DXF, DWG, STEP or PDF? Which file format for what

As soon as manufactured parts are involved, four abbreviations start flying about: DXF, DWG, STEP and PDF. In everyday use they are often lumped together, yet they were made for quite different purposes. Knowing the differences saves you queries and gets the right thing supplied first time.

The basic split: 2D and 3D

Before individual formats, one distinction is worth making because it orders everything else. Some formats describe a surface, others describe a body.

A surface format knows only length and width. It describes a silhouette, the way the outline of a cut-out biscuit does. For anything cut from a sheet — metal, acrylic, plywood — that is exactly right, because the third dimension is the same everywhere anyway: the material thickness.

A body format, by contrast, describes a three-dimensional object with all its faces, fillets and undercuts. You need that for milling, turning and 3D printing — and whenever a part is not made from a flat sheet.

DXF: the standard for cutting

DXF is a 2D exchange format and the normal case in cutting. Autodesk published it so that CAD programs from different vendors could pass drawings on, and it is openly documented. That is why practically every machine controller reads it.

Strengths: extremely widespread, lean, immediately usable for cutting processes. Weaknesses: it knows no material thickness, no tolerances and no material specification — and it stores coordinates without a dependable unit, which regularly produces parts at the wrong size.

DWG: AutoCAD's own format

DWG is AutoCAD's native format. It holds everything DXF holds and program-specific information on top. In exchange it is binary, not openly documented, and read directly by fewer controllers.

In practice that means DWG is fine when you work with a design office that also uses AutoCAD. For the route to the cutting machine it is almost always turned into a DXF. If you have a DWG, do send it along — just do not assume the shop can put it straight into the machine.

STEP: the format for bodies

STEP is a vendor-neutral 3D format laid down in the ISO 10303 standard. It describes complete bodies and is therefore the counterpart to DXF: where DXF supplies a silhouette, STEP supplies a component.

For a flat sheet metal part STEP is needlessly elaborate — and depending on the construction, the flat pattern has to be generated from it again. For milled or turned parts, on the other hand, there is no way around it. One common constellation: the customer has a STEP of an enclosure and needs the flat blanks for the sheet metal from it.

PDF: fine for viewing, awkward for making

PDF is a presentation format. It was made so a document looks the same everywhere, not so a machine can compute tool paths from it. It is nevertheless what private customers supply most often — and that is fine, as long as it is clear what you are dealing with.

The decisive difference lies between a vector PDF and a raster PDF. A vector PDF comes from a drawing program and contains real geometry; a clean contour can often be derived from it directly. A raster PDF is an embedded image, usually a scan, and has to be traced in full. You cannot tell either apart by looking — you have to zoom in.

Other formats you will meet

Occasionally SVG, AI or EPS turn up. These are vector formats from the graphics world. Geometrically they are fine, but they often lack dependable units and work with contours that have a stroke width — which can produce duplicate lines in manufacturing. For engravings and lettering they are usable; for dimensionally accurate parts they should be converted into a DXF.

STL, meanwhile, is a 3D format consisting purely of triangles. For 3D printing it is the standard; for machining it is useless, because exact radii and planes can no longer be reconstructed from the triangles.

What you should supply

The pragmatic answer: everything you have. Sending several files costs nothing, and which one is most useful to the shop is often decided only on looking. If you have a DXF, send the DXF. If you have a STEP, send the STEP. If all you have is a sketch, send the sketch — and state the dimensions with it.

Only one thing should be avoided: converting a file without checking the result. A converter that turns a scan into a DXF often produces thousands of tiny line segments instead of clean arcs. Such files look right at first glance but make the machine judder and produce visibly faceted curves. Sending the original template along is worth more in that case than the conversion.

A note on file naming

An unremarkable point that saves a lot of confusion in practice: name files so they can be told apart without opening them. Not »drawing.dxf« and »drawing_final.dxf«, but »bracket-3mm-alu.dxf«. Where several revisions are in circulation, a date or revision number belongs in the name.

This sounds like a trifle but is the cause of one of the more expensive mistakes there is: the wrong revision gets cut because two files are named almost identically. On a single part that is annoying; on a batch it gets expensive.