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Picture of How to make a custom rubber band gun
This instructable will give you, hopefully, ideas on how to make a rubber band gun from beginning to end.
We will start with the design and several different examples to give you an idea of what kind of rubber band gun to create.
 
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Step 1: Designing

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Ok to start things off, you'll need to design your rubber band gun. 

Start out with what material you want to make the gun out of and how to "cut" it.  One way is to use laser cutting and cut acrylic sheets or even metal, however with metal it's a bit more expensive and limited for sheet thickness.  Another solution to metal cutting is water jet cutting.  This method they could use nearly any thickness of metal.  For this instructable, however, will be using 1/8" thick acrylic sheets that will be laser cut from Ponoko.

Next measurements.  Measure out your hand so that you could design the gun so that it fits your hand.  Then you could measure out how long you want the gun to be.  Make sure that the rubber band you plan on working with are able to stretch long enough so that it could fit the gun without breaking.

Next is the actual design portion.  This is often the hardest part of the whole process, I've even heard that rubber band gun designing can be even more difficult than actual gun designing.

To help start here are a few places where they make rubber band guns. 

Site 1: OGG CRAFT http://oggcraft.jp/eng/eng_ratchet.html

This site is great.  It has so many different, innovative, rubber band gun designs.  The site link I've put on here is actually a list of they're different versions of ratcheting systems to deliver the rubber band.  Release mechanism : this link shows several of their different rubber band release mechanisms.  Love coming to find inspiration for a rubber band gun.

Site 2: Over Boost!? http://yappaps3.blog.shinobi.jp/Entry/47/

This site actually became internet famous at one point for their crazy awesome full metal rubber band guns.  The site link shows several different guns that they've created.  If you click on, I think, any of the guns, there's is a small gif that shows the mechanism of the rubber band gun.  They're often hard to see, and for good reason since the owner doesn't want to simply give their gun designs away.  Personally took some time to figure out how the mechanisms work, but this site has the "classic" rubber band gun design with the rotating rubber band holder and releaser.

Sorry for the long rambling for each of the two sites, but these two sites really are awesome, definite first place I go to if I want to make a rubber band gun. 

I'll talk about my design in the next step... this step is kinda gettin long... hehe

Step 2: My Design

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My design is based off of one of Over Boost!?'s design... don't remember which... I'll try to explain it as best as I can...

Ok so... the black line with the 1 is the rubber band.  The black line with the 2 next to it is a spring.  So... when you pull the trigger, teal, it pulls the blue forward.  The spring holds the blue part down while it creates tension with the purple piece.  Once the trigger goes up to a certain part the blue part goes up, releasing the purple part.  This releases the... what color is that? violet? alright violet part to spin and release the rubber band.  To stop the spinning the tension created from the blue and purple causes the purple to shoot back and stop the violet part from spinning anymore.  And... repeat.

The rest of the gun I've designed based on different guns.  The length and design of the gun was based off of the guns in the movie Equilbirium.  The handle I made so that it could fit the Beretta 92 side grip panel. 

Well... there's my design, if you want to use my design, go right ahead.

Step 3: Drawing/Making

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This next step is dependent on the person.  If you've figured out the how the mechanism works fully and don't need to visualize how it works in 3D... or... if you know how to make it in Illustrator accurately  then you can skip this step and the next, if not then continue reading.

Depending on the different 3D programs available this process could be different.  However, the overall idea of it should be the same.  The goal is to make the 2D drawing that was made previously and make it into 3D.  I first "converted" the 2D design into digital with Adobe Illustrator and used it as a template for the 3D designing.

In your 3D program of choice, make a plane of the outline of the part.  Then make a duplicate of the plane part and add thickness to it to match the thickness of the material.  This next part is optional, but I like to see how it moves by animating the parts... completely optional.  Next select the plane objects and export it to OBJ file format.

Step 4: Pepakura to the rescue!!

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For those who don't know what Pepakura is, it's a software which converts your 3D object into flat pieces with tabs so that you could make it out of paper or anything flat.  You can get more info on how to use the software with my previous instructable "For Halloween... Why Not be Yourself?"  Another handy little feature that this program has is that it could export the cutout pieces into vector format, which is the eventual outcome we are going for.

First open the software and open the OBJ file you've made and go through all the beginning setting stuff, things like flip faces, reflect, w/e.  I just do no change through the whole thing.  Now click the button at the top to uncheck the "auto" box, then click the "unfold" next to that.  Now with your 3D or 2D program figure out the length of the gun from end to end and put that into whichever length is appropriate in the window that appears after you click the "unfold" button.

Once you do that the program starts to lay out the pieces so that it could be printed out and put together, but we're unfolding flat surfaces so there's no flaps or any kind of folding necessary.  This program essentially scales your project to the correct size and you could then export it to a vector file format.

Step 5: File format for laser cutting.

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Now depending on the service you are using, they may ask for different file formats, but these are usually a vector file format like AI, EPS, or DWG.  For the service I use, Ponoko, they have AI files that has all the settings and the sizes of their sheets.

So what to do from here?  Open the exported EPS file you got from Pepakura and organize them into the Ponoko template.  For other places I'm going to guess that you could make the document the size of the material you got and then organize the pieces on there. 

Some suggestions for organizing the pieces.  Try and get some of the pieces to share an edge to decrease cutting time.  Make sure you don't have double lines, this could lead to the laser going over the spot again, which could widen the cut patch, making the pieces not as accurate as they're supposed to be.

Once this is done you can send it on the wherever and get them to cut your pieces out and ship them on to you.  Once you get them, get the screws, springs, w/e you need for the putting together part and there you have it.  ALL DONE!!

Step 6: FINALE!!

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Alrighty now for the real deal.  I've mentioned before that the design I got made was the first version of the one pictured in stage 2.  This version had a slimmer rotating portion. 

The first images shows the innards of the rubber band gun.  Notice how the rotating part with the teeth has gray goop over it and a tooth missing?  Yea that's my attempt at fixing it and trying to get it more rigid with some epoxy and thin metal rods.  The second image is a close up.

The third images shows what I tried to do to fix the teeth so that they stayed rigid, but... yea didn't work out too well, gonna go out tomorrow and get myself a metal sheet, cut out two copies of the part and somehow attach them to the sides, making them more rigid... hope that works.

Final images shows the fully assembled look.  Gonna upload a vid once I get it all in working order.  Other than the rotation portion being broken, the gun at first worked just fine, just got unlucky with a few teeth.

You may wonder why I sent the first version in and not the second, well... I had made and sent the designs before making this instructable.  By the time I had looked it over and saw my design flaws it was too late.  Now all that's left is to save up my money and make the second version later on in life.

On another note, does anyone know of a good place that laser or water cuts 1/8" metal?  Would like it if they had a simple layout template like ponoko does.  I wanna get one made out of aluminum or steel :P

Step 7: !!UPDATES!!

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Ok so I've added the metal wheel support and now it doesn't break... however the mechanism that moves to let the wheel go kinda holds the latch a bit too long and all the rubber band shoot out at once XD... gonna have to do a lil bit more modifyin.  I believe that the new version of my design should solve that... don't remember if I changed that.  I'll keep ya'll posted with this step.
Nice step by step project. Thanks!!!