I have a sweep that I want to transition from a full radius into a 3D sweep. I am trying to model how a radius tool would cut the part if it were lifted vertically while maintaining tangency with the inside circular surface. How would I define the sweep path given the cusp location, tool geometry, and tangency surface?

In the past I have drawn a 3D sketch as the sweep path and adjusted it until it was close to the intended cusp. Constructing this sketch was tedious and was prone to locking up. Does anyone have a trick they use to model this type of geometry?

The 3d sketch in the model is able to move along the desired path but I don't know how to define that movement as a sweep path.

Thanks for any tips.