After learning about the fully mechanical nature of original air raid sirens, I decided to try to build one. The basic design lends itself to sheet metal construction, so it seemed like a good opportunity for me to develop my sheet metal design and manufacturing skills.
I CADed the whole thing parametrically in SolidWorks so I could play with sizes, clearances, and tones. I had to think extensively about realistic sheet metal part tolerances and order of operations for folding and assembly.
Finally, I used a plasma cutter to cut the aluminum parts and a waterjet to cut the thin stainless parts. Then I rolled, folded, spot welded, riveted, and screwed all the parts together, creating some jigs for repeatability along the way. I also turned a few parts on the lathe for attaching the rotors to the bench grinder shaft.