Fusion 360 & CAD

CAD modeling grew directly out of 3D printing - once you're designing your own parts instead of just downloading STLs, you need a way to hit exact dimensions. A funnel-shaped hopper adapter, modeled with a flanged rectangular opening narrowing down to a round tube.

Some of it is just modeling fasteners that don't exist off the shelf in the exact size or drive style needed - like this M6x3 bolt, iterated through a hex-head Phillips version and a hex-drive version, both dialed in thread by thread. These bolts were designed for one my son's toys which had a broken bolt and needed a replacement.