Parametric Golden Spiral Fan Duct v0.2

Updated Apr 2017
User PGent fixed the shroud wall generation code back in 2015. I owe him or her a huge apology for not merging the bugfix for a year and a half. Thank you and I am sorry.

Parametric fan duct derived from http://www.thingiverse.com/thing:139984 incorporating cross-section reduction as an exponential function of the golden ratio conjugate. Default settings result in:

40mm cpu fan form factor
M3 screw bores
approximately 25% reduction in diameter
5 fins
10mm OD ball joint articulating surface