The arms of a ballista are under great stress when the weapon is operating at realistic power levels. This means two things:
- The tiller of the arms must be near-perfect
- The fibers at the back of the arms can't be cut through
There are two ways to ensure that #2 holds:
- Use the outer surface of the wood directly as the back of the arm
- Use a split surface of a log as the back of the arm
This article describes approach #2.
The process step by step Edit
You will need an axe and and a hammer for this job:
Next take a piece of straight, well-dried and good-quality bowmaking wood. I've used elm, but any sufficiently strong and elastic wood will work just fine. If you only have medium-strength wood available (e.g. birch), make the arms slightly thicker and wider than usual. I would personally not even try using softwood arms, regardless of how thick they are made: the torsion springs make visible dents even to hardwood such as elm. A softwood arm would probably crush and break in use.
Mark the outline of the arms to the end of the log:
Next split the wood slowly and carefully into smaller and smaller pieces:
Once the log is split, carefully smoothen the back surfaces of the to-be arms and continue by planing and filing the arms into correct shape.