For a 3D printed part, send 3MF or STEP if you can, and STL if that is all you have. For CNC machining and sheet metal, send STEP, because it carries exact geometry and units. For a flat laser, waterjet or plasma part, send a DXF at full scale. Avoid IGES when STEP is available. The difference is what each format keeps: units, exact curves, colours and multiple parts are present in some files and silently missing in others.
Key points
- STL and OBJ are meshes without units. A part drawn in inches can arrive 25.4 times too small.
- 3MF is a mesh with units, multiple parts, colour and material, built for additive manufacturing.
- STEP is exact geometry with units and assemblies: the right file for machining and sheet metal.
- DXF is a 2D drawing for flat cutting; draw at full scale and set its units.
- IGES is an older exchange format that often arrives as loose surfaces. Prefer STEP.
Why the file format matters for a quote
A quote is only as good as what the shop can measure from your file: volume, size, wall thickness, holes, flat faces. Some formats keep all of that exactly; others keep an approximation; some lose the scale altogether. The wrong file does not usually fail outright. It produces a quote that looks fine and is wrong, which is worse.
The formats at a glance
| Format | What it stores | Units | Colour and material | Several parts | Best for |
|---|---|---|---|---|---|
| STL | Triangles only | No | No (not in the standard) | Multi-solid in ASCII only, rarely used | 3D printing, when nothing better is available |
| OBJ | Polygons, optional normals and texture coordinates | No | Yes, through a separate MTL file | Yes, as named objects or groups | Coloured models, visual work |
| 3MF | Triangles in a zipped XML package | Yes | Yes | Yes, with placement on the plate | 3D printing |
| STEP | Exact surfaces and solids (planes, cylinders, splines) | Yes | Limited | Yes, as assemblies | Machining, sheet metal, any process |
| IGES | Exact surfaces, often without solid topology | Yes | Limited | Yes | Legacy exchange; replace with STEP |
| DXF | 2D lines, arcs, polylines and splines | Declared in the header, often unset | Layers and colours | Yes, as separate contours | Laser, waterjet, plasma and routing |
Meshes: STL, OBJ and 3MF
A mesh describes a surface as many small flat triangles. That is all a printer needs: the slicer cuts the triangles into layers. But a mesh is an approximation. A cylinder becomes a prism with many flat sides, and how many sides it gets was decided by whoever exported it.
STL is the oldest and most common. It stores triangles and nothing else: no units, no colour in the standard, no part names. It is universal, which is its strength, and it has no units, which is its weakness. The file units problem is real enough that it has its own article: STL has no units, and it will cost you money.
OBJ is a mesh format from computer graphics. It can carry colour and texture through a companion material file, and named objects. It also has no units.
3MF was designed for additive manufacturing. It is a zip package holding the mesh, the unit, several objects with their placement on the build plate, colours and materials. Most current slicers write it. If your CAD tool or slicer exports 3MF, it is usually the best mesh to send for printing.
Exact geometry: STEP and IGES
STEP (ISO 10303) stores the part as its CAD system knows it: planes, cylinders, cones and free-form surfaces, joined into a closed solid, with units and assembly structure. A machinist can read a hole’s exact diameter and axis, a flat face’s exact position, and a fillet’s radius. That is why machining and sheet metal shops ask for STEP: their work is measured in those features. For printing, a shop can turn STEP into a mesh at whatever resolution it wants.
IGES is an older neutral format. It also stores exact surfaces, but files often arrive as a set of loose faces rather than a closed solid, with small gaps between them. Turning that into something machinable or printable means repair. STEP has largely replaced it, and if your CAD system can export STEP, do that.
2D drawings: DXF
A flat part for laser, waterjet or plasma cutting only needs its outline and holes. DXF carries exactly that, as lines, arcs and polylines, which the shop reads to measure cut length and pierces. Three habits avoid most problems:
- Draw at 1:1 scale and set the drawing units before export.
- Close every contour; an open outline cannot be cut as a part.
- Remove title blocks, dimensions and construction lines, or keep them on a layer the shop is told to ignore.
The design for sheet metal guide covers what a flat pattern should look like before you export it.
How much accuracy does a mesh lose?
When CAD exports a mesh, it replaces each curve with straight segments, keeping the gap between them below a chord tolerance. For a hole, the flats sit inside the true circle, so the meshed hole is slightly smaller.
Worked example: a 20 mm diameter hole exported as a mesh at three resolutions.
The gap between a flat segment and the true circle is r × (1 - cos(180° ÷ n)) for n segments, and the hole measured across the flats is 2r × cos(180° ÷ n). Here r = 10 mm.
| Segments around the hole | Largest gap to the true circle | Diameter across the flats |
|---|---|---|
| 12 (a coarse export) | 10 × (1 - cos 15°) = 0.34 mm | 20 × cos 15° = 19.32 mm |
| 23 (chord tolerance 0.1 mm) | 10 × (1 - cos 7.83°) = 0.09 mm | 20 × cos 7.83° = 19.81 mm |
| 64 (a fine export) | 10 × (1 - cos 2.81°) = 0.01 mm | 20 × cos 2.81° = 19.98 mm |
With 0.1 mm chord tolerance, you need at least 23 segments for this hole. A coarse export loses more than a third of a millimetre at the flats, which is the difference between a pin fitting and not.
For printing, a fine export is usually fine: the printer’s own accuracy is often coarser than a good mesh. For a precise bore that will be reamed or machined later, the exact size belongs on a drawing, not in the mesh. The design for 3D printing guide covers how holes and clearances behave once printed.
Which file for which process?
| Process | Send | Also helpful |
|---|---|---|
| FDM, resin, SLS, MJF | 3MF or STEP; STL if that is what you have | The intended units, if sending STL or OBJ |
| Metal 3D printing | STEP | A drawing with critical dimensions |
| CNC milling and turning | STEP | A drawing with tolerances, threads and finish |
| Sheet metal | STEP of the bent part | A DXF flat pattern and a drawing with bend directions |
| Laser, waterjet, plasma | DXF at full scale | Material and thickness in the file name or notes |
| Injection moulding, casting | STEP | A drawing with tolerances, parting line and texture |
The design for CNC machining guide explains why a machining quote depends on the exact features only STEP keeps.
A clean file is also a closed one
Whatever the format, a printed part’s mesh needs to be a closed surface, a watertight mesh, so its volume can be measured. Holes in the surface, flipped triangles and overlapping shells make volume uncertain, and volume is what most printing prices are built on. Most CAD exports are clean; scans and models from graphics tools often are not.
Which formats an instant quote can read
Quotometry’s quoting widget reads STL, OBJ, 3MF, PLY, AMF, glTF and STEP for printed, machined, moulded and cast parts, and DXF and SVG where a process needs a flat drawing. For CNC, a STEP file can price instantly, while a mesh goes to the shop’s review because its holes and pockets are unknown. It takes units from formats that declare them, guesses them for unitless files, and shows the buyer the dimensions it read so a wrong scale is easy to spot. An IGES file is refused by name with the export to make instead, and the widget page shows what a buyer sees after uploading.
Questions people ask
Is STEP or STL better for 3D printing?
Either works for printing, because the printer only needs a surface. STEP has the advantage of exact geometry and stated units, so the shop can mesh it at the resolution it wants and there is no doubt about scale. STL is universal but has no units and its resolution is fixed at export. If you have both, send STEP, or 3MF if your tool exports it.
Can a CNC shop machine from an STL file?
Sometimes, but it is a poor input. An STL is a set of flat triangles, so cylinders and holes arrive as polygons and the exact radii, hole centres and flat faces the programmer needs are lost. Most machining shops will ask for STEP and a drawing. Send STEP from the start and the quote will be faster and more accurate.
Why does my DXF come out the wrong size?
DXF drawings often do not state their units clearly, or state a unit that differs from the one the numbers were drawn in. A part drawn in inches and read as millimetres comes out 25.4 times too small. Set the drawing units before exporting, draw at full scale, and check the overall size the shop reports back against your design.
What is wrong with IGES files?
Nothing is wrong with the format for what it was designed for, but IGES often carries loose surfaces rather than a closed solid, so a shop may receive a part with gaps between faces that has to be repaired before it can be machined or meshed. STEP carries solids and assemblies more reliably and has largely replaced it. Export STEP instead where you can.
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