Basic Miniature Desktop Trebuchet [3D-Printed]

The Trebuchet is a medieval siege weapon which uses a counterweight to hurl a projectile. Trebuchets date back to the 4th century BC in China, and were used throughout medieval Europe as siege weapons used to attack large fortresses such as castles. The trebuchet is an impressive piece of engineering and a great display of physics, which can be easily replicated and built on a smaller scale today. I mainly used this post from comsol.com written by Mats Danielsson which contains great information on the physics behind an ideal trebuchet, such as the proper ratios of counterweight mass and arm length.

Building this small model of a trebuchet is a basic, fun, and engaging way to see how physics works. This model stands at 8 inches tall is made to work, but I left room for tinkering and improvement. It launches small projectiles such as M3 nuts tied to a string. The trebuchet is able to throw 5 gain projectiles about 5 feet. The model was made in Onshape and uses metric millimeters. All of the 3D models are available down below and were printed in about 3 hours collectively.

Supplies

• Requires a 3D printer
• (1x) M3x30mm hex socket head cap screw
• (1x) M3x16mm hex socket head cap screw
• (5x) M3 nuts
• (1x) 5mm x 13mm x 4mm ball bearing
• Super Glue
• Small and heavy objects to be used as a counterweight.
• String
• Tape

[Recommended screws and nuts]

[Ball bearings used]

Step 1: Understanding How Trebuchets Work

Trebuchets work by converting gravitational potential energy stored in a counterweight to launch a projectile in a sling attached to the end of the arm by converting the potential energy into kinetic energy as the trebuchet is fired. I have made a series of crude MS Paint drawings to better demonstration what I mean by this. For a better explanation, reference back to the comsol.com post by Mats Danielsson. Or you can also read the Wikipedia article on trebuchets.

Step 2: Printing the Components

As I said, all of the component .stl files are available above. You will need to print 2 frames and 2 braces. The printing process will vary depending on your settings, but it took me about 3 hours. I recommend using brim supports of 7 to 5 mm to help prevent warping, especially with wider prints like the frames.

Step 3: Constructing the Counterweight

Now that you have printed the counterweight, you will need to fill in the inside with some sort of heavy material. Small materials like fishing weights, lead shot, sand, or pebbles would work best. Use a 100/1 ratio of counterweight mass to projectile mass to better find your desired weight. I used lead in mine, but sand would produce a similar effect due to its efficient space usage.

After you have added your weight, glue the top (handle) part of the counterweight to the opening on the body of the counterweight, as shown in the image.

Step 4: Creating the Arm

The arm of the trebuchet is made in 2 parts, both are glued to the outer part of the ball bearing. First glue the (smaller) counterweight arm to the ball bearing, after it has dried, glue the (longer) arm to the other side of the bearing. When that has finished drying, I recommend putting a small amount of glue between the points on the bearing where the two arms meet in order to create a stronger connection.

Step 5: Connecting the Counterweight to the Arm

Connecting the counterweight to the arm requires the 3Mx16mm screw and 2 of the nuts. First you put the screw though one of the loops on the counterweight, put the end of the counterweight arm through the screw aswell, then screw on 1 nut between the counterweight arm and the loop. Push the screw through the remaining loop and fasten it with a nut. See the images above for reference.

Step 6: Connecting the Completed Arm and CW to the Frame

To connect the Arm and CW to the frame, first push the M3x30mm through the one frame hole, make sure that the extended circle in facing up or towards where the arm will go (see pictures). Screw on 1 nut all the way down to secure the screw in place. Next put the inner hole of the ball bearing (fulcrum of the arm) onto the screw. Then screw another nut down to hold the Arm in place, you will know its in when the ball bearing begins to turn when you rock the arm. After this put the other frame on over the screw, with the extended circle facing inwards towards the arm (opposite direction of the other frame), then hold the frame together with a nut on the end of the screw.

Step 7: Gluing in the Braces

Your almost done! Now just add glue to the ends of the braces and place them between the 2 bottom parts of the frame. I would recommend putting them where the supports reach the bottom part (see pictures). After adding both of the braces, place the trebuchet on its side (see picture) and leave it there for the glue to dry.

Step 8: Making the Projectiles

As the last of the glue is drying on the trebuchet, it is a good idea to make projectiles to fire. The projectiles are made from string, about 10cm in total, tape, glue, and a nut. A loop is to be made at the end of the string and be 8mm in width and 20mm in length (see picture). The remaining length of the string is to be 45mm and have the nut tied on to the end. I recommend using glue on the knot to better secure the connection.

Step 9: Lay Siege!

You've finished everything, now its time to fire! First you have to put the loop of the projectile over the end pin of the trebuchet. Then you pull down the arm, all while keeping the projectile in towards the frame. When the arm is all the way down, the projectile should lay flat, perpendicular to the middle support beam of the frame. Hold the end of the arm pin down with your finger until you're ready to fire. When its time, let the arm go and watch as the arm swings up and launches the projectile. I've embedded a video of what the launch should look like.

Step 10: [Optional] Optimize Your Trebuchet

Now that your trebuchet is finished, it is now time to optimize its performance. The main ways of doing this are my adjusting the weight of either the counterweight or the projectile. I performed an experiment to see if a projectile traveled further with 1 nut tied to the string or 2. The data shows that having 2 nuts (10 grains) as the projectile makes the projectile travel less far (about 8 inches shorter than 1 nut or 5 grains).

• If the projectile doesn't go far, or slams into the ground?
  • Try adjusting the weight of the projectile, if that doesn't work, change the length of the string. It may also be a problem of the release hook, it may not have enough room to catch the string of the projectile. A telltale sign of this is that the projectile loop falls down over the arm on launch. Try sanding down the inside of the hook at the end to make the gap larger, therefor increasing the chance of it hooking the string.

I purposely made the trebuchet deconstruct into the 3 main parts (Counterweight, Arm, and Frame) so that any of the 3 parts could be changed, re-printed, and put back together. Therefore you wont have to make an entirely new trebuchet if you want to change something. If and whatever you decide to change about it, I hope it goes well.