Onager Catapult: Reverse Engineering Project

I have always wanted to design a mini-catapult that works and I thought, what's the fastest way to learn than to learn from others. In this Instructable, I will walk through on my reverse engineering process to break down a mini-catapult design that I have gotten online and have an attempt to reverse engineer the parts and explain how each component works. Additionally, I will show a successful replication of the design that works!

Who knows, maybe I should create a series of such "reverse engineering" pieces! There's so much to learn!

*Disclaimer: Just to be precise, the onager was a Roman torsion powered siege engine. It is commonly depicted as a catapult with a bowl, bucket, or sling at the end of its throwing arm. Thank you one of the members for clarifications!

• Reference 2D Image
• 3D CAD Software (Rhino 5.0; In my case)
• 3D Spatial Awareness
• Some mechanical intuition

One of the key steps of reverse engineering is to figure out the components of the catapult. For my case, I broke down off-the-shelf catapult schematics from an online image of a figurine and laid them out for visualization. Using a top-down image basically shows the entire blueprints of the design.

Next, open up illustrator or Rhino 5.0, and make traces of the image using vector lines and curves. A tip is that I had to size a relative dimension of the size of the design which I want and scale the design accordingly.

A few pointers to note:

• Interfacing elements (e.g. slots) has to match the thickness of the material sheet
• Tolerances for certain snap-fits have to be fine-tuned as well
• Fortunately, with a lasercutter, iterative prototyping is extremely fast

From tracing and iterative process of prototyping, I managed to understand how each component interacts with the other. The concept of interlocking joints, laser-cut collar shafts and bearings are really interesting concepts to take note of. This will definitely bring me to the next level to create new toys from these learnings.

A few pointers to take note of:

• Wooden materials are isotropic, making sure that you cut along/across grain depends on the mechanical load that the component is to take.
• Compliant snaps should not have thicknesses that are less than 1.5mm else the beam will have a high risk of breakage (have to negotiate the clearances and tolerances)
• The catapult slots have to be a tight fit to snug in place else it will be loose and glue is required (undesirable)
• Once your blueprints are sorted you may scale it to other materials too!

As some of you have noticed, I have quite a few failure pieces. The point about reverse engineering is not about the outcome but the process of learning from it.

For convenience, I have attached a graphic on the different components of an Onager Catapult. (By Rpanjwani3 - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1...)

With all that effort, here's the final video of the success of the design process! The current design dimensions are about 75mm x 75mm in footprint. A cute palm-sized design for any portable fun! Check out the video to enjoy the assembly process!

Feel free to drop down a comment, if you think I should scale this bigger or bring this further! Drawing inspiration from online material, I have some plans to design target boards as well as creating projectile balls (which is already proven to be doable) and possibly make this a carnival game for my future kids? What do you think?

And with that, we have completed a basic introduction on how to reverse engineer some of the available 2D designs. I am very satisfied with how it turned out. If you find this Instructable useful, do LIKE and SHARE this post and I will be submitting it to the Make It Fly design challenge!