CH718075A2 - Process for manufacturing a metal part, part obtained by this process and horological article comprising such a part. - Google Patents

Abstract

The invention relates to a method of manufacturing a metal part comprising the following steps: a) forming a blank (10a) by metal additive manufacturing according to a metal 3D printing technique, whereby a blank of the part having a first level surface condition with a first roughness value Rt1, b) finishing of at least certain faces of the blank by the technique of material removal by electrochemical machining (ECM for „Electro-Chemical Machining “), whereby a part (10b) is obtained whose said faces having undergone the finishing step have a second level surface condition with a second roughness value Rt2 that is smaller than the first roughness value Rt1. The invention relates to the manufacture of parts for watchmaking, in particular clasp parts for watch straps.

Description

Technical area

The present invention relates to a method of manufacturing a metal part, a part obtained by this method and a horological article comprising such a part. In many cases, for complex mechanisms, in particular in micromechanics, and in particular but not only in the horological field, a large number of parts must be produced before assembling them to form a functional assembly. This is very often due to the complexity of the shape of this functional assembly.

[0002] For example, but without limitation, a clasp, such as a folding clasp, for a bracelet for a wristwatch, is a functional assembly comprising a multitude of small parts, which requires multiple manufacturing steps, in generally by machining then finishing, before meticulous assembly.

[0003] Also, the manufacture of a multitude of small parts to be assembled to form a functional assembly such as a folding clasp, requires good compliance with the manufacturing dimensions. Added to this is the fact that some of these small parts are partially visible from the functional assembly. It is therefore appropriate for the visible areas of these parts to obtain high precision surfaces, and with an irreproachable aesthetic appearance. It is known that performing deburring effectively with the achievement of a good quality finish requires time and proven know-how on the part of the operator(s) performing these tasks.

[0004] This situation entails significant manufacturing costs, and this for aesthetic results that are often diminished due to the appearance of certain parts or certain zones of parts of the functional assembly. Indeed, this lower-level aesthetic aspect of certain portions of the functional assembly may be inherent to the manufacturing processes implemented, and to the impossibility of accessing certain zones of parts during the finishing steps.

State of the art

[0005] Most metal parts for watchmaking and micromechanical parts are today manufactured by machining using machining centers having up to 5 interpolated axes, by stamping, by cutting, by bending, etc. ,

[0006] Finishing steps such as deburring, rolling and polishing then take place, in particular to respect the narrow dimensional tolerances. If you want to have a top-of-the-range surface condition, meticulous work is involved, very often artisanal and carried out by a highly qualified craftsman, for example the operations of beading, chamfering, molding, polishing, satin-finishing, microblasting or PVD deposits. . Finally, there are assembly steps, such as riveting, driving in, brazing, which are very often complex and delicate operations.

Brief summary of the invention

[0007] An object of the present invention is to provide a process for manufacturing metal parts free from the limitations of known processes.

Another object of the invention is to provide a method of manufacturing metal parts which reduces the number of operations to manufacture a functional assembly.

[0009] Another object of the invention is to provide a process for manufacturing metal parts making it easier to obtain one-piece metal parts of complex shape.

Another object of the invention is to provide a method of manufacturing metal parts making it easier to obtain metal parts with a satisfactory quality of finish for at least part of the faces of said part.

Another object of the invention is that of improving the repetitiveness and the reproducibility of the operations leading to the production of the finished product.

[0012] According to the invention, these objects are achieved in particular by means of a method for manufacturing a metal part comprising the following steps: a) forming a blank by metal additive manufacturing according to a metal 3D printing technique , whereby a blank of the part is obtained having a first level surface condition with a first roughness value Rt1 (or respectively Ra1), b) finishing of at least certain faces of the blank by the technique of material removal by electrochemical machining (ECM for "Electro-Chemical Machining"), whereby a part is obtained whose said faces having undergone the finishing step have a second level surface condition with a second roughness value Rt2 ( or respectively Ra2) smaller than the first roughness value Rt1 (or respectively Ra1).

[0013] This solution has the particular advantage over the prior art of benefiting from low tool wear and reproduction precision, and this with a high surface quality, for parts that may have complex shapes.

[0014] Indeed, by step a) of additive manufacturing by a metal 3D printing technique, it is possible to obtain a single and unique part of complex shape, in the form of a blank, there or previously by machining it was necessary to produce several separate parts, by conventional machine tool machining techniques and then to assemble them by processes such as welding, brazing, screwing, etc.,

[0015] With step b) of finishing by electrochemical machining, the initial surface condition of the blank is greatly improved, on the face or faces of the part requiring a good surface quality, namely having a second roughness value Rt2 or Ra2 smaller than the first roughness value Rt1 or Ra1 obtained at the output of step a) of 3D printing.

In the present text, we consider for the roughness, i.e. Ra, namely the arithmetic mean deviation Ra, which is the integral mean of the deviations in absolute value over the base length (line) of the profile of the surface considered , or Rt, namely the peak-to-peak roughness, which can still be defined as the maximum roughness, corresponding to the sum of the distance from the average line to the highest peak and the distance from the average line to 'at the deepest bottom, for the full length of evaluation.

According to a possible embodiment of the manufacturing process, during step b) of finishing, a superfinishing of said faces of the part is carried out by the technique of material removal by precision electrochemical machining (PECM for „ Precision Electrofinishing - Chemical Machining“).

[0018] With step b) in the form of super finishing by precision electrochemical machining, the initial surface condition of the blank is further improved, on the face or faces of the part requiring an excellent quality of surface, namely having a very low roughness Rt2 or Ra2, so that on the side or sides of the part thus treated, a maximum surface quality is obtained.

According to a possible embodiment of the manufacturing process, between step a) of forming a blank and step b) of finishing, an intermediate step a1) of recovery by machining by removal of chips from the blank. In fact, in some cases, to produce certain parts of the part with improved tolerances and/or surface conditions compared to those of the blank resulting from step a) of metal additive manufacturing according to a metal 3D printing technique, Also, or alternatively, in some cases, this intermediate step a1) of machining is used to produce certain precision geometries, such as the formation of cavities or openings in the part, such as milling, drilling and/or tapping.

The present invention also relates to a part resulting from the manufacturing process described in this text. By way of example, such a part belongs to the group comprising a watch movement part, an oscillating weight, a barrel, a toothed wheel, a pinion wheel, a column wheel, a balance wheel, an escapement part, a pallet, a plate, an axle, a stem, a watchmaker's exterior part, a case, a dial, a hand, a crown, a link and a bracelet clasp part.

[0021] The invention also relates to a horological article comprising at least one part obtained according to the manufacturing process described in the present text. By way of example, said article belongs to the group comprising: a bracelet clasp, a wristwatch bracelet clasp, a folding clasp, a wristwatch and an object intended to be worn on the wrist.

Brief description of figures

Examples of implementation of the invention are indicated in the description illustrated by the appended figures in which: Figure 1 illustrates the successive steps of several variants of an embodiment of the manufacturing method according to the invention, FIG. 2 schematically illustrates the improvement in the surface condition of the part obtained according to the manufacturing method of the invention, Figures 3a, 3b and 3c show examples of parts obtained according to the method of the invention, FIGS. 4a and 4b show the various parts constituting an example of a functional assembly, respectively obtained by a conventional manufacturing process and by a manufacturing process in accordance with the present invention, and FIG. 5 illustrates the successive steps of another variant of an embodiment of the manufacturing method according to the invention.

Example(s) of embodiment of the invention

We refer to Figure 1 showing the different steps of different variants of the method of manufacturing a part according to the present invention.

According to a first step a), (left in Figure 1, step 100a), one begins by forming a blank 10a by a metal additive manufacturing technique. Various metal additive manufacturing processes are known, in particular metal 3D printing techniques such as powder bed fusion techniques which can be used in the context of the present invention.

According to one possibility within the scope of the present invention, the metal 3D printing technique is a powder bed fusion technique, such as selective laser melting (SLM for "selective laser melting.") or direct laser sintering of metals (DMLS for “Direct Metal Laser Sintering“, selective laser sintering (SLS for “Selected Laser Sintering“), a directed energy deposition technique (DED for “Directed Energy Deposition“), a molten wire deposition technique (FDM for „Fused Deposition Modelling“) or a metal deposition technique by laser (LMD for „Laser Metal Deposition“).

In particular, by this metallic 3D printing technique, a blank 10a is obtained which has a peak-to-peak surface roughness Rt1. On this subject, FIG. 2 represents an example of surface S of a blank 10a obtained by a metallic 3D printing technique, with a target dimension C1 and a roughness Rt1. One obtains for example a precision on C1 of the order of ±3σ1=±40 μm and peak-to-peak surface roughnesses Rt1 of the order of about 40 μm. In general, the roughness Rt1 obtained by a metal 3D printing technique during step a) (which is henceforth called the first roughness value Rt1), is between 30 and 60 μm, or even between 35 and 50 µm, even between 35 and 45 µm.

[0027] Metal 3D printing technology is, for example, laser fusion technology on a powder bed which, thanks to the use of powder and a small-diameter laser spot, makes it possible to produce parts which combine precision, quality of surface, material health and low level of internal stresses.

[0028] This may be selective laser melting (SLM for "selective laser melting"): the blank of the metal part is formed using high-power lasers, causing the metal to merge gradually and locally, c ie selectively, metal powder in a controlled atmosphere. Thus, the blank 10a of the part is created by addition by agglutinating metal powder particles, layer by layer, by a process of total melting of the fine particles by laser.

It may also be direct metal laser sintering additive manufacturing technology, translation of Direct Metal Laser Sintering (DMLS). In this case, it is a technique in which thermal energy (laser or electron beam) is used to sinter or fuse, layer by layer, well-defined particles of a powder bed. This additive manufacturing process formed by the direct laser sintering of metals (or DMLS) allows high printing precision. It is therefore suitable for the manufacture of parts with complex geometries and structures, with thin walls and hidden voids or channels, but also for the design of very small objects.

Directed Energy Deposition (DED) technology can also be used, which is an additive manufacturing process in which energy is used to melt the materials as they are deposited. This directed energy deposition (DED) process is a process in which the material is directly deposited and fused to a metal part thanks to a flow of directed energy (laser beam, electric arc, transferred plasma arc or other). The raw material can be in the form of powder or wire. The process is performed in a machine, where an actuator moves the part and/or the material deposition system. Geometry and material are generated, layer by layer. Directed deposition technology makes it possible to produce complex 3D structures, without any support.

It is also possible to use fused wire deposition technologies (FDM for “Fused Deposition Modeling”) or metal deposition by laser (LMD for “Laser Metal Deposition”) which have principles similar to the aforementioned examples.

[0032] The blank 10a is then (second step from the left in Figure 1) processed during a recovery by machining: it is precision machining by chip removal to form a part 10a1 at the outcome of this step 100a1. In the example illustrated, a tool 21 makes a hole. According to other possibilities, during the intermediate step a1) of machining the blank, at least one of the following operations is carried out: a milling operation, a drilling operation, and a tapping operation. This re-machining step can also include the machining of certain faces of the blank 10a to come closer to the target dimension of the desired finished part, namely to bring certain parts of the part into a tolerance and a state improved surface compared to those of the blank 10a resulting from step a) 3D printing.

This step 100a1 of resuming machining of the blank 10a is optional, and does not take place according to a variant embodiment of the method of the invention in which step a) of forming by printing 3D in step b) of finishing which will now be described.

The blank 10a (or the part 10a1 if a step 100a1 of resuming machining of the blank 10a has taken place) is then (third step from the left in FIG. 1) subjected to a step b) of finishing by electrochemical machining (ECM for „Electro-Chemical Machining“ or PECM for „Precision Electro-Chemical Machining“). Electrochemical machining (ECM for „Electro Chemical Machining“), also called material removal by electrochemical machining (UEC), is a method which makes it possible to finish the surfaces of the part using the anodic dissolution of the metal. This method allows the machining of electrically conductive materials whatever their state of treatment, in surface machining and in deburring of holes that are difficult to access. The tool is the electrode 22 forming the cathode (-) which acts under direct current and in the presence of an electrolytic fluid to create the anodic reaction which removes the surface material of the part 10b forming the anode (+) of precise way. Thus, in electrolytic machining, the metal is removed without direct contact between the tool and the workpiece. The electrolytic machine uses a controlled electric current, while a suitable conductive electrolyte solution circulates between the tool 22 (cathode-) and the workpiece 10b (anode +). The exchange of charges between the poles leads to the removal of matter, atom by atom, on precisely defined zones. It is understood that the profile of the tool 22 constitutes a negative of the face of the part treated by electrochemical machining. This step b) (100a1) makes it possible to quickly and precisely machine the faces thus treated with a tolerance conforming to or close to the desired final tolerance. This technique makes it possible in particular to process the precision defects and the roughness both on the exterior faces of the part and on the internal faces such as the internal faces of the part resulting from drilling, tapping or milling, resulting or not from step 100a1.

In particular, by this finishing technique by electrochemical machining, a part 10b is obtained which has a peak-to-peak surface roughness Rt2 in the faces treated during this finishing step b). On this subject, FIG. 2 also represents an example of target dimension C2 and a peak-to-peak surface roughness Rt2 targeted at the end of this step b) of finishing by electrochemical machining. One obtains for example a precision on C2, called ±3σ2 of the order of ±5 μm and surface roughnesses Rt2 of the order of about 50 nm. In general, the roughness obtained by an electrochemical machining technique (ECM or PECM) during step b) (which is henceforth called second roughness value Rt2), is between 0.05 and 2 μm. It is possible to achieve a roughness Rt2 which is of the order of 0.125 μm to 0.25 μm with a roughness Ra2 which is of the order of 0.025 to 0.05 μm (it can be considered that Rt is approximately equal to 5x Ra). Thus, this second roughness value Rt2 is much lower than the first roughness value Rt1. Note that in general, C1 = C2 + 6σ1 + Rt1, i.e. the target dimension for step a) 3D printing is equal to the target dimension C2 for step b) finishing plus twice the manufacturing tolerance of step a) 3D printing and the peak-to-peak roughness obtained by this step a) 3D printing.

During step b) of finishing, precision electrochemical machining PECM is preferably used, which in particular has a minimization of the inter-electrode space between the electrode 22 forming the tool (cathode -) and the machined part 10b (or 1c) (anode +)., through which the electrolyte solution flows and the exchange of the electrolyte itself. Indeed, preferably, in the case of the PECM method, the inter-electrode space is varied in a sinusoidal manner and the current is pulsed advantageously with this spatial variation. Thus, PECM precision electrochemical machining ensures particularly efficient and precise material removal. With this step b) of superfinishing by electrochemical precision machining PECM, it is possible to obtain a surface state of the part 10c equivalent to fine polishing. For example, this step b) of superfinishing by electrochemical precision machining PECM makes it possible to obtain a mirror surface by selective removal of material from the face thus treated, such as for example a steel surface.

Thus, whether it is the ECM technique or the PECM technique, a charge exchange between the cathode and the anodically polarized workpiece takes place in an aqueous electrolytic solution and it is this charge exchange which generates the machining of the part. The material removed during the process separates from the electrolyte solution as metal hydroxide. The machining is done independently of the microstructure of the metal. The possibility exists to machine materials hardened or not. The components of the installation, including the part, are not subjected to any thermal or mechanical stress.

Note that the electrolyte can be: an NaCl solution (all concentrations, an NaNO3 solution (all concentrations), an HCl solution (all concentrations), an HF solution (all concentrations), an H2SO4 solution (all concentrations) , a citric acid solution (all concentrations) AND any combination of these solutions.

[0039] Optionally, this precision electrochemical machining step (PECM) forming a step b) of super finishing, is followed by a step d) of treatment of at least one zone of the part to modify the appearance of said additional zone (step c) represented in FIG. 5 as being produced by a tool 21). For example, this processing step c) comprises at least one of the following techniques: hand polishing: Polishing is done by hand and on the fly, that is to say without support, using discs of different sizes, covered with abrasive paste. This very difficult technique makes it possible to obtain a soft and shiny surface. This meticulous work can take several hours per piece, or even more; satin finishing: in other industries this process is called brushing. In watchmaking, the execution being much finer, we speak of satin finishing. This technique consists of coating a metal surface with a set of extremely fine, parallel stripes. We obtain a soft and silky finish as well as subtle shades of color that highlight the design and the volumes of the piece; Micro-blasting: micro-blasting is a finishing technique with a grainy finish. It consists of bombarding the surface of the case with small metal balls, each leaving micro-hollows which, by their number and density, appear regular. It gives the room a matte look that smothers the light; Sandblasting: sandblasting is identical to microblasting but with grains of sand. This finish appears to achieve a more velvety surface; PVD treatment (Physical Vapor Deposition) is a process that allows a thin film of material to be deposited on a part, by vacuum vaporization. By this surface treatment, it is possible to obtain “tinted” cases in pink gold, yellow gold or even black for an affordable price. It is this technique that is used to apply DLC (Diamond Like Carbon), a dark gray to black coating with exceptional hardness and scratch resistance properties); and Laser texturing such as femtosecond laser engraving, oxidation coloring, Deep Black, beading, ...

The method according to the present invention makes it possible in particular to manufacture a part having at least one opening passing through the entire thickness of the part. Examples of such parts with opening(s) are illustrated in Figures 3a, 3b and 3c, with for the case of Figure 3a a flat part whose central band is perforated with multiple through openings forming between them a pattern which has a aesthetic role.

According to a non-limiting possibility, the metal blank 10a (and therefore the part 10b) is in one of the following materials: steel, stainless steel, austenitic stainless steel (for example a 316L, 904L or other steel), maraging steel (for „martensitic aging“, that is to say with maturing of the martensite), a precious metal, in particular gold or platinum, or an alloy of one of these metals.

It is understood that the method according to the invention makes it possible to manufacture complex parts in a single piece, whereas previously it was necessary to precisely manufacture several parts, very often of very small size, and to assemble them to form an equivalent part. . To illustrate the situation, reference is made to Figures 4a and 4b show the various parts constituting an example of a functional assembly in the form of a watch strap clasp, respectively obtained by a conventional manufacturing process (Figure 4a) and by a manufacturing method according to the present invention (FIG. 4b). It can thus be seen that the manufacturing method according to the invention makes it possible to significantly reduce the number of parts required for the manufacture of a watch clasp. The manufacturing method according to the invention also makes it possible to significantly reduce the complexity of most of the individual parts of a clasp.

It is understood that the method according to the invention makes it possible to adjust and optimize the mechanical performance of each part, component or subassembly obtained. Indeed, the process makes it possible to obtain complex parts with given rigidities and hardnesses. It is then possible to adjust and optimize these characteristics by global or local thermal processes. Preferably, not exclusively, the rigidities and the hardnesses will be optimized. To do this, according to a variant (not shown in the figures) of the methods described above, at least one energy treatment is also carried out on the part 10b, said energy treatment belonging to the group comprising heat treatment, light treatment, by laser, electron beam treatment.

[0044] The invention also relates to a computer data medium comprising instructions which, when executed by a 3D printer, allow said 3D printer to produce a blank 10a according to one of the manufacturing methods described in the this text.

The invention also relates to a computer data carrier comprising data intended for the additive manufacturing of a blank 10a of a part, said data representing the shape of a blank, which is determined taking into account the removal of material resulting from a step of finishing at least certain faces of the blank by the technique of material removal by electrochemical machining (ECM for "Electro-Chemical Machining").

Reference signs used in the figures

Rt1 First peak-to-peak roughness value (corresponds to Ra1) Rt2 Second peak-to-peak roughness value (corresponds to Ra2) C1 Target dimension of step a) of forming the blank by 3D printing C2 Target dimension of step a) for finishing the part by electrochemical machining 3σ1 Tolerance on C1 3σ2 Tolerance on C2 S Surface of the blank 10a Blank 10a1 Part after re-machining 10b Part after finishing by ECM or PECM 21 Tool 22 Electrode

Claims (15)

1. Method for manufacturing a metal part comprising the following steps: a) formation of a blank (10a) by metallic additive manufacturing according to a metallic 3D printing technique, whereby a blank of the part is obtained having a first level surface condition with a first roughness value Rt1, b) finishing of at least certain faces of the blank by the technique of material removal by electrochemical machining (ECM for "Electro-Chemical Machining"), whereby a part (10b) is obtained whose said faces having undergone the finishing step have a second level surface condition with a second roughness value Rt2 smaller than the first roughness value Rt1. 2. Method according to claim 1, in which during step b) of finishing, a superfinishing of said faces of the part is carried out, by the technique of material removal by precision electrochemical machining (PECM for "Precision Electro Finishing - Chemical Machining“). 3. Method according to claim 1 or 2, in which an intermediate step a1) of recovery by machining by removal of roughing shavings. 4. Method according to claim 3, in which during the intermediate step a1) of machining the blank, at least one of the following operations is carried out: a milling operation, a drilling operation, and a tapping operation. 5. Method according to any one of claims 1 to 4, in which, after step b) of finishing, a step d) of treatment of at least one zone of the part is also carried out to modify the appearance. of said area. 6. Method according to any one of claims 1 to 5, in which the metallic 3D printing technique is a powder bed fusion technique, such as selective laser melting (SLM for “selective laser melting.”) or Direct Metal Laser Sintering (DMLS), Selected Laser Sintering (SLS), Directed Energy Deposition (DED) , a fused wire deposition technique (FDM for “Fused Deposition Modelling”) or a laser metal deposition technique (LMD for “Laser Metal Deposition”). 7. Method according to any one of claims 1 to 6, in which the blank is made of one of the following materials: steel, stainless steel, austenitic stainless steel, maraging steel (for “martensitic aging“, i.e. i.e. with martensite maturation), a precious metal, in particular gold or platinum, or an alloy of one of these metals. 8. Method according to any one of claims 1 to 6, in which at least one energy treatment is also carried out on the part (10b), said energy treatment belonging to the group comprising a heat treatment, a light treatment, a treatment by laser, electron beam treatment. 9. Part obtained according to the manufacturing method of one of claims 1 to 8, belonging to the group comprising a watch movement part, an oscillating mass, a barrel, a toothed wheel, a pinion wheel, a column wheel, a balance wheel, an escapement part, an anchor, a plate, an axis, a stem, a watchmaker's exterior part, a case, a dial, a hand, a crown, a link and a bracelet clasp part. 10. Part obtained according to the manufacturing method of one of claims 1 to 8, said part having at least one opening passing through the entire thickness of the part. 11. Watchmaking article comprising at least one part obtained according to the manufacturing method of one of claims 1 to 8. 12. Article according to the preceding claim, said article belonging to the group comprising a bracelet clasp, a wristwatch bracelet clasp, a folding clasp, a wristwatch and an object intended to be worn on the wrist. 13. Computer data medium comprising instructions which, when executed by a 3D printer, allow said 3D printer to produce a blank (10a) according to the manufacturing method of one of claims 1 to 8. 14. Computer data carrier comprising data intended for the additive manufacturing of a blank (10a) of a part, said data representing the shape of a blank, which is determined taking into account the removal of material resulting from a step of finishing at least certain faces of the blank by the technique of material removal by electrochemical machining (ECM for "Electro-Chemical Machining"). 15. Computer data medium according to claim 13 or 14, in which said blank is capable of forming a part belonging to the following list: a watch movement part, an oscillating weight, a barrel, a toothed wheel, a pinion wheel , a column wheel, a balance wheel, an escapement part, an anchor, a plate, an axis, a stem, a watchmaker's exterior part, a case, a dial, a hand, a crown, a link and a bracelet clasp part.