One of our design goals was to have a mechanism that would provide a "snapping" motion which triggers a fast jaw biting motion after the user has placed a coin on the mouth of the coin bank.

Additionally, we needed a modular way to power the Jaw mechanism without relying on knowing what the final design would turn be. This design philosophy led us to develop an integrated Gearbox and Geneva Timing mechanism that provides extra gearing for torque plus the mechanism needed to provide the desired snapping motion

The gearbox needs to accomplish many different goals:

Provide the Jaw Mechanism power in a flexible manner
Provide extra torque if needed
Give the jaw the ability to have regular jaw motions and to "snap" at a particular configuration.
Enable an easy way to reload the jaw to prepare for the next "snap"
Have a flexible way to control the snap strength

Version 1: Naive Approach with Issues

To accomplish (1),(2) and (3), a modular gearbox implementation seemed obvious.

Since the crank of the Jaw mechanism only needs a torque input, any power transmission that gives this torque would be acceptable. (1) is accomplished by making the gearbox modular, and it powers the jaw mechanism via a timing belt.

The gearbox can be mounted anywhere so long as the L-brackets can be fixed. The belts can be tension by simply moving the gearbox mounting point on a slot.

(3) is accomplished using a unique type of geneva mechanism. In essence, the final output of the gear is modified such that a) it is spring loaded somehow and b) it has missing gear teeth to decouple the input from the motor and let the spring "fire" the output gear.

This unique gear configuration enables the mechanism to "snap" at any desired configuration since the start and end of the "snap" motion is dictated by the position and number of missing gear teeth.

The idea was that (4) was accomplished by the virtue of the design. After the mechanism snaps, the mechanism is reloaded again since the teeth of the gears would re-engage with the input gear. However, the design did not enable this action.

(5) is accomplished In this design by having a wheel with holes for the extension spring. The idea was to have multiple holes that go out radially which the extension spring can mount onto. This enables changing the preload force of the spring to provide more snapping force if needed.

Manufacturing

Only requires a band saw, a laser cutter, and a lathe. The band saw is to cut the aluminum shafts, the laser cutter is to cut the two gearbox plates and the lathe is to make the standoffs that rigidly connects the gearbox together