PlanSweeper is unique as a spline extruder in that it favors the straight-aways rather than the corners. This makes all the straight segments the same corrections while the bevel sections vary. While this works well for cornices, etc., it is problematic in the Lathe where, as you reduce the number of segments, the section is essentially shifted in toward the center, eventually overlapping the pivot.
To compensate for this, we can shift the section to the right just before sweeping along the circular plan so that it is at the radius defined by the corner vertices. In the figure to the right, dx, the amount of the compensating shift, is calculated.