I'm working on some demonstrations to teach mechanical engineering concepts to 11 - 17 year olds.
The first item that I decided to build was a Geneva Wheel. I made mine using a Shopbot.
The Geneva Wheel or Maltese cross is a gear mechanism that translates a continuous rotation into an intermittent rotary motion. The rotating drive wheel has a pin that reaches into a slot of the driven wheel advancing it by one step. The drive wheel also has a raised circular blocking disc that locks the driven wheel in position between steps.
The name derives from the device's earliest application in mechanical watches, Geneva, Switzerland being an important center of watchmaking. The Geneva drive is also commonly called a Maltese cross mechanism due to the visual resemblance when the driven wheel has four spokes. Since they can be made small and are able to withstand substantial mechanical stress, these mechanisms are frequently used in watches.
They are also used in film movie projectors, bank note counters, and assembly lines.