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Here's my guide on how to build a small catapult out of simple wood scraps. Once you have the parts, you could be 30 minutes away from flinging small objects at friends, family, and coworkers! 

TechShop Website

Step 1: Materials

All measurements are approximate. Also, the picture has extras of some materials (I'll notify which ones)

1 baseboard (1" thick, 6" x 9")
1 lever arm (0.5" thick, 1.5" x 12")
2 side supports (1" thick, 2" x 8")
2 sturdy L-braces and appropriate screws
1 hinge (same width as lever arm) and appropriate screws
1 small eye-screw (extra one in pic)
handful of rubber bands (find some with varying elasticities to allow for adjustments in catapult strength)
(ignore the spring in the picture, turns out I didn't need it)

Power drill (or screwdriver if you're feeling adventurous)
Staple gun (and staples, obviously)
Sandpaper (if you have options, use rough sandpaper to give you the shape you want quickly, and then fine sandpaper to make it really smooth. if you don't really have many options, I would place a strong preference on fine sandpaper)

Step 2: Attach Side Supports

Attach the two side supports to the baseboard using the L-braces. Place them as close to one end of the baseboard as the L-braces will allow. Make sure the L-braces are in-between the side braces and the edge of the baseboard (like in the pic). They need to be on the backside of the catapult for maximum support. Make sure both side braces are sturdy and firmly attached.

For me, the L-braces alone were enough to secure the side braces down, but if you have smaller L-braces and they don't quite do the job you can improvise with screwing the side supports into the baseboard (just be careful not to split any of the wood)

Step 3: Attach Lever Arm

Use the hinge to attach the lever arm to the end of the baseboard that is opposite from the L-braces. Make sure to orient the hinge so that the lever arm can fold up over the baseboard. It doesn't have to be able to fold flat against the baseboard if the hinge screws prevent it from doing so.

Step 4: Restrict the Lever Arm's Movement

Warning: The measurements of this step are entirely dependent on the rubber band you use. Any numbers I give here are just a frame of reference based on my materials.

Screw the eye-ring screw into the lever arm a couple inches up from the hinge. Push the rubber band through the hole in the screw and loop it through itself to secure it to the eye screw. 

Here's the tricky part: 
Depending on the elasticity of the rubber band you use, you'll need to place this next screw somewhere along the center line of the baseboard so that the rubber band stops the lever arm from falling flat on the table when you fire something from the catapult. I recommend using a screw with a very wide head and leaving enough space between the top of the screw and the baseboard so that you can loop the rubber band around it and it won't come undone accidentally. (See photos if the wording of that doesn't really make sense)

Step 5: Quality Control and Testing

Loop a rubber band around the top of the two side supports and set up the catapult like in the picture. Test the catapult to make sure the following things happen:

- The catapult doesn't fall apart
- The lever arm swings easily
- The rubber band safety catch successfully stops the lever arm from falling flat on its face, but also doesn't restrict its forward movement too much (readjust the screws holding the rubber band if it doesn't work)
- It doesn't kill people (if it does, consult a lawyer and then adjust the catapult to not kill any more people)

If your catapult works successfully, you'll probably notice that it's destroying your rubber bands little by little with every practice launch. Guess what we do in the next step?

Step 6: Sand the Lever Arm

Find the section of the back of the lever arm that comes in contact with the rubber band and sand the corners and back face to make them nice and round and smooth. Just to be safe, you should sand way more area what will actually come in contact with the rubber band. If you find yourself unable to stop stroking the silky smooth wood, you've done your job correctly. 

Step 7: Secure the Rubber Band

Using the staple gun, secure down a rubber band across the tops of the two side supports. Make sure not to actually puncture the rubber band with the staple, as that will severely weaken the rubber band and inevitably make it snap. If you hold the rubber band flat against the wood and center the staple gun over it perpendicularly, it should place the staple so that goes around the rubber band and permanently secures it in place. If the rubber band breaks from extensive use, just staple another one down and it's as good as new.

Step 8: Drill an Ammo Cavity (Optional)

If you have the right kind of drill bit, you can drill an indentation in the lever arm to give you a secure place to load your ammunition. Make it as deep as you like, but remember that too much drilling could split the wood and ruin your lever arm.

ALTERNATIVELY: You can glue a plastic bottle cap to the top of the lever arm to give you a nice little ammo slot without sacrificing potential wood strength. 

Step 9: World Domination

With the power of your new catapult, you will easily be able to smite your foes, conquer nations, overthrow governments and regimes, wreak global havoc, and take over the world. Your enemies will tremble where they stand and thoughts of you and your deadly, unprecedented weaponry and unmatched mechanical skill will keep them up at night.

If world domination isn't your thing, on the other hand, it's great for flinging coins and small objects at those inattentive enough to walk in front of your mighty catapult.
If I scale the parts I think I could 3D print
Looks like perfect size on our board room conference table. ;-)
it looks so cool I am going to menards to get all the supplies

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