Hello all.  This is a challenge that my company has struggled with for the better part of a decade.  Among other things, we make twist tracks: guides that rotate a part about its own center for orientation in automated equipment.  We make these on a 4-axis mill, using a cutter (or set of cutters) that approximate the part shape as they pass axially through a cylinder.  But we can't actually model the shape in CAD.  Drawings for fabrication describe the shape of the cutter, its path, and how the cylinder must turn, but there's no actual geometry.  We'd like to change that. 

For a part with any real thickness, the profile of the path is quite different than the outline of the part.  This would seem like a good place to use a solid sweep cut, but unlike feedscrews, these cuts are on the inside of the cylinder.  Also unlike feed screws, the path is ideally straight for a portion, then blends into a helix, then blends out into a straight portion.  Thus we rapidly run into the limits of that feature's ability. 

Attached is a file showing the part in question (a stepped cylinder) and the cylinder body from which the track is to be cut.  I have pattered the part (curve driven) to show how it will move through the track.  The path is a fit spline built on a helix and a couple straight sections of 3D sketch.  I have been unable to get any solid sweep cut to follow that path, interior, exterior or otherwise.  Note that the part stays oriented to the axis of the cylinder; it does not pitch with the path. 

Does anyone have any idea how I could create this shape?  It'd save an awful lot of trouble over here, and settle more than one bar bet. 

Thanks!

Dave