How to Edit an STL File: The Right Free Tool for Each Change

To edit an STL file, match the tool to the change. Your slicer (Bambu Studio, OrcaSlicer or PrusaSlicer) already cuts, scales, adds text and subtracts simple shapes. Tinkercad combines models and drills holes in a browser, Blender sculpts and cleans up, and Fusion or FreeCAD turn the mesh into a solid when a dimension has to be exact.

All of these are free for hobby use. The hard part is knowing which one fits the job, and what an STL lets you change at all.

Why an STL is hard to edit

An STL file is a list of triangles. Each triangle stores three corner points and a direction, and nothing else: no units, no colors and no record of how the shape was made.

A CAD file remembers "a 40 mm cylinder with a 6 mm hole through it". The STL of that part is a few hundred flat triangles that happen to look like a cylinder with a hole. To make the hole 8 mm, no program can simply change a number. Something has to cut away triangles and build new ones.

That gives the first rule: if you have the original design file, edit that and export a fresh STL. Edit the mesh when the source is gone, belongs to someone else, or the change is small: a cut, a hole, a label, a new size.

A 3MF file works the same way. It adds units, colors and several objects per file, but the shapes inside are still triangle meshes.

Pick the tool by the change

Measure before and after every edit. The free STL viewer shows the X, Y and Z size in millimetres or inches, the triangle count and the volume, without uploading the file.

Edit an STL in your slicer

Bambu Studio, OrcaSlicer and PrusaSlicer are free, and they cover most everyday edits. OrcaSlicer grew out of Bambu Studio and PrusaSlicer, and Bambu Studio itself forked from PrusaSlicer, so the three share many tools under slightly different names.

Cut a model into pieces

Use this when a model is taller than your build volume or prints better in parts.

1. Select the object and press C, or pick the Cut tool in the toolbar (Bambu Studio and PrusaSlicer).
2. Choose Planar for a flat cut or Dovetail for an interlocking joint.
3. Drag the plane, rotate it, or type an exact height.
4. If the parts will be glued, add connectors: Plug, Dowel or Snap. Dowel cuts matching holes in both halves and creates a separate pin to print.
5. Choose how each half lands: keep its orientation, place it on the cut face, or flip it.
6. Perform the cut. If non-manifold edges remain afterwards, Bambu Studio shows a pop-up that offers to repair them.

Each piece becomes its own object. Export them with File, Export, Export all objects as STLs in Bambu Studio, or File, Export, Export Plate as STL/OBJ in PrusaSlicer. OrcaSlicer has a Cutting Tool too and exports STL from File, Export.

Add raised, engraved or flush text

All three slicers place text directly on a surface, flat or curved.

The flush option is the one for multicolor prints: the letters stay level with the surface and only change the filament in that area. Bambu Studio's surround modes wrap text around curved faces, and PrusaSlicer's Use surface option does the same up to about 90 degrees of curvature.

For a logo, PrusaSlicer's SVG tool (right-click, Add part, SVG) offers the same choice as Join, Cut or Modifier, and OrcaSlicer imports SVG files and extrudes them into 3D.

Cut holes and slots

Right-click the object and choose Add negative part (Bambu Studio) or Negative volume (PrusaSlicer). Pick a cube, cylinder or sphere, or load your own STL, then move and scale it into place.

Know the catch before you rely on it. Both Bambu Lab and Prusa state that the subtraction happens only during slicing and the model's geometry does not change, so you cannot export an STL with the hole in it. That is fine if you print from the project file. If you need the edited STL, use a boolean.

Make the change permanent with Mesh Boolean

Bambu Studio and OrcaSlicer have a Mesh Boolean tool with three operations:

• Union joins overlapping objects into one.
• Subtraction (Difference in OrcaSlicer) removes object B from object A.
• Intersection keeps only the overlap.

In Bambu Studio, put the model in box A and the cutter in box B. Tick Keep original models if you want the inputs for another operation. Bambu Lab advises working on two objects at a time for predictable results. The output is real geometry, so it exports to STL.

Split, simplify and scale

• Split to Objects or Split to Parts: an STL can hold several separate shells, and Bambu Studio loads them as one object. Splitting gives each shell its own object (to arrange or print separately) or its own part (to give it another filament in the same print).
• Simplify model: OrcaSlicer's right-click Simplify reduces the triangle count of heavy scans and AI models.
• Scale: OrcaSlicer's Scale tool takes a percentage or an exact size in millimetres, with Uniform Scale keeping the proportions.

OrcaSlicer's nightly builds go further. An experimental Design tab, switched off by default, can import an STL or OBJ as a solid body and report whether it is closed. It ships only in builds newer than the 2.4.2 release, so treat it as a preview.

Tinkercad: the free online STL editor

Tinkercad is Autodesk's free 3D design web app, and it is the main way to edit an STL in a browser. It edits by combining shapes, not by moving triangles.

1. In a 3D design, click Import and choose an STL or OBJ file under 25 MB. Set the scale before it loads.
2. The model becomes one solid shape. Move it, resize it, or switch it to a Hole.
3. Add shapes from the Shapes panel: boxes, cylinders, hole shapes or the Text shape.
4. Select everything and press Union Group (Ctrl or Cmd + G). Solid shapes add material; hole shapes take it away.
5. Click Export, choose everything or only the selected shapes, and pick STL. OBJ and GLTF are also offered.

The limits are real. You cannot edit individual triangles, an imported model cannot be ungrouped into separate parts, and every export comes out grouped as one part. Tinkercad's own printing advice is walls of at least 1.5 mm.

Blender: sculpting, cleanup and everything else

Blender is free and open source and can make any mesh edit, at the cost of a learning curve.

• Import: File, Import, STL. Tick Validate Mesh to check for and fix corrupt data on the way in.
• Modes: Object Mode moves, rotates and scales whole objects. Press Tab for Edit Mode to change vertices, edges and faces. Sculpt Mode is a separate toolset for shaping meshes, the natural fit for organic models.
• Booleans: a Boolean modifier joins, subtracts or intersects another object. The Exact solver handles overlapping geometry best. The faster Manifold solver only works on closed meshes.
• Separate: in Edit Mode, press P and choose By Loose Parts to turn each shell into its own object.
• Export: File, Export, STL. Keep Apply Modifiers on so boolean results are exported, and use Selection Only to export one part.

A clean hole through a part takes four steps:

1. Import the STL and add a cylinder where the hole goes, sized to the hole.
2. Select the model, add a Boolean modifier, set Operation to Difference, Object to the cylinder and Solver to Exact.
3. Hide the cylinder and inspect the result.
4. Export with Apply Modifiers on.

For print checks, add the free 3D Print Toolbox extension (Blender 4.2 and later). It measures volume and area, flags bad geometry and fixes it with Make Manifold, hollows models, lays a chosen face flat on the bed and scales a model to set bounds or a set volume.

Fusion and FreeCAD: when a dimension must be exact

Fusion

Fusion can edit a mesh directly or convert it into a solid you can dimension. Autodesk makes it free for hobbyists and students; check the current terms for your use before you rely on it.

1. In the Design workspace, open the Mesh tab and choose Create, Insert Mesh. It accepts STL, OBJ and 3MF. Set the Unit Type and use Move To Ground.
2. If a warning icon appears next to the body in the Browser, run Prepare, Repair.
3. Choose Modify, Convert Mesh. Faceted turns each mesh face into a face on the solid. Prismatic merges groups of faces into single faces and suits parts that were first modeled in CAD. A watertight mesh becomes a solid body; an open one becomes a surface body.
4. Edit it with the normal solid tools, then right-click the body and choose Save As Mesh (3MF, STL or OBJ, with a unit type).

The third method, Organic, needs the Fusion Design Extension, which is a paid add-on. Without converting at all, the Mesh tab can cut a mesh with a plane (and fill the cut so it stays watertight), combine, smooth, shell, remesh, reduce, separate and scale mesh bodies.

FreeCAD

FreeCAD is a free, open-source parametric modeler. It treats a mesh as raw material to convert:

1. Import the STL. Optionally run Meshes, Analyze, Evaluate and repair mesh first.
2. Choose Part, Create shape from mesh. Tick Sew shape only if the mesh has small gaps.
3. Run Part, Create a copy, Refine shape to remove needless edges on flat and cylindrical faces.
4. Run Part, Convert to solid.

The solid then works with FreeCAD's booleans and modeling tools. Because the shape starts from the mesh's flat triangles, this route suits simple mechanical parts far better than dense, organic sculpts.

What about Meshmixer?

Meshmixer was the classic free STL editor. Autodesk's own listing now says it is no longer in development and will not be supported, that Fusion contains many of its features, and that there are no current plans to retire it. It is still a free download and still does plane cuts, hollowing, hole filling and auto-repair, but new projects are better started in tools that get updates.

Can AI edit an STL file?

Not in the way most people hope. A chat assistant cannot open your STL and move its triangles with any precision. What it can do is write code for a program that edits meshes:

• OpenSCAD can import() an STL and use difference() to subtract a cylinder from it, which makes a hole at exact coordinates. Its manual warns that the STL must be manifold, with no holes or self-intersections, for this to render.
• Blender has a Python API, so an assistant can write a script that adds a boolean or rescales a model.

You still run the code, check the result and correct what it gets wrong. AI 3D generators are a different thing: they create a new model from a prompt or a photo rather than editing yours. See how to make a 3D model from an image with AI for that route.

Free browser tools for the small jobs

Some edits are really conversions or checks. These run in your browser and never upload the file:

• Measure: the STL viewer shows size in mm or inches, triangle count, volume and surface area.
• Convert: the STL converter moves between STL, OBJ, GLB, PLY and 3MF, and the 3MF to STL converter unpacks slicer projects.
• Repair: the STL repair tool fills holes, fixes flipped normals and removes stray shells after an edit.
• Add a text or logo part in another color: make the lettering with Text to STL or SVG to STL, skip the base plate, then right-click your model in Bambu Studio or PrusaSlicer, choose Add part, Load and place it (PrusaSlicer shows Load in Advanced and Expert mode). As a part of the same object, it can print in a second filament.

Next step

After any edit, open the new STL in a viewer and check its size against what you meant to make. If your slicer then reports open or non-manifold edges, the edit left a gap or a crack. How to fix STL files goes through each error and the free fix for it.