Method for producing a clock pivoting shaft and clock pivoting shaft

Abstract

The present invention relates to a method for producing a balance shaft made of non-magnetic and electrically conductive material by means of a machine comprising a rotary gripping member (1) and a wire EDM machining system (2) mounted on a numerically controlled actuator (3). The method comprises the following steps: - providing a bar (4) of said material extending along a longitudinal axis (A), - arranging the bar (4) along its longitudinal axis (A) on the rotary gripping member (1), - shaping the bar (4) by EDM machining to obtain a balance shaft comprising a pivot at each end, this step comprising continuously rotating the bar (4) around its longitudinal axis (A) for shaping said pivots - optionally carrying out a tribological finishing treatment.The invention also relates to a balance shaft, in particular made from a material chosen from the families of austenitic stainless steels, metallic glasses, tungsten-based alloys, intermetallic materials or even conductive ceramics.

Description

Technical field

[0001] The present invention relates to the field of watchmaking. It relates, more particularly, to a method for producing a watch pivot shaft and more particularly to a method for producing a balance shaft. State of the art

[0002] In a mechanical watch movement, the escapement and the balance spring are important elements in determining the regularity of the rate. The balance spring is assembled on a shaft, called a balance shaft, pivoting in bearings. The escapement generally includes an anchor mounted on a shaft, called an anchor tigeron, and an escape wheel mounted on an escape pinion. The gear train, allowing the transmission of energy from the barrel to the escapement, also includes numerous pivoting shafts. The quality of the pivoting has a great influence on the performance of the movement, in terms of precision, efficiency and durability in particular.

[0003] The industrial production of watch pivot shafts, and more specifically balance shafts, is particularly complex. Indeed, the shaft is turned from a bar, the material of which must be soft enough to be machined precisely and quickly, without causing premature wear of the cutting tools. But the areas interacting with the bearings, called pivots, must also be hard enough to resist wear during operation. In addition, the surface condition of the pivots must be particularly worked and is currently obtained by rolling operations, which require a material with a hardness of at least 600Hv.

[0004] These rolling operations not only allow the desired surface finish to be obtained, but also the required shapes for the pivots with well-controlled dimensional precision, with a tolerance of the order of a micron. The turning is therefore carried out according to intermediate dimensions, which take into account the material removed during rolling. On the figure 2 , we have represented along line 10, the shape of a shaft resulting from bar turning, and along line 12, its shape after rolling. Rolling makes it possible to obtain the desired geometry for the pivots. Optionally, an additional tribological finishing operation, such as polishing, is still carried out. The latter causes additional abrasion of material and makes it possible to obtain the dimensions along line 14, which are the final dimensions.

[0005] The material chosen must therefore be easily machined, and then be able to be hardened by quenching or tempering treatments. The machinability of a material can be directly correlated to its specific cutting force, measured by manufacturers by the parameter Kc, generally given in N / mm 2< or MPa. Carbon martensitic steels including lead and manganese sulfides are generally used to improve their machinability. A steel of this type used in watchmaking is known under the reference 20AP.

[0006] Although satisfactory in terms of machinability and surface finish, the steels currently used have the disadvantage of being sensitive to exposure to a magnetic field, which can then disrupt the running of the watch.

[0007] Currently, watchmakers are therefore looking for alternative alloys with properties similar to those of martensitic steels, namely good machinability, a capacity for hardening by various treatments, and which are as non-magnetic as possible. One of the aims of these attempts is to preserve the machining techniques mastered until now, to reproduce the proven and satisfactory pivot shapes.

[0008] Unlike the approach currently followed, the present invention aims to change the paradigm and aims to propose a new method for producing a watch pivot shaft, particularly a balance shaft. Disclosure of the invention

[0009] More specifically, a first aspect of the invention relates to a method for producing a watch pivot shaft, for example a balance shaft, from a non-magnetic and electrically conductive material. The method is carried out using a machine comprising a rotary gripping member and a wire EDM machining system mounted on a numerically controlled actuator. The method according to the invention comprises the following steps: a. providing a bar of said material extending along a longitudinal axis, b. arranging the bar along its longitudinal axis on the rotary gripping member, c. shaping the bar by wire EDM machining to obtain a pivot shaft comprising a pivot at each end, step c. comprising continuously rotating the bar around its longitudinal axis to shape said pivots. d. Optionally, carrying out a tribological finishing treatment.

[0010] The questioning of traditional machining techniques (turning, rolling) and the use of a new technique, with unsuspected possibilities, makes it possible to explore new fields and new possibilities in terms of materials.

[0011] Another aspect of the invention relates to a balance shaft made from a non-magnetic alloy chosen from the family of austenitic stainless steels, metallic glasses (Bulk Metallic Glass or BMG in English), tungsten-based alloys (tungsten carbide with nickel binder for example), intermetallic materials or even conductive ceramics. Brief description of the drawings

[0012] There figure 1 represents a schematic view of a machine comprising a rotating gripping member and a wire EDM machining system mounted on a numerically controlled actuator. figure 1also shows a bar arranged on the rotating gripping member.

[0013] There figure 2 attached represents the geometries of a balance shaft during its production using a traditional process, while the Figures 3a, 3b and 3c propose balance shaft geometries made by the method according to exemplary embodiments of the invention. Method of carrying out the invention

[0014] The invention proposes to use a new manufacturing technique, based on a numerically controlled machine, the actuator of which is equipped with a wire EDM machining system. Electro-erosion, also called EDM (electrical discharge machining in English), is a machining process which consists of removing material from a part using electrical discharges. There are different types of electro-erosion including: Die-sinking EDM, in which an electrode with a shape complementary to the shape to be machined is driven into the part. This technique is generally not suitable for machining precision parts in series because the shaped electrode exhibits significant wear. Empirically, we can consider that the material removal during die-sinking machining is divided into 80% material removal on the part to be machined and 20% material removal on the electrode, i.e. wear of the electrode. Wire EDM, where a conductive wire driven by a plane and / or angular movement cuts a part along a ruled surface.

[0015] The numerically controlled machine also comprises a rotating gripping member intended to receive a bar in a material to be machined. The bar extends along a longitudinal axis. Typically, the turning bars for the production of balance shafts are cylindrical and have a diameter between 1.5 mm and 3 mm, preferably 2 mm; and a length between 1 m and 3 m, preferably 2 m. The bar is mounted on the rotating gripping member along its longitudinal axis, that is to say that the rotation of the bar is done around its longitudinal axis like a traditional turning machine. This arrangement makes it possible to produce watch pivot shafts by electroerosion machining.

[0016] There figure 1represents a schematic view of a machine comprising a rotary gripping member 1 and a wire EDM machining system 2 mounted on a numerically controlled actuator 3, as well as a bar 4 arranged on the rotary gripping member 1. The bar 4 extends along a longitudinal axis A.

[0017] According to the invention, a watch pivot shaft is produced, comprising a pivot at each end, with a wire EDM machining system. The wire 2 has, for example, a diameter of 0.02mm to 0.30mm, preferably a diameter of 0.02mm to 0.25mm, even more preferably a diameter of 0.02mm to 0.15mm. Advantageously, the wire 2 may be made of a material chosen from the group consisting of: Brass CuZn36, CuZn37, CuZn40 without coating Brass CuZn36, CuZn37, CuZn40 with coating Zn, Cu 5 Zn 8 , Cu Coated copper CuZn50, Cu 5 Zn 8 Coated steel CuZn40 Copper alloy Nickel titanium alloy Coated tungsten.

[0018] According to a first embodiment, the wire EDM machining system 2 is used to machine a part from a bar 4 set in continuous rotation in a rotary gripping member 1, like a lathe. The continuous rotation of the bar 4 makes it possible to create a part consisting of different surfaces of revolution. Among these surfaces of revolution we can find pivoting zones or bearing surfaces allowing the assembly of different components. For a balance shaft, these bearing surfaces make it possible to assemble the balance rim, the plate or the collet for example.

[0019] According to a second embodiment, certain machining operations on the shaft are carried out with incremental (non-continuous) rotation. In this case, during wire EDM machining 2, the bar 4 is not driven in continuous rotation. The bar 4 can then rotate through a given angle and be machined again by wire EDM 2. The parts of the shaft produced in this way then have one or more flats or a polygonal surface. For example, a first rough machining step can be carried out by machining the bar 4 times at 90°, 6 times at 60°, 10 times at 36° or more generally X times at Y°, with X*Y° =360°, in order to remove a significant amount of material. The presence of incremental rotation machining makes it possible to structure the bar. This incremental rotation machining can concern the bar over its entire length, with the exception of the ends, or one or more parts of the bar.It is thus possible to prepare a shaft whose ends are machined by continuous rotation and the longitudinal part machined by incremental and / or continuous rotation. The shaft resulting from this machining can therefore include a part having a square, octagonal section, or flats. Incremental rotation can consist of carrying out a first machining without rotation, rotating the bar without machining, carrying out a second machining without rotation until the desired shape is obtained. Advantageously, an additional step of continuous rotation EDM machining is then implemented on the bearing surfaces requiring a surface of revolution, for example on the pivots. Some bearing surfaces can remain polygonal, in particular non-functional surfaces or assembly surfaces (the driving out of the plate or the ferrule can be done on a polygonal surface for example). Thus, step c.of the process may include one or more electro-erosion machining operations. The presence of several electro-erosion machining operations may in particular make it possible to produce a pivot shaft having one or more flats or a polygonal shape. Step c. therefore includes continuous rotation machining and, optionally, incremental rotation machining which may be followed by another continuous rotation machining operation.

[0020] Advantageously, the use of a machine comprising a wire EDM machining system 2 mounted on a numerically controlled actuator 3 combined with a rotary gripping member 1, makes it possible to produce any shape on the shaft, including shapes which are not of revolution, provided that the rotation of the bar 4 is incremental.

[0021] Thanks to the directions that the wire can take, it is possible to consider making shafts and pivots with conventional shapes, i.e. shapes currently produced by bar turning and rolling. Other shapes are also possible, which would not be possible by bar turning or rolling. The targeted geometries are obtained directly during step c. of wire EDM machining.

[0022] For example, the figure 3 gives three possibilities, respectively to Figures 3a, 3b and 3c .

[0023] The end of the pivot illustrated in 3a is flat. It is described as a right circular cylinder of diameter D (axial symmetry). The base of this cylinder has a 90° circular arc of radius A, between 5 and 25 um, starting from the generator of the cylinder. Tangentially and continuously, a radius of curvature R greater than 0.5 mm, extends radius A to the center of the base of the cylinder.

[0024] The end of the pivot illustrated in 3b is convex. It is described as a right circular cylinder of diameter D (axial symmetry). The base of this cylinder has a 90° circular arc, of radius A between 5 and 25 um, starting from the generator of the cylinder. Tangentially and continuously, a radius of curvature R between 0.1 and 0.5 mm, more generally between D / 2 and 0.5 mm, extends radius A to the center of the base of the cylinder.

[0025] The end of the pivot illustrated in 3c includes a concave notch E. The pivot is described as a right circular cylinder of diameter D (axial symmetry). The base of this cylinder has an arc of a circle of approximately 90°, with a radius A of between 5 and 25 μm, starting from the generator of the cylinder. The pivot end has a generally convex shape (presence of a radius at the pivot end) with concave machining. This machining is, for example, a notch E produced by the EDM wire 2 on the pivot end. This shape, which is not a shape of revolution, makes it possible to modify the contact point of the pivot when the pivoting is done at the pivot end.

[0026] The watch pivot shaft (advantageously balance shaft) has a pivot diameter D preferably between 0.03mm and 0.20mm, advantageously between 0.04mm and 0.09mm.

[0027] The examples given above are not limiting and the shapes described between the cylindrical part and the flat part, respectively convex, respectively concave, can be adapted, as well as the radii of curvature of the convex, respectively concave parts. In addition, the pivot can have a slightly conical shape instead of the cylindrical part.

[0028] Particularly advantageously and unexpectedly, it was observed that this wire EDM turning technology made it possible to obtain very good process control and manufacture parts in the dimensions used for watch pivots, with not only very reduced tolerances but also remarkable surface finishes.

[0029] As for the surface conditions obtained, they are similar or even better than those obtained after conventional bar turning. At the end of bar turning, a roughness Ra is measured advantageously between 0.08 µm and 0.15 µm after wire EDM machining, whereas it is generally 0.2 µm after conventional bar turning. The quality of the surface condition can make it possible to do without subsequent rolling. The method according to the invention can therefore be without a rolling step after wire EDM machining.

[0030] A rolling step may however be provided after the wire EDM machining step. This rolling step may be particularly useful when the wire EDM machining machine 2 has a single rotary gripping member 1 and the shaft is separated by cutting from the rest of the bar 4, this cutting being carried out at a pivot of the shaft. Generally speaking, the surface condition at the cut surface is quite complex to control and a reworking of the cut surface by rolling on a dedicated machine may therefore be provided. Alternatively, a reworking of the cut surface by EDM, after having turned and replaced the shaft in the rotary gripping member 1, may also be implemented.

[0031] An additional tribological finishing / polishing step can be planned to obtain a surface finish that meets the required specifications, for example to further refine the surface finish. This type of operation is typically carried out in bulk, by stirring in a medium loaded with abrasive particles. A large number of parts can therefore be treated very efficiently. No change in geometry is observed during this operation, but only a homogeneous reduction in dimensions, of a few um, depending on the duration of the operation as well as a reduction in roughness.

[0032] Even more interestingly, tests were carried out with materials not used for this type of application, these materials being known for their poor machinability with traditional tools, such as bar turning chisels. Indeed, machining these materials leads to rapid wear of carbide cutting tools, which makes them unusable industrially. In addition, given the small dimensions of the shafts and pivots, the mechanical constraints imposed during bar turning make it almost impossible to produce parts in this type of material.

[0033] The material used in the wire EDM process must be electrically conductive in order to be suitable for EDM machining. Advantageously, the material has an electrical conductivity greater than 1 Sm -1< (1 Siemens per meter), even more advantageously greater than 100 Sm -1< .

[0034] The appropriate material is chosen from non-magnetic metals, advantageously austenitic and electrically conductive stainless steels, titanium or titanium alloys, metallic glasses (Bulk Metallic Glass or BMG in English), or even tungsten-based alloys.

[0035] It is also possible to choose a non-magnetic and electrically conductive material from: Intermetallic materials Conductive ceramics, for example, materials consisting of a ceramic base (e.g., ZrO 2 , Si 3 N 4 , Al 2 O 3 ), enriched with a dispersed conductive phase (e.g., TiC, TiN, TiCN, WC, or TiB 2 ). Metal matrix composites, for example, tungsten carbide with a nickel binder Silicon, for example, boron- or phosphorus-doped silicon Titanium carbide (TiC)

[0036] These materials have the advantage of being non-magnetic and electrically conductive.

[0037] Thus, the invention also relates to a watch pivot shaft, for example a balance shaft made of such a non-magnetic and electrically conductive material. Although this is not excluded, the shaft according to the invention does not need to receive an external coating to modify its tribological properties.

[0038] Advantageously, said material has a hardness (before and after wire EDM machining) greater than 450HV, preferably greater than 600HV. Advantageously, said material has a hardness (before and after wire EDM machining) between 450HV and 2000HV, even more advantageously between 600HV and 1200HV.

[0039] When the chosen material is a metal alloy, the latter can possibly be heat treated, for example to modify certain properties, such as its hardness. These heat treatments, called aging for this type of alloy, are advantageously carried out before wire EDM machining, which makes it possible to avoid any dimensional changes that may occur during heat treatments.

[0040] The alloys mentioned being particularly hard, possibly after an ad-hoc heat treatment, one can therefore do without hardening steps generally implemented after machining. Indeed, with traditional bar turning, heat treatment operations such as quenching or tempering are carried out after machining, which requires taking into account the thermal stresses and deformations undergone.

[0041] Thanks to the possibilities offered by the machining according to the invention and thanks to the precision that this technique makes it possible to obtain, it is therefore possible to machine the watch pivot shaft (advantageously a balance shaft), including any pivots, with the final geometry. A simple tribological finishing or polishing step is preferably carried out, while retaining the defined geometry, that is to say while retaining the angles or radii, a simple contraction by regular material removal can be observed. The process is therefore much better controlled.

[0042] It has thus been possible to produce, by laser turning, watch pivot shafts, for example balance shafts, including pivots, in different materials having average to poor machinability by cutting tool, for example materials having specific cutting forces greater than or equal to 2400 N / mm 2< . Surprisingly, wire EDM turning as described above makes it possible to obtain the desired shapes with the required precision and particularly good surface finishes.

[0043] It is emphasized that the pivots are shaped during the turning operation. All the geometric characteristics of the shaft, including the pivots, are well realized with extremely low dispersion. The dimensions obtained take into account, where applicable, the removal of material linked to a finishing step, such as polishing or other.

[0044] The shafts can be used without the addition of an external coating. This results in shafts that are homogeneous in terms of material and properties.

[0045] An interesting aspect of the manufacturing process is that the wire EDM machining system 2 can work along the entire length of the shaft, except for the pivot end that connects the shaft to the bar 4. By modulating the parameters of the wire EDM machining system 2, it is thus possible to shape all areas of the shaft, gradually refining the surface finish. Unlike a traditional bar turning process where the balance shaft is made by longitudinal section, as the bar advances, the wire EDM machining system makes the shaft by working its entire length and reducing the diameters, then refining the surface finishes, progressively.

[0046] The method may comprise the following blocks of steps, each comprising a plurality of steps, the details of which may be modified depending on the know-how of the person skilled in the art and the specific features of the machine used: Roughing the bar: consists of reducing the diameter of the bar to a diameter of 1 to 3 mm, Profiling the balance shaft: consists of giving the general shape of the balance shaft (plate, general geometry···), Stitching the rivet: either a classic stitch (via classic concave machining or via sinking EDM) or the production of crimping tabs by wire EDM, the tabs then being folded down by plastic deformation in order to secure the balance rim to the balance shaft, this production of tabs can be done with the wire EDM system, Shaping the ends of the pivots: consists of producing the extreme part of the pivot, Finishing treatment, for example by bulk tribofinishing.

[0047] According to an advantageous embodiment, the manufacturing method comprises: shaping a first end of the shaft while the shaft is held by the machine on the side of a second end of the shaft, where appropriate by means of the bar 4, gripping by the machine of the shaft by the first end previously shaped, and shaping the second end while the first end is held by the machine.

[0048] Between the second and third steps of this embodiment, the shaft is advantageously separated by cutting from the rest of the bar 4, this cutting being carried out at the second end. Advantageously, the cutting is carried out by the wire EDM machining system 2. In other words, the method uses a machine equipped with two spindles, so as to produce a first part of the shaft on a first spindle, a transfer to a second spindle and machining of a second part of the shaft following the transfer to the second spindle. The loading and unloading of the shafts could be carried out automatically.

Claims

1. Method for producing a watch pivot shaft made of non-magnetic and electrically conductive material by means of a machine comprising a rotary gripping member (1) and a wire EDM machining system (2) mounted on a numerically controlled actuator (3), said method comprising the following steps: a. providing a bar (4) of said material extending along a longitudinal axis (A), b. arranging the bar (4) along its longitudinal axis (A) on the rotary gripping member (1), c. shaping the bar (4) by wire EDM machining to obtain a watch pivot shaft comprising a pivot at each end, step c. comprising continuously rotating the bar (4) around its longitudinal axis (A) for shaping said pivots.

2. Method according to claim 1, characterized in that the method further comprises a step d. of tribological finishing treatment carried out after step c.

3. Method according to one of the preceding claims, characterized in that said material has a hardness greater than 450HV, preferably greater than 600HV.

4. Method according to one of the preceding claims, characterized in that said material has an electrical conductivity greater than 1 S. m -1 , advantageously greater than 100 Sm -1 5. Method according to one of the preceding claims, characterized in that said material is chosen from the families of austenitic stainless steels, metallic glasses, tungsten-based alloys, intermetallic materials or conductive ceramics.

6. Method according to one of the preceding claims, characterized in that the watch pivot shaft has a pivot with a diameter between 0.03mm and 0.20mm, advantageously between 0.04mm and 0.09mm.

7. Method according to one of the preceding claims, characterized in thatstep c. comprises: - shaping a first end of the shaft while the shaft is held by the machine on the side of a second end of the shaft, - gripping by the machine of the shaft on the side of the first end previously shaped, and - shaping the second end while the shaft is held by the machine on the side of its first end.

8. Method according to one of the preceding claims, characterized in that step c. comprises one or more steps of wire EDM machining, by incremental rotation of the bar 4, to: - produce one or more flats or a polygonal shape.

9. Method according to one of the preceding claims, characterized in that wire 2 has a diameter of 0.02mm to 0.30mm, preferably of 0.02mm to 0.25mm, more preferably of 0.02mm to 0.15mm.

10. Method according to one of the preceding claims, characterized in that the bar 4 of step a. has a diameter of between 1.5 mm and 3 mm; and a length of between 1 m and 3 m.

11. Method according to one of the preceding claims, characterized in that said material is a conductive ceramic selected from ZrO2, Si3N4 or Al2O3 ceramics, enriched with a dispersed conductive phase selected from TiC, TiN, TiCN, WC and TiB2.

12. Watch pivot shaft produced by the method according to one of claims 1 to 11 comprising a pivot at each end, the diameter of the pivots D being between 0.03mm and 0.20mm.

13. Watch pivot shaft according to the preceding claim, comprising a convex pivot end having a radius of curvature R of between D / 2 and 0.5 mm.

14. A balance shaft according to claim 12, comprising a flat or convex pivot end.

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