Converting Solid to SMesh
To convert a Solid into a smesh, just toggle the option in the 3D properties of the object. When converting from a Solid into a smesh, a set of facets will be generated to become facets on the smesh. This can result in unpredictable results. The following is an example of going directly from Solid to smesh.
You can improve the predictability by first converting the Solid to a TC Surface then to a smesh. The following is an example of going from Solid to TC Surface to smesh.
Changing between Solid and smesh for 3D primitives like a box will generally produces more predictable results as well.