The decision
A shop, a supplier or a print service asks for a 3D file, and the two formats on offer are STEP and STL. Both describe the same part, but not the same information. The choice decides whether the receiver can measure a hole's true diameter, change a feature, or only slice and print a shape. This page compares the two on the same criteria and says when each is the right file to send. Both are neutral formats, readable by many systems, and the comparison names no product.
STEP: an exact solid
STEP is the ISO 10303 family of standards. A STEP part file describes a boundary representation (B-rep): faces that lie on exact surfaces, such as planes, cylinders, cones and free-form NURBS surfaces, bounded by edges and joined into a closed solid. A 10 mm hole is a cylinder of radius 5 in the file, not an approximation of one. The file states its length unit. Application protocols set what else it carries. AP242 (managed model-based 3D engineering) succeeds the older AP203 and AP214 and can add 3D annotations, such as tolerances and datums, in a form software can read.
STEP usually carries the shape without the CAD system's feature history. The receiver gets a solid they can measure, machine and modify with direct editing, not the original sketches and parameters.
STL: a triangle mesh
STL describes a surface as a list of triangles, each with three vertices and a normal, in ASCII or binary form. It has no curves, no faces in the CAD sense and no unit: a value of 10 can be 10 mm or 10 in, and the receiving software assumes one. A cylinder becomes a ring of flat facets, and how closely they follow the true surface depends on the chordal and angular tolerances chosen at export. Finer facets follow curves more closely and make larger files.
STL is simple, and almost every 3D-printing slicer reads it. Newer mesh formats, such as AMF (ISO/ASTM 52915) and 3MF, keep the mesh approach and add units, color and other data that STL lacks.
Criteria compared
| Criterion | STEP (B-rep) | STL (mesh) |
|---|---|---|
| Geometry | Exact surfaces, edges and a closed solid | Flat triangles approximating the surface |
| Units | Stated in the file | None; the receiver assumes them |
| A hole's diameter | Exact, measurable as a cylinder | Faceted; depends on the export tolerance |
| Editing in CAD | Direct editing of faces; usually no feature history | Hard; most CAD treats a mesh as a reference body |
| Machining (CAM) | Toolpaths on true features: holes, planes, pockets | Possible for free-form surfacing; prismatic features come out faceted |
| 3D printing | Read by many slicers, or converted to a mesh first | Read by almost every slicer |
| Inspection planning | Nominal geometry for CMM programs and comparison | Usable for comparing scans; no exact features to probe |
| Tolerances and datums | Can carry 3D annotations (AP242) | None; the drawing must travel with the file |
| File size | Compact for prismatic parts | Grows with finer facets |
When to choose each
- Send STEP when the part will be machined, quoted from its features, inspected on a CMM, modified, or used in an assembly. Send the released drawing with it, because the drawing's tolerances govern.
- Send STL when a slicer, a print service or a viewer asks for a mesh, and the faceting is acceptable for the process. State the unit in the order, because the file cannot.
- Send both when one supplier prints a prototype and another machines production parts. Make the STL from the same STEP solid, and record the export tolerance.
- Avoid converting an STL back into a STEP solid for machining. The result is a solid made of triangles. A true cylinder cannot be recovered from facets without remodeling it from the drawing.
The review each file still needs
Neither file shows that the part is right. A STEP file can hold a well-formed solid with a wrong dimension, and an STL file can be a clean mesh of the same mistake. Check the solid against the drawing before you export it, as the guide to verifying CAD against a drawing describes. For STL, also check the unit and the export tolerance on the largest curved features. A file that opens without errors has passed a format check, not an engineering one.
What TruePart exports
TruePart exports the exact B-rep solid as STEP, and a mesh of it as STL, once the solid is measured against the specification. A STEP export is not manufacturing approval: the drawing's tolerances, material and process still need a qualified person's review.
Questions
Can a machine shop work from an STL file?
Some can, for free-form shapes, but holes and flat faces come out faceted, and nothing in the file states the unit. Most shops ask for STEP and the drawing for machined parts.
Does an STL file have units?
No. It stores bare numbers, and the receiving software assumes millimeters or inches. State the unit wherever you send one.
Can I convert STL to STEP?
A converter can wrap the triangles in a STEP file, but the faces stay flat triangles. To get true cylinders and planes, the part has to be remodeled from the drawing or the original CAD.
Which STEP version should I send?
Ask the receiver which application protocol their software reads. AP242 is the newest of the three common ones and can carry 3D annotations; AP214 and AP203 are widely read.
Sources
- ISO 10303-21, Industrial automation systems and integration, Product data representation and exchange, Part 21: Implementation methods: Clear text encoding of the exchange structure · ISOThe STEP file format.
- ISO 10303-242, Application protocol: Managed model-based 3D engineering · ISOAP242, the successor to AP203 and AP214, including 3D annotations.
- ISO/ASTM 52915, Specification for additive manufacturing file format (AMF) · ISO/ASTMA mesh format for additive manufacturing that states units, unlike STL.
What TruePart does today
- TodayTruePart exports the exact B-rep solid as STEP, and a mesh of it as STL, once the solid is measured against the specification.
- LimitA STEP export is not manufacturing approval: the drawing's tolerances, material and process still need a qualified person's review.