Tiny Online Tools
What a slicer needs from an STL
An STL file is a bag of triangles with no structure: no shared vertices, no unit, no idea which side is out. Slicers cope with that by welding it themselves and hoping the result is a closed solid. When it is not — when there are holes, edges shared by three faces, or triangles wound the wrong way — you get missing walls, mystery infill, or a flat refusal to slice. This tool does the repair up front, so what reaches the slicer is unambiguous.
The repair pass, in order
Weld merges vertices that sit at the same position within a tolerance. Nothing else can work before this: in raw STL soup, every triangle owns its own three corners, so every edge looks like the boundary of a hole.
Degenerate and duplicate triangles are removed next. Zero-area faces carry no surface, and a triangle stored twice doubles a surface the slicer then has to disambiguate.
Winding is unified by flooding across the surface: two correctly-wound neighbours traverse their shared edge in opposite directions, so a face that agrees with its neighbour's direction is flipped.
Holes are filled last, by fanning triangles from the centroid of each boundary loop. Doing this before welding would try to fill every edge in the file; doing it before the winding fix would leave the new triangles to be reasoned about.
Optimising for the print
Simplify runs quadric edge-collapse decimation. A 3 M-triangle sculpt sliced at 0.2 mm layers gains nothing from detail below the nozzle width, and a smaller file loads and slices far faster. The ratio is a request, not a promise — the simplifier stops early rather than exceed its error budget, so the achieved count is what gets reported. Lock border keeps the rim of any remaining hole intact.
Rescale either fits the largest dimension to a target in millimetres or multiplies by a factor — the usual fix for a model exported in centimetres or inches that lands on the bed 10× or 25.4× too small.
Drop to the build plate centres the part and sits its lowest point at zero, which is where most slicers expect to find it.
Binary or ASCII
Binary STL is roughly five times smaller and is what you want unless something downstream specifically needs readable text. Both are offered.
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