This clever device is a factory machine for making screws. Three cog wheels turn three shafts of steel. The middle shaft is the rod being made into a screw by a cutting surface located in the cross-beam, while the other two shafts ensure that it will be cut evenly. The size of the wheels determine the pitch, or turns per inch, of the screw. Leonardo has drawn replacement wheels, which can be seen under the machine. The bigger the wheel, the more threads per inch in the screw.
