UNRUHWELLE

Description

Technical field

[0001] The present invention relates to the field of watchmaking. It relates, more particularly, to a balance shaft, State of the art

[0002] In a mechanical watch movement, the balance spring is an important element in determining the regularity of the rate. The balance spring is mounted on a shaft, called the balance staff, which pivots in bearings. The quality of the pivoting has a significant influence on the performance of the movement, particularly in terms of precision, efficiency, and durability.

[0003] The industrial production of 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 (Vickers).

[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 1 , the shape of an axis resulting from turning is shown along line 10, and its shape after rolling along line 12. Rolling allows the desired geometry for the pivots to be obtained. Optionally, an additional tribological finishing operation, such as polishing, is still carried out. The latter results in additional material abrasion and allows the dimensions to be obtained along line 14, which are the final dimensions.

[0005] The material chosen must therefore be easily machined, and then be capable of being 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] Documents CH1025860A4 and DE1207289B disclose balance shafts made from iron, nickel and chromium alloys.

[0009] Unlike the approach currently followed, the present invention aims to change the paradigm and aims to propose a new balance axis. Disclosure of the invention

[0010] One aspect of the invention relates to a balance shaft as defined in the appended claim. Brief description of the drawings

[0011] There figure 1 attached represents the geometries of a balance shaft during its production using a traditional process, while the Figures 2a, 2b and 2c propose balance shaft geometries produced by the method according to the invention. Method of carrying out the invention

[0012] Recent technological developments have proposed the use of a new manufacturing technique, based on a 5-axis numerically controlled machine, whose actuator is equipped with an ultrashort pulse (USP) laser. This type of laser produces a uniform laser spot of around 25 µm. This laser is used to irradiate a workpiece, in the form of a bar rotated in a rotating gripping member, similar to a bar turning machine.

[0013] A very high amount of energy is applied to the workpiece for a short period of time. Melting or thermal changes do not occur because the material is vaporized instantly. In addition, the laser is rotated with what is called trepanning optics. This allows beams with different angles to be sent to the workpiece, automatically changing the rotation diameter and the angle of incidence.

[0014] As a result, thanks to the directions that the beam can take, it is possible to consider producing axes and pivots with conventional shapes, i.e. shapes currently produced by turning and rolling. Other shapes are also possible, which would not be achievable by turning or rolling. The targeted geometries are obtained directly during the laser machining stage, without rolling operations.

[0015] So, for example, the figure 2gives three possibilities, respectively to Figures 2a, 2b and 2c .

[0016] The end of the pivot illustrated in 2a is flat. It is described as a right circular cylinder with a diameter D between 50 and 100 µm (axial symmetry). The base of this cylinder has a 90° circular arc with a radius A, between 5 and 25 µm, 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.

[0017] The end of the pivot illustrated in 2b is convex. It is described as a right circular cylinder with a diameter D between 50 and 100 µm (axial symmetry). The base of this cylinder has a 90° circular arc, with a radius A between 5 and 25 µm, 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.

[0018] The end of the pivot illustrated in 2c is concave. It is described as a right circular cylinder with a diameter D between 50 and 100 µm (axial symmetry). The base of this cylinder has a 90° circular arc, with a radius A between 5 and 25 µm, starting from the generator of the cylinder. Tangentially and continuously, a radius of curvature R less than -D / 2, preferably less than -0.35 mm, extends the radius A to the center of the base of the cylinder.

[0019] 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.

[0020] Particularly advantageous and unexpectedly, it was observed that this laser 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.

[0021] Given the relative movements of the bar to be machined and the trepanning head, achieving sufficient precision seemed a priori to be a challenge. To give an idea, the diameter of the axis at the pivot is approximately 50 µm and the tolerance is of the order of µm.

[0022] As for the surface finishes obtained, they are better than those obtained after conventional turning. At the end of the turning process, a roughness Ra of between 0.025 and 0.1 µm is measured after laser machining, while it is 0.2 µm after conventional turning. The quality of the surface finish means that subsequent rolling is not required.

[0023] Thus, to obtain a surface finish that meets the required specifications, a simple tribological finishing / polishing step is used to further refine the surface finish. This type of operation is carried out in bulk, by stirring in a medium loaded with abrasive particles. A large number of parts can therefore be processed very efficiently. No change in geometry is observed during this operation, but only a homogeneous reduction in dimensions, of a few µm, depending on the duration of the operation.

[0024] 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, these materials have specific cutting forces greater than or equal to 2400 N / mm 2< and cause rapid wear of carbide cutting tools, which makes them unusable industrially. In addition, given the small dimensions of the axes and pivots, the mechanical constraints imposed during bar turning make it almost impossible to produce parts in this type of material.

[0025] Thus, tests were carried out in particular with alloys from the Inconel family comprising at least 50% Nickel, between 13 and 30% Chromium and elements chosen from Fe, Mo, Nb, Ta, Co, Mn, Cu, AI, Ti, Si, C, S, P and B. These alloys have the advantage of being non-magnetic. Thus, the invention relates to a homogeneous balance staff made from such a non-magnetic alloy. The term "homogeneous" means that the material has the same mechanical properties throughout. Although this is not excluded from the protection, the staff according to the invention does not need to receive an external coating to modify its tribological properties.

[0026] More specifically, the alloys used may be an Inconel 718. The alloy comprises between 50 and 55% Nickel, between 17 and 21% Chromium, between 2.8 and 3.3% Molybdenum, between 4.75 and 5.50% Niobium, between 0 and 1.0% Cobalt, between 0 and 0.35% Manganese, between 0 and 0.30% Copper, between 0.20 and 0.80% Aluminum, between 0.65 and 1.15% Titanium, between 0 and 0.35% Silicon, between 0 and 0.08% Carbon, between 0 and 0.015% Sulphur, between 0 and 0.015% Phosphorus, and between 0 and 0.006% Boron, and the balance of Iron.

[0027] More generally, different materials with a specific cutting force greater than or equal to 2400 N / mm 2 can be used. The chosen alloys can optionally be heat treated before being shaped, for example to modify certain properties, such as their hardness. These heat treatments, called aging for this type of alloy, are carried out before laser machining, which avoids any dimensional changes that may occur during heat treatments.

[0028] Thus, the following materials can be used: ISO P: High-alloy steels (> 5% alloying elements), annealed, quenched and tempered; ISO S: Heat-resistant superalloys excluding titanium (Iron base, Nickel base, Cobalt base), annealed, aged, cast; Austenitic stainless steels: 1.4472 and Biodur ®< 108, for which it has been demonstrated by tests that their specific cutting force Kc is greater than or equal to 2400 N / mm 2< (although part of the ISO M group); ISO H: Quenched, quenched and tempered steels, shell iron (cast or cast and aged).

[0029] The alloys mentioned are particularly hard, possibly after an ad-hoc heat treatment, so it is possible to do without hardening steps generally implemented after traditional 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.

[0030] Thanks to the possibilities offered by laser machining and the precision that this technique allows to obtain, it is therefore possible to machine the balance shaft, including the pivots, with the final geometry. A simple tribological finishing or polishing step is preferably carried out, while maintaining the defined geometry, that is to say by maintaining the angles or radii, a simple contraction by regular material removal can be observed. The process is therefore much better controlled.

[0031] It was thus possible to produce, by laser turning, balance shafts, including pivots, in different materials with specific cutting forces greater than or equal to 2400 N / mm2. Surprisingly, laser turning as described above makes it possible to obtain the desired shapes with the required precision and particularly good surface finishes.

[0032] It is emphasized that the pivots are shaped during the turning operation. All the geometric characteristics of the axis, 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.

[0033] The axles can be used without adding an external coating. This results in axles that are homogeneous in terms of material and properties.

[0034] An interesting aspect of the manufacturing process is that the laser can work along the entire length of the shaft, except for the pivot end that connects the shaft to the bar. By modulating the laser parameters, it is 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 laser makes the shaft by working its entire length and reducing the diameters, then gradually refining the surface finishes.

[0035] Typically, during the different machining stages, the laser parameters may vary within the following ranges: Average power: between 10 and 100 W; Energy per pulse: between 20 and 400 µJ; Frequency: between 100 and 1000 kHz; Pulse duration: between 500 fs and 20 ps.

[0036] The method comprises 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, rivet, general geometry, etc.), Stitching the rivet: consists of performing the stitching, which requires oblique positioning of the shaft, to perform concave machining on the plate, Shaping the ends of the pivots: consists of producing the final surface finish on the extreme part of the pivot.

[0037] Carrying out this final step by the laser requires that the ends be free of access for the beam. Consequently, the manufacturing process includes: a first step of machining, particularly shaping, of a first end of the shaft while the second end is held by the machine, where appropriate by means of the bar, a second step of resumption by the machine of the shaft by the first end previously machined, and a third step of machining, particularly shaping, of the second end while the first end is held by the machine.

[0038] Between the first and second stages, the axis is separated by cutting from the rest of the bar, this cutting being carried out at the second end. Advantageously, the cutting is carried out by laser.

[0039] Advantageously, the use of a laser head combined with a 5-axis numerical control machine allows any shape to be produced on the axis, including shapes that are not of revolution, as long as the rotation of the bar is stopped.

Claims

1. Homogeneous balance shaft made of a non-magnetic alloy comprising, in mass percent, between 50 and 55% nickel, between 17 and 21% chromium, between 2.8 and 3.3% molybdenum, between 4.75 and 5.50% niobium, between 0 and 1.0% cobalt, between 0 and 0.35% manganese, between 0 and 0.30% copper, between 0.20 and 0.80% aluminum, between 0.65 and 1.15% titanium, between 0 and 0.35% silicon, between 0 and 0.08% carbon, between 0 and 0.015% sulfur, between 0 and 0.015% phosphorus, and between 0 and 0.006% boron, with the remainder being iron.