A coarse screw-thread is formed in the upper part of the inner cup, and this screws on to the end of the iron bolt by which it is supported.
Each control is operated via a large knurled knob which turns a screw thread to effect motion.
Some flat bits have a screw thread instead of the center point and this helps the drill pull itself through the timber.
The deep spiral groove means that waste is removed quickly and the center screw thread helps the drill to pull itself into the material.
As the glass cylinder, driven by a motor, revolves upon its axis while also advancing (by means of a screw thread on the axis), all portions of the picture are successively brought under the beam or pencil of light and cause a beam of varying intensity to fall on the selenium cell.