surface of revolution, in order to maintain control of the contact area between the pivot and the bearing. It is therefore essential that the initial roughness before this step be low enough to allow for a tribofinishing step of reasonable duration. The roughness Ra of the final surface of revolution obtained is, for example, on the order of 10 to 15 nm.

[0030] This process provides an alternative to grinding ceramics and involves machining them using femtosecond laser ablation without compromising their mechanical strength. The lasers used have ultrashort pulses (femtosecond lasers) so as not to thermally affect the material being machined, and are used at wavelengths specifically chosen to avoid altering the material's structure.

[0031] Advantageously, the process allows the realization not only of the surface 20 of revolution of the watch component 1, but of the entire surface of the pivot including said surface 20, or even of all the surfaces of the watch component.

[0032] Various tests were carried out on different balance staff materials, such as zirconia, yttria zirconia, single-crystal alumina (sapphire), or an alumina-zirconia combination (ATZ). Specifically, 3% yttria zirconia was used.

[0033] Preferably, the laser beam emits in the infrared with a wavelength between 800 nm and 1100 nm, ideally 1030 nm ± 5 nm or in the green with a wavelength between 500 nm and 540 nm, ideally 515 nm ± 2.55 nm or in the blue with a wavelength between 400 nm and 480 nm or in the ultraviolet with a wavelength less than 400 nm, ideally 343 nm ± 25 nm.

[0034] Preferably, the machining step includes a turning phase, that is, a machining step in which the watch component is rotated around an axis of rotation and in which the laser beam is movable relative to this axis of rotation. Any other kinematic approach is possible as long as the laser beam scans a surface of the component.

[0035] Preferably, the laser beam has an average energy per pulse (also called fluence) between 0.001 mJ and 2 mJ, preferably between 0.01 mJ and 0.5 mJ, or even between 0.04 and 0.05 mJ for the materials and dimensions tested.

[0036] The lateral overlap rate, i.e., perpendicular to the carriage direction or perpendicular to the component axis, is defined by the workpiece rotation speed and the laser frequency, and can be set between 0% and 99.9%. It is preferably between 20% and 99.9%, ideally between 99.6% and 99.8%.

[0037] The longitudinal overlap rate is defined by the beam scanning or carriage speed and the laser frequency, and can be set between 0 and 99.9%. It is preferably between 20 and 99.9%, ideally between 0% and 80.8%. It should be noted that a longitudinal overlap rate of 100% corresponds to parting off the workpiece.

[0038] Preferably, the laser beam scans the first surface along a helical trajectory.

[0039] Preferably, the laser beam is applied in directions tangent to the first surface, or in directions substantially tangent to the first surface. Alternatively, the laser beam can be applied in directions normal to the first surface, or in directions substantially normal to the first surface.

[0040] Preferably, the laser beam has a diameter between 5 µm and 100 µm, preferably between 10 µm and 60 µm and ideally between 15 µm and 25 µm for the materials and dimensions tested.

[0041] Preferably, the duration of the tribofinishing step is less than 20 hours, preferably less than 10 hours.

[0042] Preferably, the tribofinishing step includes a bulk polishing step. Preferably, the tribofinishing step includes the use of diamond abrasive particles with a size on the order of 1 µm. Preferably, the tribofinishing step includes the use of a ceramic carrier in the form of beads with a size between 125 and 250 µm, mixed with water and an additive.

[0043] Various tests were also carried out by varying the nature of the laser beam. In particular, several lasers operating in the infrared (IR, wavelength 1030 nm), green (wavelength 515 nm), or ultraviolet (UV, wavelength 343 nm) ranges were tested, using equipment that allowed for parameterizing the relative speeds of the laser beam scan and the rotation of the watch component. However, the parameters remained identical (indicated as "E5") for all the tests presented below, especially regarding the laser fluence, which was kept constant between the different beams, as well as the beam scan speeds and the rotation of the watch component.

[0044] As shown in the graph of the figure 2 The green (Ref. 515) and UV (Ref. 343) lasers allow for satisfactory surface roughness of around 50 nm after machining for various technical ceramics. The IR laser (Ref. 1030) allows for surface roughness of around 50 to 100 nm after machining for various technical ceramics, provided that the equipment configuration allows for optimization of parameters compared to state-of-the-art equipment and parameters (Ref. 1030-C).

[0045] As indicated on the figure 3 Watch components made of 3% yttria zirconia, machined with green (Ref. 515), UV (Ref. 343), and IR (Ref. 1030) lasers and tribofinished by mechanical stressing with an abrasive mixture, vibrated in a tank, using diamond abrasive particles approximately 1 µm in diameter combined with a ceramic carrier in the form of beads with diameters between 125 µm and 250 µm, with the addition of water and an additive, can all achieve the appropriate final roughness of 10 to 15 nm. This appropriate final roughness is achieved after 10 hours of tribofinishing using watch components machined with green (Ref. 515) and UV (Ref. 343) lasers. Twenty hours of polishing are required on the machined watch components using the IR laser (Ref. 1030) to achieve equivalent roughness. This results in, as can be seen on the figures 4a, 4b et 4c (representing a balance staff pivot in 3% yttria zirconia respectively after IR laser machining (Ref. 1030), after 10 hours of tribofinishing following said machining and after 20 hours of tribofinishing following said machining), a modification of the dimensions and geometry of the components, particularly at the pivot ends, thus affecting the operation of the watch component. figures 6a et 7a These represent the appearance of a 3% yttria zirconia post immediately after the machining stage, respectively with a green laser and with a UV laser. figures 6b et 7b represent the appearance of a 3% yttria zirconia post after the tribofinishing step applied respectively to posts as illustrated in figures 6a et 7a The final roughness of the components illustrated on the figures 4c , 6b et 7b are satisfactory, but the end of the pivot of the figure 4c is rounded; this change in geometry will influence the chronometric performance of the watch equipped with such a pivot.

[0046] The results of the observations and dimensional measurements were confirmed by flexural strength measurements to ensure that the function of the axis-type watch components could be fulfilled.

[0047] As shown in the graph of the figure 5 The bending resistance of the pivots, measured by the force applied by a knife at a fixed distance from the pivot end and related to the cross-sectional area of ​​the pivot at the point of knife application, is greater for watch components machined with a green laser (Ref. 515) compared to watch components machined with an IR laser (Ref. 1030) or a UV laser (Ref. 343). This test was performed on watch components made of 3% yttria-stabilized zirconia.

[0048] It therefore seems advantageous to work in the green light range (515 nm) or UV (343 nm) to obtain the lowest possible roughness at the machining output and the greatest flexural strength.

[0049] To simplify and clarify the description, we call the component produced by the implementation of the process whatever its stage of realization, that is to say we call the component before laser machining, after laser machining or even after tribofinishing, although the component undergoes modifications during the process.

[0050] The surface of revolution 20 is described here specifically at the pivot of a balance staff, but any other surface of revolution of a watch movement axis for which a particular geometry and / or roughness must be achieved can also be produced using the same method. In particular, the method described above allows for the production of an escapement axis, a seconds axis, or any other axis that is part of a finishing chain in a watch movement.

[0051] Such a shaft may include a first toothed portion 30. This portion 30 may be formed from the same material as the shaft. Thus, the shaft may take the form of a shaft-mounted pinion. Such a shaft-mounted pinion may include a second portion 40 for receiving a wheel plate. In particular, such a shaft-mounted pinion may be made entirely of ceramic.

[0052] Alternatively, the first toothed portion 30 can be attached to a third receiving portion of the shaft, for example by pressing. In this case, the first toothed portion 30 can be metallic.

[0053] The maximum diameter of the first toothed portion 30 is, for example, less than or equal to 2 mm. The teeth of the first portion 30 may have a conventional profile, such as a Treybal profile.

[0054] Compared to current state-of-the-art methods that rely on cutting tools, the use of a femtosecond laser beam (green or UV) offers time savings and optimizes the properties of the final component, provided certain machining conditions are met. Furthermore, it is possible that a femtosecond laser beam operating in the infrared (IR) range could also achieve satisfactory results under certain conditions.

[0055] Throughout this document, "ceramic" means a homogeneous or substantially homogeneous material, including at the microscopic level.

[0056] Preferably, the ceramic is homogeneous in at least one direction, or even in all directions, over a distance greater than 6 µm, or even greater than 10 µm, or even greater than 20 µm.

[0057] Preferably, the ceramic does not have any non-ceramic material in at least one direction, or even in all directions, over a distance greater than 6 µm, or even greater than 10 µm, or even greater than 20 µm.

[0058] Preferably, the fact that the component and / or part of a component is ceramic means that this component or part is entirely ceramic. Even more preferably, the fact that the component and / or part of a component is ceramic precludes the component or part of a component from being made of a material composed of ceramic grains bonded together by a non-ceramic matrix, such as a metallic matrix.

[0059] The described process is particularly well suited to the manufacture of a ceramic component or part of a component. Although not part of the invention as defined by the claims, a metallic component, for example a component, in particular a shaft, made of tungsten or tantalum or any other hard metal that is difficult to machine by conventional means, could benefit from such a process.

Claims

1. A method for producing a surface of revolution (20) of a clock or watch component (1), the surface of revolution being made of ceramic, and characterized in that the method comprises: - a step of machining, with a femtosecond laser beam, a first surface of the clock or watch component so as to obtain a second surface, in particular so as to obtain a second surface whose roughness Ra is less than 100 nm, or less than 70 nm, and then - a tribofinishing step applied to the second surface so as to obtain said surface of revolution.

2. The method as claimed in claim 1, wherein the clock or watch component is an arbor, notably a balance staff or an arbor of an escapement wheel or an arbor of a seconds wheel, and / or wherein the diameter of the component is less than or equal to 2 mm.

3. The method as claimed in one of the preceding claims, wherein the surface of revolution is a surface of a pivot of the clock or watch component and / or wherein the diameter of the surface of revolution is less than or equal to 200 µm, or even less than 100 µm.

4. The method as claimed in the preceding claim, wherein the ceramic is a zirconia, an yttriated zirconia, notably a 3% yttriated zirconia, a monocrystalline alumina or an alumina-zirconia combination.

5. The method as claimed in one of the preceding claims, wherein the machining step comprises a turning phase.

6. The method as claimed in one of the preceding claims, wherein the laser beam is an infrared laser beam, notably an infrared laser beam having a wavelength between 800 nm and 1100 nm, in particular a wavelength of 1030 nm ± 5 nm, or a green laser beam, notably a green laser beam having a wavelength between 500 nm and 540 nm, in particular a wavelength of 515 nm ± 2.55 nm, or an ultraviolet laser beam, notably an ultraviolet laser beam having a wavelength under 400 nm, in particular a wavelength of 343 nm ± 25 nm, or a blue laser beam, notably a blue laser beam having a wavelength between 400 nm and 480 nm.

7. The method as claimed in one of the preceding claims, wherein the laser beam has an energy between 0.001 mJ and 2 mJ, preferably between 0.01 mJ and 0.5 mJ, or even between 0.04 and 0.05 mJ.

8. The method as claimed in one of the preceding claims, wherein the laser beam scans the first surface following a helical trajectory.

9. The method as claimed in one of the preceding claims, wherein the laser beam has a diameter between 5 µm and 100 µm, preferably between 10 µm and 60 µm, or even between 15 µm and 25 µm.

10. The method as claimed in one of the preceding claims, wherein the tribofinishing step takes less than 20 hours, preferably less than 10 hours.

11. The method as claimed in one of the preceding claims, wherein the tribofinishing step comprises a step of bulk polishing and / or comprises the use of diamond abrasive particles and / or the size of which is of the order of 1 µm.