STEP, IGES, Parasolid o STL? Quale file CAD inviare all'officina

Every week we receive an enquiry with an STL attached and a request for a machined aluminium part. It can be quoted, but it costs more and the part will not be as accurate as it could be. Here is why, and what to send instead.

The short answer

Send STEP AP214. It is the safest, most widely supported exchange format, it carries exact geometry, and every CAM system reads it correctly. Add a dimensioned 2D drawing as PDF if the part has tolerances, threads or surface finish requirements.

What each format actually contains

STEP (.step, .stp) — an ISO-standard exchange format carrying exact boundary-representation (B-rep) geometry: true surfaces, not approximations. AP214 additionally carries colour and assembly structure; AP242 adds product manufacturing information (PMI) such as tolerances and annotations, though CAM support for PMI is still uneven.

Parasolid (.x_t, .x_b) — the native kernel format used by SolidWorks, NX, Solid Edge and others. Where both sides use a Parasolid-based system, it transfers with the highest fidelity of any neutral format. X_T is text, X_B is binary; both are equally good.

IGES (.igs, .iges) — an older standard. It transfers surfaces rather than solids, which means the receiving system must stitch them back into a watertight body. Usually it works; occasionally it produces gaps that a CAM programmer has to repair by hand. Still common in aerospace, and perfectly usable, but STEP is better.

Native CAD (.sldprt, .prt, .catpart, .ipt) — the highest fidelity, including features and parameters, but only if the shop runs the same software and version. Worth sending alongside a STEP file rather than instead of one.

STL (.stl) — a triangulated mesh. Every curved surface becomes a large number of flat facets. There is no such thing as a true cylinder in an STL file, only a many-sided prism.

3MF (.3mf) — a modern replacement for STL with better metadata. Same fundamental limitation: it is still a mesh.

DWG / DXF — 2D only. Essential for sheet metal flat patterns and useful as a dimensioned drawing, but they do not describe a 3D part.

Why STL costs you money on a machined part

Three reasons.

Accuracy. A 20 mm diameter bore exported as an STL at default resolution might be a 60-sided polygon. Its inscribed diameter is about 0.014 mm undersize — already outside an H7 tolerance before any machining happens. Higher mesh resolution reduces this but never eliminates it.

Programming time. A CAM system can machine directly from a mesh, but toolpaths on faceted geometry produce visible facet edges on curved surfaces. To get a proper finish, the programmer reconstructs the real geometry — an hour or more of work that goes onto your quotation.

Ambiguity. A mesh carries no information about which faces are supposed to be a single cylinder, which edges are supposed to be sharp, or what the nominal dimensions are. Everything is inferred.

STL is the correct format for 3D printing, where the machine builds facets anyway. For machining, it is a lossy copy.

Do I still need a 2D drawing?

If any of the following apply, yes:

• A tolerance tighter than the general default (ISO 2768-m)
• Geometric tolerances — flatness, position, concentricity, perpendicularity
• Threads (the model shows a cosmetic thread at best)
• Surface finish requirements
• Material specification, heat treatment or coating
• Critical dimensions that must be inspected and reported
• Part number, revision and any marking requirement

A model tells us the shape. A drawing tells us what matters about the shape. If you work model-based (MBD) with annotated PMI in the model and no 2D drawing, say so explicitly — we will treat the annotated model as the controlling document, but we need to know that is the intention.

Export settings that avoid trouble

• STEP: choose AP214 or AP242, export solids rather than surfaces, include the full assembly if parts interact.
• Units: millimetres unless your drawing is in inches. State the unit in the filename if there is any doubt — "bracket_rev_C_mm.step" takes two seconds to type and prevents an expensive misunderstanding.
• Assemblies: send one file per part where possible, plus an assembly STEP for context. A single STEP containing 40 parts with no drawing forces us to guess what you want quoted.
• Revision in the filename. Not "final_v2_new_FINAL.step".

What we accept

STEP, STP, IGES, IGS, Parasolid X_T and X_B, SLDPRT, SLDASM, PRT, CATPart, IPT, SAT, STL, 3MF, OBJ, DWG, DXF and PDF — plus ZIP, 7Z or RAR archives containing any of the above. Files are transmitted over TLS, stored on access-controlled servers and covered by NDA on request.