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Extend Face

In one sentence: Extrapolate a selected face beyond its current boundary by extending it along its parametric U and V directions.

Overview

Workbench: Surface
Menu: Surface → Extend Face
Shortcut: none

Takes a single face from a solid or surface body and extrapolates it beyond its boundary edges. The extension is tangent to the original surface at the join. Used to make surfaces overlap for subsequent trimming operations.

Intuition

A NURBS surface is defined by a mathematical function over a rectangular parameter domain. That domain can be extended beyond its current boundaries simply by evaluating the function at parameter values outside the original range. Extend Face does exactly this — it evaluates the surface function slightly outside the current domain to produce an extended surface.

Think of it like pulling the edge of a rubber sheet: the sheet stretches beyond its original boundary, staying tangent at the pull point. The farther you pull, the more the extended portion may deviate from the "natural" extrapolation.

When to use it

  • You need a surface to overlap another surface so you can trim them together to a clean intersection line.
  • A surface doesn't quite reach an edge it should meet, and you need to push it past that edge before trimming.
  • You want to see how a surface's mathematical shape continues beyond its current boundary.

When NOT to use it

  • For large extensions — NURBS extrapolation degrades quickly with distance. Extensions larger than 10–20% of the surface size often produce unexpected curling or looping. Use a dedicated surface extension approach (creating new geometry) for large extensions.
  • For adding material to a solid — this produces a surface, not a solid feature. Use Part Design tools for solid operations.

Step-by-step

  1. Select a single face on a solid or surface object.
  2. Choose Surface → Extend Face.
  3. A Surface::Extend object is created and the extended surface appears, overlapping the original.
  4. In the Properties panel, adjust Tolerance if needed.
  5. Drag the boundary handles in the 3-D view (if available) to control the extension distance.

Parameters

Parameter Type Default Description
Face Reference — The source face to extend
Tolerance Float 0.1 Maximum tangency deviation at the extension join

Common mistakes and pitfalls

Extension curls, loops, or has extreme curvature

Cause: NURBS extrapolation is mathematically valid but not geometrically predictable for large extension distances.
Fix: Keep extensions small (5–10% of the surface dimension). For larger extensions, build new geometry rather than extrapolating.

Cannot select an interior face

Cause: Face selection requires clicking directly on the face in the 3-D view; clicking on an edge selects the edge, not the face.
Fix: Click clearly in the interior of the face, away from edges.

Python API

import FreeCAD as App

doc = App.ActiveDocument

ext = doc.addObject("Surface::Extend", "ExtendFace")
ext.Face = (doc.getObject("Body"), ["Face6"])  # replace with target face
ext.Tolerance = 0.01
doc.recompute()

See also