Turning profile selection from 3D
From v5.x is possible import turnable profiles from 3D step models.
In turning context, the maching geometries will not be defined directly by 3D model features.
It will be generated the 2D TURNABLE PROFILE of the 3D model.
You are going to works with this 2D profile, not with 3D model directly.
For example see 3D part model below. 
Now I want to turn one side of the external profile of the part.
From MENU > LATHE > EXTERNAL TURNING > [ From 3D Model ]
Now the software will calculate the turnable closed profile of the part .
It will also add the profile to the newly added turning machining operation.
You can see the elaborated turnable profile here in RED .
The turning toolpath will be created for all the lenght of the part.
If I want to machine only a part of the external profile, I can set the TOOLPATH LIMITS.
From machining options, enable the Z- Limit and enable the mouse snap checkbox.
Click on the middle of the part. 
Now the toolpath is limited in Z- Direction.
