Universal Catapult for Seej

by BrianEnigma, published

Universal Catapult for Seej by BrianEnigma Jul 4, 2012


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In the late 1700s Eli Whitney, the designer of the cotton gin, developed muskets with interchangeable parts to help the United States military. The manufacture of objects with interchangeable parts was soon a key factor in the industrial revolution. Mr. Whitney is perhaps a little anachronistic when brought alongside castle and siege warfare, but we can use the principles he championed to build a better piece of siege machinery.

In this spirit, and on July 4th, America's Independence Day, I would like to share the Universal Catapult for Seej. (More information about Seej can be found here: http://www.zheng3.com/seej/ ). Some friends and I started to play Seej, using the basic set ( http://www.thingiverse.com/thing:24013 ). Maybe it was our technique, maybe it was the rubber bands we had on hand, but we found the rubber-band-as-pivot to be woefully inaccurate and nearly unplayable. I set off to build a better catapult.

After two rounds of revisions, I'm posting the v03 Universal Catapult design. This catapult has three main features going for it:

Tension Keys - The tension keys pull the rubber bands instead of twisting them. This leads to fewer twisty knots and better linear tension.

Adjustable Strike Plate - I only have the one strike plate for now, but in theory, you should be able to swap them out for plates of varying depth. This allows for more fine control of the angle of release.

Modular Base - Everything sits atop a base with modular holes. If you would like to design a trebuchet, ballista, or even gatling gun or death-laser, then feel free to skip reinventing the base and use this one. See the base detail image for exact measurements.

The OpenSCAD source files are also attached.

For the blog post announcing this model, read http://netninja.com/2012/07/04/the-universal-catapult-for-seej/

For the blog post presenting the earlier revisions that led to this one, read http://netninja.com/2012/08/12/the-evolution-of-a-catapult/


Print the following pieces in these quantities:

  • • Base, x2, base.stl, white in the image
  • • Base Struts, x2, strut.stl, gray in the image
  • • Rubber-band tension keys, x2, key.stl, yellow in the image
  • • Sides, x2, side.stl, brown in the image
  • • Arm, x1, arm.stl green in the image
  • • Strike plate, x1, brace.stl, blue in the image

I used the Replicator defaults, but bumped up the number of shells to 2 on the arm and keys.

Assemble the plastic parts as pictured. With heavy office rubber bands, I use two -- one from the catapult arm to the peg of each key. For longer, thiner rubber bands, I would suggest looping the rubber band from the peg of one key to the catapult arm and over to the peg of the other key. Once the rubber bands are in place, the keys can be turned to adjust tension. Pull the keys up slightly, turn, and re-seat to adjust the catapult strength.

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Since you were shooting (pun) for accuracy . . . why did you stick with rubber bands? Why not springs or repelling magnets? Rubber bands deteriorate gradually with use. Springs as well, but much less. Magnets . . . generally you wouldn't notice a changed in this century. Just curious.

Given that this was designed to be a kid's toy (a different catapult design for the Seej game), I went with common household items — just like the original game. Not everyone has a spring with a specific length and tension coefficient, nor does everyone have neodymium magnets of a specific size and strength lying around in their junkdrawer at home.

dues it shoot

Xerxes' observation is pretty much spot-on. Printed every part in PLA in one go, they all came out spectacular except for a tiny bit of warping in one of my base plates, although it has no effect on the model itself. Every part goes together fantastically, although the tensioner keys are a tad loose in their slots and sometimes forcefully eject when a projectile is launched. I just put a layer of blue tape around the circular peg that sits inside the base piece and they stay put now. Coins are thrown about straight, maybe a five or ten degree angle. Strike plate experimentation is in order. Oh, and one last thing - the slot is too small for an american penny. I just put a penny in, used a torch on it, then squished it into the slot. Works like a charm.

This printed great! Snapped together solidly. Tension keys are very functional and easy to use. It shoots pretty much straight on, which is fine. I'll probably make the strike plate wider (towards the arm) so it will launch a little more vertically--and the modular design makes for easy switching of the strike plate. A longer arm would also be nice since the tension keys are close to either side of the arm, cramping the room to hold the arm down. Overall, though, a very nice design and I will be replacing my kids' arsenal with these.

I've found string works much much better than elastic bands with the original design - just wrap it between the keys a few times, then tighten (fiddly). I went from pennies going half a metre to
gt;5m :)