Abstract: A conversion unit for attachment to a machine tool constructed to operate normally in English units of measurement so that the machine tool may be used directly in metric units, or vice versa. The unit comprises an externally toothed gear wheel which may be fitted to a screw of the machine tool to turn with it, and an internally annular gear eccentrically mounted to mesh with the other gear, the number of teeth on the gears being chosen so that the correct conversion ratio is produced, the measurements being readable from graduated scales.BACKGROUND OF THE INVENTION.