Overview#
Imported STEP/IGES files, converted meshes, and over-trimmed models often arrive as surfaces that will not form a solid. Repair is a diagnostic routine: find the free edges, decide whether the defect is a gap, an overlap, a flipped face, or a bad face, and then choose the smallest fix.
Where to find it#
- Diagnosis: Surface > Modify > Stitch (free edges), Inspect panel analysis tools, Properties.
- Fixes: Stitch, Patch, Untrim, Extend, Trim, Reverse Normal, Unstitch, Delete Face, and Split Face, all under Surface > Modify or Create (Delete via Delete key on faces).
Step by step: a general repair routine#
- Unify: if a STEP import arrives as many bodies, select them all and run Stitch with the default tolerance. See what stays open.
- Count the free edges: the Stitch dialog shows the free-edge count, and its Unstitched Edges table lets you click through each gap. Note each location.
- Classify each defect.
- Tiny gap (less than tolerance): raise tolerance slightly and restitch.
- Larger gap in a flat area: Patch with Connected continuity.
- Gap with a missing curved face: Patch with tangent edges, or loft between the edges.
- Overlapping or extra faces: Unstitch, delete the extra, restitch; or Trim.
- Hole in a face (bad trim): Untrim with Internal Edges.
- Face too short: Extend then Trim.
- Reversed face: Reverse Normal.
- Restitch and verify the free edge count dropped.
- Verify with Properties (volume positive and plausible) and Section Analysis.
- Analyze quality with Zebra or Curvature Map in the repaired region.
Specific problems#
Inside-out solid or negative volume#
After stitching, check Properties. A strange volume or a body that Boolean-fails indicates reversed normals. Unstitch, use Reverse Normal on the faces that point inward, and restitch.
Sliver faces and tiny edges#
Imports often contain faces a few microns wide. They block Fillet and Shell. Options: delete the sliver face and Patch the hole, or Unstitch and rebuild the area with a single Patch.
Converted mesh bodies#
Mesh bodies are not surfaces. Convert them with the Mesh tab > Modify > Convert Mesh, which can produce a solid or surface body. Methods: Faceted (each mesh face becomes a face, so heavy), Prismatic (merges groups of faces into prismatic features), and Organic (an organically shaped solid or surface; part of the Fusion Design Extension). Operation is Parametric or Base Feature. Reduce the mesh first (Mesh Reduce) if face counts are high.
Trim loops that will not go away#
If a trim edge is bogus, Untrim at that edge, then re-trim cleanly.
Rippling analysis#
If Zebra shows jitter across a large face, the surface itself is poorly parameterized. Rebuild: extract the boundary edges, then Patch or Loft a clean replacement.
Options reference#
| Tool | Use for | Notes/default |
|---|---|---|
| Stitch (Tolerance) | Joining, finding free edges | Raise tolerance gradually |
| Patch | Filling holes with continuity | Per-edge continuity; see Patch page |
| Untrim | Removing bad trim loops, filling holes | All, Internal, External, Selected |
| Reverse Normal | Flip face orientation | Required for inside-out geometry |
| Unstitch | Break quilts to rework | Loses solid status |
| Delete Face | Remove a bad face | Healing behavior is not confirmed in Autodesk's documentation |
| Extend / Trim | Grow and cut back faces | Standard gap closing |
| Split Face | Divide a face for independent edits | Surface > Modify |
Tips and pitfalls#
Tip: Work in a copy. Duplicate the body into a new component before repairs so you can compare.
Tip: Repair the biggest defects first; many tiny free edges vanish once a big gap is closed and the quilt re-forms.
Warning: Raising the stitch tolerance to a large value (such as 1 mm or more) hides defects by distorting geometry. Accurate parts for machining need exact fits.
Warning: Repairs in a history-off (direct modeling) document can't be rolled back through the timeline. Save a version first.
Warning: Importing a STEP from a CAD system with a different units setting can produce tiny or huge geometry. Check units before repair.
Tip: If a face is beyond repair, extract its boundary edges via Offset or by projecting, and build a new face. Rebuilding is often faster than patching a damaged face.
Worked example: repairing an imported housing#
You import a STEP file of a 90 x 60 x 35 mm housing that arrives as 48 surface faces and will not become a solid.
- Count free edges. Run Stitch with the default tolerance. Fusion keeps the body as a surface, and the Stitch dialog reports the free edges. Suppose there are three open loops: one around a missing 12 mm hole cap, one at a sliver gap, and one along a seam.
- Measure each gap with Inspect > Measure. The seam gap is 0.03 mm, the sliver gap is 0.4 mm, the hole is a true missing face.
- Fix in order of cheapness. Re-stitch at 0.05 mm tolerance: the seam closes. For the 0.4 mm sliver, delete the two sliver faces and Patch the opening with the boundary edges selected. For the hole, Patch with a flat (G0) cap.
- Restitch. Free edge count drops to zero and the body becomes a solid.
- Verify. Properties should show a positive volume close to the expected value (a 90 x 60 x 35 box is 189,000 mm3; with walls the housing might read 40,000 mm3). A Section Analysis through the middle should show a clean wall with no internal faces.
Always verify volume against an independent estimate. A solid with the right topology but the wrong volume usually has an inverted region.
Triage table#
| Symptom | First tool | Second tool |
|---|---|---|
| Free edges after stitch | Measure the gap | Tolerance, then Patch |
| Negative volume | Reverse Normal | Unstitch and restitch |
| Tiny faces or sliver | Delete Face (heal) or Stitch with larger tolerance | Rebuild the area with Patch |
| Ragged trim edges | Untrim then Trim again with a clean sketch | Rebuild the face |
| Wavy reflections | Zebra | Rebuild from fewer sections |
| Mesh import | Convert Mesh, then repair | Rebuild with surfaces over the mesh |
Failure modes and fixes#
| Symptom | Cause | Fix |
|---|---|---|
| Stitch keeps failing at one corner | Three-way junction with a tiny gap | Zoom in, measure, patch the corner, or delete one face and rebuild. |
| Patch creates a bulge | Opening is large and the continuity too high | Use G0 or G1, add guide curves, or split the opening. |
| Solid appears hollow in Section Analysis | Inner faces from a duplicate shell | Remove the duplicate body. |
| Repair works but the file is slow | Thousands of tiny faces from a mesh convert | Reduce the face count by rebuilding with larger patches. |
| Reverse Normal does not change appearance | The orientation change is not obvious in the shaded view | Check the result by stitching and reading the volume in Properties. |
A systematic routine#
- Save a copy before repairs.
- Stitch and count free edges.
- Measure the smallest gaps first; clear them with tolerance.
- Rebuild larger defects with Patch, Extend, or Trim.
- Restitch. Aim for zero free edges.
- Check volume, Section Analysis and Zebra.
- Note the settings (tolerance, patched areas) in a comment or a timeline name, so another person can follow your repair.
When to give up and rebuild#
A repair that involves more than a few dozen faces is often slower than rebuilding. As a general rule of thumb (not Autodesk data), if after 30 minutes you have not reached zero free edges, trace the key profiles from the import and surface them freshly; the new surfaces will be cleaner and lighter.
Understanding free edges and where they come from#
Most open edges fall into four origins. Gaps happen when two surfaces were trimmed with different tools, so their edges do not coincide. Missing faces occur when a face failed to import or was deleted. Overlaps arise from duplicated geometry left from earlier operations. Slivers are faces with near-zero width, often created by trimming with a tool nearly coincident to an existing edge. Knowing the type tells you the fix: tolerance for gaps, Patch for missing faces, delete for overlaps, and delete plus rebuild for slivers.
Repair budget#
Estimate the effort before starting. A single gap with a measured size under 0.05 mm takes seconds; a missing face takes minutes with Patch; a damaged region of a few dozen faces may take hours. Use free edge counts as your meter: if each fix reduces the count steadily, continue; if fixes create new free edges elsewhere, you are probably fighting a bad base surface and should rebuild.
Documenting a repair#
Rename repaired bodies or timeline features with notes such as 'Patch_hole_12mm' or 'Stitch_tol_0.05'. When a collaborator reopens the file, the repair history explains why geometry looks the way it does, and you can change tolerance values with confidence later.
Practice exercise#
- Import (Insert > Insert Mesh or open a STEP) a model with open faces, or create your own: stitch a surface box, then Unstitch it, delete the top face and nudge another face 0.5 mm away with a move.
- Stitch all: it stays open at two seams.
- Fix the 0.5 mm gap with a small increase in tolerance and see if it closes; then undo and fix it correctly with Extend plus Trim.
- Patch the missing top. Stitch. Expected: a solid with the right volume.
Stretch challenge#
Take any imported STEP or STL-to-solid file (or create your own by building a box from surfaces, deleting one face, shifting another by 0.03 mm and inverting a third). Perform the routine above and record: free edge count after each step, the tolerance used, the final volume and the expected volume. Expected result: zero free edges, a volume within 1% of the expected figure, and a log of at least three distinct fixes.
Related#
- Stitch and Unstitch
- Patch
- Trim and Untrim