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Chapter 3. BallistaMail > Foreign Words: Fabricating the Epizygis and Modiolus - Pg. 105

MAKE: PROJECTS Eccentric Cubicle Figure 03.106: The bow arms at rest, showing the bowstring making firm contact against the rear skein plate supports. The Harrowing Process of Optimizing Bowstring Length ! CAUTION: If you want your ballista to shoot straight, Pay Attention! Place your assembled trigger sled mechanism on the guide rails, lock the jaws around the bowstring and pull it back towards the rear of the ballista body while holding the loose end of the string in position on the bow arm. The arms will be pulled back under tension into their cocked position. When the sled reaches the back guide rail mounting blocks, adjust the string length so that the arms are in a fully retracted position. Cut the string to length and crimplock it. Release the string from the trigger and allow the arms to return to their resting position. [Figure 03.106] Now, adjust the position of the arm shafts in the skeins inwards or outwards so that the bowstring is snug against the rear rack support posts. Mark the position of the skein bundles on the arm shafts, and then drill and set a a short length of 1 / 8 " brass rod into the middle ply of the arm shaft at that point, to serve as arm position locks. [Figure 03.107] I used 100 lb.-test braided picture-frame-hanging wire to cable the winch. This is a more pliable material than the bike cable used for the bowstring, making it more suited to repeated winding/unwinding around the winch spool. It's less resistant to friction-related trauma, though, so resist the urge Figure 03.107: The bow arm position locks: cunningly deployed bits of brass rod to use it as a substitute bowstring. The pliability allows it to be easily knotted, which is the best way to secure it. Consult your Girl Scout knot-tying guide or Shibari Girl Scout tying guide for surgeon's knot instructions, and load up your winch with about 24" of wire. You'll need four epyzigis lock bolts, which are just ¾" lengths of ¼-20 threaded rod with a thumbnut stuck in place on one end with thread-locking glue. Thread them into the appropriate holes on the modioli after skein tensioning to keep the bars from slipping and releasing tension. [Figure 03.108] The projectile is a 6" length of high visibility orange 1" ID PVC sprinkler pipe with hardwood end caps spun with the invaluable drillpress lathe bodge (see Chapter 7). Aesthetically, it's kinda out of place in the wood `n brass surroundings of BallistaMail, but the low mass improves efficiency significantly and the cheerful, eyecatching hue makes finding errant shots a lot easier. The endcaps press-fit 94