STL to 3MF

An STL is a bag of loose triangles with no units and no name. preslice opens one as it is, and Convert to 3MF saves a .3mf that has both.

How do I turn an STL into a .3mf?

Drop the .stl on the viewer, or click to browse. Binary and text STL both work. It opens in 3D View. Click Convert to 3MF in the file list, and bracket.3mf appears beside bracket.stl and opens. If a bracket.3mf is already in the list, the new one is bracket (2).3mf. It holds one object named after the file, in millimetres.

Which mesh formats does preslice open?

STL and OBJ. See OBJ to 3MF. For STEP or anything else, export an STL from the program that made it first.

Why is my model 25.4 times too small?

An STL stores bare numbers with no unit, and preslice reads them as millimetres, like every slicer. A part drawn in inches comes out 25.4 times too small. Check the Size row under the mesh in 3D View before you convert: if a part you drew 100 mm wide reads 3.94, scale it by 2540 % in your slicer. See a model opens at the wrong size.

Do the triangles change?

The vertices do. An STL repeats each corner once per triangle it touches, and the .3mf stores each corner once, which is what slicers expect and what makes the file smaller. A triangle whose corners collapse into one point or a line has no area, and it is left out. The shape you see does not change.

Where do the print settings come from?

Nowhere, at first: an STL carries none, so the new file is geometry alone. Open Convert, pick a slicer, and the file gets that slicer's default printer, filament and process settings.

Can I aim it at my own printer?

Yes. In Convert, tick printer, nozzle, bed or filament and choose from the list. Anything you saved under My print settings is marked in it.

What has the .3mf gained?

Units, an object name, a plate and somewhere to keep settings. The new file declares unit="millimeter", one of the six units the 3MF Core Specification allows on a model, so nothing downstream has to guess the scale. That is the whole difference between the two formats, and it is why a slicer would rather have the .3mf. See 3MF against STL.