After import the model is (for FreeCAD), just an assembly of faces. You might want to convert the model into a shape FreeCAD can recognize and that could be altered in FreeCAD.
To do this:
- Switch to the Part Workbench
- Select the mesh, and go to the Part → Create shape from mesh or press the Part ShapeFromMesh button
- Click in the dialog
- Select the newly created shape
- Go to Part → Convert to solid
- Select the newly created solid
- Go to Part → Refine shape or press the Part RefineShape button
Note: The last step is not necessary, but it will clean the solid of most of its residual edges on planar and cylindrical surfaces.
"cannot convert because shape is not a shell"
Well, your shell seems to have errors, maybe it is not closed (has holes). Since the capabilities of the mesh workbench in FreeCAD are a little limited at the moment, you might want to try examining and repairing of your model with third-party software. After doing that, you may try importing and converting of your model again.
Third Party Programs
- License: Open Source (GPL V2)
- Runs on Windows 32/64 bit, Linux and macOS
- netFabb basic
- License: Freeware
- Runs on Windows XP/7/8, Linux and macOS