I bought a spring assortment from Amazon on a whim. I've never actually used springs on any of my experiments, so I decided to design and build a little toy catapult as a weekend project.
Step one was picking a suitable spring, since that would determine all other dimensions. I used the highly scientific method of stretching each variant and feeling the sproing. Most of them were way too tight, but I found one that sproinged just right.
A toy catapult is stupidly simple, yet it still involves lots of design choices.
My lever would pivot on a brass rod, since I already had a piece of stock laying around. For the initial prototype, I just hacksawed an extra long piece. For the final version, I cut more precisely and used press-fit caps to keep it captive.
I had to figure out how to attach the spring. I determined that an M4 screw and washer through the ring at each end would work. My first thought was using a threaded brass insert for strength. Then I realized I could just go all the way through and use a nut instead. I made long slots rather than holes to allow for adjustability
I puttered my way through FreeCad and printed my first prototype in PLA:
The prototype worked surprisingly well. The slot for screws worked as designed. The lever pivoted smoothly on the brass rod. Durability tests (going pew pew over 200 times) showed that wear was minimal. I also realized the brass rod needed to be a bit further back rather than directly under the lever-stopping-bar.
Another hour in FreeCad and I had my final design. I added a triangle for stability and to provide space to slide the pivot rod back. I made it wider because it looks cooler. I slimmed some body parts to make it less clunky.
Here's the final version:
