I want to model a Spiral Phase Plate (as shown in figure SPP.png).
Here the thickness of the object varies with the azimuthal angle 
as in 
.
I have tried using a parametric surface to draw the faces of the plate and create a closed solid (as shown in figure SPP2.png) and then convert to a solid, however I keep on getting the message ´The operation resulted in an empty geometry´.
What could be the issue here? Is there a way around to create the spiral phase plate?