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STEP to STL: choosing mesh quality

The 3D Armory · Updated October 27, 2026

STEP can describe precise CAD surfaces. STL approximates those surfaces with triangles. The useful question is how much detail your next application needs.

Choose a starting tolerance

After you select a STEP or STP file, choose STL output. The mesh-quality control sets the linear deflection requested from the CAD engine: Standard is 0.1 mm, Fine is 0.01 mm, and Coarse is 0.5 mm. These are tessellation settings, not a printer's layer height or a guarantee of manufactured accuracy. The engine also applies an angular-deflection setting.

Start with Standard, inspect the result, and use Fine if curved edges look too coarse for your purpose. A tighter tolerance generally creates more triangles and increases conversion time and file size. For a rough fit check or a large visualization model, Coarse may be enough. A flat-sided part may look the same at every setting because planar faces do not need extra triangles to approximate curvature.

Worked example: checking size

  1. Download the sample STEP cube and select it in the converter.
  2. Leave the output as STL and mesh quality as Standard · 0.1 mm.
  3. Convert and download the STL. Open it in a slicer or mesh editor and interpret the coordinates as millimeters.
  4. Verify a 10 × 10 × 10 mm bounding box. The sample is already faceted, so it is useful for checking dimensions, not comparing curved-surface quality.
  5. For a quality comparison, repeat with your own curved part at Standard and Fine. Compare the same curved region and the output size; do not judge smoothness from a shaded preview alone.

Units and assemblies

STEP and IGES imports request millimeter coordinates from the CAD engine. STL itself has no reliable unit field, so the receiving application must interpret those coordinates as millimeters. If a part opens at the wrong size, check the receiving application's import setting before rescaling it.

All imported parts are combined into one output mesh file. Colors, material definitions, assembly structure, and CAD feature history are not retained. Separate components can remain disconnected inside the file. If you need individual part files, export those parts separately from your CAD application.

Before printing or fabrication

Reopen the exported mesh, measure a known feature, and inspect holes, thin walls, and contacting surfaces. Review the slicer's layer preview for missing regions. Successfully writing an STL does not prove that an imported surface model is a closed printable solid.

The upload selector accepts files up to 25 MB; imported CAD meshes are limited to 2 million triangles, and a conversion stops after three minutes. If a complex model fails, try Coarse or export a smaller set of parts. See the troubleshooting guide for recovery steps.