Hi,
Here are a couple of ways I would do it. First the over build method is relatively easy and produces perfectly smooth results but is less accurate relative to some of the design curves. It will also require some point editing of the volute surface to dial in the shape. Second the patch method (it does not use the surface patch command) requires complicated steps with lots of rebuilding and matching of surfaces. The results are more accurate but tend to be less smooth. In this case Rhino was unable to fillet the edge. Which required additional steps to trim out sliver surfaces and then blend everything together. 
