How to Prepare a File for 3D Printing
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Most printing problems start in the file: wrong units, faceted curves, walls too thin to print, holes that come out tight. Ten minutes of checks before you send a file can save a reprint.
Which format to send
- STEP is the best choice. It's exact geometry, so we can measure it, add machining stock to a critical face, or thicken a wall without remodeling the part.
- 3MF is a good mesh format. It stores units and can carry color and several parts in one file.
- STL works everywhere, but it has no units and stores curves as flat triangles. Export it carefully (below).
- OBJ works too. It mostly comes from scanning and 3D modeling software.
Native CAD files from SolidWorks, Fusion, Inventor and the rest are useful as an extra, but send a STEP with them.
Units and scale
An STL file has no units, just numbers. A part modeled in inches and opened as millimeters comes out 25.4 times too small: a 4 inch bracket becomes 4 mm. Model in millimeters when you can. If not, say which units the file uses, and put the overall dimensions in your email so the scale can be checked.
STL export settings
An STL approximates curved surfaces with triangles. With coarse settings, holes print as polygons and smooth curves show flats. Most CAD programs have a resolution setting: pick fine or custom, with a chord (deviation) tolerance around 0.01 to 0.02 mm and an angle tolerance of 5° or less. Going much finer makes the file huge without improving the print.
Export each part as its own file. If you export an assembly as one STL, the parts end up in one file, and parts that touch can fuse together when printed.
Check that the mesh is closed
A printable mesh has to be watertight: no gaps, no flipped faces, no surfaces passing through each other. Files exported from solid CAD are usually fine. Meshes from scans, sculpting tools or game models often aren't. Most slicers and mesh tools have a repair function that will flag and fix the common problems.
Design rules that matter most
| Rule | FDM | SLA | SLS / MJF | Metal |
|---|---|---|---|---|
| Thinnest wall | 1 mm | 0.5 mm | 0.8 mm | 0.4 mm |
| Smallest hole | 2 mm | 0.5 mm | 1.5 mm | 1.5 mm |
| Overhangs | Supports past 45° | Supports needed | No supports | Supports past 45° |
| Gap between moving parts | 0.5 mm | 0.5 mm | 0.3 mm | Machine to fit |
| Drain holes in hollow parts | Not needed | 4 mm | 5 mm | 5 mm |
These are typical minimums, and thicker walls and bigger holes are always safer. Walls are the most common problem: a thin rib that looks fine on screen can vanish in the slicer or print as one fragile line.
Holes, threads and inserts
Holes print slightly undersize in most processes. For a hole that has to fit a pin or a bearing, model it at nominal size and mark it, and it gets drilled or reamed after printing. Small modeled threads print poorly in plastic, especially in FDM. Model a plain hole instead: a tap drill hole (4.2 mm for M5) or a hole sized for a heat-set insert from the insert maker's chart. Threads in metal parts are tapped or machined after printing.
Tell us what matters
A file shows the shape, not which parts of it are important. Add a short note or a drawing with:
- Dimensions that need a tolerance, and what the tolerance is
- Cosmetic faces, where support marks would be a problem
- Faces that mate with other parts
- Loads, heat or chemicals the part will see
- Quantity, color and finish
That lets the part be oriented and supported correctly the first time.
Name files so they can't be mixed up
Use the part name, revision and quantity, like bracket-rev3-qty10.step. When you send a revision, change the name. It's the simplest way to avoid printing the old version.
When the file is ready, send it through our 3D printing service. No CAD? We can model it from a sketch or scan the part.