Chasing Dial

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A worm is used to reduce speed. For each complete turn of the worm shaft the gear shaft advances only one tooth of the gear. As the speed is reduced the power to the drive increases correspondingly. Worm gears are a compact, efficient means of substantially decreasing speed and increasing power.
In this case, with a thirty tooth gear, the speed is reduced by a factor of thirty. Also, the axis of rotation is turned by 90 degrees.
Unlike ordinary gears, the motion is not reversible, a worm can drive a gear to reduce speed but a gear cannot drive a worm to increase it.

When the half nut lever is in the up position it is disengaged; in the down position, engaged. If the leadscrew is turning when the half nut is engaged the carriage will move towards the headstock or tailstock depending on the direction of leadscrew rotation as determined by the reverse lever. You must pay attention when cutting under power and disengage the half nut as the cutting tool approaches the chuck. You can reduce this risk in some cases by reversing the leadscrew direction so that you are cutting away from the chuck.
If the leadscrew is rotatating but the half-nut is not engaged, the thread dial turns slowly. Once the half-nut is engaged, the thread dial does not turn.