So I've always wanted to build a mechanical iris, and this 3D design contest sounds like the perfect way to work the bugs out.
We'll call this step 0 (maybe? it is my first instructable...) - Research! You need to figure out what you want to make and if its ever been made before. What problems they have had, or success stories. So hit up google images and search for mechanical iris.
the first image is what i actually ended up with!
Disclaimer: I know the mechanical iris has been instructabled to death, but this is focusing on the CAD design of it, not the mechanism itself.
Step 1: Draw It!
Step 2: Check It
After I made sure it would function the way I wanted, I took the time to focus on how i would make it. Since I haven't wont the replicator yet, I would have to make this by hand. look at all those complicated cuts? So I made a 3rd version with less intricate cuts.
Always remember that you can draw anything, model most things, but you're limited to what you can actually build. That's why I want a 3D printer!