A fusee is a device used in clocks and watches to equalize mainspring
torque, since mainsprings supply heavy torque when fully wound and less torque as
they run down. In contrast, weight-drive provides constant torque.
This is a reverse fusee for a clock I'm building. "Reverse"
because the fusee is on the same arbor as the
mainspring. This permits making it with a simple cone shape instead of a fancy curve, and because it permits changing the amount
of torque delivered to the movement.
(more details..)
The picture shows the fusee/mainspring and the drum in a test frame. Shown here with the fusee/mainspring arbor wound up and pulling on the drum. The mainspring is coiled up inside the barrel which is screwed to the fusee.
The cord from the fusee/mainspring wraps around the drum on the right, which will eventually hold the main wheel. To increase
torque to the train, you'd increase the drum diameter -- reduce drum
diameter to decrease torque. As shown, the drum is 3/8" in diameter and holds 22 turns. The mainspring/fusee has ball bearings on both ends. There is also a ratchet on that arbor which permits a "pre-wind" on the fusee spring.
If you do a search on Google for reverse fusees, you don't find much. I suspect they weren't used a lot because of the extended
length of the fusee/mainspring arbor -- too long to go between the
plates of a watch.
You can't cut it all at once, so you need to make several passes over to cut the groove. I ran the lathe as slowly as it would go.
John Shadle, CMC
Here is the tool I used to make the grooves in the fusee. It's made from heavy 1/2" x 2" steel, with the halves hinged with cone bearings. It mounts on the crossfeed of my Chinese 9 x 20 lathe. The bit is a round-nosed bit with a negative rake. The lathe carriage is power-driven at 8 threads per inch, or a pitch of 1/8". Pressure is applied to the bit by hand to make the cut.
