A Convergent Body is created when the edge(s) of a B-Surface are Extruded.
The expectation is that a standard Analytic body is created.
Solution This behavior is controlled by the Modeling Preference 'Treat Degree 1 Spline
as Polyline' (under 'File -> Preferences - > Modeling -> Convergent').
When this option is checked, 3D geometry created with single-segment splines
is created as convergent geometry. When it is unchecked standard analytic
geometry is created.
Notes and References
Hardware/Software Configuration Platform: INTL64
OS Version: 764SP1