Overview#
Trim splits intersecting surfaces and removes unwanted sections of a surface or quilt. You select a surface body intersected by an extended surface, then select the section to remove. Untrim does the reverse: it extends a trimmed surface back out toward its underlying natural boundary, filling holes and removing trim loops.
Surface modeling is mostly "make surfaces larger than you need, then trim them to shape". Trim is therefore among the most used surface commands.
Note: Trim also exists as a sketch command (Trim in the sketch palette), and in solid modeling the Split Body command is similar. This page covers the surface Trim command.
Where to find it#
- Surface > Modify > Trim
- Surface > Modify > Untrim
- Toolbar search: S, then type Trim or Untrim.
Step by step#
Trim#
- Create two surfaces that cross each other (for example, a surface extruded from a curve and a second extended surface through it).
- Start Trim from the Surface > Modify panel.
- Select the surface body that is intersected by the extended surface.
- Select the section of the extended surface body to remove.
- Click OK. The unwanted region disappears, leaving the remaining surface with a new trimmed edge.
- Hide the cutting surface if it remains visible.
Note: not confirmed in Autodesk's documentation: whether Trim accepts a sketch curve or a plane directly as the cutting tool. The help describes trimming with an intersecting (extended) surface. If you need a sketch-shaped cut, extrude the sketch into a surface first.
Untrim#
- Start Untrim.
- Select the trimmed surface.
- Choose the untrim type: All Edges (all internal and external edges), Internal Edges, External Edges, or Manual (manually selected edges).
- Optionally specify an Extend distance for the exterior edges of the untrimmed surface, and check Keep Copy to preserve a copy of the original surface.
- Confirm with OK. Holes are filled or external boundaries extended toward the underlying surface's natural limits.
Tip: Hold Ctrl (Windows) or Command (macOS) to suppress the tool preview and modify the selection set.
Options reference#
Trim#
| Option | What it does | Notes |
|---|---|---|
| Surface body | The surface body intersected by the extended surface | Select first |
| Section to remove | The section of the extended surface body to delete | Chosen from the divided regions in the preview |
Untrim#
| Option | What it does | Notes/default |
|---|---|---|
| Face(s) / Surface | The trimmed surface to restore | Select the surface body or face |
| Untrim Type: All Edges | Removes all trim loops, restoring the full underlying surface | Inner holes and outer cuts |
| Untrim Type: Internal Edges | Fills only holes | Keeps outer trim |
| Untrim Type: External Edges | Extends only the outer boundary | Keeps holes |
| Untrim Type: Manual | Chooses specific edges | Use when only one cut should be reversed |
| Extend | Extends the exterior edges of the untrimmed surface by a distance | Optional |
| Keep Copy | Preserves a copy of the original surface | Optional |
Tips and pitfalls#
Tip: Make the trimming surface larger than the target. If it ends before the target does, the split is incomplete and the region you want to select does not appear as a separate piece.
Tip: For symmetrical parts, trim once and then mirror the surface body rather than trimming both sides separately. You will get a cleaner join at the plane of symmetry.
Tip: Trim then Stitch is a common sequence: trim surfaces so they meet exactly at their intersection, then stitch along those shared trimmed edges.
Warning: Untrim extends only to the surface's natural, underlying definition. A surface created by a Patch or Loft may have a natural boundary that is smaller than you expect, so Untrim cannot invent new geometry. Use Extend if you need beyond-boundary geometry.
Warning: A trimmed surface keeps its original parameterization. Very small remaining fragments of a big, wild untrimmed surface can look fine yet behave badly in analysis. If zebra stripes ripple only near trimmed regions, consider rebuilding the surface with a fresh Loft or Patch.
How Trim decides what to remove#
Trim splits intersecting surfaces and removes sections of a surface or quilt. Understanding that the cutting surface divides the target into regions, and you choose which regions to delete, avoids most surprises.
The cutting surface must fully cross the target. If it stops short of the boundary, the surface is not divided and nothing can be removed (general guidance).
Worked example: a 30 mm window in a curved skin#
A 120 x 80 mm curved skin needs a rounded rectangular window 30 x 20 mm, centered.
- Sketch the window on a plane parallel to the skin's mean plane, offset 5 mm above it, and surface-extrude the window profile through the skin to make a cutting surface.
- Run Trim, select the skin, then select the inside region cut out by the window surface.
- The window edge follows the cutting surface, so its chord is 30 mm. Measure the width on the surface: because the skin curves, the arc length across the window is slightly more than 30 mm. For a skin curved with a 200 mm radius across the 30 mm chord, the arc length is 2 x 200 x asin(15 / 200) = 30.03 mm.
If you need exactly 30 mm along the surface, project the curve onto the surface first (Sketch > Create > Project/Include > Project to Surface) and build the cutting surface from that, or adjust the sketch width to about 29.97 mm.
Untrim#
Untrim removes trim boundaries from a face so the underlying surface shows again. It can restore a hole, restore the outer boundary, or both. It only works if the face still has the original surface definition behind the trimmed outline, so it can recover a trimmed surface but not a face that has been rebuilt.
Use Untrim to:
- Heal an unwanted inner hole in an imported face.
- Recover surface area you trimmed by mistake outside the timeline.
- Extend a trimmed face back to its natural boundary before extending or patching.
Failure modes and fixes#
| Symptom | Cause | Fix |
|---|---|---|
| Trim shows no regions to select | The tool does not cross the surface fully | Extend the tool or the sketch beyond the boundary. |
| Trim removes the wrong side | Picked the wrong region | Edit the feature and choose the opposite region. |
| Edge is jagged after trimming | The tool has tiny segments or a loose tolerance | Use cleaner sketch geometry; avoid fit-point splines with many points. |
| Result has a sliver face | Tool nearly coincident with an existing edge | Move the tool 0.1 mm or use the existing edge as the tool. |
| Stitch fails after trimming | Trim edges of neighbors do not match | Trim both with the same tool so edges coincide. |
| Untrim does nothing | Face has no hidden surface data (rebuilt) | Rebuild the face with Patch. |
| Cut location differs from the sketch | The cutting surface was extruded along the sketch plane normal | Check the sketch plane orientation, or use projected curves. |
Trimming both sides with one tool#
When two surfaces meet, trimming both with the same sketch or surface makes the new edges coincide. This is the reliable way to get a surface that stitches. If you trim each with a different tool, the edges will differ slightly (often by more than the stitch tolerance), and the result will not close.
Workflow advice#
- Extend first, then trim; trimming an exactly meeting surface is fragile.
- Keep the sketch behind the cutting surface simple and fully constrained so edits do not break the trim.
- Check the trimmed edge in a section, not only in the shaded view.
- Avoid trimming and untrimming repeatedly on the same face; once the history is messy, rebuild the face.
- After a big trim, run Stitch to confirm the neighbors still close.
Practice exercise#
- Draw a 100 mm sketch line on the Front plane and surface-extrude it 50 mm to make a wall. On the Top plane draw a circle of 40 mm centered on the wall and surface-extrude both directions (long cylinder crossing the wall).
- Use Trim with the cylinder as the trim tool and remove the part of the wall inside the cylinder. A circular hole should appear.
- Run Untrim with Internal Edges on the wall: the hole fills back in.
- Re-run Trim, this time removing the part of the cylinder outside the wall. Observe that the remaining cylinder fragment is short. Explain which pieces you selected.
Expected: after step 3 the wall is whole again; after step 4 you have a hollow cylindrical section embedded in the wall.
Stretch challenge#
Make a curved skin from a 200 mm radius arc extruded 100 mm. Sketch a 30 x 20 mm window and Trim it with a cutting surface extruded straight from the sketch, then again with a cutting surface built from a curve projected onto the skin. Measure the arc width of each window along the surface (the first should be close to 30.03 mm if the chord is 30 mm; the second close to 30 mm). Use Untrim to restore the second window and Trim again with the same sketch but a different region choice. Expected result: two measured widths differing by roughly 0.03 mm, and a restored surface after Untrim with no leftover trim edges.
Related#
- Extend
- Stitch and Unstitch
- Surface Repair