Parametric case fan model

Parametric fan design remixed from MiseryBot's version. As with the original, this is not actually a plan for a working fan, it just simulates the look and feel for integration/design modeling purposes.

This parametric design proportionally resizes all aspects reasonably well, excepting really small or really large scale. You can adjust:
- Fan dimensions and thickness in mm
- Housing mount tab thickness ratio
- Fan blade count, thickness, twist direction and twist intensity
- Fan hub-to-housing diameter ratio and blade-to-housing gap
- Mount hole diameters for various metric screw sizes (WIP)

You can also download the OpenSCAD file and remix it yourself if you need to dial it in further.