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Stand the 2 uprights so they’re side by side. Nail 3 5 in (13 cm) pieces of wood across the 2 sides. Screw an eye hook into 1 end of a 16 in (41 cm) piece of wood. Mark 3 distances from the end of the 16 in (41 cm) piece of wood. Drill holes through the wood at the marks. Insert an eye hook and a nail at an angle on the other end of the wood. Measure 3 inches (7.6 cm) from the top of the uprights and drill a hole. Cut a 6 in (15 cm) length of wire hanger with wire cutters. Slide the wire hanger through the uprights and swinging arm.
Hold the 2 uprights next to each other. Try to line them up as evenly as you can. Both supports should be facing the same direction so they’re lined up correctly. Place 1 crosspiece at the very front of the 2 uprights and align them so they’re even. Use a hammer and nail the crosspiece to the 2 uprights. Then, nail a crosspiece just behind the posts of both uprights. Nail 1 more piece of wood at the very end of the 2 uprights to connect them together. Aim carefully and use small hammer strokes to drive the nails without bending them or cracking the wood. Take an eye hook and use your hands to twist it into the wood. Use constant pressure and rotate it clockwise. Continue twisting it until only the loop of the hook is exposed and the entire screw is inside of the wood. Don’t yank or force the eye hook too hard or it could crack the wood. Use a ruler or tape measure and mark 3 inches (7.6 cm), 4 inches (10 cm), and 5 inches (13 cm) from the eye hook at the end of the wood. Mark the locations with a pencil or marker. Make the lines dark so you can clearly see them. Use a small drill bit, such as a 40mm, to bore holes through the marks on the wood. Start the drill before you press it onto the wood and maintain steady pressure as you drill through them.  The 3 distances allow you to adjust the trebuchet to launch projectiles different distances. Remove any sawdust or splinters so the swinging arm will be able to rotate smoothly. At the opposite end of the piece of wood, screw in an eye hook at an upward angle so the look as on the side of the piece of wood and the screw is near the end of the wood. Then, take a nail and insert it into the end of the wood at a downward angle so it’s pointing towards the loop of the eye hook. Leave about 1⁄2 inch (1.3 cm) of the nail sticking out of the end of the wood. Use a small nail that doesn’t have a thick head on it. Use a ruler or tape measure to measure from the very tops of the uprights. Mark the location with a pencil or marker and place a dot at the center of the line. Then, take a power drill and use a small drill bit to bore through both of the uprights. Hold the uprights steady with 1 hand when you drill them so you don’t snap or crack the crosspieces. Untwist a wire hanger and measure out a segment of straight wire. Use a pair of wire cutters to clip the hanger and remove the piece. You can also use a smooth metal rod that fits inside of the holes. The 16 inches (41 cm) piece of wood with the eye hooks will serve as the swinging arm of the trebuchet. Push the wire through 1 upright, then slide it through the bottom-most hole of the swinging arm, then insert it into the other upright. You will now have a straight bar that the arm can swing around. Test the arm by rotating it back and forth to make sure it moves smoothly without any sticking or wobbling.