The Ballista is a Torsion weapon invented by the Greeks and widely used throughout the roman empire as a siege weapon. It survived through to the middle ages and was used along side Medieval siege weapons such as catapults and trebuchets. Its design is rather simple; It is two torsion springs made out of rope or sinew with the prods of the bow arm placed within them. Beyond this its mechanism is similar to the crossbow. The ballista uses a windlass to cock the mechanism and a quick release mechanism to fire the bow. It can be used to fire bolts or round shot.
Step 1: Materials
24"X48" "Handy panel" of MDF Particle Board
1/8" Braided nylon cord (48') (for torsion engines)
thin nylon cord (for bow string)
1/4" steel rod (36") X2
1" diameter PVC Pipe
small piece of leather (for bullet pocket)
The material cost for this project is surprisingly low, hovering around the $25 mark for all building materials.
Mill machine with travel of around 24"X48"
Solidworks 3D design software
Mastercam mill path/design software
set of hand files
hack saw with metal blade
Step 2: 3D Models
Step 3: Drawing Sheets
A basic guide for hand tools
Drill out all holes with a 1/4" drill bit
For pockets, drill a hole of a larger diameter than your jig saw blades width, then cut out the pocket with the jig saw
cut the parts out with the jig saw
Use a router to put the channel in the two body pieces
If you do not own a router, carve a path for the bolt track on each side with wood working tools
The first zip file contains the draft sheets saved as .SLDDRW for Solidworks and in .Jpg. The second zip file contains the draft sheets saved in .pdf
Step 4: Making the Parts
Design the mill paths based on the 3D models by opening the files in Mastercam and adding support geometry.
Mill multiple sets of the supports, as these parts are struck by the bow limb during firing and can fail over time.
Step 5: Assembly
The lock mechanism fits at the back of the crossbow. Follow the 3D assembly to build it. The remaining 2 3" pins go in the back 2 holes on the stock and the side plates of the lock slide over them. The 2 parts of the internal mechanism go inside and line up with the other two holes on the lock plate. The 2 parts of the trigger mechanism are held together by 2.25" pins.
Step 6: Bow Limbs
Cut the 1" PVC pipe to two pieces approximately 10.75"
Use a hack saw, band saw, file, etc. to make a knock approximately .5" deep on one side of each bow limb.
Step 7: Torsioning the Bow
Step 8: Stringing the Bow
Step 9: Conclusion
Video of ballista firing a bolt at target bag
video of ballista firing a rubber ball
video of ballista firing a bolt through a box. range distance- 40ft