STEP vs STL? How to choose the right file format

STEP vs STL – solid or mesh

Every 3D file basically belongs to one of two families. A solid model, also called B-rep, describes the geometry mathematically. A hole is an exact cylinder, a face is an exact plane and a fillet has a defined radius. STEP, IGES and Parasolid work this way.

A mesh model describes the surface with thousands of small triangles. A hole becomes a polygon that only looks round. STL, OBJ and 3MF are mesh formats. A mesh is perfectly adequate for 3D printing, but you cannot measure an exact diameter or easily change a dimension.

This is the most important difference. From a solid you can always create a mesh, but from a mesh you do not get a true solid back without remodelling the part. So the original file should always be kept as a solid.

STEP – the standard format for CAD and CNC

STEP (ISO 10303) is the format that almost all CAD programs and workshops can read. The file is called .step or .stp and contains exact solids, surfaces and assemblies. Choose STEP when the part is to be milled, turned, cast or injection moulded, or when someone else will continue the design.

STEP comes in different application protocols. AP203 is the oldest and contains geometry only. AP214 added colours and layers and is still common in the automotive industry. AP242 is the newest and can also carry product and manufacturing information (PMI), such as tolerances linked to faces. If you do not know which to choose, AP214 works almost everywhere.

The feature history does not travel in a STEP file. The recipient gets a “dumb” solid without sketches or constraints. It can be edited, but not as easily as in the native file.

STL, 3MF and OBJ – for 3D printing

STL is the oldest and most widespread format for 3D printing. The file contains only triangles – no units, no colours and no material. That is why a part drawn in inches sometimes turns up 25.4 times too small in the slicer. Always check the size when you open an STL.

3MF is the modern alternative. The file contains units, multiple parts, colours and often print settings as well. It is also smaller than an equivalent STL. If your slicer can read 3MF, it is usually the better choice.

OBJ is used mostly in graphics and games. It can carry colour and texture, but has no fixed units. Need an STL turned into an editable model? Read about converting STL to STEP.

When a solid is exported to STL, the software decides how many triangles a curved surface gets. With too coarse a resolution, a 5 mm hole becomes visibly faceted and measures smaller than it should. With too fine a resolution, the file becomes unnecessarily large without the print getting any better.

A guide value for 3D printing is a chordal deviation of 0.01–0.05 mm and an angular tolerance of a few degrees. That gives smooth surfaces and files of a few megabytes. Never export a mesh from a mesh – always go back to the original solid model. An example: a 150 mm cover with many fillets typically becomes an STL of a few megabytes with sensible settings.

IGES, Parasolid and native files

IGES (.igs, .iges) is an older exchange format from the 1980s. It handles surfaces well but solids less well, and an IGES file often arrives as loose surfaces that have to be stitched together. Use STEP if the recipient can read it, and IGES only when it is explicitly requested.

Parasolid (.x_t, .x_b) is the geometry kernel that SolidWorks, Solid Edge and Autodesk Fusion, among others, are built on. A Parasolid file therefore often comes through as a very clean solid in those programs.

Native files, such as .f3d, .sldprt or .ipt, contain the full history but can only be opened in the same program, and sometimes only in the same version. Between different CAD systems, STEP or Parasolid is the safe choice.

DXF and DWG – 2D for laser, waterjet and sheet metal

DXF is the most common format for flat profiles. Laser cutting, waterjet cutting, plasma cutting and routing of sheet materials almost always start from a DXF. The file should be at 1:1 scale, in millimetres and contain closed profiles without duplicate lines. Text and dimension lines should be on a separate layer or removed.

For sheet metal parts to be bent, the workshop needs the flat pattern. It is calculated from sheet thickness, bend radius and K-factor. Read more about sheet metal design.

DWG is AutoCAD's native format. It is often used for drawings and layouts, but for cutting DXF is safer because every program can read it.

The PDF drawing – still essential

A 3D file shows what the part looks like, but not how accurately it must be made. Tolerances, threads, surface roughness, material and surface treatment are on the drawing. Most workshops therefore want both a STEP file and a PDF drawing to give an accurate price.

If the drawing and the 3D model give different dimensions, it must be clear which one applies. Read more in the guide to engineering drawings for machining.

How to export a STEP file correctly

First check the units in the document. Most programs export in the unit the model was drawn in, and millimetres are standard in Europe. Then choose the protocol – AP214 or AP242 – and whether the whole assembly or just one part should be included.

Give each part a clear name before exporting. The names are carried into the STEP file and become bodies or components for the recipient. An assembly where every part is called “Body1” is hard to work with. Hide or delete construction geometry, sketches and reference planes that should not be included.

It is a good idea to reopen the exported file, in the same or another program, and check a couple of dimensions. It takes a minute and reveals most problems: wrong units, missing parts or surfaces that did not become a solid. For 3D printing, do the same check with the STL or 3MF file in the slicer.

Common file format mistakes

The most common mistake is sending an STL to a CNC shop. It cannot be programmed properly and often results in a higher price or a refusal. The next most common is wrong units: a DXF in inches or an STL without scale.

Other mistakes are DXF files with open profiles, assemblies exported as a single body and STEP files sent without a drawing when tolerances matter. Unsure what your supplier needs? Send us the file and we will check it.

File formats compared

STL and OBJ have no units – the recipient must know whether the file is in millimetres or inches.

Frequently asked questions