Edit and repair STL online, convert STL to STEP

An STL from a model sharing site, a 3D scan or an export from another program is a mesh of triangles. It prints fine but is hard to change: no holes, edges or dimensions, just thousands of flat triangles. In the MESH tab you load such a file, check, repair and simplify it, and turn a mechanical part back into an exact solid you can fillet, drill and save as STEP.

Loading and checking

• Insert Mesh accepts STL, OBJ, PLY and 3MF files. Insert CAD or mesh file in the INSERT tab also reads glTF (GLB).
• After loading, a message shows the triangle count and the size. Mesh files store no units, so the numbers are read as millimetres. If the model comes out roughly 25 times too small, the file was saved in inches.
• Mesh Diagnostics tells you whether the mesh is watertight (has no holes) and counts holes, separate pieces, edges shared by more than two triangles and zero-area triangles. It also warns when the mesh is inside out.

Repair and simplify

• Repair Mesh merges vertices that should be shared, removes degenerate and duplicate triangles and flips the ones facing the wrong way. For scanner files, a higher weld tolerance helps.
• Fill Holes closes every opening, or only those up to a given perimeter, so wanted holes stay open.
• Reduce Mesh keeps the share of triangles you choose and reports how far the shape moved.
• Separate Shells makes each loose piece its own body, and Plane Cut slices off what you do not need.

Repair, Fill Holes, Reduce and Plane Cut change the mesh in place, off the timeline and without undo, so keep your original file. You can save the result as STL, OBJ, PLY or 3MF.

From mesh back to a solid

Convert Mesh offers three methods:

1. Prismatic recognises planes, cylinders, cones and spheres. A block with holes comes back with real flat faces and round holes, ready for fillets, joining with other bodies and STEP export.
2. Merged turns triangles lying in one plane into single faces and leaves the rest.
3. Faceted turns every triangle into its own face: faithful, but heavy.

The message afterwards says how many faces were recognised exactly. Conversion is a timeline step and the source mesh stays in the project. You cannot change the method later with Edit: delete the step (the mesh comes back) and convert again.

The other route is Section to Sketch: a plane cuts the mesh and the outline lands in a new sketch as lines and arcs, ready to redraw with dimensions and extrude.

Things to keep in mind

• A rounded edge along a curve, such as the rim of a cylinder, stays made of small flat faces after conversion and is not an editable fillet.
• A mesh with holes converts into an open shell, not a closed solid, so repair it first.
• Figurines and face scans have no planes or cylinders to recognise, so keep them as meshes.
• A mesh body takes no part in solid operations and does not go into STEP or other exact formats until converted. Move and Scale work on it directly: they change the mesh itself, without a timeline step, so a file saved in inches can be scaled right after import (factor 25.4).
• Convert, Repair, Fill Holes and Reduce run on the server and need a connection. The mesh travels there in one piece of up to 128 MB, roughly 7 million triangles, and an operation longer than 2 minutes is stopped. Above 150,000 triangles the convert dialog warns of a wait of tens of seconds, so reduce big meshes first. Diagnostics, plane cuts, separating and sections run in the browser.