STEP, DXF, STL… which files should you send your manufacturer?

Whatever the technique, a manufacturer needs two things: the shape of the part and your requirements. The 3D file conveys the shape accurately. The dimensioned drawing conveys what the 3D model does not say: tolerances, surface finishes, material, treatments, critical dimensions, threads. For a machined part, the safest combination is therefore a STEP file accompanied by a drawing in PDF format.

The native format of your software (SLDPRT, IPT, CATPart…) is not ideal for sending: it only opens with the same software, often in the same or a newer version. A neutral exchange format is read by the vast majority of CAM software. The right format then depends on the process: machining, 2D cutting, 3D printing or printed circuit boards.

STEP (.step, .stp): the format to favour

STEP is an international standard (ISO 10303), supported by all the major CAD software. It exists in several application protocols:

• AP203: geometry and assembly structure. Enough to convey a shape.
• AP214: adds colours and layers, among other things. Very common in default exports.
• AP242: the most recent, which incorporates and extends the previous two, and can carry 3D annotations (dimensioning and tolerancing) when your software exports them.

For machining, AP214 or AP242 is perfectly suitable. Export solids rather than surfaces, one part per file, then reopen the resulting file to check that it is complete.

IGES (.igs, .iges): a fallback format

An old format oriented towards curves and surfaces, whose specification has been frozen for a long time. Exporting unstitched surfaces can leave gaps or unconnected edges that complicate CAM. Keep it for cases where STEP is not possible.

Parasolid (.x_t, .x_b)

Siemens’ modelling kernel, used by several programs including SolidWorks, NX and Solid Edge. The .x_t file is in text format, the .x_b in binary format. Faithful to the geometry, it is a good alternative to STEP if your manufacturer accepts it.

Native formats (SLDPRT, IPT, CATPart, PRT, F3D…)

They can be sent in addition to the STEP file if the manufacturer uses the same software and has confirmed this to you. Otherwise, do not send the native format alone.

DXF and DWG: 2D cutting

For laser, waterjet or plasma cutting or punching, the workshop uses the flat outline of the part. DXF is an Autodesk exchange format read by most cutting software; DWG is AutoCAD’s native format, which varies between versions. A few rules ensure a usable file:

• drawing at 1:1 scale, in millimetres;
• closed outlines, with no duplicates or overlapping segments;
• text converted to outlines if the marking is cut;
• dimensions, annotations and bend lines on layers separate from the cutting outlines.

For a bent part, supply the flat pattern in DXF and the bent part in STEP. See sheet metalwork and cutting.

The dimensioned PDF: essential for tolerances

Export the PDF from your CAD software (a vector document that can be read at scale) rather than attaching a photo or a scan. The title block, compliant with ISO 7200, should state the reference, revision index, material, scale, general tolerances (for example ISO 2768-mK) and treatments. Add the critical dimensions, threads, surface finishes and projection method. The 3D file serves as the reference for the shape, but it is the drawing’s notes that carry your requirements. The guide machining tolerances helps you dimension it correctly.

Printers work from a mesh of triangles. STL is its historical form: universal, but it contains no units, colours or materials, and the smoothness of the surface depends on the resolution chosen at export. A too-coarse export leaves visible facets on curved surfaces; also check that the mesh is closed and has no inverted faces.

3MF, defined by the 3MF Consortium, is a more modern format: it embeds units, colours, materials and several objects in a single archive, and removes the ambiguities of STL. Other formats exist (OBJ for colour, AMF standardised as ISO/ASTM 52915), less common among suppliers. Some also accept STEP and do the meshing themselves, particularly in metal printing; ask them.

Add a dimensioned PDF drawing if some dimensions are critical, if surfaces are to be machined after printing or if you expect precise tolerances. To choose between 3D printing and machining, read 3D printing or CNC machining; for suppliers, see 3D printing.

For a bare printed circuit board, the standard is Gerber (RS-274X, in its extended X2 variant which adds attributes): one file per layer (copper, solder mask, silkscreen, outline), accompanied by the drill file in Excellon format. The ODB++ and IPC-2581 formats bring all this information together in a single package, which limits file-assembly errors; check that they are accepted.

State in a separate document the characteristics that the files do not always carry: number of layers, board thickness, surface finish (HASL, ENIG…), solder mask colour, electrical test. For an assembled board, add the bill of materials (BOM) with manufacturer part numbers, the component placement file, the schematic and the assembly drawing. The suppliers concerned are presented on the electronics page.

Many projects start from an existing part, a sketch or an old paper drawing. Several solutions exist, from the simplest to the most complete:

• A scanned drawing or a dimensioned sketch. It is sometimes enough for a simple part, provided the dimensions, material and quantity are legible. Take clear photos and indicate the critical dimensions.
• The part to be reproduced. Measuring it or 3D scanning it produces a reference mesh. This mesh cannot be machined directly: it has to be rebuilt as a solid model.
• Modelling by a design office. It turns your sketch or sample into a 3D model and a dimensioned drawing, which you will reuse for all your quote requests.

In every case, state what is functional (bearing surfaces, bores, centre distances) and what can be adapted. The design office page presents the suppliers who can finalise your drawings.

• Units. Work in millimetres and state it on the drawing and in the covering message.
• Versions. A single revision index, identical on the drawing and the 3D file. Remove old versions from what you send.
• Naming. Adopt a simple rule, such as REF_index_description (for example 1234_B_motor-bracket.step), with no spaces or accents.
• One file per part. Do not send the complete assembly when only one part is to be made; for an assembly, attach the bill of materials.
• Checking. Reopen the export in a neutral viewer to check that it is complete, at the right scale and with no missing faces.
• Archive. Group the files for a request in a compressed folder (zip) of reasonable size, with a summary text file if needed.
• Prevailing document. State it explicitly, generally the drawing, in the event of a discrepancy with the 3D model.

On Meaflow, posting a request is free and you can attach your files to it; your drawings are only visible to logged-in members.