These parts — weird screws, camshafts, strange gears, or simply a shaft with a keyway (or two) — can really only be made with a rotary axis on a CNC machine. Sure, you could buy a rotary axis ...
Nowadays it is even possible to get a 3D printer kit ... machine build. He had previously built a CNC Router and decided it was best to add a hot end and extruder to the already built 3 axis ...