Introduction: Make a Desktop Trebuchet

About: "If you want to build a ship, don't drum up people to collect wood and don't assign them tasks and work, but rather teach them to long for the endless immensity of the sea." - Antoine de Saint Exupéry. I long …

A trebuchet is a gravity powered catapult. This one is my own design. It's made to be laser cut (or scroll sawed) out of 1/8th MDF or plywood.

This kit is an intermediate difficulty. There are lots of parts to glue and some need to be lined up by hand. It takes me about 30 minutes to go from a pile of parts to a fully working trebuchet.

Here it is firing some little wooden "bullets".

Step 1: Gather Materials

You will need:
  • One set of wooden parts. Use attached PDF.
  • One cloth sling. Use attached Basket.pdf for a pattern.
  • Some ballast for the counter weight. (nuts, small bolts, BB's, lead shot, etc)
  • Wood glue
  • strong thread or thin string
  • A long twist tie, some masking tape, small wood clamps or other way of securing some of the parts while the glue dries.
  • Some 400 grit sandpaper (optional)
For the wood parts you can use the attached files to create your own or send them off to a laser cutter to make them. It should work fine in 1/8th MDF, Baltic Birch and probably acrylic or lexan (use the appropriate glue of course). You can open the .svg files in Inkscape.

Feel free to make as many kits for personal use as you want. (It's not alright to sell kits without my permission, OK?) The basket on the counterweight is a "living hinge" design. It bends where the laser scored it. There are 2 on the template since it's the most likely part to break during assembly. It's probably not possible to do on a scroll saw. You will need to modify the design somewhat.

Step 2: Hints and Tricks

  • Use full strength wood glue.
  • Use glue sparingly. It doesn't take much to solidly stick this model together.
  • Wipe up excess glue right away. Some parts might not fit if you let glue dry in random places.
  • Be gentle with the small parts. They can break during assembly.

Step 3: Glue Axles Together

There are two axles in the design. The parts look like tuning forks and slip into each other. Start with a dab of glue on each side close to the notch. I use a paintbrush to apply the glue. Put glue on both sides of both halves and slide together. Wipe away any visible excess glue. If you let a drop of glue dry on one end you will have trouble later.

Glue the small axle the same way. Set them aside for the glue to dry.

Step 4: Glue Arm Stablizers Together

Find the 4 torus (doughnut)  shaped parts. Lay them all scorched side down. You will build two stacks of two stabilizers.
  • Put a little glue on one ring and stack another on top of it. Make sure they are lined up as well as you can. Roll them around on the table to make sure.
  • Do the second set the same way.
  • Set aside and let dry.

Step 5: Glue Counterwieght Basket Sides to Bottom

The basket goes together in two steps. The fist step is to glue the sides onto the bottom in the center.
  • But glue on the center tabs.
  • Stand both sides up on the tabs. This is a little tricky. You might have to support the sides against a box or a book.
  • Set aside and let dry, but make sure it dries strait as possible.
Once the first course of glue dries you can finish the basket. You can do some other steps while you wait.

Step 6: Glue the Stabilizers to the Arm

The stabilizers should be dry enough to fit on the arm now.
  • Glue one set of stabilizers to each side of the arm. Be sure to remove excess glue from the center of the holes.
  • Test fit the big axle to ensure the stabilizers are on correctly.
  • Remove the big axle and set arm aside to dry.

Step 7: Sand the Big Axle

Optionally you may sand the big axle for smoother movement. Take some 400 grit sandpaper and wrap it around the axle. Turn the axle with your fingers.

The goal here is to knock down the corners a little, but not to make the axle round. If you sand it too much, the guides (circles with + shaped holes) will slide on too far. A little bit of sanding will make the lever turn much smoother.

Step 8: Fit One Arm Guide

Find one of the guides (circle with a + shaped hole in it.) Slide the guide onto the big axle. Make a note of where is stops. Slide it off and add a little glue to where it will sit. Slide it back on and wipe away any excess.

Step 9: Finish the Basket

This step it's preferable to use more than just a tiny bit of glue. Don't just glop it on of course. Use a thick bead instead of a thin film.

  • Add glue to the edges of the basket sides from the center tab to the notches on both sides.
  • The glue should be a bead about 3mm wide along the edge.
  • Put some glue in the notches.
  • Carefully stretch each side of the bottom into the notches.
  • Clamp and set aside to dry.

Step 10: Wait About 10 Minutes for the Basket to Dry

So, yeah.
How you been?
Me too.
Dum de dum de dum dum dum de dum.
Maybe you should get a snack now or something?

Step 11: Add the Basket to the Arm

The arm only works one way up. It's got a little notch where it sits in the basket.
  • The small axle attaches the basket to the arm.
  • Push the small axle into one of the holes in the basket from the outside.
  • Put some glue into the notch on the arm. Use a good amount so it stays wet for a bit.
  • Put the arm into the basket and slide the small axle into it.
  • Work the small axle into the hole in the other side of basket.
  • Carefully pull (or push with a pen or something) the axle out a bit.
  • Put some glue on the ends and the center where the arm attaches.
Make sure the arm is at right angles to the basket as it dries.

Step 12: Add the Axle and Second Guide to the Arm.

  • Slide the big axle into the arm.
  • Slide the second guide onto the axle.
  • Make sure the axle spins freely. Don't get the second guide on too tight.
  • put a little glue on the outside of the joint where the axle meets the new guide. wiggle the guide in and out a bit to work in the glue.
  • Double check the axle spins well.

Step 13: Get Ready, the Next Few Steps Need to Happen Before Any Glue Dries

Take a deep breath. Read the next few steps before you try them. It's not hard, but keep moving once you start.

The goal is to get the arm, the base and sides all put together quickly. This lets you make sure it's all lined up before the glue dries. It's not a disaster if you don't but your model might be a little lopsided on close inspection.

Step 14: Glue Base to One Side

  • Put a bead of glue along one edge of the base.
  • Put the side on. I like to put them on scorched sides out. ( they are symmetrical, so no worries.)

Step 15: Glue the Big Axle to Both Sides and the Second Side to the Base

This is the final glue assembly step. You need to put the last side on the base and axle at the same time.
  • Put a bead of glue on the edge of the base.
  • Put some glue in the + shaped holes in each side.
  • Push the big axle into the already glued side. Make sure the basket is down and the hook is up.
  • Push the second side onto the base and the axle at the same time.
  • Sit the model upright and make sure it's all strait.
At this point the sides might slide in on the axle a little. Push them back out so they are flush on the outside. Hold until the glue sets enough to hold it all.
  • Let the glue dry for a few minutes while you admire your creation.

Step 16: Assemble the Basket

The length of the "ropes" will determine how your trebuchet fires. Too short and it will fire mostly up. Too long and it will fire in a downward trajectory. It's easier to start too long and shorten as you test.
  • Measure a bit of thread as long as the arm to the axle.
  • Double it.
  • Add about 1 inch for knots.
  • Cut the thread.

My design has the basket attached so it can slide along the thread. This makes it easy to center as you adjust the length of the rope.
  • Thread the thread through the holes (you don't need holes, you can just use the needle to sew the basket).
  • Weave the thread though the holes on one side of the basket.
  • Starting on the same side you just exited, weave the thread back through the other side of the basket.
  • Put a loose end through the hole in the arm and tie the ends together. (A double overhand or figure eight knots works great.)
  • trim the loose ends.

Step 17: Add the Firing String

This trebuchet uses your finger as a release mechanism. You will need something to hang onto.
  • Measure a piece of thread that's about 1/3rd longer than the base.
  • Tie a figure eight knot or a triple overhand knot in one end.
  • Thread the other end through a needle.
  • Pull the needle with the thread doubled up though the very center of the basket from the inside.
  • Hang the loose end of the rope on the hook of the arm.
  • Pull the arm down with the string, keeping tension on it so the rope doesn't slip off the arm.
  • Pull the firing string under the arm and out the other side of the trebuchet.
  • Tie a loop in the end of the string where it comes out. Don't make the loop too long or it will get caught during firing.
  • Cut off the excess.

Step 18: Fill the Basket With Ballast

Find something heavy to fill your basket. Washers or nuts will work will. BB's or lead shot will need to be secured in the basket so they don't dump out during firing.

Your trebuchet is now ready to fire! Congratulations.

Step 19: Fire and Tune

Wondering what all those left over little circles are for? They are ammunition!

  • Loop the rope over the hook.
  • Load a bullet in the basket.
  • Carefully pull the string though the base until the arm is all the way down.
  • Aim, and let go to fire.

  • If your bullet goes too high up, your ropes are too short.
  • If your bullet shoots on a downward trajectory, the ropes are too long.
  • If it fires backwards your string or basket is too stiff.
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