Old screw-cutting lathe / fusee engine

Old screw-cutting lathe / fusee engine

This screw-cutting lathe, probably of early nineteenth century date, is of a type designed in the mid eighteenth century for cutting fusees for clock motions.

When the mandrel which carries the fusee blank is turned by the handle (not original), a pinion on it rotates. The pinion is in mesh with a rack which is thereby caused to travel at right angles to the axis of the lathe. The rack-frame carries a bar, the inclination of which is adjustable. The cutting tool is carried by a slide-rest which is restrained by guides to move parallel with the mandrel. The slide-rest also carries a saddle at one end which passes over the inclined bar. Consequently when the rack travels across the bed of the lathe, it causes the tool-holder to move along the bed at a rate which depends on the inclination of the bar and which thus determines the pitch of the thread cut in the blank.