An English Boxford VSL lathe showing the clever but simple lever-operated mechanism by which the direction of drive to the screwcutting gearbox can be reversed.
In its neutral position it allows the headstock spindle to revolve without driving any of the gears. Lifting the lever up or down (the spindle must be stationary of course) engages one or other of a pair of gears below the spindle causing the next gear in the train to revolve either clockwise or anticlockwise.
By this means the entire gear train can be reversed in direction and the leadscrew (and power-feed shaft if fitted) made to revolve so that the carriage is moved towards the headstock or away from it.