Introduction: How to Make a Model Roman Ballista (Torsion Powered)

About: I'm an engineer and a dad who has a love for designing and making toys, STEM projects and anything electronicy.

After the success of the Model Roman Catapult and, with our Year 6s learning about the Romans, I thought I'd try recreating a different siege engine.

Follow these instructions to make your very own torsion (wound up) powered Model Roman Ballista!

See the video below for a step-by-step live guide to this instructable!

Step 1: Tools and Materials

For this project, you'll need these tools and materials:


  • Junior hacksaw
  • Benchhook
  • G-clamp
  • Goggles
  • Hot glue gun (or white PVA/wood glue and card triangles)
  • Pencil and ruler
  • Drill with 3 or 4mm drill bit


  • Square cross-section wood (you can order in bulk from TTS or Consortium) or square dowel from a hardware store
  • 5mm round dowel (you can either order this from TTS/Consortium or pick it up at a hardware store)
  • A metre of strong string/garden twine (I found the green plastic garden twine to be strong enough)
  • Spare dowel for ballista bolts!!

Step 2: Cutting Your Wood to Length

Cut your wood to these lengths:

Square cross-section

  • 3 x 20cm (base)
  • 2 x 12cm (front support)
  • 2 x 5cm (back support)
  • 5 x 3cm (front support and back support)

5mm round dowel

  • 3 x 10cm (arms and back winder)
  • 4 x 2-3cm (tighteners for front support)

Step 3: Drill and Glue the Front Support

First, take your 2 x 12cm pieces and use a pencil and ruler to mark 2 points 2.7cm in from either end.

Next, use a drill with a 4mm drill bit to drill holes at your marks.

After that, take your 2 x 12cm pieces and 4 of your 2-3cm pieces and line them up line in the first photo, making sure there's a 1cm gap between the middle 2 (3cm) pieces.

Add a dab of hot glue to the end of each piece of wood and press them together.

Optional: Add card triangles to each join for extra step.

Step 4: Threading the String for the Arms

First, cut yourself 2 x 15cm pieces of string. Now for the tricky part!

I found in hindsight that wrapping a short piece of sticky tape around the end of each piece of string (so it looks like a shoe lace) helps stop the string unravelling and eases threading it through the holes).

Thread one end of the string through one hole, through the space between and out the other hole.

Next, loop the string around one of the 2-3cm pieces of dowel and thread it back down the same hole is just came out of. Look at the photos for a guide here!

Pass the string through the gap underneath and through the original hole you started with.

Now, tie the two ends of the string over another 2-3cm piece of dowel, knot it and trim the excess string.

Repeat on the other side.

Finally, gently push two of the 10cm round dowel between each of the paired pieces of string (so one's on either side) and twist the 2-3cm pegs (in the same direction!) so the string begins to tighten around the dowel. Count the turns as you do this and, once you can pull the arm back and it springs back into place, repeat on the other side with the other arm.

Step 5: Glue the Long Base

Next, take one of the 20cm pieces and glue it in the centre of the bottom of the front support.

As you glue it, take care that it forms a right angle with the front support.

Optional: Add a card triangle on the join for strengthening.

Step 6: Make and Attach the Back Support (with Optional Winder)

Next, take your two 5cm pieces of square wood and the remaining 3cm piece of square wood.

Take the two 5cm pieces of square wood and drill a 5mm hole (or even 5.5mm to allow for movement) ~1cm from the end of each piece.

Then, fit the 5cm and 3cm pieces together in the photo, allowing a 1cm gap at the bottom of the 5cm pieces to create feet for the back support to rest on.

Use a dab of hot glue to secure the pieces in place and add card triangles to strengthen the joins if you want to.

Step 7: Attach the String for the 'bow'

Now, take your string and stretch it between both ends of the firing arms.

Measure the length and add around 10cm to make tieing off easier.

Tie each end of the string to the arms, making sure the arms are slightly under strain when let go. This will help the arms to not lose force when you let go of them.

Optional: use a knife to cut a little groove into the firing arm for the string to sit in, or add a dab of hot glue to hold it in place.

Step 8: Build Up the Base

Now you know how high the string is, glue the remaining 20cm pieces onto the long base to build it up until it rests just under the string.

This will allow your ballista bolts to sit snugly on the base.

Step 9: Fire Away!

Congratulations, you're finished!

Why now use some remaining dowel, sharpen it with a pencil sharpener and make some home-made ballista bolts?

Thanks very much for checking out my instructable. If you did like it, then please do leave a comment below and I'd love to see any photos you take when you make your own!

Reflections and improvements
I found that garden twine made a good string for the project.

If you added two 20cm lengths of dowel as 'guide rails' on either side of the 20cm base, the dowel would slide and release much straighter.

The project could easily be scaled up to make a 50cm or 1m version, with thicker string or more turns around the front support.

Wood Contest

Participated in the
Wood Contest