How to Convert a 3D Scan to a Solid Body (BREP) in Fusion 360

The Problem

3D scanners produce meshes — STL, OBJ, or PLY files made up of thousands (or millions) of triangles that approximate the surface of the scanned object. For 3D printing, meshes work fine. For everything else — CNC machining, CAD design, manufacturing drawings, dimensional inspection — you need a solid body.

A mesh is just a skin of triangles. It has no volumetric data, no faces you can dimension or constrain, and no features you can modify parametrically. Try to add a hole to a mesh and you'll understand the problem immediately. You need to convert the mesh into a BREP (Boundary Representation) solid — a model defined by surfaces, edges, and vertices that CAD software can actually work with.

There are three main approaches, each suited to different situations.

Approach 1: Parametric Modelling Over the Mesh

Best for: Engineered parts with defined geometry — brackets, housings, shafts, gears, anything with flat faces, cylindrical features, and standard dimensions.

This is the method covered in detail in our guide to converting scans to STEP files and our gear reverse engineering guide. Import the mesh as a reference, then model the part from scratch using standard CAD features (extrude, revolve, loft, fillet, chamfer, hole).

It's the most work, but produces the best result — a clean, fully parametric model that's easy to modify and manufactures perfectly. For engineered parts, this is almost always the right approach.

Approach 2: Form (T-Spline) Tools

Best for: Organic shapes, complex curves, and parts that don't have simple geometric features — think intake manifolds, body panels, ergonomic handles, sculpted housings, and artistic forms.

Fusion 360's Form environment uses T-spline surfaces that you can sculpt and shape freehand. The workflow is to start with simple primitive shapes and deform them to match the scanned mesh.

Step by Step

1. Import the mesh — Insert > Insert Mesh. Position it at the origin and lock the body so it doesn't move.

1. Enter the Form environment — Create > Create Form. This switches Fusion into its sculpting mode.

1. Start with primitives — Add a box, cylinder, sphere, or torus as your starting shape. Choose the primitive that most closely matches the overall form of your part.

1. Subdivide to add resolution — Select faces and use Insert Edge or Subdivide to add more control points where you need finer detail.

1. Pull vertices, edges, and faces to match the mesh — Use the Edit Form tool to grab geometry and drag it into position. Work from the broad shape inward to the details. The scanned mesh should be visible behind your T-spline as a reference.

1. Use bridge, crease, and merge — Bridge connects two faces across a gap (useful for holes and openings). Crease sharpens edges where you need defined transitions rather than smooth blends. Merge joins vertices to close up the surface.

1. Finish Form — When the T-spline matches the scan to your satisfaction, click Finish Form. Fusion converts the T-spline surface into a BREP solid body.

1. Clean up with solid modelling tools — Back in the regular modelling environment, add fillets, chamfers, precise holes, flat faces, and other features that are easier to do with standard tools than with sculpting.

Tips for Form Modelling

• Work from coarse to fine. Get the overall shape right before adding detail.
• Fewer control points is better — a simpler T-spline with 200 faces is easier to control than one with 2,000.
• Use symmetry. If the part is symmetrical, model half in the Form environment and mirror it after finishing.
• T-splines always produce smooth, G2-continuous surfaces. If you need sharp edges, apply creases.

Approach 3: Mesh Sectioning

Best for: Parts with clear cross-sectional profiles — ducting, aero surfaces, housings with consistent cross-sections, and parts that change shape gradually along one axis.

This technique uses the mesh itself to generate cross-section curves, which you then loft together to create a solid.

Step by Step

1. Import the mesh and position it in the workspace.

1. Create sketch planes at intervals — Space construction planes along the length of the part. For a 300mm part, planes every 20-30mm gives you enough sections to capture the shape.

1. Generate section curves — Use Mesh > Mesh Section (or Inspect > Section Analysis) to create curves where each plane intersects the mesh. These curves represent the cross-sectional profile at each location.

1. Fit clean geometry to the section curves — In a sketch on each plane, trace over the section curves using splines, arcs, and lines. Clean up any noise or irregularities from the scan.

1. Loft between sections — Use the Loft tool to create a smooth solid body that passes through all the cross-section profiles.

1. Add end caps and features — Close off the ends and add any holes, bosses, or details that the loft didn't capture.

This method works particularly well for aero surfaces, intake runners, complex curved housings, and any part where the geometry flows smoothly from one end to the other.

Direct Mesh-to-BREP Conversion

Fusion 360 can directly convert a mesh to a BREP body (right-click the mesh > Mesh to BRep), but be aware of the limitations:

• Triangle count limit — Only works reliably on meshes under approximately 10,000 triangles. Most scans have hundreds of thousands or millions.
• Poor geometry — The result is a body where every triangle becomes a flat face. It's technically a solid, but it has no clean surfaces, no editable features, and produces enormous file sizes.
• Reduce first — If you want to try this, use Mesh > Reduce to bring the triangle count down before converting. You'll lose detail, but it may be acceptable for very simple shapes.
• Only practical for simple geometry — Spheres, boxes, and basic organic forms. Anything with fine detail or thin walls will fail or produce unusable results.

For anything beyond a rough reference body, the three methods above are far better options.

General Tips

• Keep the mesh visible but transparent while modelling. Reduce the mesh body opacity in the browser panel so you can see your solid model through it.
• Use Inspect > Deviation Analysis to quantify how closely your solid model matches the original mesh. This is especially useful for quality-critical parts.
• Don't chase every ripple in the scan. Scans include noise, surface imperfections, and artefacts from the scanning process. Model the design intent, not the scan artefacts.
• For symmetrical parts, model half and mirror. This halves the work and guarantees symmetry.
• Save versions as you go. BREP conversion from Form is not always easily reversible. Save a version before finishing the Form so you can go back if needed.

Need Help Converting a Scan?

We offer scan-to-CAD conversion services — send us your STL or OBJ file and we'll produce a clean, solid STEP model ready for manufacturing. Get in touch for a quote, or check out our 3D scanning services if you need the part scanned first. If you need a scan to work with, browse our 3D scan library — popular conversion projects include engine scans and gearbox scans. Ready to manufacture? Get an instant quote on your finished STEP file.