Pile 'o Polyhedrons: the Improved Screw

When I tried to make a throated worm screw ( http://www.thingiverse.com/derivative:9733 ), I ran into an issue: the thread profile was being extruded along the right curve in space, but there was no skewing of the profile to fit the curve.

You can see what I mean by comparing the images for this thing to the images for its parent. The tip of the thread remains vertical, no matter how the thread tapers or widens around the shaft. In the case of a throated worm screw, this meant that the worm