Converting CAD Models and Exporting for Printing
CAD programs store exact curved surfaces; printers need triangles. Either send a STEP file to a slicer that converts it on import, or export STL or 3MF yourself and choose the tessellation, the maximum distance and angle between the mesh and the true surface, where finer means smoother curves and larger files.
Exact surfaces versus triangles
Engineering CAD programs describe a part with exact geometry: a hole is a true cylinder, a fillet a true curve. Slicers work on triangle meshes. Somewhere between the CAD file and the printer, every curved surface is turned into flat triangles. That step is called tessellation, and you can let the slicer do it or do it yourself. The wider chain from model to G-code is in CAD, Slicing and the Path to a Print.
Option 1: send STEP to the slicer
Some slicers open STEP files directly. Prusa states that PrusaSlicer triangulates the STEP file during import. This keeps you out of the export settings, and the STEP file is a good exchange format if someone else will slice the part. The trade-off is that the slicer chooses the tessellation, with whatever control it offers. If your slicer does not accept STEP, or you want to control the mesh, use option 2. STL, 3MF, OBJ, STEP and G-code compares the formats.
Option 2: export a mesh yourself
Fusion
Save As Mesh (right-click a body or component in the Browser) writes 3MF, STL (binary or ASCII) or OBJ. Set the Unit Type to millimeter for STL, and choose One File or One Body Per File. For control over the triangles, the Tessellate tool (Mesh tab, Create) converts a body to a mesh with these refinement settings:
| Setting | What it limits |
|---|---|
| High / Medium / Low | Presets for all the values below |
| Surface Deviation | The maximum distance between the original surface and the mesh |
| Normal Deviation | The maximum angle between the normals of the mesh faces |
| Maximum Edge Length | The longest any triangle edge may be |
| Aspect Ratio | How long and thin each triangle may be |
Tick Preview to see the mesh and a live triangle count as you change the values.
FreeCAD
In the Mesh Workbench, Meshes, Create mesh from shape opens a Tessellation panel with a choice of mesher. The Standard mesher has Surface deviation (maximum linear deviation from the surface), Angular deviation (maximum angle from one mesh section to the next, used on curved surfaces) and an option to make the surface deviation relative to each edge's length. The Mefisto mesher uses a maximum edge length, and the Netgen mesher a fineness setting from very coarse to very fine. Then export the new mesh object.
Other programs
Most CAD programs expose the same two ideas under different names: a distance tolerance (chordal deviation, sag) and an angle tolerance. Look for them in the export dialog or its options.
Choosing the resolution
Prusa warns that a modeling program can show a smooth curve while the geometry has only a few sides, and that the print can turn out as a polygon rather than a circle. The distance setting controls exactly that. For a circle split into equal straight segments, the gap between each segment and the true arc is:
r is the radius and N the number of segments around the circle. Example: a 10 mm radius with 32 segments deviates by about 0.048 mm; with 16 segments, about 0.19 mm. The deviation halves roughly every time you add about 40 percent more segments, and the triangle count rises accordingly.
Holes are affected in a useful-to-know way. When the corners of the polygon sit on the true circle, the flat sides sit inside it, so a faceted hole measures smaller across its flats than the design:
d is the design diameter and N, the number of segments, is even, so the across-flats size is measured between two opposite flat sides. Example: a 10 mm hole with 16 segments measures about 9.81 mm across the flats; with 64 segments, about 9.99 mm.
Practical guidance Rule of thumb: keep the deviation well below what your printer can resolve, so the printer, not the mesh, sets the surface quality. Past that point, finer tessellation only makes the file bigger and the slicer slower. If you are unsure, export a test part at two settings and compare the prints Measure it.
Export checklist
- Units set to millimeters, or export 3MF; see Model Scale and Units.
- Threads and other details that are cosmetic in CAD changed to real geometry.
- Separate bodies exported the way you intend: one file, or one per body.
- Tessellation fine enough for your smallest curved feature, not finer.
- The exported mesh opened in the slicer with no warnings; see Meshes, Manifolds and Normals.
- The CAD file kept. If the mesh has problems, fixing the source and exporting again beats repairing the mesh.
- Supported file formats Prusa Research
- Convert a solid body to a mesh body Autodesk Fusion Help
- Save bodies in a component as 3MF, OBJ, or STL files Autodesk Fusion Help
- Mesh FromPartShape FreeCAD Documentation
- Modeling with 3D printing in mind Prusa Research
Last reviewed 2026-11-05