project, Woodworking, Armory

Rubber band gun - MODOTCHAO clone

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IMG_20120819_110434.jpg

I would love to claim that I designed my latest project, but all I did was find this video, and be inspired to try to copy the design. I have always been a fan of making things with mechanisms, and so the incredible collection of rubber band guns that ogg craft has designed and built really grabbed my interest. Plus I figured a rubber band gun is a great thing for the office environment (to compliment the flying monkeys and the quadcopter)

Ogg craft has a great array of images and video, but no specific plans, so when I set out to recreate the MODOTCHAO I basically started by tying to draw what I could see into a schematic and fill in gaps or guesses about how things work. In particular I had to think about what measurements mattered versus which could be fairly arbitrary.

Here are my sketches

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The obvious thing here is that the ratchet mechanism implies a few important measures. The gap between ratchets needs to be the same as the gap between the pins holding the bands. Each pull back on the ratchet is just enough to release one more band. This also implies that the trigger needs to be able to have a travel enough to pull us back that amount, and slightly over, but not enough to move two positions.

I arrived at the measure by some guesses at how thick the rear pins needed to be to support the load of the rubber bands, whilst not so think that everything becomes too big. I figured that 3mm made sense, having decided that I sketched out the rear and all the pins and arrived at measurements for that piece of the gun. Then the gaps between the pins at 45 degrees determined the side of the ratchet teeth.

Next I stretched a few rubber bands to their limit to get an estimate for the total gun length, it seemed like 300mm was the upper limit on how far you really want to stretch a band, so I used that to restrict the maximum length (basically the length when loading the bands from the front to the back of the last pin)

with all these guesses and some drawings I figured I was in pretty good shape, amongst my musings I figured I had a pretty good idea how the trigger needed to work. Clearly it was effectively 2 thin sides, connected at points, with a gap in the middle for the latch of our ratchet. In order to move back and forth there needed to be a recess in the handle that would let it pull back, I knew something needed to stop the trigger from just falling out of the gun, so at the back I figured I'd sink another pin into the handle piece and have the trigger slide onto that. At the front the trigger is captured between the sides and the trigger guard piece which has a small blade piece which extends up between the sides of the trigger.

I wasn't sure where I would find 3mm 'pins' and at one point thought I'd just turn them. But as I walked around my local supermarket I happened to spot some bamboo skewers that looked perfect. They actually required a little sanding to get the right diameter, but they were cheap, strong and ended up being what holds the whole gun together (well, with a little glue to help)

For the rest of the materials I used various bits of off-cut I had laying around, the dark wood is mahogany, the handle is beach, the main shaft I think was some sycamore off-cut. Basically whatever I had laying around that was roughly the right size for the pieces I needed to make.

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Here are the pieces dismantled as I applied wood wax

What became apparent in this gun is that there are a lot of forces at work, and getting the right balance is crucial to it operating correctly. Obviously when fully loaded there is the force of 8 rubber bands holding everything in place, the first pull on the trigger has to push the side fins back against that force. In my original attempt I was concerned that the side fins are only attached in one place, onto the main top 'barrel' right at the top corner of their shape. Its obvious that the rubber bands are going to excerpt a torque on this joint. So in my design I drilled two small holes to put some bamboo pins through which I hoped would combat that torque. However it really wasn't enough, even glued, the joint was weak. So I decided that what was required was a hidden rail on each side of the gun. The side pieces would get a corresponding groove cut on the inside, so they would just slide along these rails. This was a relatively straight forward modification, and for all I know the original ogg craft gun has exactly this, since it doesn't have to be visible that the rail exists. This modification made the whole mechanism much more robust feeling.

The second point of concern is the actual trigger hook which pulls against the ratchet teeth. This is quite a thin piece, and I ultimately made 4 different attempts at it. the first was ok, but I put the pivot point too high. I assumed that the pin which holds the rubber band for trigger return went through the trigger and also provided the pivot for the hook piece. However this was too high, pulling back on the trigger wanted to make the hook pop out of the ratchet teeth and so it was hard to pull with sufficient force to release the last few bands. To fix this I made  new hook piece with a pivot point at the bottom. Actually I made a couple, the first snatched away from me on the belt sander and was shattered, the second was made but the wood grain was too weak and  the hook snapped off in tests. The last I made from metal, not exactly in keeping, but I was tired of making wooden variants that broke. Initially I tried a pivot point at the bottom of the trigger, figuring this way when I pull back the force is pulling the hook up into the teeth and giving better hold. However that was too much force upwards, it meant that the hook didn't release from the teeth on the forward stoke, it just bound into the tooth and stuck there. And so I adjusted again to put the pivot in the centre of the trigger. This way when you pull back you can flex to the top half of the trigger, and when you want to release you can flex down and assist the hook coming free.

The hardest part is actually releasing the last band, this is where the rubber band providing the return force is at its biggest stretch, so whilst there is just one more band to pull against, the whole gun want's to pull back to the start position, making it hard for the hook to come forwards fast enough to catch the last tooth before it pulls forwards a little. Getting the strength of this return band right is key, too strong and you can't fire the last shot, too weak and it doesn't have enough force to overcome the friction of the wood, and the return doesn't work.

Here is a video of mine in action, in the first you can see I have to use my other hand to help get the last band fired, in the second I show just a clear view of the return mechanism. To be honest that bit is about the most satisfying part of the design, I find myself just happily setting the gun to the fired state and just enjoy triggering the return slide. It feels solid and satisfying as a motion.

http://www.youtube.com/watch?v=9xs2IHXySvs&feature=youtube_gdata_player

http://www.youtube.com/watch?v=yDtJGk5ystw&feature=youtube_gdata_player

I went through a bunch of tests, assembling and disassembling, so most of the bamboo pegs holding it together are not glued in place. Here you can see me testing it before cutting the bamboo to length

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