If a manufacturer is going to cut, mould or bend your part, send a STEP file. If it's going on a 3D printer and nothing else, STL is fine.
That's the short version, and it's right about 90% of the time. The other 10% is where people lose a week, so it's worth knowing why the two formats behave so differently.
What's actually inside each file
An STL is a bag of triangles. That's all it is. No units, no colours, no feature tree, no idea that a group of triangles was once a hole. The surface is approximated, and how well depends entirely on the export settings someone picked on the day.
A STEP file stores the real geometry: exact surfaces and edges, units, and (if you export it that way) the assembly structure with each part as its own body. A cylinder stays a cylinder.
The difference is easy to put numbers on. Take a 30 mm hole. Exported as an STL with 24 segments around the circle, the polygon sits 0.13 mm away from the true circle at the middle of each flat. With 64 segments that drops to about 0.018 mm. Neither is wrong, exactly, but if your drawing says H7 on that hole (a tolerance band of 0.021 mm on a 30 mm bore), the 24-segment file can't even describe the feature you're trying to specify.
Which matters more than people expect.
Why CNC, sheet metal and moulding quotes need STEP
A machinist's CAM software wants surfaces it can measure and offset. Hand it a mesh and the first thing it does is try to find the planes, cylinders and fillets again, and it often finds them slightly wrong. A sheet metal shop needs the thickness and the bend lines, which a mesh doesn't carry at all. A toolmaker quoting an injection mould wants to check draft angles face by face, and you can't measure draft on a face that has been chopped into 40 slivers.
It's not that a shop can't work from a mesh. Many can. It's that they'll spend time reconstructing what you already had, and they'll price that time, or add a safety margin, or quietly guess.
A founder once sent me an STL for a DFM review. It had been "tuned" for his printer: holes oversized by 0.2 mm, walls offset to compensate for shrinkage. Nothing in the file said so. I reviewed the wrong part for the best part of an hour before I asked for the original CAD. The review took five minutes after that.
When STL is the right answer
Printing. FDM, SLA and SLS services quote and slice from meshes, and most accept STL (or 3MF, which is better because it carries units). If the part will never be anything but a printed prototype, STL is fine and STEP is a bonus.
Scans are the other case. A scanner gives you a mesh, so a mesh is what exists. The honest next step is rebuilding it as proper CAD, which is what the page on STL to STEP conversion is about. Automatic mesh-to-solid tools exist, but they usually hand back a STEP that is technically solid and practically useless: millions of tiny faces, one per triangle, impossible to edit and no better for a machinist than the mesh it came from.
So don't expect a button to turn an STL into a model you can manufacture from.
3MF, OBJ, IGES and the other formats you'll be asked about
3MF is the sensible upgrade from STL for printing: units, colours and multiple bodies travel with the file. OBJ is a mesh format built for rendering and games, so I skip it for anything mechanical. IGES is the old neutral CAD format. It still works, but it loses things STEP keeps, so I only reach for it when a supplier's software can't read STEP. Native files (SLDPRT, F3D, IPT) only help if the shop runs the same system, and often the same version.
Why guess what a supplier prefers when one email settles it? Ask.
If you're moving between CAD systems rather than sending to a shop, the page on CAD conversion to SolidWorks-compatible files covers what usually survives the trip.
STEP isn't the whole package
Here's the part that gets missed. A STEP file gives the shop your geometry, but it doesn't reliably carry your tolerances, threads, surface finish or inspection points. If the drawing says nothing, many shops fall back on a general tolerance like ISO 2768-m, which allows ±0.2 mm on a 20 mm dimension. That might be fine for a bracket. It's not fine for a bearing seat.
So the quote you get back is for the part they assumed, not the part you meant.
Send a PDF drawing alongside the STEP and call out what matters: the three or four dimensions that control fit, any threads, the finish, the material. The manufacturing drawing checklist walks through what a supplier expects to see on it. You don't need to dimension everything. You need to dimension the right things.
Two export mistakes I see constantly
The first is the fused solid. You export an assembly as a single body, so the STEP arrives as one lump with no separate screws, no separate gasket and no way to tell where one part ends and the next begins. For an assembly, export the structure, with each component as its own body. For a single part, check there isn't a hidden second body sitting inside it.
The second is units. STL has none, so a model in inches opened by someone who assumes millimetres arrives 25.4 times too small or too large. STEP carries units, but it's still worth stating the overall size in your email ("about 85 mm long") so both sides can sanity-check it.
Both take thirty seconds to avoid and a day to untangle.
What I'd actually send
If I were sending a part out for quotes tomorrow, I'd attach these and nothing more:
- A STEP file per part, plus one assembly STEP if there's more than one component.
- A PDF drawing with the critical dimensions, tolerances, material and finish called out.
- An STL only if the shop is quoting printing.
- One line with quantity and the process you're aiming for: CNC, sheet metal, moulding or print.
I'd leave out the native CAD file unless asked. It tells the shop nothing the STEP doesn't, and it invites them to open it and wonder why the feature tree looks the way it does.
If you're not sure the part is ready for a quote at all, the DFM checklist is a decent ten-minute sanity pass before you send anything. And if you'd like a person to look at it, I do a free initial review of one part: send it through the review request form and sign an NDA first if the CAD is confidential. More on how that works is on the DFM review page.