- First, modify the 1.25-inch PVC crosses so they slide easily down the 1.25-inch PVC vertical pipes. In addition, you'll have to modify the 1.25-inch PVC T-fitting so it rotates freely on the falling cross arm. To do this, use a rotary tool such as a Dremel with a sanding drum or better, an electric spindle sander to remove enough PVC from the interior of the crosses and T so the pipe slides through it easily. Sanding down the PVC isn't hard, but it is a messy job, so wear a dust mask while you work.
- Once all the PVC pipe pieces are cut and the crosses are modified so they slide easily, attach the pipe flanges to the plywood base securely using screws or bolts. Then, dry fit all the pipe and pipe fixtures to make sure they fit correctly, adjusting the alignment of the PVC fittings as necessary for smooth operation.
- When all the pieces are perfectly aligned and slide easily, permanently assemble your trebuchet by solvent-welding the parts together, as shown in the assembly diagram, using PVC primer and cement.
- Next, build the wooden strike bar and attach it to the plywood base using deck screws. This is merely a wooden bar that flips the throwing arm over when it falls. You can experiment with the positioning of the strike bar for best performance.
- To get the most distance from your trebuchet, put your projectile in a sling. That increases the length of the lever arm and gives you more throwing power. While medieval catapults used a sling attached to the throwing arm, getting those to work properly is tricky, so it's easier to simply attach the sling to the projectile. You can try golf balls, small water balloons, or other items of your own choosing for ammunition.
- To unleash the power of your machine, raise the weighted crossbar and insert a wooden prop to hold it. Tie a rope to the prop. To fire, give it a tug.