Forming and cutting

Use planes to position cuts, cross-sections, and tapers. Use face selectors to choose the surfaces to modify. The recipes below remain reusable after each operation; native geometry is built by Smith.evaluate/1.

Draft about a neutral plane

Draft tilts walls, often so a part can leave a mold. The neutral plane is the reference at which the original footprint stays fixed.

alias Smith.{Plane, Selector}

sides = Selector.type(:plane) |> Selector.exclude(Selector.parallel(:z))
tapered = Smith.box(20, 16, 10)
  |> Smith.draft(faces: sides, neutral: :xy, angle: 5, count: 4)
{:ok, result} = Smith.evaluate(tapered)

At the neutral plane, the footprint stays fixed. Positive angles remove material on the pull side. direction: defaults to the plane normal; it must not lie in the neutral plane. Angles are degrees and must be strictly between −90 and 90. Zero validates and copies the body. Selected faces must be planar, cylindrical, or conical. OCCT may propagate the draft across tangent-connected faces; count: checks the explicit selection before that propagation. Disappearing faces and other topology transitions can fail.

Split a solid or reuse its section

plane = Plane.xy(z: 6)
snapshot = Smith.from_result(result)
lower = Smith.split(snapshot, plane, keep: :negative)
upper = Smith.split(snapshot, plane, keep: :positive)
both = Smith.split(snapshot, plane)

cap = snapshot
  |> Smith.section(plane)
  |> Smith.translate({0, 0, -6})
  |> Smith.extrude({0, 0, 2})

Positive means the side toward the plane normal. The default keep: :both keeps separate solids; it does not fuse the cut back together. These operations accept solids or compounds containing only solids. A missed section or discarded side returns a valid empty compound. Tangent edges and points do not create solids or section faces. A section coincident with a planar boundary retains that face.

section produces filled faces with normals matching the plane, preserving holes and disconnected regions. It is not a projection. Extrude these faces with a world vector that has a nonzero component normal to every face. Multiple regions become separate solids. Scalar extrusion remains a sketch operation.

Extract, offset, and thicken surfaces

wall = Smith.cylinder(10, 12) |> Smith.surface(Selector.type(:cylinder))
sleeve = wall |> Smith.offset(2) |> Smith.thicken(-1)
{:ok, sleeve_result} = Smith.evaluate(sleeve)
{:ok, volume} = OCEx.volume(sleeve_result.shape)
true = abs(volume - :math.pi() * (144 - 121) * 12) < 1.0e-5

expanded = Smith.box(20, 16, 10) |> Smith.offset(2, join: :intersection)

surface selects faces and sews their shared boundaries. An empty selection fails. Connected faces become shells; disconnected regions stay separate. The cylinder wall above has outward normals, so the offset increases its radius from 10 to 12. Negative thickening builds inward, closing a solid wall between radii 11 and 12.

offset moves a surface along its normals. On a solid it expands or contracts the boundary; on a planar sketch it moves the face out of its plane. It does not expand the sketch outline in 2D. thicken turns a face or open shell into a solid, closing its boundary. It rejects existing solids and closed shells. To hollow a solid by removing selected faces, use Smith.shell/2.

Both operations accept join: :arc or :intersection. Arc joins round gaps; intersection joins extend adjacent surfaces. Offset defaults to arc and thickening to intersection. Signed distances must have magnitude greater than 1.0e-7 mm. Compound members are processed independently; the result is not automatically fused.

These algorithms require suitable smooth surfaces and can fail at narrow gaps, large offsets, or self-intersections. Self-intersection repair is disabled. Successful BREP validation is not a complete self-intersection check. Inspect sections and measure the resulting walls before relying on a part's fit.

Preview and export

The forming Livebook shows these operations in stages, checks analytic volumes and section area, and exports a three-part assembly. Each preview supports fullscreen and PNG download. Use verified export for STL and 3MF fabrication files; open surfaces must first become solids to pass printable-mesh checks.