Helicoid Half Twist

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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 varies between -1 and 1. The parameter t ranges from 0 to Pi for the half-twist. Additional information is available at http://mathvis.academic.wlu.edu/2015/07/27/helicoids/.