Function blend_faces 

pub fn blend_faces(
    model: &mut Model,
    solid: &Shape,
    a: &Shape,
    b: &Shape,
    radius: f64,
    tol: Tolerances,
) -> OgeomResult<Built>

Blend two faces of one solid with a rolling ball of the given radius.

The two faces need not touch. What they must do is face each other across a corner: their planes must meet, both must reach the stretch of that meeting line the blend will sit on, and the material must fill the dihedral between them, which is asked of the solid rather than assumed from the normals, because normals cannot tell a step from a slot.

Planar supports only. A curved face-face blend needs the marching seat (the spine that is the two offset surfaces’ own intersection), which is recorded as owed in docs/PLAN.md rather than guessed at here.

Errors

OgeomError::Construction if either face is not planar, the planes are parallel, the faces do not both reach the meeting line, or the radius is not a usable length.