CRYSTAL-FITTING PRESS
The crystal-fitting press designates the workbench tool used for the fitting and removal of the crystal in a watch case. It takes the form of a small lever press, the frame of which receives at each of its two ends interchangeable dies, of profile and diameter adapted to the shape of the crystal and the geometry of the bezel. The function consists, depending on the case, of slightly compressing the crystal to insert it into its seat, of fitting a tension-ring crystal in place, or of removing an existing crystal for replacement. The diversity of case constructions — snap-fit crystal, tension-ring crystal, screw-in crystal, glued crystal — and the variety of materials used justify the existence of dedicated tooling.
The instrument consists of a vertical frame of machined metal or cast aluminium, and of a slide controlled by a lever or a screw. The slide and the base of the frame each receive a die — a circular support piece with a concave or convex active face, supplied as a set to cover the range of crystal and case diameters. During fitting, the case rests on the lower die, which holds it flat; the crystal, presented on the upper die, is progressively brought into contact with the bezel by lowering the slide. The pressure applied is metered by hand through the lever, and the watchmaker visually controls the even seating of the crystal around its entire periphery.
The dies form the functional core of the press and are supplied as sets comprising several dozen pieces. Concave dies are used for fitting domed crystals, which lodge in the concavity without suffering any central marking; flat or convex dies are used for flat crystals and for certain removal operations, when pressure must be transmitted to the shoulder of the glass rather than to its face. Specific dies are provided for tension-ring crystals, the fitting of which requires prior compression of the crystal-ring assembly before its insertion into the bezel. The dies are generally made of synthetic material or aluminium, so as not to mark the crystal, and their diameter is engraved on their lower face for easy identification.
The crystal-fitting press is used at every operation of replacing or restoring a crystal: substitution of a scratched acrylic crystal, fitting of a new mineral crystal on an overhauled case, or mounting of a sapphire crystal with a sealing gasket. The safety of the operation lies in the choice of a die of the exact diameter of the seat, in the perfect centring of the case and crystal on the press axis, and in the regularity of the pressure applied by the lever: excessive or off-centre force will crack a sapphire crystal, split an acrylic crystal, or mark the edge of the bezel. Like the other hand-operated tools dedicated to casing-up, the crystal-fitting press does not participate directly in the movement, but conditions the water resistance and the final quality of the assembled timepiece.