Patent Publication Number: US-2021170629-A1

Title: Jewel, in particular for an horological movement, and manufacturing method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to European Patent Application No. 19214883.1 filed Dec. 10, 2019, the entire contents of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The invention relates to a method for manufacturing a jewel, in particular for an horological movement. 
     The invention also relates to a jewel, in particular of an horological movement, for example an industrial jewel or a technical ceramic. 
     The invention also relates to an horological movement including such a jewel. 
     The invention also relates to a pressing device for implementing the method. 
     BACKGROUND OF THE INVENTION 
     In the prior art of watchmaking, ruby, ZrO2 or sapphire-type jewels are used in particular to form counter-pivots or guide elements, called bearings, in timepieces. These counter-pivots and guide elements are intended to be in contact with the pivots in order to make the latter movable in rotation, with minimal friction. Thus, they form, for example, all or part of a bearing of an axis mounted in rotation. The guide elements generally comprise a through hole for inserting the pivot pin therein. 
       FIG. 1  is a representation of a bearing  1  for a pivot  2  of a rotating mobile according to the prior art. The bearing  1  comprises a bearing block  3 , wherein is arranged a guide element  4 , which is here a jewel. The jewel includes a through hole  5  to receive the end  6  of the pivot  2 . Thus, the pivot  2  can rotate in the hole  5 . 
     Normally, synthetic industrial jewels are used in horological movements. In particular, the method of the Verneuil type is known for manufacturing monocrystalline-type jewels. There are also poly-crystalline-type jewels, which are made by pressing a precursor in order to obtain a green body of the future jewel from a pressing tool. The jewels are then sintered, machined to obtain a finished shape with the desired dimensions. In particular, concerning the guide elements made of poly-crystalline jewel, the pressing tool is for example provided with a wire participating in the construction of a hole blank. 
     However, these techniques for machining these poly-crystalline jewels do not allow small holes to be obtained. In particular, diameters of 0.11 mm can be reached, thanks to the usual techniques known today. But it is not possible to go below this value. In order to be able to go below, it is necessary to resort to laser technologies, which are difficult to implement industrially, and which do not directly provide a quality surface finish of the hole. 
     SUMMARY OF THE INVENTION 
     The purpose of the present invention is to overcome all or part of the disadvantages mentioned above, by proposing a method for manufacturing a jewel on a large scale allowing the production of a very small diameter hole. 
     To this end, the invention relates to a method for manufacturing a poly-crystalline-type jewel, in particular for a timepiece, the jewel comprising, for example, poly-ruby of the Al2O3Cr type or Zirconia ceramic of the ZrO2 type, the method comprising a first step of producing a precursor. 
     The method is remarkable in that it comprises a second step of pressing the precursor in order to form a body, the pressing being carried out using a pressing device provided with an upper die and a lower die defining a pressing space wherein the precursor is disposed, the upper die comprising a concave portion of oblong shape, the device being provided with a wire at least partially traversing the lower die to open into the pressing space, the lower die being able to slide around the wire, the pressing being carried out by bringing the lower die and the upper die closer together to form a body comprising an upper face provided with a dome and a lower face provided with a hole extending at least partially into the dome. 
     Thus, this method allows to form green bodies which, after sintering and machining, will give jewels with a hole of very small diameter, in particular less than or equal to 0.1 mm. This method is further implemented by an easy-to-use pressing device, the pressing device used being an improvement of a conventional device for manufacturing a green body. The invention therefore allows to manufacture these jewels industrially on a large scale, without having to resort to systems that are expensive and complicated to be implemented. 
     According to a particular embodiment of the invention, the pressing step is carried out by displacing the lower die towards the upper die around the fixed wire. 
     According to a particular embodiment of the invention, during pressing, a circular recess is formed in the upper face of the body around the dome, the upper die being provided with a flange delimiting the concave portion. 
     According to a particular embodiment of the invention, the upper die displaces upwards under the effect of that of the lower die, the displacement of the upper die being less than that of the lower die. 
     According to a particular embodiment of the invention, the method comprises a third step of sintering said body in order to form the mineral body. 
     According to a particular embodiment of the invention, the method comprises a fourth machining step to remove an upper portion of the dome from the mineral body, in order to obtain a hole traversing the jewel. 
     According to a particular embodiment of the invention, the method comprises a fifth finishing step, for example a lapping and/or brushing and/or polishing of the mineral body. 
     The invention also relates to a jewel, in particular for an horological movement, formed from a poly-crystalline-type body and comprising, for example, poly-ruby of Al2O3Cr type or Zirconia of ZrO2 type, the body can be obtained by the method according to the invention. The jewel is remarkable in that the body comprises a lower face provided with a hole whose diameter is less than 0.11 mm, the jewel comprising a dome on the upper face, the hole partly extending into the dome. 
     According to a particular embodiment of the invention, the dome forms an angle comprised within an interval ranging from 30° to 90°, preferably from 50 to 70°, or even 60°. 
     According to a particular embodiment of the invention, the diameter of the hole is comprised within an interval ranging from 0.2 to 0.8 mm, or even from 0.4 to 0.6 mm. 
     According to a particular embodiment of the invention, the hole is a through hole, an upper portion of the dome being removed. 
     According to a particular embodiment of the invention, the upper face of the jewel comprises a circular recess arranged around the hole. 
     According to a particular embodiment of the invention, the jewel comprises a flare around the hole on its lower face. 
     The invention also relates to a timepiece comprising such a jewel, in particular for a balance bearing. 
     The invention also relates to a pressing device for the manufacture of a jewel, in particular for a timepiece, the device comprising a casing defining a housing, an upper die and a lower die configured to be able to displace in the housing, the dies defining a pressing space wherein a precursor can be disposed, the upper die comprising a flange, the device being provided with a wire at least partially traversing the lower die to open into the pressing space, the wire being fixed relative to the lower die and centred on the flange of the upper die, the lower die comprising an orifice for receiving the wire, the die being able to slide around the wire. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages will emerge clearly from the description which is given below, in an indicative and non-limiting manner, with reference to the appended drawings, wherein: 
         FIG. 1  is a schematic representation of a bearing for a pivot according to an embodiment known from the prior art; 
         FIG. 2  is a block diagram of a method for producing a jewel according to the invention; 
         FIG. 3  is a schematic representation of a portion of a pressing device according to the invention; 
         FIG. 4  is a schematic representation of the portion of  FIG. 4  with the precursor; 
         FIG. 5  is a schematic representation of the pressing device according to the invention; 
         FIG. 6  is a schematic representation of the pressing device according to the invention during pressing; 
         FIG. 7  is a schematic representation of a green body obtained after the pressing step thanks to the method according to the invention; 
         FIG. 8  is a schematic representation of a mineral body obtained after part of the machining step thanks to the method according to the invention; 
         FIG. 9  is a schematic representation of a jewel obtained thanks to the method according to the invention; 
         FIG. 10  is a schematic representation of a system for manufacturing a jewel comprising a pressing device according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As explained above, the invention relates to a method for manufacturing a jewel capable of forming a guide element of a timepiece. The jewel is for example intended to be in contact with a pivot, also called a trunnion, for example of a balance axis, in order to make the latter movable in rotation with minimal friction. It is therefore understood that the present invention allows in particular to produce a jewel which can form all or part of a bearing of an axis mounted in rotation, such as that shown in  FIG. 1 . 
     The jewel is formed from a precursor, modelled as a green body, which becomes a mineral body of the poly-crystalline type for sapphire, the body comprising, for example, poly-ruby of the Al2O3Cr type or Zirconia ceramic of the ZrO2 type. The mineral body is cut to become the final jewel. 
     In the embodiment 5 of the method, shown in  FIG. 2 , such a method includes a first step  7  of producing a precursor from a mixture of at least one powder material with a binder. This material can be, in a non-limiting and non-exhaustive manner, ceramic. This step  7  is intended to form a precursor from a ceramic-based powder taken in the binder. 
     In this context, the ceramic-based powder can include at least one metal oxide, one metal nitride or one metal carbide. For example, the ceramic-based powder may include aluminium oxide in order to form synthetic sapphire or a mixture of aluminium oxide and chromium oxide in order to form synthetic ruby, or else zirconium oxide. In addition, the binder can be of various natures such as, for example, of polymeric types or of organic types. 
     The embodiment then includes a second step of pressing  8  the precursor from an upper die and a lower die of a pressing device, to form a green body of the future jewel. The pressing step is shown in  FIGS. 4 to 7 , which are described later in the description. The pressing step  8  allows to obtain a green body provided with a dome and a hole extending at least partially inside the dome. It is therefore understood that the shape of the dome is provided by the concave portion  23  of the upper die  22 , and the shape of the hole is provided by the shape of the wire  17  of the lower die  16  of the pressing device  20 . 
     The method comprises a third step of sintering  9  said green body in order to form the mineral body of the future jewel in said at least one material. 
     The material can be, as mentioned previously, ceramic. In other words, this step  9  is intended to sinter the green body in order to form a ceramic body of the future pierced jewel. Preferably according to the invention, the sintering step  9  may include a pyrolysis, for example by thermal debinding. 
     The method  10  comprises a fourth machining step  11 , in particular for removing a portion of the dome from the mineral body, in order to obtain a hole traversing the jewel. Machining involves planing the upper portion of the dome. Thus, by removing the upper portion, the hole is opened in the upper face of the green body to obtain a through hole. The machining step  11  also comprises a sub-step of shaping the upper face and the lower face to obtain a predefined jewel thickness. 
     The method comprises a fifth finishing step  12 , for example a lapping and/or brushing and/or polishing of the mineral body. This finishing allows to give the jewel a surface state compatible with its use. Such a finishing step also allows the adjustment of the final dimensions and/or the removal of edges and/or the local modification of the roughness. 
     In  FIGS. 3 to 6 , the pressing device  20  comprises a casing  15  provided with a chamber, inside which an upper die  22  and a lower die  16  can slide. Each die  16 ,  22  is fixed on a double-acting press. The upper die  22  and the lower die  16  define a pressing space  25  wherein the precursor  21  is disposed. 
     In  FIGS. 3 and 4 , only the lower die  16  is shown. The device  10  is further provided with a wire  17  at least partially traversing the lower die  16  to open into the pressing space  25 . The wire  17  is fixed relative to the lower die  16  and centred on the lower die  16 . The lower die  16  comprises an orifice  19  for the passage of the wire  17 . Thus, the lower die  16  slides around the wire  17 . The wire  17  is therefore stationary relative to the lower die  16 . 
     The lower die  16  is further provided with a domed portion  18 , preferably of conical shape with a large opening angle, for example comprised within an interval ranging from 60° to 140°, preferably comprised between 90° and 120°. The domed portion  18  is centred on the lower die  16 , so that the passage  19  and therefore the wire  17  are arranged at the top of the domed portion  18 . 
     The precursor  21  is positioned in the pressing space  25 , as shown in  FIG. 4 . Then, the upper die  22  is positioned in the housing on the precursor  21 . 
     The upper die  22  comprises an oblong concave portion  23 . The concave portion  23  is centred on the axis of the upper die  22 . The concave portion  23  preferably has a conical shape with a circular base, the top of the cone defining the bottom of the concave portion  23 . The bottom of the concave portion has for example a rounded shape. The cone has, for example, an opening angle comprised within an interval ranging from 30 to 90°, preferably from 50 to 70°, or even 60°. 
     The upper die  22  is further provided with a flange  24  delimiting the concave portion  23 . The flange  24  is circular and preferably has a substantially rounded profile. Thus, during pressing, a circular recess is formed in the upper face of the green body around the concave portion. The minimum diameter of the flange  24  is greater than that of the circular base of the concave portion  23 . Preferably, the minimum diameter corresponds to that of the circular base of the concave portion  23 . Thus, the recess  33  formed in the upper face  36  borders the circumference of the concave portion  23 . The internal wall  38  of the recess  33  forms the base of the concave portion  23 . 
     The pressing  8  is carried out by bringing the upper die  22  and the lower die  16  closer together, so as to compress the precursor  21  in the pressing space  25 . Preferably, the pressing  8  is carried out by displacing the lower die  16  towards the upper die  22  around the fixed wire  17 . Thus, the precursor  21  is packed against the upper die  22  to give the green body a shape corresponding to the pressing space  25  once the two dies  16 ,  22  are brought closer together. The green body therefore takes the shape of the upper  22  and lower  16  dies for the upper  36  and lower  37  faces of the body. 
     Thus, such a pressing step  8  is intended to compress the precursor  21  in order to form the green body of the future pierced jewel with a dome on the upper face and a hole on the lower face. 
     Preferably, the upper die  22  displaces under the pressure effect of the lower die  16 , the displacement of the upper die  22  being less than that of the lower die. Thus, the risk of breaking the wire  17  during pressing  8  is reduced. 
       FIG. 7  shows the green body  30  thus obtained. The green body  30  comprises an upper face  36  provided with a dome  31  and a recess  33  around the dome  31 . The dome  31  preferably has a conical shape with a circular base corresponding to that of the concave portion  23  of the upper die  16 . The dome  31  is prominent from the upper face  36 . In other words, it extends at least partially beyond the rest of the upper face  36 . Beyond the recess  33 , the upper face  36  of the green body comprises a substantially planar circumferential surface  35 , the height of which is less than that of the dome  31 , but greater than the recess  33 . 
     The green body  30  comprises a lower face  37  provided with a hole  32 . The hole  32  was formed by the wire during pressing. The hole  32  has a cylindrical shape. The hole  32  has a depth selected so that it extends through the jewel at least partially in the dome  31 . The depth is for example selected to go beyond the bottom of the recess  33 , from the lower face  37 . At this stage, the hole  32  is not a through hole, but includes a bottom disposed in the dome  31 . Thanks to the method, a hole  32  is obtained with a very small diameter, which may in particular be less than 0.1 mm, or even less than 0.05 mm. 
     The lower face  37  of the green body  30  is provided with a flared portion  34 , the flared portion  34  bordering the hole  32 . The flared portion  34  has a conical shape. This flaring then forms a cone for engaging the pierced jewel  40 . The cone  12  is preferably circular. The cone has a first opening  39  at its base and a second opening  41  at its top. The first opening  39  is larger than the second one 41, and is formed in the lower face  37  of the body  30 . The connection of the cone  34  and the hole  32  is performed through the second opening  41  to form an edge. Thus, the flare  34  allows to easily insert the pivot of an axis of a part movable in rotation, in particular in the event of an impact. The angle of the cone is selected to prevent the edge formed by the top of the cone and the hole  8  from protruding too much. For example, an angle comprised between 60° and 140°, preferably comprised between 90° and 120°, is selected. 
     The green body  30 , once formed, is subjected to the sintering step to obtain a mineral body, which maintains an identical shape. 
       FIG. 8  shows the mineral body  30  after part of the machining step  11 , during which the upper portion  48  of the dome  31  has been removed to obtain a through hole  32 . The mineral body  30  is provided with the upper  36  and lower  37  faces which are of different shapes. Indeed, the lower face  37  has a conical shape bordering the hole  32 , while the upper face  36  has a recess around the hole  32 . Such a through hole  32  comprises a first opening  49  defined in the mineral body and opening into the lower face  37 . The through hole  32  also comprises a second opening  51  defined in the mineral body  30  and opening into the upper face  36 . Such a jewel has, for example, a thickness of 0.18 mm and a diameter of 0.8 mm, and a hole of diameter less than 0.1 mm. Such dimensions allow the use of very small diameter pivots. Preferably, the entire upper face  36  has the same height. Thus, the upper face  36  of the body is flat, outside the hole  32  and the recess  33 . Material can also be removed from the circumferential surface  35 , to obtain a desired jewel thickness. 
     The machining step  11  can also comprise a sub-step of planing the peripheral face  52  of the mineral body  30 , in order to give it a determined diameter. The machining step  11  can also comprise a sub-step of planing the lower face  37 , or even of widening or cutting the hole  32 . 
       FIG. 9  shows an example of a jewel  40  obtained after all the steps of the method  10 . Such a jewel  40  can be used as a guide element mounted in a bearing, such as that of  FIG. 1 . However, such a jewel cannot be limited to the watchmaking field and can be applied to any element movably mounted relative to a bearing, or to an industrial jewel (water jet nozzle, etc.), or a technical ceramic (insulator, etc.). The jewel  40  comprises the features described in the method above. The jewel  40  is traversed by a hole  42  intended to receive a pivot. The jewel  40  includes an upper surface  46  and a lower surface  47 , one of which comprises a functional element, here a cone  44 , communicating with the through hole  42 . The upper face  36  comprises a recess  43  and the other side of the through hole  42 . In other words, the hole  42  communicates with the upper face  46  and with the lower face  47 . 
     Of course, the present invention is not limited to the example illustrated but is susceptible to various variations and modifications which will be apparent to the person skilled in the art. In particular, other types of functional elements formed during the pressing step can be advantageously considered according to the invention. 
     Referring to  FIG. 10 , the invention also relates to a jewel manufacturing system  60 . This system  60  comprises the following various devices:
         a device  51  for producing a precursor from a mixture of at least one powder material with a binder;   a device  20  for pressing the precursor material as defined above;   a device  53  for sintering said green body, and   a machine  54  for machining the body  30  of the future jewel  8  resulting from the sintering of the green body.       

     It will be noted that at least two of these devices  20 ,  51 ,  53  and  54  can together form the same entity of the system  60 . Such a system  60  is capable of implementing the method for manufacturing the jewel  40  shown in  FIG. 9 , by going through the steps in  FIG. 2 .