Ballistas are basically large crossbows mounted on mechanisms that allow them to be aimed since they are usually too unwieldy for human use. Shown above are a few designs as well as a physical model, but as you can see, there are variations in the designs, especially compared to the final product. This instructable is meant to introduce you to the general concept, but it can be taken in many different directions. At first, my group was going to create the ballista to be electronically controlled, which influenced a lot of design choices. The presumed design is the drawing with the red bands in it, but midway through the project, we decided to push forward the development of the electronics and focus on a purely mechanical design (this is better represented by the second rougher drawing). If there are any inconsistencies in the pictures, feel free to reference the final step to see how our project specifically turned out, which you can use to crossreference and figure out the specific actions that should be done.

Supplies

Tools

Drill: bits about the size of the inner rod of the screw for pre-holes

Impact Driver: Phillips bit for screws

Miter Saw: for cutting 2x4s and PVC tubing to length

Sandpaper: for smoothing out rough edges and sizing holes properly

Adhesives/Retention

Wood Glue, PVC Primer, PVC Cement, Duct Tape

Main Pieces

2x4, 4ft lengths for the "X" shaped base (x2)

2x4, 1ft lengths for verticle reinforcement (optional, x2)

2x4, 8in lengths for the "U" shaped pivot section (x3)

2x4, 3ft length for the crossbow front section

Large 2ft or 3ft Plywood Circle

Blue Plate (or any flat moderately strong material) for aiming section

2in PVC, 4ft tall verticle pipe for pivot section

2in PVC, 4ft long horizontal pipe for the barrel

2in PVC Coupler (for the "X" base)

1.75in PVC Insert Cap

1.75in PVC Coupler

Additional Parts

Strong foot long rod for the shaft of the pivot

Large Washers for feet (optional, x4)

Wooden 6in Dowels (x3)

2ft long Elastics, 4ft stretched (10lbs pull force for our group, x2)

3D Printed Parts

Rollers (x4) (quantities below are for each roller)

Standardized Skateboard Ball Bearings (1 each)

3/4in long bolt (any thickness so long as it fits in the bearings, holds the bearing, 1 each)

1.5in long bolt (this is to attach the print to the "X" base, 2 each)

Corresponding nuts (3 each)

Side Pulleys (optional, x2)

Wooden Dowel (already listed above, 1 each)

Projectile1 (optional, as many as you want)

Step 1: Start With the Base

1. Cut two 2x4s to an arbitrary size stable enough to hold up the head. I recommend at least four feet.

2. Carve out a notch in the middle of two 2x4 pieces and form them together in an "X" pattern. Secure with screws and glue if available.

3. 3D print four rollers and attach bearings. These parts bear the brunt of the weight of the upper body and allow for a smooth rotational motion.

4. Drill a hole into the center of the piece, large enough for the PVC pipe to fit through a rotate smoothly. I recommend using a PVC coupler since the fit is very good, but it might require some sanding.

5. (Optional) Add feet to the base. I used large washers and some superglue, but you could use some 2x4 offcuts and some screws if desired.

Step 2: Assemble the Pivot

1. Get a large circle (about 2 or 3 feet in diameter) and cut a hole through the middle.

2. Mount the verticle PVC pipe through the hole. I recommend using duct tape to tighten the seal if you make the hole too large. Secure the pipe with screws.

3. (Optional) add verticle constraints, that being two 2x4 offcuts, to reinforce the verticle shaft and prevent a large amount of wobble later on.

4. Cut a strong rod to length (we used a 0.5cm thick hard plastic rod as our shaft) and put it through three 2x4s arrange into a "U" shape. You can use a screw force through the pipe into the side of the 2x4 to secure the angle, but a more permanent and easy to use solution would be to have a flat piece of material act as a hold for the specific angles (like the blue plate in the final product). But if you wish to motorize the project, then the plastic plate could be replaced with a gear section like the one in the drawing at the beginning of the instructable.

5. Place the pivot on top of the base. It should slide into the inner circle easily and rest upon the bearings on the base.

Step 3: Construct the Crossbow Head

1. Cut a slot into the side of the second PVC tube so the eye-hooks can ride along those channels. Then add a hole from the top to the bottom, about the size of a dowel, a few inches in front of the back of the slot to hold the carrier in place once the ballista is drawn back to fire.

2. Make a carrier to move along inside of the PVC tube. Ours is made of a PVC interior coupler and a threaded hole insert to create a flat surface. Install the eye-hooks inside of them 180 degrees apart so the elastics can attach to the carrier and launch a projectile in the chamber. If you follow the instructions exactly, then I recommend PVC cementing the threaded insert so that it does not come apart while it is pulled.

3. Cut a hole in the center of another 2x4 piece so that the PVC launch tube can slide through and be secured. Then, if you want to mimic a compound bow, you can add sliders on the side of the 2x4 by cutting a cubic notch, adding the 3D printed rollers on a rod, then screw in the eye-hooks on the front side of the bow.

Step 4: Bring All of the Pieces Together

1. This is the completed ballista with all of the accouterments our group added.

2. Add two 2x4s to the front of the head of the crossbow section then add another 2x4 at the bottom of the head at the center of mass (figure that out by holding the head and shifting until it is balanced). This will allow you to screw in the head to the pivot, completing the ballista once you add the whole upper portion to the base.

3. 3D print a projectile or find something roughly the size of the outlet, insert it into the chamber, draw back the bands, lock the dowel into place to hold the carrier, then aim the ballista and remove the dowel to fire.