Under the Loupe/Repairing the Hook on a Low Mainspring for a Manually Wound Watch
Repairing a Mainspring Hook
Following is a sequence of images portraying the repair of a low or flat mainspring that has had its original hook, which latches to the barrel wall, broken off. To carry out this repair, the end of the mainspring where the hook was originally located is bent over and clamped onto another small piece of spring steel the same height as the mainspring.
The first step is to anneal the end of the mainspring where the hook was originally located. A length of 10-12mm from the end is a suitable amount to be annealed. To anneal, heat the metal to a cherry red colour and allow it to cool slowly at room temperature.
Forming the Initial Bend
A Close Up of the Initial Bend
Heating the Bend
The initial bend is heated again. This eases the mechanical stress introduced in bending the spring and also facilitates easier creation of a more acute bend in the spring while minimizing the risk of fracturing the metal.
Working the Bend
Slowly and patiently work the metal to shape, heating as necessary. Patience and a gentle (yet firm) touch are the keys to success. Once the bend is close to the shape necessary to accommodate the new hook, introduce the hook into the fold and clamp the metal firmly over it.