Figure 1: Exploded view of an assembled balance wheel
Figure 2: Plan of a collet
In general terms, the word “collet” refers to a slotted washer that can rotate more or less freely around the axis on which it is fitted. For example, collets can be found on the arms of certain variable-inertia balance wheels.
In mechanical movements, the collet is the connecting element between the hairspring and the balance staff. It is one of the components of the regulating organ and plays an essential role in adjustment operations.
It consists of a slotted brass washer with an off-center transverse hole designed to receive the hairspring’s inner terminal (the “hook”). When the hairspring hook is correctly formed, it is inserted into the transverse hole of the collet. Once properly positioned, it is secured by a small brass pin (traditional method) or glued/crimped (industrial method).
The collet is fitted onto the upper part of the balance staff. The friction fit must be firm enough to ensure that the collet remains perfectly fixed to the staff when the watch is running. A broad chamfer is provided under the collet to facilitate its fitting onto the staff and to allow easy removal without damaging the hairspring. The slot in the collet enables fine angular adjustment during regulation or when setting the position of the “beat,” particularly in watches without a movable stud carrier.
The collet is an easy component to manufacture. A simple lathe and a small sensitive drill are all that’s needed to produce it.
The collet is an easy component to manufacture. A few machining operations (turning, drilling, and slot milling) are sufficient, involving a production cycle of only a few seconds.