This is a model of part of a helicoid with parametric equations x(t)=u.Cos(t), y(t)=u.Sin(t), and z(t) = 2t/3, where the parameter u goes between 0.25 and 1.25. The parameter t goes between 0 and 2Pi.
(BTW the spot on the .stl is a quirk of the viewer and vanishes when you see the object in Thingview.)
This model was designed and printed by my WLU summer research student Emily Jaekle ('16). See http://mathvis.academic.wlu.edu/2015/07/27/helicoids/.
We printed this helicoid on a FormLabs Form 1+ liquid resin printer in clear resin (0.2mm slices). The print was excellent. The .form file is included.