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: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.