MICROMETRIC JEWELLING TOOL
WHAT IS A MICROMETRIC JEWELLING TOOL?
The micrometric jewelling tool designates the workbench instrument used for fitting, removing and adjusting in depth watchmaking jewels as well as bushings. The widespread use of jewelling in mainplates and bridges, and even more the requirement of a precise dimension for the axial play of the staff — the endshake — led to the development of a press allowing both a perfectly vertical movement and a micrometric control of the stroke. This equipment has since become the obligatory step for any careful jewelling work, whether for the replacement of a bearing or for production work.
The instrument consists of a vertical frame of machined metal or cast iron, extended by a column at the top of which a micrometric-screw head is mounted. The screw commands, through a play-free slide, the descent of a punch holder receiving the pushing punch. At the base of the frame, a support table is milled to receive the lower stake on which the mainplate or the bridge to be jewelled rests. The stakes are supplied in complete sets comprising several dozen pieces paired by diameter: the active face of the pushing punch bears on the centre of the jewel through a polished seat. The centre of the pushing punch carries a pump-centring pin whose diameter corresponds to the hole of the jewel. This centring pin ensures perfect centring of the jewel and guarantees that it is pressed in perfectly straight. The lower stake receives the mainplate without exerting any constraint on the jewel already in place on the other side.
The fitting operation consists of inserting the jewel into its housing, then progressively lowering the pushing punch until it makes contact with and centres the jewel. The rotation of the micrometric screw then drives the jewel into its bore, to the exact depth commanded by the stroke of the tool. This depth directly determines the endshake of the corresponding wheel’s pivot: too little, it prevents the free running of the gear train; too great, it generates a risk of producing poor depthings between the wheels. The precision of the micrometric adjustment allows the watchmaker to place the jewel at the nominal dimension, then to correct it if necessary by very short successive displacements. The removal operation is carried out according to the same principle, although care is generally taken to move the jewel always in the same direction (from the inside outwards).