In this design, 12 teeth on the spur are theoretically in constant mesh with the worm gear, although manufacturing tolerances result in less-than-perfect contact. The gear reduction is 72:1 (72 revolutions of the worm gear produce one revolution of the spur gear). Both the worm and spur are constructed using true involute gear tooth profiles.
The worm gear is horizontally mounted on a pair of knife-edge bearings, and the spur is fastened to a vertical shaft using a c-clip.
This model makes an excellent hands-on demonstration of worm gears for a wide range of students. It can also be used as a challenge. i.e. computing and modeling the worm gear geometry is non-obvious.