Make the arm from the ½”×3″×36″ pine board. Drill a ¼” hole centered 20″ from one end. This will be the finger end of the arm. Taper the arm from a full 3″ at the axle to 1″ at the finger end. We used a band saw to cut the board, but a handsaw would work.

Now determine the length of the counterweight end of the arm. Figure B shows the counterweight end extending 14¾” from the axle hole. That was the length that worked for us. That distance may need to be different for your arm, depending on the height of your stack of weights, because the weights need to clear the triangle. To determine how long to make the counterweight end of your throwing arm, measure the height of your stack of weights and subtract that number from 17¼”. That should give you nearly 1″ of clearance for the counterweight to swing through the triangle. If you allow too much clearance, then your weight doesn’t fall as far, so you’re giving up energy.

Using the length you just calculated, measure from the center of the axle hole toward the non-tapered end of the board. Make a mark and cut off the board.

The point here is to build a strong triangle with a base 20½” inches from the axle. The triangle needs to be wide enough so that the counterweight can safely swing through without hitting it, and it needs to be strong enough to withstand extreme forces during the launch. To keep it simple, we made an equilateral triangle with plenty of extra room; feel free to make your triangle a little narrower.

Lay the three 36″ square dowels on a floor and arrange them in a triangle. Lay a ½” scrap on top of the triangle legs as shown, and use it to mark the locations where you’ll cut. It’s important that this scrap is ½” thick because it represents the thickness of the throwing arm.

Now mark the axle hole. This mark should be perpendicular to the cut marks that you already made, and, therefore, perpendicular to the throwing arm.

Without moving the legs of the triangle, shift the triangle base toward the axle mark until the bottom of the base is 20½” from the axle mark. Make sure you still have an equilateral triangle; the legs should be the same length. Our legs ended up being about 233/8″ inches, when measured as shown.

Keeping the legs in this position, mark both legs and the base for cutting. Then cut off both ends of the triangle legs, and both ends of the base.

Drill ¼” axle holes, centered in each leg, parallel to the lines you drew. It’s easiest to drill the side where the hole will be perpendicular to the wood. To be extra sure he drilled in the right spot, my son first carefully made a pencil mark where he wanted to drill. Next he made an indentation with a center punch, and then he pre-drilled with a 1/8″ bit before finally drilling with the ¼” bit.

Hot-glue the legs to the base, then secure these joints with drywall screws. First drill 1/8″ pilot holes, then screw 2″ drywall screws into the pilot holes

. You’re drilling near the ends of the dowels, so you may split the wood. We split one end, then glued and clamped it. We’ve had no problems with it.

Drill a ¼” hole 4″ from the axle hole, on the finger side of the arm. Our hole was about ¾” from the edge of the arm.

Drill a ¼” hole centered near the end of a popsicle stick. To reduce the chance of splitting, clamp the popsicle stick firmly against a piece of waste wood, and drill through the stick and into the wood. You may want to drill a small pilot hole first, to make sure your hole is in the center of the stick. (Popsicle sticks are handy but they’re barely big enough to drill a ¼” hole. Any thin, strong, drillable material would work well here. It just needs to hold up the projectile.) Repeat this with one more popsicle stick. While you’re at it, make some extras.

Use the 1″ hex bolt and the wing nut to attach the popsicle sticks to the arm.

We used fabric for our design because it is readily available. For my son’s competition trebuchet we used a piece of net, which seems lighter and has less wind resistance.

Cut a 10″×10″ square of fabric. Nylon works well because you can melt the holes in it and seal frayed edges. Draw a diagonal line from one corner to the opposite corner. Find and mark the center of the diagonal line.

Draw two pairs of dots. Each pair should be about 1½” apart, and 1½” from the diagonal line.

Now make holes where you drew the dots. Because we used nylon, we could melt holes with a nail that we heated with a propane torch. My son held the nail firmly with a pair of vise-grip pliers.

At each end of the diagonal line, tie the corner of the fabric in a knot, tight and close to the edge of the fabric. Then use cable ties to cinch each pair of holes together. Clip off the cable tie excess.

Test your pouch. Place your projectile of choice in the pouch. We used a “high bounce Pinky” ball for the projectile. A small apple or just about any other roundish object should work. See if the projectile stays in place when you hold both ends of the pouch. Then let go of one end of the pouch and see if the projectile rolls out. If the projectile either won’t stay in or won’t come out, cut the cable ties and make new pairs of holes. Spacing the holes more widely (before cinching with cable ties) should hold the projectile more securely in place. More closely spaced holes should help it roll out. Placing the holes closer to the centerline may also help the projectile roll out more easily.

Trim off excess fabric.

Cut the head off one of the nails, so that the remaining headless nail is about 2″ long. This will be the trebuchet finger. It looks like a round dowel in this drawing, but for this small-sized trebuchet we only need a nail.

Drill a small pilot hole into the finger end of the arm. The pilot hole should be a little narrower than the nail that will become the finger. The purpose of the pilot hole is to prevent splitting as you hammer in the nail. It would be a good idea to try a practice hole on some waste wood first.

Nail the pointy end of the clipped nail into the pilot hole, so that ½” sticks out. If the end that’s sticking out is sharp, file it down until it is rounded. You don’t want it to scratch someone.

Tie a long length (maybe 30″) of cord next to one of the sling knots. The knot in the sling fabric keeps the knot in the cord from slipping off.

In the same way, attach another length of cord to the opposite corner.

Drill a hole through the throwing arm, barely larger than the cord, centered 2″ from the finger end of the arm. This hole needs to be far enough from the end so that it doesn’t hit the buried end of the nail that forms the finger.

Insert one sling cord through the hole so that it enters the arm on the flat side and comes out on the tapered side. Place your projectile in the sling pouch, and arrange both cords. Snug the projectile into its position beneath the platform (popsicle sticks) and make the cords taut. The popsicle sticks should stick out perpendicular to the arm.

Mark the point where the sling cord exits the tapered side of the arm, and then tie a knot about ½” beyond that point. The purpose of this knot is to prevent the cord from pulling through the arm. After you make the knot, check to see that the sling cord is the correct length. If it’s too long, shorten it by tying one or more additional knots near the first one.

Take the other sling cord, loop it around the finger, and tie a knot to create a loop at the end of the sling cord.

At this point, you have created a weapon called a “staff sling.” A video on the Instructables page for this project shows my son testing his, to make sure the projectile platform and sling work properly. Don’t overdo it on the first try. My daughter tried it, and our Pinky ball is now lost in the woods.

GHT BASE

The counterweights need to have a strong base connecting them to the arm. To make it, cut three 3½” lengths of 2×4 lumber. Sandwich two of the 2×4s around the end of the arm, and screw the sandwich together with 3″ drywall screws. Don’t put a screw in the center, because it may interfere with the lag screw that will hold the weights in place.

Attach the third 2×4 to the ends of first two. Don’t put a screw in the center of this one either, because that’s the location of the lag screw. You’ve built your counterweight base.

Now drill a hole into the center of the counterweight base, a little bit smaller than the lag screw. I think we used a 5/16″ bit for the ½” lag screw, but a 3/8″ should also work.

You may want to drill a test hole in some scrap wood first, and try the screw’s fit. You’ll want it to be nice and tight.

Stand up the arm on its finger end. Stack the weights with the smallest (2½lb) on the bottom. The 5lb goes next and the largest (10lb) goes on the top. Why? You want the largest weight to fall as far as possible, to give your projectile the most energy.

I cut a section of ¾” PVC pipe and placed it in the weights’ holes. That helped keep the weights aligned, but it isn’t necessary.

Place the ½” washer on the 6″ lag screw. Tighten the screw to clamp the weights in place by screwing it tightly into the counterweight base. The screw should take a ¾” or 19mm wrench.

The spikes run 45° diagonally through the triangle base, corner edge to corner edge. Make these holes about 6″ from the triangle legs (far enough so the legs won’t interfere with your drill or hammer).

To make drilling into the edge easier, first sand, cut, or file a notch into the triangle base where you want to drill. My son also likes to use a center punch to make an indentation before drilling. Drill holes small enough to keep the spikes tightly in place, but big enough to prevent splitting; we drilled 1/8″ holes.

Hammer the remaining two nails through the holes. File or cut the ends of the spikes so they’re not so sharp and scratchy. They just need to be able to stick into grass or dirt.

Mark each leg 15″ from the base corner, as shown in Figure Bb. At each mark, drill a hole barely larger than the cord.

Cut a length of cord at least 24″. Tie a knot in one end, insert the other end through both leg holes, then tie a knot in the free end so it won’t pull back through, so that the length of cord between the two legs is 16″.

13. ATTACH TRIANGLE TO ARM

Attach the triangle to the arm using the 2½” hex bolt. Use a washer on each side of each piece of wood — 4 washers in all. Turn the nut by hand until it begins to tighten; don’t overtighten it. At this point, the total weight of the trebuchet should be around 20 pounds.

Using a file or a saw, make a series of notches on the back of the arm. These notches are used to hold the tuning string in place. To begin, position the string so that the triangle makes a 45° angle with the arm, and the first notch here. Make other notches on other sides of this notch. It’s helpful to label the notches so that you can keep track of where you set the tuning string during each launch.

Your Walking Arm Trebuchet is complete!