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Sections

In one sentence: Loft a smooth NURBS surface through a series of cross-section edges in order.

Overview

Workbench: Surface
Menu: Surface → Sections
Shortcut: none

Creates a Surface::Sections object by interpolating a NURBS surface through an ordered list of cross-section edges or wires. Each profile is a section; the tool constructs the smoothest surface that passes through all of them in sequence.

Intuition

Imagine a ship hull drawn as a set of frame cross-sections, each one a curved line at a different station along the hull's length. Sections stretches a smooth NURBS skin through all those frames in order, interpolating the curves between them. The result is a surface that passes exactly through every section.

This is similar to Part Loft, but it produces a surface (not a solid) and works with existing geometry edges rather than sketches.

When to use it

  • You have a series of cross-section curves (edges from existing geometry) and need the surface skin that connects them.
  • You are doing surface-based styling (automotive, consumer products) where section curves are the design intent and the surface derives from them.
  • You need the first or last edge to blend tangentially into an adjacent surface (G1 continuity at the ends).

When NOT to use it

  • For a solid loft, use Part Loft or Part Design Additive Loft — Sections produces a surface only.
  • For a 2-, 3-, or 4-curve patch, Fill Boundary Curves or Filling are more direct.

Step-by-step

  1. Select the cross-section edges in order from first to last (Ctrl+click). The order matters — the surface interpolates between them in the selection sequence.
  2. Choose Surface → Sections.
  3. In the task panel, verify the section list. Use the up/down arrows to reorder if needed.
  4. For each section edge, use the reverse button (⇄) to flip its direction if the surface appears to twist.
  5. Optionally set Tangent edges at the first and last sections for G1 continuity with adjacent surfaces.
  6. Click OK.

Parameters

Parameter Type Default Description
NSections Reference list — Ordered list of cross-section edges/wires (first to last)
Tangent Edges Reference list — Edges on the adjacent surface at the first or last section, used to enforce G1 continuity at the ends

Compared to Part Loft

Feature Surface Sections Part Loft
Input Edges and wires Sketches and wires
Output NURBS surface Solid shell
End tangency Yes (via tangent edges) Yes (via profile tangency)
Closed result No Optional
Editable after creation Yes (parametric) Yes (parametric)

Common mistakes and pitfalls

Surface twists between profiles

Cause: Adjacent section curves run in opposite directions.
Fix: Use the reverse button (⇄) in the task panel to flip profiles until all run in the same direction.

Sections not in order

Cause: Sections were selected in an incorrect order and the surface folds back on itself.
Fix: Use the up/down arrows in the task panel to reorder the sections from one end of the loft to the other.

Tangent edge continuity has no effect

Cause: The adjacent surface at the tangent edge is planar or the tangent direction is perpendicular to the section.
Fix: G1 continuity only works when the tangent edge is on a smoothly curved surface and is geometrically compatible with the first or last section.

Python API

import FreeCAD as App

doc = App.ActiveDocument
surf = doc.addObject("Surface::Sections", "Sections")

# NSections is a list of (object, edge_name) tuples
surf.NSections = [
    (doc.getObject("Sketch"), "Edge1"),    # first section
    (doc.getObject("Sketch001"), "Edge1"), # second section
    (doc.getObject("Sketch002"), "Edge1"), # third section
]
doc.recompute()

See also