If you use the carriage bolt, you'll have to grind it down so that it's smooth. To make the firing pin, you're going to need a sturdy hinge, some more steel pipe, and any carriage bolt or smooth piece of metal. You'll use this later to pull down the arm, then unhook it to prepare to fire. Tie one end of the nylon rope around the spool and the other end around the S-hook. Weld another pipe onto the spool, as to create a means to wind the spool with some sort of crude metal handle. Cut and place a metal pipe into the holes, and leave a bit sticking out on each end - this will be the spool. Step 7: Make the Winding Spool, Rope, and Firing PinĪ few feet in from the back of the catapult, on the upward sloping braces, drill a hole through each brace, so that a metal pipe can fit though the holes. Slide out the aluminum pipe, and slide it back on so that the Rotating Arm Holder is able to Rotate about it. The holes should be a bit bigger than your carriage bolts, and will be used to hold the arm in place. I would take this opportunity to drill two holes, one 1/3 up the 4" pipe and the other 2/3 up the 4" pipe, one perpendicular to the other. Now, compare it to the width of your 'pult, center it, mark the edges, and cut off the excess 2.5" ABS. When you're done, it should look similar to the 2nd to last picture in this step. The parts are going to be a tight fit, and the cement sets relatively quickly, so you have to apply a lot of pressure to make sure your members are in their correct places. You want a nice, even coat on both members, and don't apply too much. Unscrew the applicator from the can and wipe it on the edge of the can - the applicator tends to pick up way more cement than you actually need, and you don't want to waste any, do you? Spread the cement on the inside of the T adapter, and on the outside of whatever you're putting inside the adapter. To use ABS cement, first try and make sure that your surfaces are nice, clean, and smooth. You might want to wear a respiratory mask and GLOVES for this one, too - that cement stuff is narsty! In this case, we're going to take the 4" pipe and cement it to the base of the "T", and on each side of the "t" we'll cement in a 4"-2.5" adapter and half of the 2.5" pipe (2' in each side). Sounds like a job for the Rotating Arm Holder! Get your ABS T-adapter, 2 sections of ABS pipe, and ABS cement. Now that we have a nice, smooth axle, we need something that will rotate about the axle and hold the arm. Add a ton of padding to the cross brace - you'll need this if you don't want the catapult/arm to break. Also, on one side, install the other U-bolt, facing inwards, through one of the posts - this will be a tie-off point. It's pretty scary when one of those snaps at full tension. Our tubing ripped a few times because the holes had sharp wood daggers eating the tubing. This is there the surgical tubing will go, so try and make it nice and smooth. say 1.5") holes into the cross-brace on both sides. Turning lag bolts in 4x4's is pretty annoying, isn't it? Drill two huge (but not too big. Keep turning until the little pointy part sticks out of the other end of the wood, then re-clamp it and finish up. You can take it down from the posts now if it makes it any easier. Now, start screwing your lag bolts into the cross-brace piece. The cross-brace's top should be flush with the top of the two posts, and should be closer to the shorter side of the base of the frame. To install a cross-brace, first pre-drill four holes in that leftover 3' 4x4 that you cut earlier (two on each side), after clamping it to the top of the two 4x4 posts we put up in the last step. Otherwise, follow the instructions below. If you don't want a cross-brace, you'll have to get two more u-bolts and bolt them to the top of the posts so that they face eachother. If you want to go true catapult, the cross-brace is essential, but I highly advise against it. Later on, we found that this really didn't work, and ended breaking a really nice fiberglass pole because of it (and the insane momentum it had). So we originally planned to have a cross-brace, which would stop the catapult arm from aiming down and force the release of the projectile at a 90 degree angle to the ground.
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