Overview#
Extend lengthens a face or surface by selecting one or more edges and moving them away from the face. It is the tool that makes surfaces "big enough" to intersect with neighbors so Trim can cut them cleanly. It is also used to grow a surface past a boundary for a later blend, or to repair a surface that falls just short of its neighbor.
Where to find it#
- Surface > Modify > Extend
- Toolbar search (S): type "Extend".
Step by step#
- Start Extend.
- Select the edges on the surface body to extend. With Enable Chaining checked, adjacent tangent-connected edges are selected with them; uncheck it to select a single edge.
- On the Feature tab, choose the Extend Type, which determines the direction of the new surface: Natural, Perpendicular or Tangent.
- For Natural, choose an Edge Alignment: Free Edges (G0 point continuity) or Align Edges (G1 tangent continuity).
- Drag the Distance manipulator or enter an exact value.
- Optionally pick an Analysis Type (None, Zebra, Curvature Map, Isocurve Analysis) on the Analysis tab.
- Click OK.
Autodesk's tip: you can extend edges through another shape and then remove the excess with Trim.
Extend types explained#
- Natural: extends the existing face, continuing the underlying surface as if the same equation simply ran on. Choose Free Edges (G0) or Align Edges (G1) for the edges of the extended surface. Best for planes, cylinders, cones and well-behaved surfaces. For wild freeform surfaces, natural extension can bend unexpectedly because the underlying NURBS continue along their own tangents (general guidance).
- Tangent: creates new flat faces tangent to the existing face. Useful for general freeform surfaces when natural extension is wobbly.
- Perpendicular: creates new faces perpendicular to the existing face.
Options reference#
| Option | What it does | Notes |
|---|---|---|
| Edge | Edges on the surface body to extend | Multiple allowed |
| Enable Chaining | Selects adjacent connected edges with the one you pick | Uncheck to select a single edge |
| Extend Type | Natural, Perpendicular or Tangent | Sets the direction of the new surface |
| Edge Alignment | Natural only: Free Edges (G0) or Align Edges (G1) | Aligns the edges of the extended surface |
| Distance | Length of the extension | Manipulator or exact value |
| Analysis tab | None, Zebra, Curvature Map, Isocurve Analysis | Previews the result in context |
Tips and pitfalls#
Tip: Extend slightly more than you need. After trimming, the excess is gone, but an extension that falls short leaves a sliver gap that never closes.
Tip: When extending a surface into an area where you will later trim, extension quality hardly matters. When the extension remains visible in the final model, check it with zebra stripes.
Warning: Extending a surface with a tight inner curve (a radius smaller than the extension distance) is a classic failure: the surface folds over itself. Use a smaller distance or a different approach.
Warning: Natural extension of a trimmed surface uses the underlying untrimmed surface, but only to the extent the underlying definition exists. For surfaces made from a Loft or Patch with limited definition, Extend may reject large distances. Rebuild with a longer loft instead.
Note: not confirmed in Autodesk's documentation: whether Extend can be applied to faces of a solid body. The help describes it for faces and surface bodies.
Tip: Self-intersection check: after a big extension, orbit and look for creases or flips. If the surface has pinched, reduce the distance.
How Extend actually behaves#
Extend lengthens a face or surface by moving selected edges away from the face. You select edges (or a chain of edges), a distance, and an extend type. Autodesk's help does not say whether an edge shared with a neighbor (a stitched seam) can be extended; as general guidance, work from free edges, and Unstitch first if an edge is refused.
The three types answer different questions:
| Type | What the new surface does | Use it when |
|---|---|---|
| Natural | Extends the existing face using its own mathematical definition, so a cylinder continues as a cylinder and a plane as a plane. Edge Alignment is Free Edges (G0) or Align Edges (G1). | You want to overshoot a trim, or a cylinder should simply be longer. |
| Tangent | Creates new flat faces tangent to the existing face. | You want a flat lip off a curved surface, or a controlled ramp. |
| Perpendicular | Creates new faces perpendicular to the existing face. | You need a run-off that stands perpendicular to the existing face. |
Natural extends can diverge badly on freeform splined surfaces: a loft with a strong flare keeps flaring, and a 30 mm extension can end up very far from where you want it. Tangent is safer for freeform shapes over short distances.
Worked example: overshooting for a trim#
You have a 100 x 60 mm loft surface and want to cut it with a cylinder of radius 40 mm that sits 5 mm beyond the surface edge. The trim fails because the cutting surface does not fully cross the surface.
- Select the free edge and choose Extend with type Natural and a distance of 10 mm. That is 5 mm of required overlap plus 5 mm of safety.
- Use the extended surface as the trim target and trim to the cylinder.
- If the part needs the original outline back afterwards, trim again with the original boundary as a sketch tool (or trim at exactly the cylinder edge); remember that any extension left on the body will show up in later stitching or thickening.
As a rule of thumb (general guidance, not Autodesk data), choose an extension of 10 to 20 percent more than the minimum required. A longer extension on a freeform surface can start to fold, so keep it modest and check with a section.
Failure modes and fixes#
| Symptom | Cause | Fix |
|---|---|---|
| Extend produces a surface that curls or self-intersects | Natural type on a strongly curved freeform; distance too large | Use Tangent, reduce the distance, or extend in two smaller steps and check each. |
| The command rejects the edge | Edge is shared (not free), or the edge is a seam of a closed surface | Unstitch first, or choose a different edge. |
| Corners show gaps when extending a chain | Adjacent edges extend in different directions | Select the whole chain (Enable Chaining), then check corner behavior in the preview. |
| The extended surface has a visible crease at the original edge | Tangent or Perpendicular selected where the surface curves away, or Natural with Free Edges (G0) | Use Natural with Align Edges (G1), or apply Patch with curvature continuity to the seam instead. |
| Extension seems to do nothing | Distance is tiny relative to units or the preview is clipped | Check units (mm versus cm) and the distance value. |
| Later Thicken fails on an extended surface | The extension created a very thin sliver or self-overlap | Delete the problem part of the surface with Trim and rebuild with a smaller extension. |
Extend versus alternatives#
- Offset a boundary or loft a larger surface when the freeform shape is important to you; Extend only guesses at how the surface continues.
- Ruled Surface is better for a flange of known angle and length off an edge: it gives you angle and distance control.
- Patch fills a gap between two edges; Extend grows towards, not between.
- Replace Face (a Solid tool) handles solid faces by moving them to a surface and trims or extends the body, so for solids you rarely need Extend.
Workflow advice#
- Extend only as far as needed, then trim. The best surface is the smallest one that serves the purpose.
- Do the Extend before stitching to neighbors, so the edges you need are free.
- Check with an edge-on section view that the extension did not bend.
- When extending several edges by the same distance, do it in one command so the result is computed together.
- Zebra the result for kinks at the original boundary, particularly if the original was G2 with its neighbor.
Extending a chain of edges: corners and consistency#
When you select several connected edges (Enable Chaining), Fusion extends them together. Autodesk's help does not describe how corners between chained edges are computed, so preview the corner at high zoom before accepting. If it looks wrong, extend the edges in separate operations and then Patch or Trim the corner.
Sizing an extension from a trim requirement#
Work out the extension from the geometry instead of guessing. To trim a surface with a 25 mm radius cylinder whose axis sits 3 mm outside the surface edge, the cylinder must cross the surface entirely, so extend by at least the 3 mm gap plus a margin of 5 mm, 8 mm total. Also confirm that the extended region does not meet other geometry you do not want to cut. A quick check is a section view through the extended area.
Practice exercise#
- Create two surfaces that nearly meet: a vertical surface-extruded wall and a sloped surface-lofted panel that ends 5 mm short of the wall.
- Use Extend on the sloped panel's end edge by 20 mm using Tangent.
- Use Trim to cut both surfaces at their intersection and discard the overlaps.
- Stitch the two remaining pieces. Expected: a single quilt with one shared edge and no free edge at the junction.
Stretch challenge#
Create a spline-profile extrusion surface 80 mm wide and 40 mm tall. Extend its top free edge by 15 mm using each of the three types and compare the new edge positions using Inspect > Measure (expect roughly 15 mm for the flat extensions; Natural on a curved surface can differ, which is the point of the comparison). Then use the Natural extension as an overshooting trim target for a 25 mm radius cylinder and verify that the trim succeeds where it failed before extending. Expected result: three bodies with measured edge offsets recorded in a short table, and one successful trim.
Related#
- Trim and Untrim
- Stitch and Unstitch
- Continuity G0 to G3