Abstract:
The invention relates to a system ( 9 ) for securing a part ( 7 ) made of a material with no usable plastic domain, including a securing device ( 11 ) on an arbour ( 3 ) passing through a hole ( 8 ) in said part. According to the invention, the securing device ( 11 ) includes a recess ( 4 ) that communicates with the hole ( 8 ) in said part and securing means ( 6 ) attached to the arbour ( 3 ) and at least partially matching the shape of the recess ( 4 ), so as to make the movement of the part ( 7 ) integral with that of the arbour ( 3 ). 
     The invention concerns the field of timepiece manufacture.

Description:
[0001]    This application claims priority from European Patent Application No. 09174421.9 filed Oct. 29, 2009, the entire disclosure of which is incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The invention relates to a system for securing a part made of a material that has no usable plastic domain, i.e. which cannot be driven in and, more specifically, a part of this type formed of silicon-based material. 
       BACKGROUND OF THE INVENTION 
       [0003]    It is known, in watchmaking, to mount parts on arbours by driving in said parts, i.e. via plastic deformation of the part and/or the arbour. The advent of parts made of silicon, makes it impossible to secure said parts by driving them onto arbours because of the risk of breakage thereof. In current fabrication systems, a polymer type adhesive material is used to secure the silicon parts to their arbours or another locking system involving a third mechanical part. However, these solutions are not satisfactory because, in the first solution, the adhesive materials age randomly and sometimes very prematurely and the second solution is too complex to implement. 
       SUMMARY OF THE INVENTION 
       [0004]    It is an object of the present invention to overcome all or part of the aforementioned drawbacks by proposing a system for securing a part made of material with no usable plastic domain onto an arbour, which uses neither driving in nor bonding techniques. 
         [0005]    The invention therefore relates to a system for securing a part made of material with no usable plastic domain, which includes a securing device on an arbour, said arbour passing through a hole in said part, characterized in that the securing device has a recess that communicates with the hole in said part and securing means attached to the arbour and at least partly matching the shape of the recess in order to make the movement of the part integral with that of the arbour, without driving in or bonding the part. 
         [0006]    Advantageously, the part made of material with no usable plastic domain is not stressed in its plastic domain (no driving in) but simply moved mechanically by the assembly comprising the arbour and securing means. 
         [0007]    According to other advantageous features of the invention:
       the recess is blind to form a slot;   the recess has no edge;   the securing means has an insert made of metallic material;   the metallic insert is obtained by partially melting said arbour or by melting a solder or by shrink fitting said metallic material in amorphous form;   the securing device further includes a collar mounted on the arbour for use as a shoulder for said part;   the material that has no usable plastic domain is silicon-based.       
 
         [0014]    The invention also relates to a timepiece, characterized in that it includes a securing system in accordance with any of the preceding variants. 
         [0015]    Finally, the invention relates to a method of securing a part made of material with no usable plastic domain onto an arbour, characterized in that it includes the following steps:
       a) fabricating the part with a hole for receiving said arbour and a recess that communicates with the hole;   b) fabricating said arbour;   c) assembling the arbour in the hole of the part;   d) filling the recess with a metallic material so that it matches the shape of the recess and part of the external diameter of said arbour;   e) hardening the metallic material to secure the part to the arbour without driving in or bonding.       
 
         [0021]    According to other advantageous features of the invention:
       step d) is achieved by localised melting of the arbour or by melting a solder using high energy radiation;   step d) is achieved by shrink fitting said metallic material in amorphous form;   step c) continues until the part abuts against a collar of the arbour formed during step b);   the material with no usable plastic domain is silicon-based.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    Other features and advantages will appear more clearly from the following description, given by way of non-limiting illustration, with reference to the annexed drawings, in which: 
           [0027]      FIG. 1  is a diagram of a sprung balance resonator according to the invention; 
           [0028]      FIG. 2  is a diagram of a balance spring according to the invention; 
           [0029]      FIG. 3  is a partial diagram of  FIG. 2 ; 
           [0030]      FIG. 4  is a cross-section of  FIG. 3 ; 
           [0031]      FIG. 5  is a similar diagram to  FIG. 4  showing a securing device according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0032]    In the example illustrated in  FIG. 1 , the invention will be explained with reference to a sprung balance resonator, generally referenced  1 . However, the invention can also apply, for example, to a wheel set or to pallets within or outside the field of watchmaking. 
         [0033]    Sprung balance resonator  1  has a pivoting arbour  3 , a balance  5 , a balance spring  7  and a securing system  9 . Pivoting arbour  3 , which may also be called the balance staff in the example of  FIG. 1 , is used for rotatably mounting balance  5  and balance spring  7  between two bearings, which are, for example, arranged respectively in a bridge and a plate (not shown). 
         [0034]      FIG. 2  shows an enlarged view of balance spring  7  which, for the purposes of the explanation below, is a part made of material with no usable plastic domain, such as, for example, crystalline silicon, crystalline silica, crystalline alumina, silicon nitride or even silicon carbide. However, balance  5  can also be fabricated from this type of material and, consequently, can also be mounted on arbour  3  by means of a second securing system  9 . 
         [0035]    Preferably, balance spring  7  has a collet  2  which is integral therewith. This balance spring  7  may, for example, be obtained from a plate which is etched throughout, by photolithography of a photosensitive resin and then etching the parts of the plate that have no resin. 
         [0036]    According to the invention, securing system  9  is for joining balance spring  7  to arbour  3 . Thus, securing system  9  includes a securing device  11  that has a recess  4  and securing means  6 . Recess  4 , in the example of  FIG. 2 , communicates with the arbour hole  8  of collet  2 . Recess  4  thus forms a slope  10  between the approximately vertical inner wall of arbour hole  8  and the approximately horizontal top surface of collet  2 , as more clearly illustrated in  FIG. 4 . Slope  10  thus forms a blind recess, which has no edge, and which surrounds securing means  6 . This slope  10  can thus be obtained from the teaching of CH Patent No. 1549/09 in the Applicant&#39;s name, which is incorporated herein by reference. 
         [0037]    Securing means  6  are intended for attachment to arbour  3  and to at least partially match the shape of recess  4 . Preferably, securing means  6  includes an insert made of metallic material which, according to three embodiments, can be obtained by partially melting arbour  3  or by melting a solder or by shrink fitting said metallic material in amorphous form. 
         [0038]    Thus, as visible in  FIG. 5 , the hardening of the insert allows the preferably metallic arbour  3  to be attached but also forms a stud that cooperates with recess  4 , which then acts like a mortise. 
         [0039]    It is thus clear that securing means  6  do not have to be attached to collet  2  but only to arbour  3 . However, the rotation movement of arbour  3 , which imparts the same movement to securing means  6 , also drives collet  2  mechanically, i.e. balance spring  7 , via said stud—mortise connection. Consequently, advantageously according to the invention, balance spring  7  made of material with no usable plastic domain, is not stressed in its plastic domain. 
         [0040]    Preferably, securing device  11  further includes a collar  13  mounted on arbour  3  to act as a shoulder for balance spring  7  in proximity to collet  2 . Indeed, this configuration allows collet  2  to be locked relative to the length of arbour  3  via contact between collar  13 , which forms a shoulder, and securing means  6 , which form a stud. 
         [0041]    The three embodiments of the fabrication method will now be explained. According to a first step a), the method starts with fabrication of part  7  with a hole  8  for receiving arbour  3  and a recess  4  that communicates with hole  8  as illustrated in  FIG. 2 . 
         [0042]    According to a second step b) which may be carried out prior to, during or after step a), the method continues with fabrication of arbour  3  preferably with shoulder  13 . In a third step c), the method continues with the assembly of arbour  3  in hole  8  of balance spring  7 , preferably until it abuts against collar  13  of arbour  3 . 
         [0043]    The method then includes a fourth step d) for filling recess  4  with securing means  6  such as, for example, a metallic material so that it matches the shape of the recess and part of the external diameter of arbour  3 . Finally, the method ends with a fifth and final step e) of hardening the metallic material so as to secure balance spring  7  to arbour  3 . 
         [0044]    According to a first embodiment, step d) is achieved by localised melting of arbour  3 , i.e. arbour  3  is partially melted at zone  12  so that one liquid part of arbour  3  runs into recess  4  as illustrated in  FIGS. 4 and 5 . The melting in step d) is, preferably, achieved by high energy radiation, for example, by means of a laser source. 
         [0045]    According to a second embodiment, step d) is achieved by melting a solder, i.e. a metallic material between zone  12  and recess  4  is melted so that the solder is made liquid and runs into recess  4  in a similar manner to the first embodiment. The melting according to the second embodiment is also preferably achieved by high energy radiation, for example, by means of a laser source. 
         [0046]    According to a third embodiment, step d) is achieved by shrink fitting said metallic material in amorphous form, i.e. the metallic material is heated to between its vitreous transition temperature and crystallisation temperature, then pressed into recess  4  so that it is moulded to the shape of said recess  4 . 
         [0047]    Thus, whichever embodiment is used, in the fifth and final step e), securing means  6  cool down and can be attached to arbour  3  to form said stud which will allow balance spring  7  to be driven in rotation via its collet  2 . Thus, advantageously, even if the part, i.e. balance spring  7 , is made of a material with no usable plastic domain, it is possible to secure it to an arbour  3  without having to drive in or bond said part. 
         [0048]    Of course, the present invention is not limited to the illustrated example, but is capable of various variants and alterations which will be clear to those skilled in the art. In particular, slope  10  can be replaced by a vertical wall and a horizontal wall, i.e. recess  4  includes a bottom that is approximately parallel to the top surface of collet  2 . In both examples, it is clear that recess  4  is blind, i.e. it does not pass right through the part which needs to be secured. 
         [0049]    It is also clear that although the part in the above example is a balance spring  7 , whose collet  2  includes hole  8  and recess  4 , the invention can perfectly well be adapted to a wheel set type part whose hub includes the hole and recess or even to escapement pallets whose lever includes the hole and recess. As explained above, these examples concern the field of watchmaking. However, the invention can be applied to other fields.