CASE KNIFE
WHAT IS A CASE KNIFE IN WATCHMAKING?
The case knife, or watchmaker’s knife, designates the simplest of the instruments used to open a watch case. It is intended for snap-back cases, whose press-fit closure relies on the watertight engagement of a peripheral lip into a groove in the middle part (case middle). The action consists of inserting the tip of the blade into this groove, then exerting a slight prying movement so as to disengage the case back and free access to the movement.
The instrument is composed of a flat blade, cut from stainless steel, mounted on a handle. The blade is intentionally thick and rigid, with a solid back, and its edge is not sharpened: it is a rounded or slightly bevelled edge, polished, whose function is not to cut but to engage cleanly in the groove of the case back without marking it. Several profiles coexist depending on the shape and depth of the groove to be released.
The operation is performed with the watch held in one hand, the case resting on a soft support or service cushion, the blade presented flat against the case middle and then engaged by a moderate rotation of the wrist until the case back lifts. The most common mistake is for the blade to slip on the side of the case middle, leaving a fine trace.