METHOD FOR MANUFACTURING A CLOCK SPRING WITH PRECISE STIFFNESS

Description

[0001] The present invention relates to the manufacture of a watch spring. The term "spring" refers to any element that can be elastically deformed to absorb energy and / or produce force or movement. Examples of watch springs include balance springs, return springs for rocker arms, levers, or hammers, jumpers, and flexible guides, particularly flexible oscillator guides.

[0002] The present invention relates more particularly to the manufacture of watch springs by engraving a wafer of material. It is now well known in watchmaking to use engraving techniques such as laser engraving, plasma engraving, deep reactive ion etching (DRIE), or wet etching to manufacture watch components in large quantities and with precision. The most common etching material is silicon.

[0003] Currently, several hundred components are typically engraved on a single sheet of etching material. The etching characteristics are not always stable over time or homogeneous in space, resulting in geometric variations between components on different sheets, or even within the same sheet. In the case of springs, this translates into a variation in stiffness. However, a precise stiffness is crucial for a watch spring to ensure accurate control of the forces at play in a watch movement. This is especially true when the spring, such as a balance spring or a flexible guide, performs a restoring function for an inertial element (balance wheel) in an oscillator that serves as the time base. The spring's stiffness, along with the moment of inertia of the inertial element, determines the oscillator's frequency and therefore has a significant impact on the accuracy of time measurement.

[0004] Patents EP 3181938 and EP 3181939 describe methods for manufacturing balance springs of precise stiffness. According to the method described in patent EP 3181938, a) a balance spring is formed with dimensions larger than those required to obtain a balance spring of predetermined stiffness, b) the stiffness of the balance spring formed in step a) is determined by measuring the frequency of the balance spring coupled with a balance wheel having a predetermined inertia, c) the thickness of material to be removed to obtain the balance spring of predetermined stiffness is calculated, and d) the calculated thickness of material is removed from the balance spring formed in step a), steps b), c), and d) being repeatable to further improve dimensional accuracy.According to the process described in patent EP 3181939, a) a spiral is formed in dimensions smaller than the dimensions required to obtain a spiral of predetermined stiffness, b) the stiffness of the spiral formed in step a) is determined by measuring the frequency of the spiral coupled with a balance wheel having a predetermined inertia, c) the missing material thickness is calculated to obtain the spiral of predetermined stiffness and d) the spiral formed in step a) is modified to compensate for the missing material thickness, steps b), c) and d) being able to be repeated to further improve the dimensional quality.

[0005] These processes give excellent results but they only apply to spirals.

[0006] Another method, described in patent application EP 3416001, allows for obtaining a flexible-guided oscillator of a precise frequency. This method consists of: a) forming a flexible-guided oscillator having dimensions different from those required to obtain a flexible-guided oscillator of a predetermined frequency, b) measuring the frequency of the oscillator formed in step a), c) from the frequency measurement, calculating a thickness of material to be added, removed, or modified on the oscillator formed in step a) to obtain the flexible-guided oscillator of a predetermined frequency, d) from the calculation performed in step c), modifying the oscillator formed in step a) to obtain the flexible-guided oscillator of a predetermined frequency.

[0007] This method also gives excellent results, but it is limited to oscillators with flexible guidance. Furthermore, it does not separate the flexible guidance from the inertial element, which complicates its implementation when, for example, one wishes to modify only the flexible guidance in step d).

[0008] Document WO2019180558 A1 discloses a method for manufacturing a spring starting from a reference spring engraved in the same plate.

[0009] The present invention aims to remedy the aforementioned drawbacks and proposes to this end a method for manufacturing a watch spring according to the attached claim 1.

[0010] Specific embodiments are defined in the attached dependent claims.

[0011] The watch spring can be a separate piece or part of a monolithic piece that is engraved into the plate.

[0012] The term "section" refers to the cross-section of the blade or blades of the spring in question (watch spring or reference spring). Similarly, the term "length" refers, in the context of the invention, to the length of the blade or blades of the spring in question. Thus, in the case of a spring comprising several blades, a first / second predetermined length is associated with each blade, this first / second predetermined length being either identical to or different from that of the other blades of the spring.

[0013] The first predetermined length and the second predetermined length can be the same or different.

[0014] Other features and advantages of the present invention will become apparent from the following detailed description, made with reference to the accompanying drawings, in which: there figure 1 is a perspective view of a flexible-guided clockwork oscillator, the flexible guidance of which can be manufactured by the process according to the invention; the figure 2 is a block diagram of the process according to the invention; the figure 3 schematically shows a plate in which parts of the flexible-guided watch oscillator and reference balance springs are engraved; the figure 4 is a cross-sectional view of an elastic blade of said parts of the flexible-guided watch oscillator or reference spring.

[0015] There figure 1 This demonstrates a flexible-guided oscillator 100 for a timepiece, for example, a watch, particularly a wristwatch or a pocket watch. The oscillator 100 comprises a support 1, a balance wheel 2, and a flexible guide 3 connecting the support 1 to the balance wheel 2. The support 1 is intended to be mounted on a fixed or movable frame of the timepiece. The flexible guide 3 is manufactured by the method according to the invention, which will be described later. The flexible guide 3 comprises first and second elastic blades 4, 5, identical but extending in parallel planes and in different directions so as to cross without contacting each other. In a planar top view, the point of intersection of the blades 4, 5 coincides with the geometric center of the balance wheel 2. The intersection of the blades 4, 5 defines a virtual axis of rotation A of the balance wheel 2 with respect to the support 1, an axis of rotation that is perpendicular to the plane of the oscillator and the balance wheel 2.The flexible guide 3 thus serves to suspend the balance wheel 2 from the support 1, to guide the balance wheel 2 in rotation relative to the support 1 around the virtual axis of rotation A and to exert on the balance wheel 2 an elastic restoring torque tending to bring it back into an equilibrium position relative to the support 1.

[0016] Unlike many oscillators with flexible guides, the oscillator 100 is not monolithic but is formed from a stack of parts, one of which comprises the first elastic blade 4, another the second elastic blade 5, and yet another the balance wheel 2, which is therefore an added component relative to the flexible guide 3. In the illustrated example, the balance wheel 2 is between an upper part 6 and a lower part 7. The upper part 6 comprises an upper stage 8 of the support 1, the first elastic blade 4, and an upper arm 9 connected to the upper stage 8 of the support 1 by the first elastic blade 4. The lower part 7 comprises a lower stage 10 of the support 1, the second elastic blade 5, and a lower arm 11 connected to the lower stage 10 of the support 1 by the second elastic blade 5. The upper and lower stages 8 and 10 of the support 1 are joined together, for example, by pins 15.The upper and lower arms 9, 11 are assembled to a diametrical arm 13 of the balance wheel 2, for example by pins 17. The balance wheel 2 can be of classic shape, typically annular.

[0017] The assembly of the upper and lower parts 6, 7 and the balance wheel 2 allows for a true physical separation of the elastic blades 4, 5, obtained more easily than with a monolithic manufacturing.

[0018] The material for the upper and lower parts 6, 7 is chosen for its good elastic properties and its suitability for micromachining. As a typical example, each of these parts 6, 7 is made of silicon by single-level deep reactive ion etching (DRIE). The silicon parts 6, 7 can be coated with a reinforcing layer, for example, a layer of silicon dioxide, to improve their mechanical strength. This silicon dioxide layer can also be of a thickness chosen to make the oscillator frequency 100 insensitive to temperature variations (typically 30°C).

[0019] The balance wheel 2 is made of a different material than the upper and lower parts 6 and 7, and therefore also of the flexible guide 3. The material of the balance wheel 2 can be a dense material such as beryllium copper, gold, platinum, nickel silver, or another dense metal or alloy. It can thus have a small diameter for a given moment of inertia. In this way, friction with the air is reduced, which increases the quality factor.

[0020] With its added balance wheel 2, the oscillator 100 allows the balance wheel 2 and the flexible guide 3 to be paired, in other words, to associate a balance wheel having a chosen moment of inertia with a flexible guide producing a chosen torque in order to obtain a desired frequency, as for a balance wheel and hairspring.

[0021] The symmetries of the oscillator 100 make it relatively insensitive to shocks and its orientation relative to gravity.

[0022] Further details on the oscillator 100 can be found in patent application EP 20185171.4.

[0023] The frequency f of oscillator 100 is given by the following formula: f = 1 2 π K I where K is the stiffness of the flexible guide 3 and I is the moment of inertia of the balance wheel 2 with all its attached components. It is understood that the accuracy of the stiffness of the flexible guide 3 influences the accuracy of the frequency of the oscillator 100 and therefore the accuracy of the time measurement by the timepiece.

[0024] The stiffness K depends on the modulus of elasticity and the dimensions of the blades 4, 5 forming the flexible guide 3. For a flexible guide of the separated crossed blade type as shown in the figure 1 It corresponds to the formula: K = 1 6 . E . he 3 L where E is the modulus of elasticity of the material used, h is the height of each blade (dimension in the direction of the axis of rotation A), e is the thickness of each blade and L is the length of each blade.

[0025] The manufacturing process for the flexible guide 3 will now be described in relation to the figures 2 à 4 .

[0026] In a first step E1, parts 6 and 7 are engraved in a plate 20 of engraving material. Parts 6 and 7 may be identical, one of them being subsequently reversed during assembly, or they may already have symmetrical shapes with respect to each other. Each part 6, 7 is, however, engraved in such a way that the cross-section, constant or variable, of its blade 4, 5 has a size that is greater than the size necessary to obtain a first predetermined stiffness, this over at least part of the length of the blade 4, 5 and preferably over the entire length of the blade 4, 5. In step E1, i.e. all the dimensions (height h and thickness e for a rectangular section; cf. figure 4 ) of the section of the blade 4, 5 are greater than the dimensions allowing the first predetermined stiffness to be obtained, or only part of these dimensions is / are greater than the dimensions allowing the first predetermined stiffness to be obtained.

[0027] The etching process in step E1 can be deep reactive ion etching (DRIE), chemical etching, focused ion beam (FIB) etching, or laser etching, for example. The etching material can be homogeneous or composite. It may be silicon-based, quartz-based, glass-based, ceramic-based, metal-based, or alloy-based. Silicon-based materials can be monocrystalline, polycrystalline, or amorphous silicon. They may be doped or undoped.

[0028] Of the techniques mentioned above, the most precise is deep reactive ion etching. However, phenomena occurring during etching or between successive etchings can induce geometric variations.

[0029] Typically, several parts 6 and several parts 7, and even a large number of these parts, are made simultaneously in the same plate 20.

[0030] According to the invention, in addition to parts 6 and 7, one or more reference springs 21, preferably several, are engraved in plate 20. These reference springs 21 have a shape different from that of the blades 4 and 5 (in top view). These reference springs 21 are typically springs whose stiffness can be easily and reliably measured with standard instruments, for example, spiral springs, as shown, or bending specimens, in particular three-point bending blades.

[0031] In the case of a balance spring, its stiffness can be measured by coupling it to a balance wheel of known inertia—for this purpose, the balance spring can be a single piece with its ferrule 21a for mounting on the balance staff—and by measuring the frequency of the balance-spring thus formed, this frequency being mathematically related to the stiffness. The balance wheel can be integrated into a counting machine on whose working axis the balance spring is mounted. The stiffness of the balance spring can be measured after the spring has been detached from the plate 20 or while the spring is still attached to the plate 20.

[0032] Preferably, in order to simplify the engraving of the blades 4, 5 and the reference springs 21 and more generally the process according to the invention, the reference springs 21 have the same section (same geometry and same section size) as the blades 4, 5 and this section of the blades 4, 5 and the reference springs 21 is constant.

[0033] When several reference springs 21 are engraved, they are preferably distributed across the entire plate 20, like parts 6 and 7. Parts 6, 7, and the reference springs 21 are attached to the plate 20 by material bridges 22 left during the engraving process. Material bridges 23 may also be left between the rigid parts of each part 6, 7 to protect the blades 4, 5.

[0034] In a second step E2 of the process, the stiffness of the reference spring(s) 21 is measured. The stiffness of all the reference springs 21 can be measured and their average calculated, or the stiffness of a sample of the reference springs 21 can be measured and their average calculated. The average stiffness determined in step E2 is considered representative of each of the reference springs 21.

[0035] In a third step E3, starting from the stiffness determined in step E2, a thickness ξ of material is calculated to be removed from the reference springs 21 over a first predetermined length, typically their entire length, so that the reference springs 21 have a second predetermined stiffness. The material removal can be designed to vary only the height h of the leaf of the reference springs 21, only the thickness e of this leaf, or both the height h and the thickness e. The second predetermined stiffness is related to the first predetermined stiffness. More precisely, the second predetermined stiffness is such that if the same thickness ξ of material is removed from each of the leaves 4, 5 over a second predetermined length, typically their entire length, each of the leaves 4, 5 has the first predetermined stiffness.

[0036] At a fourth step E4, the thickness ξ of material calculated in step E3 is removed from the blades 4, 5 over the second predetermined length, in other words the size of their section is reduced over the second predetermined length, so that each of these blades 4, 5 has the first predetermined stiffness.

[0037] In the case of silicon-based blades 4, 5, step E4 may include a first phase consisting of oxidizing the blades 4, 5 to transform the thickness of material to be removed into silicon dioxide, and a second phase consisting of removing the silicon dioxide layer thus formed. Silicon dioxide forms by consuming silicon to a depth corresponding to approximately 44% of its thickness. After removal of the silicon dioxide layer, the resulting blades 4, 5 have a reduced cross-section.

[0038] Oxidation can be carried out thermally, for example between 800 and 1200°C under an oxidizing atmosphere using steam or oxygen gas. The oxide formed on the silicon-based material can be removed in a bath or vapor phase using a chemical agent containing, for example, hydrofluoric acid.

[0039] Advantageously, these oxidation and deoxidation operations are carried out on the entire plate 20 bearing parts 6, 7 and reference springs 21, plate which is therefore placed in an oven for oxidation and then treated by a bath or chemical steam for deoxidation.

[0040] This oxidation-deoxidation method is particularly precise. However, other methods can be used to remove material at step E4, for example chemical etching, laser etching or, for parts 6, 7 made of glass, the FEMTOPRINT® process which consists of changing the properties of the glass by means of a femtosecond laser and then subjecting the glass to a wet chemical etching operation.

[0041] Steps E2, E3 and E4 can be repeated with reference springs 21 remaining on plate 20 and having undergone the same treatments as blades 4, 5, in order to refine the dimensional quality of blades 4, 5.

[0042] After step E4, or the final step E4, an additional step E5 may involve treating the blades 4, 5, or more generally the parts 6, 7, to improve some of their characteristics. In the case of silicon-based parts 6, 7, they may be coated with a layer of silicon dioxide to improve their mechanical strength. This silicon dioxide layer may also have a thickness chosen to make the oscillator frequency 100 insensitive to temperature variations, with the temperature-dependent stiffness variations of the flexible guide 3 compensating for the temperature-dependent inertia variations of the balance wheel 2. The silicon dioxide layer may be formed by thermal oxidation, for example, between 800 and 1200°C under an oxidizing atmosphere using steam or oxygen gas, or by a deposition technique such as chemical vapor deposition (CVD) or physical vapor deposition (PVD).This step E5 changes the stiffness of the flexible guide 3 but in a way that is determinable and can therefore be anticipated when choosing the first predetermined stiffness before step E1.

[0043] The process ends with the detachment of parts 6, 7 from plate 20 by breaking or eliminating the material bridges 22 that held them.

[0044] It should be noted that the method according to the invention avoids having to measure the stiffness of the blades 4, 5 during industrial manufacturing. Thanks to a previously established correlation between the stiffness of the blades 4, 5 and that of the reference springs 21, namely the correspondence between the first predetermined stiffness and the second predetermined stiffness, only the stiffness of the reference springs 21, which is easier to measure, needs to be determined.

[0045] The correlation between the stiffness of the blades 4, 5 and that of the reference springs 21 can be obtained by calculation. To do this, before step E1, a blade 4, 5, for example with a perfectly rectangular cross-section, can be theoretically dimensioned, having the first predetermined stiffness. The first predetermined stiffness is a stiffness chosen so that the flexible guide 3, assembled to a rocker arm 2 of predetermined inertia, oscillates at a predetermined frequency. Furthermore, a reference spring 21, with a cross-section, for example, perfectly rectangular and preferably identical to that of the theoretical blade 4, 5, can be theoretically dimensioned, having a length chosen so that its stiffness is within a range measurable by a standard instrument or an instrument of a known type. The stiffness of this theoretical reference spring 21 can be calculated. It is the second predetermined stiffness.

[0046] Thus, in step E1, blades 4, 5 and reference springs 21 are engraved into plate 20 based on the geometry and dimensions of the theoretical blade 4, 5 and the theoretical reference spring 21, with the exception that the cross-section of blades 4, 5 is intentionally chosen to be larger than that of the theoretical blade 4, 5, and the cross-section of reference spring 21 is intentionally chosen to be larger than that of the theoretical reference spring 21. The additional material thickness of blades 4, 5 compared to the theoretical blade 4, 5 is identical to the additional material thickness of reference springs 21 compared to the theoretical reference spring 21. The engraving process generally does not allow for the reproduction of a perfectly rectangular cross-section.The inherent uncertainties of the engraving process and the difficulty of creating vertical surfaces mean that the cross-section of the engraved elements differs from the theoretical cross-section, for example, a more trapezoidal than rectangular geometry and non-planar surfaces. The blades 4 and 5 obtained in step E4 therefore have a cross-section different from the initial theoretical cross-section, but they possess approximately the desired stiffness, namely the first predetermined stiffness. The calculation in step E3 can be approximated by assuming that the engraved blades 4 and 5 and the engraved reference springs 21 have a perfectly rectangular cross-section.

[0047] Alternatively, instead of calculating the second predetermined stiffness from the theoretical dimensioning of the blades 4, 5 and the reference springs 21, it can be determined empirically, for example in the following way: (i) parts 6, parts 7 and reference springs 21 are engraved in the same plate on the basis of the geometry and dimensions of the theoretical blade 4, 5 and the theoretical reference spring 21, with the exception that the cross-section of the blades 4, 5 is chosen to be larger than the cross-section of the theoretical blade 4, 5 and the cross-section of the reference springs 21 is chosen to be larger than the cross-section of the theoretical reference spring 21, the thickness of material of the blades 4, 5 in addition to the theoretical blade 4, 5 being identical to the thickness of material of the reference springs 21 in addition to the theoretical reference spring 21; (ii) pairs of parts 6, 7 are detached from the plate and assembled successively to a balance wheel 2 of known inertia to determine the stiffness of each blade 4, 5 by measuring the frequency of the oscillator 2, 6, 7; (iii) we calculate the average of the stiffnesses thus measured;(iv) From the average stiffness thus determined, the thickness of material to be removed from the blades 4, 5 over the second predetermined length is calculated so that they have the first predetermined stiffness; (v) this thickness of material is removed from the second predetermined length of the blades 4, 5 remaining on the plate and from the first predetermined length of the reference springs 21, for example by oxidation-deoxidation of the entire plate; (vi) optionally, steps (ii) to (v) are repeated one or more times; (vii) the stiffness of the reference springs 21 is measured, whether still attached to the plate or not, and its average is calculated; this average constitutes the second predetermined stiffness, corresponding, for each blade 4, 5, to the first predetermined stiffness.

[0048] This empirical method is more complicated to implement than the analytical method, but it can be more precise. It is only implemented once, to determine the second predetermined stiffness. Then, during industrial production (steps E1 to E5), carried out using a new plate, 20, the blades 4, 5 no longer need to be detached from the plate and assembled to a balance wheel since it is the stiffness of the reference spring 21 that is measured.

[0049] In a variant of the empirical method, in step (i) parts 6 and parts 7 are engraved into the plate based on the geometry and dimensions of the theoretical blade 4, 5, except that the cross-section of the blades 4, 5 is chosen to be larger than the cross-section of the theoretical blade 4, 5. Reference springs 21 are also engraved into the same plate with a geometry and dimensions chosen so that their stiffness falls within a range measurable by a standard instrument or an instrument of a known type. Once the correspondence between the first and second predetermined stiffnesses has been established (step vii), step E1 is carried out based on the same geometries and dimensions as in this variant of step (i).

[0050] Throughout the detailed description above, the stiffness of the blades 4, 5 is adjusted by removing material, in other words, by reducing the cross-sectional area of ​​the blades 4, 5. Alternatively, the invention can be implemented in reverse by adding material (increasing the cross-sectional area of ​​the blades 4, 5). Thus, in step E1, each part 6, 7 can be engraved such that the cross-sectional area of ​​its blade 4, 5 is smaller than the size required to obtain a first predetermined stiffness. In step E3, a thickness of material to be added rather than removed is calculated, and in step E4, this thickness of material is added to the blades 4, 5, or even to the entire plate 20. The added material may be the same as the material of the blades 4, 5 engraved in step E1, or it may be different. The method for determining the second predetermined stiffness is adapted accordingly.

[0051] Various methods can be applied to add material, such as thermal oxidation, galvanic growth, physical vapor deposition, chemical vapor deposition, atomic layer deposition, or any other additive method. For example, chemical vapor deposition can be used to form polycrystalline silicon on 4, 5 monocrystalline silicon wafers.

[0052] As an alternative to adding or removing material, at step E4 a material thickness calculated in step E3 can be modified without necessarily changing the dimensions of the cross-section of the blades 4, 5. In particular, the structure can be modified according to a predetermined depth of the external surface of the blades 4, 5. As an example, if amorphous silicon is used to form the blades 4, 5, it can be planned to crystallize to a predetermined depth to form an amorphous silicon core covered with a layer of polycrystalline silicon in order to obtain the first predetermined stiffness.

[0053] In another variant, at step E4 the composition can be modified to a predetermined depth of all or part of the external surface of the plates 4, 5. For example, if monocrystalline or polycrystalline silicon is used to form the plates 4, 5, it may be provided for to dope or diffuse interstitial or substitution atoms to a predetermined depth to form a monocrystalline or polycrystalline silicon core covered with a layer doped or diffused using different silicon atoms in order to obtain the first predetermined stiffness.

[0054] The present invention makes it possible to obtain blades 4, 5 with precise stiffness despite the variations in engraving, without having to measure the stiffness of these blades during industrial production. This would require either assembling the parts 6, 7 to each other and to a balance wheel, or resorting to a non-standard stiffness measuring instrument and / or one less reliable than a balance wheel-hairspring frequency measurement. It goes without saying, however, that the invention is not limited to the manufacture of flexible oscillator guide blades or to the use of hairsprings as reference springs.It can be applied to any watch spring such as flexible guide, flexible guide part, rocker return spring, lever or hammer spring, jumper, in particular to watch springs for which the watchmaker does not have a reliable stiffness measuring instrument, or may have one (to empirically establish the correspondence between the first predetermined stiffness and the second predetermined stiffness) but not repeatedly (for industrial production).

Claims

1. Method for manufacturing at least one timepiece spring comprising the following steps: a) etching (E1) in a single plate (20) at least one timepiece spring (4, 5) with a leaf or leaves and at least one reference spring (21) with a leaf or leaves, the cross-section of the or each leaf of the timepiece spring (4, 5) having dimensions different from the dimensions necessary for the timepiece spring (4, 5) to have a first predetermined stiffness, b) measuring (E2) a stiffness characterising the reference spring (21) etched in step a), c) from the stiffness measured in step b), calculating (E3) a thickness (ξ) of material to remove, to add or to modify on the reference spring (21), over the whole length of its leaf or of each of its leaves, so that the reference spring (21) has a second predetermined stiffness, the second predetermined stiffness being such that if said removal, addition or modification of said thickness (ξ) of material is effected on the timepiece spring (4, 5) over the whole length of its leaf or of each of its leaves, the timepiece spring (4, 5) has the first predetermined stiffness, d) modifying (E4) the timepiece spring (4, 5) over the whole length of its leaf or of each of its leaves according to the calculation effected in step c) so that the timepiece spring (4, 5) has the first predetermined stiffness, characterised in that the reference spring (21) is of a shape different from that of the timepiece spring (4, 5) when seen from above.

2. Method as claimed in claim 1, characterised in that the timepiece spring (4, 5) is a flexible guide, a part of a flexible guide, a rocker return spring, lever return spring or hammer return spring, or a jumper.

3. Method as claimed in claim 1 or 2, characterised in that the timepiece spring (4, 5) is an oscillator flexible guide or a part of an oscillator flexible guide.

4. Method as claimed in any one of claims 1 to 3, characterised in that the timepiece spring (4, 5) is a flexible guide leaf for an oscillator (100) comprising a balance (2) mounted on at least two flexible guide leaves.

5. Method as claimed in any one of claims 1 to 4, characterised in that the reference spring (21) is a spiral spring.

6. Method as claimed in any one of claims 1 to 5, characterised in that step a) comprises deep reactive-ion etching.

7. Method as claimed in any one of claims 1 to 6, characterised in that the timepiece spring (4, 5) etched in step a) is based on silicon and that in step d) said thickness (ξ) of material is removed from the timepiece spring (4, 5) by oxidation and deoxidation operations.

8. Method as claimed in claim 7, characterised in that the oxidation and deoxidation operations are implemented on the whole plate (20).

9. Method as claimed in any one of claims 1 to 8, characterised in that in step a) a plurality of said timepiece springs (4, 5) are etched in the plate (20) and in that in step d) each of said timepiece springs (4, 5) is modified over the whole length of its leaf or of each of its leaves according to the calculation effected in step c).

10. Method as claimed in claim 9, characterised in that in step a) a plurality of said reference springs (21) are etched in the plate (20) and in that the stiffness measured in step b) is the stiffness of one of said reference springs (21), the average stiffness of all the reference springs (21) or the average stiffness of a sample of the reference springs (21).

11. Method as claimed in any one of claims 1 to 10, characterised in that it comprises, before step a), a step consisting of defining a theoretical timepiece spring having the first predetermined stiffness and a theoretical reference spring, and in that in step a) the timepiece spring (4, 5) and the reference spring (21) are etched on the basis of the geometry and the dimensions of the theoretical timepiece spring and of the theoretical reference spring, except for the cross-section of the or each leaf of the timepiece spring (4, 5), the dimensions of which are different from those of the theoretical timepiece spring, and for the cross-section of the or each leaf of the reference spring (21), the dimensions of which are different from those of the theoretical reference spring, the thickness of material of the timepiece spring (4, 5) more or less with respect to the theoretical timepiece spring being the same as the thickness of material of the reference spring (21) more or less with respect to the theoretical reference spring.

12. Method as claimed in claim 11, characterised in that the second predetermined stiffness is obtained before step a) by calculating the stiffness of the theoretical reference spring.

13. Method as claimed in claim 11, characterised in that the second predetermined stiffness is obtained before step a) by etching, in a single plate, timepiece springs and reference springs on the basis of the geometry and dimensions of the theoretical timepiece spring and of the theoretical reference spring, except for the cross-section of the or each leaf of the timepiece spring, the dimensions of which are different from those of the theoretical timepiece spring, and for the cross-section of the or each leaf of the reference spring, the dimensions of which are different from those of the theoretical reference spring, the thickness of material of the timepiece spring more or less with respect to the theoretical timepiece spring being the same as the thickness of material of the reference spring more or less with respect to the theoretical reference spring, by modifying the timepiece springs and the reference springs in the same way, over the whole length of the or each leaf of the timepiece springs and over the whole length of the or each leaf of the reference springs, until the first predetermined stiffness is obtained for the timepiece springs, and by measuring the stiffness of the reference springs thus obtained.

14. Method as claimed in any one of claims 1 to 10, characterised in that the second predetermined stiffness is obtained before step a) by etching, in a single plate, timepiece springs and reference springs, the cross-section of the or each leaf of the timepiece springs having dimensions different from the dimensions necessary for the timepiece springs to have the first predetermined stiffness, by modifying the timepiece springs and the reference springs in the same way, over the whole length of the or each leaf of the timepiece springs and over the whole length of the or each leaf of the reference springs, until the first predetermined stiffness is obtained for the timepiece springs, and by measuring the stiffness of the reference springs thus obtained; and in that in step a) the timepiece spring (4, 5) and the reference spring (21) are etched on the basis of the same geometries and dimensions as said timepiece springs and said reference springs etched initially.

15. Method as claimed in any one of claims 1 to 14, characterised in that the timepiece spring (4, 5) and the reference spring (21) etched in step a) have the same constant leaf cross-section.