CAD File Formats (STEP, STL, IGES, DWG)

The format determines what data survives to manufacturing. STEP and IGES carry full 3D boundary geometry; STL carries only a triangle mesh with no tolerances or assembly data; DXF and DWG carry 2D profiles; PDF carries 2D outlines. Sending the right format for the process is what lets a shop read the part correctly on the first pass.

What each format carries

Each CAD format stores a different kind of geometry, and the difference matters at the machine. STEP (ISO 10303) stores a boundary representation, or B-rep, which holds exact curves, surfaces, tolerances, and assembly structure, so it is the primary 3D format for CNC machining and sheet metal fabrication. IGES is an older 3D exchange format that carries similar geometry but is less standardized and less widely supported, so it survives only as a legacy fallback for older CAM systems. STL stores a faceted triangle mesh, which is fine for 3D printing but loses the exact curves and all tolerance data, because every surface becomes an approximation built from flat triangles.

2D and artwork formats

DXF is a 2D vector interchange format that stores polylines, lines, arcs, and circles, which is exactly what a laser, waterjet, or plasma cutter follows. DWG is the AutoCAD native 2D and 3D format; some suppliers accept it, but DXF is the safer interchange choice. PDF stores a 2D outline that a few suppliers accept for simple profiles, and EPS or AI holds the vector artwork used for silkscreen or laser marking. Raster images such as JPG and PNG are not manufacturing files at all, because a cutter cannot follow a pixel image.

Choosing by process

Match the format to what the process needs to read. CNC machining and sheet metal fabrication need exact 3D geometry with tolerance intent, which means STEP. Two-dimensional cutting needs a flat vector profile, which means DXF, with the cut outline on a dedicated layer and continuous lines for the cut path. Additive manufacturing builds from a mesh, which means STL for simple parts or STEP when the service can use it for better dimensional accuracy. Sheet metal bending wants the flat, unfolded pattern as a DXF with bend lines, never the bent 3D model, because the supplier forms the bends from the flat blank. Silkscreen and laser marking want a clean vector such as DXF, EPS, or AI.

The cost of a wrong format

The cost of a wrong format is mostly delay: an STL sent for a CNC part forces the shop to rebuild the geometry, a bent 3D model for a sheet metal part forces them to unfold it, and a raster image for a cut forces a redraw. Picking the format the process expects removes that delay.

Format reference in depth

A closer look at each format shows why the choice matters and where each one fits or fails.

STEP: the workhorse 3D format

STEP (ISO 10303) is the workhorse 3D format for machining and fabrication. It stores a boundary representation, which means exact curves, surfaces, and solids rather than an approximation, and it can carry assembly structure, tolerances through PMI, and other product manufacturing information. Because it is an ISO standard, nearly every modern CAD and CAM system reads it, which makes it the safest 3D format to send when you do not know the supplier’s toolchain. A STEP exported from native CAD preserves the design intent, so the shop can program toolpaths directly from the exact geometry instead of rebuilding it. Its main limitation is file size on very large assemblies, but for the vast majority of parts it is the right 3D choice.

STL stores only a triangle mesh: a list of flat triangles that approximate the outside of the part. That is exactly what a 3D printer slices, so STL is the universal additive format. The catch is that every curved surface becomes a faceted approximation, and the format carries no tolerance, no assembly, and no material data. The denser the triangles, the closer the mesh sits to the true surface, but the larger the file. For 3D printing this is acceptable, and a well-resolved STL prints cleanly. For CNC it is the wrong choice, because the shop would have to reverse-engineer the faceted mesh back into machinable surfaces, and any tolerance intent is already gone. If you must move from a mesh to machining, expect a rebuild step.

DXF is the standard 2D vector interchange format for cutting. It stores the polylines, lines, arcs, and circles that define a flat profile, organized into layers, and it is what laser, waterjet, and plasma machines follow. Because it is an open interchange format, it is more portable than DWG across different CAD systems, which is why most cutting suppliers prefer it. Good DXF practice keeps the cut outline on a dedicated layer with continuous lines, pushes text and dimensions onto separate layers the supplier may ignore, and exports at 1:1 scale in the stated units. For sheet metal, the DXF carries the flat unfolded pattern with bend lines marked, so the supplier knows where to form.

DWG is the AutoCAD native format. It can carry both 2D and 3D data and is widely used in drafting, but it is a proprietary format, so compatibility outside AutoCAD-based systems is less reliable than DXF. Many suppliers accept DWG, but when the toolchain is unknown, exporting a DXF as well removes the risk. The choice between DXF and DWG rarely changes the part; it changes how confidently the file opens on the receiving end.

IGES is an older 3D exchange format that predates STEP. It can carry surface and curve geometry, so it still works as a fallback for legacy CAM systems that do not read STEP cleanly. In practice, STEP has replaced IGES for almost everything, because STEP is more standardized, more complete, and more widely supported. Keep IGES as a backup option for an older machine, not as a first choice.

PDF, EPS, and AI serve the 2D and artwork end. A few suppliers accept a PDF for a simple outline, but PDF is best treated as a drawing reference rather than a cut file. EPS and AI carry the vector artwork used for silkscreen or laser marking, where a clean vector is essential and a raster image will not do.

Working across formats

Designs often move between formats on the way to manufacturing, and the conversions can quietly lose data. Exporting a native solid to STL tessellates the surfaces into triangles, which is fine for printing but discards the exact curves; moving that STL back toward machining then needs a reverse-engineering step. Exporting a 2D drawing to DXF can drop dimensions and text that live on layers the supplier ignores, so the cut path must be self-contained on its own layer. Importing a STEP into a different CAD system can shift or fail on complex surfaces if the translators disagree, so a geometry check on receipt is worth the seconds it costs.

The safe path and conversion checks

The safe path is to export from native CAD directly into the format the process needs, avoid unnecessary intermediate conversions, and verify the result before sending. When a conversion is unavoidable, check the output against the original for missing features, changed dimensions, or units drift, because those are the errors that turn into bad parts.

Checklist

• Right format for the process: STEP for CNC and sheet metal, DXF for 2D cutting, STL for printing.
• Exported from native CAD, not from a simplified or converted mesh.
• Tolerances on a 2D drawing or in the filename, not assumed from the STEP.
• Layers organized, with the cut path on its own dedicated layer.
• Units stated in the file and the filename, at 1:1 scale.

Common format mistakes

• Sending STL for a CNC part. STL loses the exact curves and all tolerance data, so the shop must rebuild the geometry. Send STEP for CNC.
• Sending a bent 3D model for a sheet metal part. The shop needs the flat unfolded pattern with bend lines. Send a flat-pattern DXF.
• Sending DWG when DXF is safer. DWG is AutoCAD native and not every system reads it cleanly; DXF is the widely supported interchange format most cutting suppliers prefer.
• Trusting embedded GD&T as the only tolerance carrier. STEP can carry GD&T through ISO 10303-518, but not every CAM system reads it, so put critical tolerances on a 2D drawing as well.
• Sending a raster image for a cut path. JPG and PNG are reference images only; a cutter needs a vector such as DXF.
• Leaving units off. A format does not enforce units, so a STEP in inches opened as millimetres scales the part 25.4 times wrong.

Tolerances

• STEP can carry GD&T through ISO 10303-518, but not every CAM system reads it. Put critical tolerances on a 2D drawing or encode them in the filename (for example, part_pm0.1mm.step), and declare the general-tolerance class in the title block.
• The format sets the ceiling on what tolerance data can travel. A STEP can carry PMI; an STL cannot carry any. Plan the tolerance communication around the format you actually send, and let the drawing carry what the model cannot.

Design rules

• Export from native CAD and avoid aggressive mesh simplification, which converts exact surfaces to triangles and loses the geometry a machined part needs.
• Keep the part outline on a dedicated layer in DXF, and put text, dimensions, and hatch on separate layers that a supplier may ignore, so the cut path is unambiguous.
• Use continuous lines for cut paths and never hidden or dashed lines, which a cutting machine may skip.
• Keep one part per file or clearly label nesting, and export at 1:1 in the stated units with no in-file scaling.