STEP or IGES? How to send your part file
Nothing wastes more time in quoting than a file that arrives incomplete or in the wrong format. The model will not open, the surfaces are loose, the scale is wrong, or the model arrives with no tolerances at all. All of it is avoided by a choice that takes a few minutes.
Short answer: send STEP
For CNC machining the right answer is almost always STEP (.step or .stp). The reason is simple: STEP carries the part as a closed solid, mathematically exact. CAM generates the toolpath from that exact geometry, with no approximation in between.
The AP242 flavour of STEP is preferred for five-axis work because it keeps surface continuity without dropping to an approximation. On multi-axis moves small surface errors accumulate and turn into gouges and collisions; AP242 reduces that risk.
What each format carries
| Format | What it carries | For CNC |
|---|---|---|
| STEP (.step / .stp) | Exact solid geometry (B-rep) | The right choice. Every CAM package reads it. |
| Parasolid (.x_t) | Exact solid geometry | A good alternative if a STEP translation produces artifacts. |
| IGES (.igs / .iges) | Usually loose surfaces | It works, but it carries risk — see below. |
| STL (.stl) | A mesh of triangles | Not suitable. It is a 3D printing format. |
| DXF / DWG | 2D geometry | For sheet cutting and 2D work; it does not define a solid part. |
| Native (SLDPRT, CATPart…) | The full model | Fine if we have the same software; otherwise send STEP. |
Why IGES carries risk
IGES is an older exchange format that STEP has superseded. It causes two problems. First, the part often arrives not as a closed solid but as separate surface patches that have to be rejoined by hand in CAM, and gaps or open edges can remain between them.
Second, curves are approximated on export. On flat faces this goes unnoticed; on curved and free-form parts it degrades toolpath accuracy. In five-axis work those errors compound along the move. If IGES is all you have, send it — but let us know, because the model may need repair first.
Why a part cannot be machined from STL
STL describes a surface as a mesh of triangles. There is no exact geometry inside it — a cylinder is not a cylinder but hundreds of small flat facets. A shop cannot generate a precise toolpath from that mesh, and where it tries, the facets show on the surface.
STL also carries no unit information. Whether the file is in millimetres or inches cannot be read from it, and if the two are confused the part comes out 25.4 times the wrong size. STL is the correct format for 3D printing, not for machining.
Is a 3D model enough? No.
The model states the shape of the part; it does not state how the part will be accepted. Some things are not in the model and must be on the 2D drawing:
- TolerancesThe model carries nominal size. How far each dimension may deviate belongs on the drawing; untoleranced dimensions fall under the general tolerance class in the title block.
- DatumsWhich face the part is referenced from. Without datums, perpendicularity and position cannot be measured.
- Surface roughnessWhich face has to be how smooth. The model does not carry it.
- Material and finishAlloy, temper, anodising or plating and what it adds to the size.
- Edge conditionDeburring, chamfers, no sharp edges.
ISO 2768 general tolerances
What applies to dimensions with no tolerance, and which class belongs in the title block.
What is a datum on a drawing?
Choosing references, the 3-2-1 rule, and why the order changes the measured result.
What is surface roughness Ra?
Ra versus Rz, the N grades, and which Ra each process can hold.
Checklist before you send
- Export the model as STEP; choose AP242 where you can.
- Confirm the unit — millimetres or inches; putting it in the file name is a good habit.
- Close and reopen the model and check it: did it come through as a solid, are any surfaces loose?
- Add the 2D drawing — tolerances, datums, surface finish, material, treatment.
- State quantity and lead time; a single piece and a repeating batch are not priced the same.
With those five in place the quote goes out the same day. If something is missing we ask; and if we see a tolerance or surface value that will cause trouble in manufacture, we say so before production starts.
- STEP
- IGES
- STL
- Parasolid
- AP242
- CAD files
- technical drawing
- Ankara Ostim
Frequently asked questions
Which file format should I send for CNC?
STEP (.step or .stp). It carries the part as exact solid geometry, every CAM package reads it, and the toolpath is generated straight from that geometry. For five-axis work the AP242 flavour is preferred. If a STEP export produces artifacts, Parasolid (.x_t) is a good alternative.
Can I send IGES?
You can, but prefer STEP if you have it. IGES usually carries the part as loose surfaces rather than a closed solid, and it approximates curves on export. That can require repair in CAM and reduces toolpath accuracy, especially on curved surfaces.
Can a part be machined from an STL file?
No. STL describes the surface as a triangle mesh with no exact geometry inside — a cylinder is a stack of flat facets, not a cylinder. STL also carries no unit information, so confusing millimetres and inches produces a part 25.4 times the wrong size. STL is a 3D printing format.
Is a 3D model on its own enough?
No. The model states the shape, not the acceptance criteria. Tolerances, datums, surface roughness, material and finish are not in the model and belong on the 2D drawing. With a model alone, dimensions are taken as nominal and that becomes a dispute at acceptance.
Can I send my native CAD file (SolidWorks, CATIA)?
You can, but add a STEP as well. A native file carries the richest information when the same software and version are available; otherwise it cannot be opened. STEP opens everywhere, which is why it is the safe one.
Send us your technical drawing and quantity; we will prepare your quote with a manufacturability review.
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