Hairspring

The hairspring earns its name. An incredibly delicate coil of metal, it combines with the the balance wheel to form the oscillating heart of the watch that determines the keeping of time. And as a student, I had to learn how work and shape it.

First, the collet. This is a c-shaped ring of brass that grips the balance staff. The inner coil of the hairspring is bent and placed in a hole in the side of the collet and secured with a tiny brass pin. The pin is trimmed and the hairspring is made to lie flat and centered around the collet, then it is assembled with the balance wheel to find the regulating point. This is done by adjusting where the outer coil of the hairspring is held firm while comparing how fast the assembly oscillates against a known standard. A specific amount of the hairspring then needs to be cut from the *inner* coil to get the regulating point and the point where the hairspring attaches to the collet to align. Once the inner coil is set and the regulating point found on the outer coil, the outer coil needs to be bent outward and shaped into a new arc to match where the watch will hold and regulate the spring. Attach the end of the outer coil to a metal anchor stud with another brass pin, make very fine adjustments so the spring lies perfectly flat and centered when the watch is assembled, and everything can finally be put to together and timed.

As an aside, the amount that needs to be cut from the inner coil of the hairspring in calculated specifically for each piece. The equation that we were given was a simple but approximate one; it worked well enough for our example hairspring, but produced a growing error the more that needed to be cut or for different sizes. Well, I wasn't an engineer for nothing, and took this as a challenge. With a bit of math, I was able to derive a new equation that was more accurate and worked for more sizes while retaining near the simplicity of the original. It is now a part of the official school curriculum.