Show the Working

3MF to STL

Your model never leaves your browser, and nothing is downloaded to make this work. A mesh is a list of points and a list of triangles; reading one and writing the other is arithmetic your browser can already do.

…or drop one anywhere on this page.

Into the format every slicer opens, and the one with the most to say about what it loses: no materials, no colour, no names, and no way to share a corner between triangles, so the file grows.

3MF is what 3D printing moved to, and the reason is that it records what its numbers mean. It is also a build plate rather than a model: several objects, each placed by its own transform, which are all put where the file says.

This conversion loses the units, and that matters

3MF is the only format on this site that records what its numbers mean: <model unit="millimeter">, or inches, or microns. STL cannot hold that. It is not converted and not approximated — there is nowhere to put it.

So what your file says is shown above, and it is worth writing down, because whatever opens the result next will have to guess. Guessing wrong is why models arrive at twenty-five times their intended size, or a twenty-fifth. Nothing is rescaled here: the numbers come across exactly as they were written, and silently multiplying your model by 25.4 would be far worse than telling you.

A 3MF is a build plate, not a model

The <build> section lists items, each naming an object and carrying a transform of twelve numbers. Objects can also be assembled out of other objects, nesting, each with its own transform. That is what makes a 3MF a plate with things arranged on it rather than a single shape.

A reader that takes the first object and ignores the build gets the right triangles in the wrong places — every part at the origin, interpenetrating — with the triangle and vertex counts perfectly correct. The transform is twelve numbers in row-major order with the translation last, which is not how glTF stores the same idea; read the other way round, a moved part goes somewhere else entirely.

An STL repeats every corner

An STL has no way to say that two triangles share a corner. There is no vertex list and no index; each triangle carries its own three points. So a corner where six triangles meet is written out six times, to full precision, every time.

A cube is eight corners, and an STL stores thirty-six. That is the whole reason the file gets bigger on this page, and it is the format doing it rather than the conversion: the box that trimesh writes is 433 bytes as an OBJ and 684 as a binary STL, and its 320-triangle sphere is 9,807 bytes against 16,084. On a scan of a few million triangles the difference is not small.

Nothing is lost by it — the shape is identical, and the corners weld back together coming the other way. It is just larger, and now you know why.

What does not come across

Everything except the triangles. An STL has no colour, no materials, no texture coordinates, no object or group names and no scene — it is geometry and nothing else, which is exactly why every slicer reads it. Faces with four or more corners are split into triangles. Face normals are recalculated from the three corners rather than copied, because a stored normal that disagrees with the geometry is worse than none and a great many files have them wrong.

Whether it is watertight is reported above, because that is what a slicer is about to ask: a closed solid has every edge shared by exactly two triangles, and one that does not leaves the slicer guessing what is inside. This is measured after welding — straight off an STL no two triangles share a corner by index, so every model on earth would look full of holes. Nothing here repairs a mesh; it tells you, and repairing is a different job.

Nothing is rotated. glTF is Y-up by its specification, COLLADA records an up axis of its own, and STL, OBJ and PLY say nothing at all — in practice arriving Z-up from CAD tools. What a file declares is reported rather than acted on, because a silent rotation is far worse than a model you can turn yourself. If your object arrives lying on its side, that is why, and it is one rotation in whatever opens it next.

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