Hmmm that wasn't as easy to draw as I hoped. It took 3 attempts, thinking it would just be two screw cut passes right and left with some blending where the hard edge meets. Not so lucky, it needed the intersection blending properly. The difference between the upper blue tool path and the lower tool path, not something I can easily achieve on the manual lathe. A 3 axis CNC mill/router would do it with one axis being a rotary, at least the tool path is now generated. The pitch is 7mm as that's the largest I can cut here, another thought would be to screw cut the shaft, and then 3D print the transition ends. 7mm pitch is still a bit slow and could do with being double that.