Small watchmaker's screw-cutting lathe or fusee engine

Small watchmaker's screw-cutting lathe or fusee engine

This small screw-cutting lathe, which was intended to be clamped in a vice, was designed for cutting fusees, spirally grooved spindles which function as an equalising force on a clock's mainspring, ensuring it imparts a constant force to the clock's motion even though the spring is steadily unwinding itself.

The cutting tool is held in a holder which is constrained to slide parallel to the mandrel, on which a screw thread has been cut. On rotating the mandrel and the work held in the chuck, an elongated nut, which communicates with the tool holder through a lever, runs along the screw and causes the tool to traverse. The pitch of the screw cut depends on the leverage employed, which is adjustable by altering the position of the fulcrum.