Overview#
Every 3D shape you exchange is represented in one of two fundamentally different ways, and confusing them is behind a large share of interoperability headaches.
BRep (boundary representation) describes a solid by its boundary: a set of faces, each lying on an exact mathematical surface (plane, cylinder, cone, sphere, torus, NURBS), bounded by edges and vertices, with topology recording which faces meet along which edges. A hole in a BRep is a true cylinder with a radius you can read. You can fillet it, offset it, change its diameter, or machine it with exact arcs. Fusion's native solids, STEP, IGES, SAT, SMT, Parasolid and all the vendor CAD formats are BRep.
Mesh describes a surface as a collection of small flat polygons, usually triangles. A "round" hole in a mesh is a many-sided polygon. A mesh has no notion of "cylinder" or "fillet"; it only knows vertices and triangles. STL, OBJ, 3MF, PLY and most scan formats are mesh. Fusion handles meshes as a separate body type, mainly through the Mesh workspace and Insert Mesh.
Neither is better in general. BRep is the right representation for design, manufacturing, and measurement. Mesh is the right representation for scans, sculpts, 3D printing slicers, rendering and anything that must be fast to display. The art of exchange is choosing correctly and converting only when necessary.
Where to find it#
| Task | Location |
|---|---|
| Insert a mesh | Insert > Insert Mesh |
| Mesh tools | Mesh workspace (workspace switcher): Repair, Reduce, Remesh, Shell, Plane Cut, etc. |
| Convert mesh to BRep | Mesh workspace or Design workspace: Modify > Convert Mesh (in Design, available for mesh bodies); also Mesh tab Convert Mesh |
| Convert BRep to mesh | Modify > Convert options, or Save As Mesh; in Mesh workspace, Create > BRep to Mesh |
| Mesh section analysis, display | Inspect tools; mesh display modes in the Mesh workspace |
| Export mesh | Right-click body > Save As Mesh (STL, OBJ, 3MF) |
Names of the conversion commands moved between releases (Convert Mesh, Mesh to BRep, and so on). Verify in your version.
Step by step#
Bringing a scanned or downloaded mesh into Fusion for measurement or reference only#
- Choose Insert > Insert Mesh, pick the file, set units (see the import options page).
- Leave the body as a mesh. Use it as a visual reference: sketch on planes aligned to it, measure with Inspect > Measure (which can snap to mesh vertices and, in some cases, to facet planes), and section it with Section Analysis.
- Model new BRep geometry around the mesh by tracing it, which keeps the final design editable and light.
Converting a mesh into an editable solid#
- Insert the mesh, then switch to the Mesh workspace. Repair it first: run Repair to close holes, remove duplicate or non-manifold faces and fix inverted normals.
- Reduce the triangle count if it is huge using Reduce or Remesh. A mesh of several million triangles will make conversion slow or fail.
- Return to the Design workspace (or use the Mesh workspace's Convert command) and choose Modify > Convert Mesh.
- Choose the Operation (Parametric, Organic/Prismatic, depending on version). The prismatic style tries to merge flat and cylindrical regions into larger faces; the organic style produces faceted faces or T-spline-style surfaces. Choose Method accordingly.
- Run the conversion and check the result. A faceted conversion produces a solid with thousands of tiny planar faces. It is a valid BRep and can be used for Boolean operations, but it is not truly clean.
- If you need clean geometry, rebuild: use the mesh as a reference and model true features over it, then compare using Section Analysis or the Inspect > Deviation-type tools if available in your version.
Converting a solid into a mesh#
- For export, use Save As Mesh (see the export workflow page), which tessellates at the chosen refinement without altering your design.
- To get a mesh body inside Fusion (for mesh-only operations such as remeshing or lattice-style work), use the BRep-to-mesh conversion in the Mesh workspace. You get a mesh body in addition to, or in place of, the solid depending on the command's options.
- Pick the refinement. Too coarse a mesh shows faceting on round surfaces; too fine creates huge files.
Options reference#
| Concept / option | What it does | Notes |
|---|---|---|
| Surface Deviation | Max distance between mesh and true surface | Lower is finer. A common starting point for small printed parts is around 0.01 to 0.05 mm; larger parts tolerate more |
| Normal Deviation | Max angle between neighboring face normals | Controls roundness on curves; typical values are a few degrees |
| Maximum Edge Length | Upper limit on triangle edge size | Avoids huge flat triangles |
| Aspect Ratio | Limits long thin triangles | High values can create slivers |
| Convert Mesh: Faceted | Each triangle (or merged coplanar group) becomes a planar face | Safe, large, not editable in the usual sense |
| Convert Mesh: Prismatic | Detects planes, cylinders and other primitives and merges triangles | Works best on mechanical parts from clean scans |
| Convert Mesh: Organic | Converts to T-spline-like surfaces | Better for freeform shapes; verify in your version |
| Repair | Closes holes, fixes normals, removes degenerate triangles | Run before any conversion or boolean |
| Reduce / Remesh | Lowers or regularizes triangle count | Reduces file size and conversion time |
When to use which#
| Situation | Use | Why |
|---|---|---|
| Sending a part to a CNC shop | BRep (STEP) | They need true arcs and edges for toolpaths |
| Sending a part to a 3D printer | Mesh (3MF or STL) | Slicers consume triangles; STEP also works with some print services but may be re-tessellated |
| Sharing an organic sculpt | Mesh (OBJ or 3MF) | There may be no meaningful BRep |
| Keeping a scan as a reference | Mesh | Convert only the features you need |
| Exchanging with another CAD user | BRep (STEP) | They want solids they can measure |
| Rendering in another tool | Mesh (OBJ, FBX) | Render engines and DCC tools work with triangles and UVs |
| Reverse engineering | Both | Scan (mesh), then rebuild (BRep) |
| Analysis meshes | Specialist | FEA uses its own meshes; STEP is the usual input |
The trouble with converting a mesh to BRep#
The conversion is inherently lossy in intent even when it is geometrically faithful. A scan or an STL has already lost design intent: the mesh does not know that a set of triangles represents a single bore. Converting each triangle into a face yields a legitimate solid that is useless for editing. Automatic feature recognition (prismatic conversion) can recover simple primitives, but threads, tapers, blends and noisy surfaces break it.
Practical outcomes you should expect:
- Clean CAD-origin STL, high resolution: prismatic conversion may recover planes and cylinders.
- Scan data: noise prevents primitive detection. Expect to rebuild manually with the mesh as underlay.
- Organic forms: conversion to T-spline or quad meshes in the Form workspace may be better than a triangle-to-face conversion. See the Form section.
- Huge meshes: reduce first, convert second.
Tips and pitfalls#
Tip: If you only need to cut a hole into a mesh-derived part or measure its size, you can often skip conversion entirely: use Mesh workspace tools like Plane Cut and Combine, or measure directly on the mesh.
Tip: Sketch on slices. Create planes through the mesh, section it, and trace the outlines with fit-point splines or lines and arcs. This is the classic reverse-engineering approach and produces clean features.
Tip: Keep the original mesh file. It is the evidence of what you rebuilt, and it lets you re-verify deviation later.
Warning: A mesh exported at low refinement and re-imported cannot be refined; the lost curvature is gone. Always export from the BRep master at the refinement you need.
Warning: STL has no units. The same file can be 40 mm or 40 inches. If you receive a mesh without context, measure a known feature before trusting it.
Warning: Meshes with open edges, self-intersections or flipped normals (non-manifold) are not "watertight". Slicers repair some of these silently; CAD conversion often fails. Always run a repair and check for holes.
Warning: Boolean operations on meshes (combine, subtract) are more fragile than on BRep. Expect failures on dirty data and plan for a repair pass.
Warning: Converting a high-density mesh can freeze the application for a long time and may use gigabytes of memory. Save first. Convert smaller sections when possible.
Practice exercise#
- Model a block with a 20 mm diameter through hole and a 2 mm fillet along one edge.
- Save As Mesh as STL at Low refinement; then again at High. Insert both into a new design.
- Measure the hole in each: look at the apparent diameter at different positions (you will see variation in the low refinement mesh).
- Run Convert Mesh on the High mesh with the prismatic option. Check whether the hole became a true cylinder (select the face and look at its type or try changing diameter with Press Pull).
- Repeat on the Low mesh and compare.
Expected result: you see concretely that mesh resolution caps what conversion can recover.
Related#
- Format landscape (format-landscape)
- Data loss and large files (data-loss-and-large-files)
- Mesh workspace and import (07-form-sculpt-mesh/mesh-workspace-and-import)
- Mesh repair, reduce, remesh (07-form-sculpt-mesh/mesh-repair-reduce-remesh)