STL to STEP that rebuilds real surfaces

They wrap the triangles rather than replace them. A mesh face becomes a planar STEP face, one per triangle, so a 200 000-triangle model becomes a 200 000-face solid: technically valid, hundreds of megabytes, and impossible to work with — you cannot select a hole, change a diameter or fillet an edge that is really two hundred edges. This recognises the surfaces the mesh was tessellated from and writes those instead. A drilled hole comes back as one cylindrical face bounded by two circles, which is what it was in the CAD that made the STL.

It depends on the part, and you find out before you spend any time on it. Machined and modelled parts — plates, brackets, housings, anything with flat faces and drilled holes — come back as real CAD. Scans, sculpts, organic shapes and threaded fasteners do not, because their surfaces are genuinely free-form and there is no plane or cylinder underneath to recover. On ten real test parts the surface recovered ranged from 15% to 93%, and the panel puts your file's number on screen before converting.

For circular features, yes, and that is not a marketing claim. A tessellated hole is a polygon inscribed in the true circle, so it measures slightly small — the more coarsely it was exported, the smaller. Fitting the circle back recovers the diameter the original CAD had, to the last digit; the verification suite asserts exactly that against parts whose radii are known. What it cannot do is invent detail the mesh never carried: a fillet exported at low resolution stays as coarse as it was exported.

Because the promise is still narrowing. It rebuilds planes, cones and cylinders — including partial ones, so the rounded corners on an ordinary bracket now come back as real geometry rather than facets. What it still cannot do is a fillet: where a rounded edge blends into two faces at once, the surface is a torus or a sphere, and neither is recognised yet. On a heavily filleted part that is most of the surface. Charging for a converter with a hole that size in it would be a refund queue rather than a business, so it is marked beta, it is free while it is beta, and the gate tells you before converting which case your file is.

Neither. Every online converter that produces real solids does it with a CAD kernel — OpenCascade, usually — either on a server or as fifty megabytes of WebAssembly. This ships zero bytes of either. The reason it can is that by the time a kernel would be called, everything a STEP file needs is already known: which triangles form a face, the loops that bound it, and which edges are shared. A kernel sews loose faces together by geometric proximity; here adjacency comes from the mesh's own edge map, so the shell is closed by construction.

Millimetres, declared inside the file. STL carries no unit at all, so the numbers in it could be anything; every CAD package reads them as millimetres by convention, and writing plain metres into the STEP header — which some converters do — makes the part arrive a thousand times too large in each direction, a billion times the volume. If your STL was authored in inches, set that in the Read as menu and the conversion is redone at the right scale.

You are told, with the numbers, and then you decide. Converting anyway is allowed and sometimes right — a facet STEP still imports, still measures, and still positions correctly in an assembly. What it will not do is behave like modelled geometry, and the difference between knowing that beforehand and discovering it in SolidWorks is most of the value here. Models made almost entirely of free-form surface are refused outright: the file would run to tens of thousands of faces and no CAD opens that comfortably.