Rat Jaw Mechanism 1st Draft
At first, it was a great idea. Then it started to fall apart.
I based my ideas on these newer drawings that show how a compound pulley system might amplify my jaw motion. The drawings show the axis of rotation for the jaw above my forehead, which would put my head in the rats neck. Not quite right. And since I was mentally referring to these drawings when I built the framework and mechanism, those proportions are also incorrect. Back to the drawing board.
The left one above is the closest to what I should have made.
This is an “artistic rendering”.
I ordered parts from Pololu, made by Tamiya, thinking they would suit the nature of the bench build of the mechanism, namely the framework and the mechanical system, but the parts turned out to be difficult to use. First of all, they’re injection molded so in order to use a part I have to cut it away with a blade. The tolerance is not great on some pieces, with too much slip. Girders and shafts can be adjusted to suit the specs of the system, but it’s destructive, and the part may not be useful for another project later. You have to break the plastic pieces and cut the metals ones with a file.Basically you have to buy a crap load of parts and hope you don’t make mistakes. The metal hardware (nuts, bolts, screws, shaft collars, and shafts– no washers!) are quality, and there’s variety, but they don’t give you enough of them to make it easy. I had to dip into other sets for more bolts/nuts and shafts. There are these really horrible plastic nuts that you thread yourself by wrenching them onto the bolts. They only work once, and they don’t work for very long. The pulley wheels are okay, but the rubber bands are not, and neither are the plastic parts that keep the pulleys on the shafts. Those parts are tiny and very similar too each other but not easily identifiable– it would have been nice for there to be a color system. And again, there’s not enough of them. My review: not great for prototyping, but the parts can be used in projects in lieu of building small parts to spec out of wood or aluminum. Legos seem to be comparably priced with a lot more yield, especially reusability.
The disaster area:
So after wallowing for a night, I set out to build the frame and mechanism out of wood and aluminum, with just a smattering of Tamiya parts. I built a curved, aluminum jaw-like part that would interface with my jaw, and placed it between two wooden blocks. To the front of that I attached another piece of aluminum, the curve of which somewhat followed the line the jaw piece draws in the air, but it straightens a bit towards the top. That piece pulls on two cables that are threaded through two pairs of pulleys before reaching the rat jaw piece. A spring causes the rat jaw to recoil back into closed position. It’s basically a third class lever with two parts acting. Behold the phallic torture device reminiscent of a cardinal tombstone:
Main problems with this design are:
- static hang: when the mechanism is in the correct orientation, the minor weight of the wood I used for the jaw pulls on the spring. This problem is exacerbated when any motion is applied because of the uncontrollable bob and dangle action.
- spring tension: as the spring is extended, the tension increases and more power is needed to extend it further. My jaw works hardest when it is closed, and has the least force when fully open.
- there is no comfort.
- it is nowhere near head mountable.
To make it better I will try making the rat jaw lower on my head, closer to mine, and fit it around my head a little closer. Since I don’t want the rat costume to be ENORMOUS and unwieldy, I should spend a lot of time making the head smaller, and have less dead space in the inner real-estate. I need to split the work of the pulleys better, widening the space between them. In v.2 it may be that there are no pulleys and the rotational axis of the rat jaw is placed exactly over my jaw’s axis. If the piece that interfaces with my jaw is melded with the rat jaw, the motion will be 1:1. That would function along the same principal as a small pulley/gear sharing the axis of rotation with a large pulley/gear: the small rotational motion is amplified in size and speed. However, this would mean I need more power in my jaw.
Video of the torture device in action:
More pics/drawings are available in this Flickr set.