The short answer
Send both a STEP file and a binary STL (or 3MF). STEP is what engineers read — it preserves exact CAD geometry, tolerances, and editability, so it’s best for quoting and design-for-manufacturing review. STL is what the printer actually consumes — a triangle mesh that every machine understands. If you only send one file, send both anyway: STEP for the engineer, STL for the machine.
What each format actually is
STL (stereolithography)
STL describes a part as a mesh of triangles — thousands of tiny flat facets approximating your surfaces. It’s been the default print format since the 1980s and every slicer, printer, and service on earth accepts it.
- Binary vs ASCII: binary STL is smaller and faster; use it. ASCII is readable but bloats files.
- Pros: universal, simple, works everywhere.
- Cons: it’s an approximation — curved surfaces are faceted; it stores no units, no tolerances, no material, no color; it loses all CAD intelligence (history, threads as features, fillet radii).
STEP (.step / .stp)
STEP is the neutral CAD exchange format. It stores exact surfaces, threads, holes, tolerances, and units as mathematical data, and it’s editable in most CAD programs.
- Pros: exact geometry, editable, carries design intent; what a manufacturer needs to review your part for machining or DFM.
- Cons: not directly printable — it must be meshed to STL/3MF first; larger files; not every consumer service accepts it.
3MF — the modern upgrade
3MF is a newer mesh format designed to replace STL. It carries color, material, and texture data, stores units, and uses a more compact representation with better precision than STL. If a service accepts 3MF, prefer it over STL.
Other formats you’ll meet
- OBJ — mesh with color/texture maps; common for resin and multi-color work.
- AMF — XML-based mesh with material/color; rare in practice.
- Native CAD files (.sldprt, .step native) — services rarely accept these; convert to STEP.
- PDF — for review only; never for printing.
When STL is fine
For quick functional prototypes, a clean binary STL is enough — especially for FDM and resin prints where the mesh resolution is far finer than the layer height. Consumer services (print shops, marketplace platforms) often accept only STL. Just make sure your export settings produce a watertight mesh (no holes), correct scale, and a reasonable triangle count.
When STEP matters most
STEP becomes the right choice in these situations:
- Any quote for machined or metal parts — STEP lets the manufacturer program with real geometry.
- Tight tolerances, threads, and snap fits — mesh approximations can shift dimensions by fractions of a millimeter that matter.
- DFM review — an engineer can open your STEP, measure, and flag problems. With STL alone they’re guessing at your intent.
- Quotation accuracy — STEP-based quoting is accurate; STL quoting is a best guess that tends to under- or over-estimate.
Rule of thumb: for a quick FDM prototype, STL is fine. For anything you’re paying a professional service to engineer and build, send STEP.
The file-prep checklist before you send anything
Get these right and you’ll avoid most rejection emails:
- Units in millimeters. Export at 1:1 scale. STL stores no unit — a part meant to be 50 mm can arrive as 50 inches.
- Watertight mesh. No holes, no non-manifold edges, correct face normals. Slicers and services check this automatically.
- Wall thickness. Respect minimums: roughly 0.8–1 mm for FDM, 0.3–0.5 mm for resin, 1–1.5 mm for SLS nylon on structural walls. Thinner walls print fragile or fail.
- Resolution. Export at ~0.01 mm chord tolerance for STL. Too coarse = faceted surfaces; too fine = 200 MB files that slow everything down.
- Threads and features. Keep threads and threaded holes as STEP features so the engineer can evaluate them properly.
- Name the files clearly —
part-name_v2.step+.stl, notfinal_v3_FINAL2.stl.
What a good service will ask you for
| Format | When to send | Who reads it |
|---|---|---|
| STEP | Quoting, DFM review, metal/machined, tight tolerances | Engineers, manufacturing |
| STL (binary) | The actual print, quick prototypes, consumer services | Slicers, printers |
| 3MF | Color/multi-material prints, modern services | Slicers, printers |
| OBJ | Textured/colored models | Resin services, art prints |
| Communication only | People, not machines |
A professional service will typically ask for STEP + STL and then confirm units, material, and finish back to you in writing.
FAQ
Q: Should I send STL or STEP to a 3D printing service? A: Send both. STEP for quoting and DFM review, binary STL for the actual print. If the service accepts only one, follow their spec — many consumer services accept STL only.
Q: What is the difference between STL and STEP files? A: STL is a triangle mesh approximation used by printers; it has no units, tolerances, or design history. STEP is a CAD data format with exact geometry and editability, used for engineering and manufacturing review.
Q: Can a 3D printer use a STEP file directly? A: No. STEP must be converted (meshed) to STL or 3MF before slicing. Most services do this conversion for you, but they need the original STEP to do it accurately.
Q: Is 3MF better than STL? A: For most modern uses, yes — 3MF stores units, color, and material data and is more precise. If a service supports 3MF, prefer it; otherwise binary STL is fine.
Q: Why was my STL file rejected? A: Usually a mesh problem: not watertight, wrong scale/units, walls below minimum thickness, or corrupted geometry. Run the checklist above before re-exporting.
Q: What tolerance should I use when exporting STL? A: Around 0.01 mm chord deviation is a good default — smooth surfaces without exploding file size. Adjust finer for very small features.
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