Import IGES Files into FreeCAD Without Losing Continuity

Importing IGES files into FreeCAD often introduces broken faces, micro-gaps, or disrupted surface continuity because the format frequently represents geometry as unstitched independent surfaces. This article explains how to configure FreeCAD’s internal Open CASCADE Technology (OCCT) import settings, adjust precision tolerances, and utilize workbench repair tools to ensure surfaces stitch correctly into continuous, solid geometries without losing curvature or tangent continuity.

Configure IGES Import Preferences

By default, FreeCAD may import IGES entities as discrete, unstitched trimmed surfaces. Configuring the import settings forces FreeCAD to sew faces during the translation phase:

  1. Open FreeCAD and navigate to Edit > Preferences.
  2. Select Import-Export from the left-hand panel, then click the IGES tab.
  3. Check the box labeled Perform sewing. This instructs the OCCT geometry kernel to automatically stitch adjacent face edges upon reading the file.
  4. Set the Sewing tolerance. The default is typically 0.01 mm. If your original model has ultra-fine details and loses continuity, reduce this to 0.001 mm. If edges fail to join, increase it slightly (e.g., 0.05 mm to 0.1 mm) to bridge micro-gaps without distorting face tangency.
  5. Set the entity reading preference to prioritize B-Rep (Boundary Representation) solids over simple surface meshes whenever the IGES file contains manifold topology.
  6. Click Apply and OK.

Verify and Repair Surface Continuity

If the imported model still displays split edges, faceted transitions, or fails to form a manifold solid, use the Part Workbench to diagnose and restore continuity:

  1. Switch to the Part Workbench.
  2. Select the imported model in the tree view.
  3. Go to Part > Check geometry. Review the report for errors like non-manifold edges, self-intersections, or face-boundary discrepancies.
  4. If the model consists of disjointed surfaces, select all individual imported sheets and navigate to Part > Compound > Make compound.
  5. With the compound selected, go to Part > Create shape from mesh or use the Part > Connect > Connect objects tool to merge coplanar and tangent faces.
  6. Alternatively, open the Part > Shape Builder utility, choose Shell from faces, select all the imported IGES faces, check Refine shape, and click Create. This binds adjacent edges while preserving G1 (tangency) and G2 (curvature) continuity across shared boundaries.

Optimize Export Source (When Applicable)

Surface continuity loss often originates in the exporting CAD software rather than FreeCAD:

  • Export as Solid/Manifold: Ensure the source software writes the IGES file using IGES 5.3 specifications with Manifold Solid B-Rep Object (MSBO) entities (Entity Type 186), rather than disconnected Type 143/144 trimmed surfaces.
  • Match Unit Scales: Set the unit scale in both the source software and FreeCAD (Preferences > Units) to identical scales (e.g., millimeters) prior to import to prevent floating-point calculation errors along boundary edges.