A converted mesh can come out with thousands of unnecessary tiny faces. Here's what the smoothing slider actually does about that, and how to pick a value with confidence.
A mesh that looks perfectly smooth in a 3D viewer can still be made of an enormous number of nearly flat, nearly co-planar triangles. This is especially common with two sources: phone or handheld 3D scans, where the scanning software oversamples flat surfaces just to be safe, and certain slicers or export pipelines that re-tessellate a shape into far more triangles than its geometry needs. Convert that kind of mesh into a STEP solid with no changes at all — see the STL vs. STEP guide for what "converting" a mesh actually means — and you get a technically valid solid where a single flat wall is represented as hundreds of separate tiny CAD faces instead of one. It behaves correctly, but it's unpleasant to work with: selecting "the wall" in your CAD software selects one facet out of hundreds, and the file is needlessly large.
Smoothing looks for groups of triangles that are already nearly flush with each other — nearly co-planar, within a small angular tolerance — and merges them into a single larger flat face. A surface that was 400 tiny triangles pretending to be one wall becomes one real CAD face representing that wall. This is a simplification of the mesh, not a reconstruction of some "true" curved surface the mesh never actually described — merging flat facets doesn't invent curvature that wasn't there, it just stops representing a flat area as needlessly many separate faces.
The part that makes this safe rather than lossy is a hard displacement limit: when merging triangles, the converter never moves a vertex further from its original position than the maximum tolerance you set. Every merged face still passes through the mesh's original surface within that bound. A wall that was flat to begin with ends up exactly flat, at the same position. A wall that had genuine small bumps or scan noise gets smoothed toward flat, but only up to the distance you allowed — it can't drift arbitrarily far from where the original mesh actually was.
In the converter, this is the "Maximum smoothing allowed per vertex" slider, ranging
from 0 mm to 0.5 mm, defaulting to 0.1 mm. Setting it to 0 disables
smoothing entirely — every original triangle becomes its own tiny CAD face, a fully literal,
face-for-face conversion of the mesh. Raising it lets the converter merge triangles as long as no
point moves more than that many millimeters from where it started.
| Situation | Suggested setting |
|---|---|
| High-precision mechanical part, tight-fit surfaces, exact reference geometry | 0 mm — keep a literal, unaltered conversion |
| Model downloaded from a marketplace, reasonably clean mesh | Default (0.1 mm) is usually fine |
| Phone or handheld 3D scan with visible surface noise | 0.1 mm–0.3 mm, checked against the 3D preview |
| Purely cosmetic model, file size or face count matters more than exact geometry | Higher end of the range (up to 0.5 mm) |
When in doubt, start at the default, look at the result in the converter's 3D preview, and only change it if you can actually see a problem — either leftover facet noise (raise the tolerance) or a feature that looks softened when it needs to stay sharp (lower it, down to 0 if necessary). The preview reflects the exact solid you'll download, so there's no guessing involved.