Pre and Post Expo

Well hello everyone!

Ethan here, and we finally made it through the final day of the crossbow project, at least as far as our grade is concerned.

It was nail-bitingly close, as several things seemed to go wrong all at once, and everything needed to be addressed.

Starting off, we had to order a new motor for the crossbow, which didn't arrive until Wednesday, the 10th. And low and behold, it didn't fit the snazzy new gears we had acquired from an old Engineering teacher of mine who I shall keep anonymous for privacy reasons. The gears, as well as the housing that I built for them, are comprised of parts from a company called VEX, and they not only held up much better than the previous gears, but provided a more solid foundation for the gearbox itself, as well as making it easier to position and mount.

This left the body to be made, so Tuesday night, I grabbed a bunch of cardboard, some duct tape, and made the first body the prototype would be shown in:

 Scruffy here was kind enough to model the ADC V1.0 (Automatically Drawn Crossbow), and as you can see, its not the prettiest frame, but it was what I had lying around.

 Just a side profile here

And a top view to finish it off.

Alas, the cardboard frame brought its own problems. While it held everything, it was not very sturdy. Drawing the bow would cause the cardboard to deform slightly, which resulted in a loss of energy in attempting to draw the crossbow. A few inches was better than an immediate stall, but this still would not do. Luckily, Scruffy remembered he had some wood lying around at his house, and took the bow home to construct a more sturdy body.

Meanwhile, the motor was having its own issues with the design. The housing we had purchased for the motor did not line up with the holes in the VEX plates, so there was no way to mount it. Whats more, the motor shaft was 3mm in diameter, just small enough to not gain any purchase on an 1/8" square hole. So a mounting plate and a coupler had to be designed and 3D printed before the expo on Friday.

Mounting Plate in Solidworks

The motor shaft side of the coupler in Solidworks

The axle side of the coupler in Solidworks

Thankfully the parts were able to get fabricated and were picked up at around 1PM on Friday, the 12th, just one and a half hours from the expo.

Now not only had Scruffy been able to make a simple wooden base for the crossbow, but he also used a double pulley I still had lying around along with one of his own, making a pulley system to increase the advantage of the gearbox and make it even easier to draw the bow. It was only able to draw between 6-8 inches before we were uncomfortable with letting it run, but that's still a sizable amount.

 Now you might notice the hammer lying on the table here. That's not just randomly there. Our firing mechanism was made up of a metal pin going through the crossbow, and that hammer. The hammer was hooked onto the pin portion under the crossbow and levered to pull the pin through until the string released. This firing system turned heads, but had them quickly looking the other way to watch the cardboard tube soar across the expo room.

We are proud that we were able to get this up and running, but it doesn't stop here. A better motor, better gears, a custom gearbox, a more optimized frame, and we could have something quite feasible here. But for now, our professor liked it, onlookers liked it, and it was a blast to shoot around and show off the mechanics. Prototype successful as we see it.

So until more work gets done on this, I bid you all adieu,

Ethan Weigold