How to prepare an STL for 3D printing (and get an accurate quote)

Most files we receive print fine. The ones that don’t usually fail for one of a handful of reasons, and every one of them is easy to fix before you send the file. This is the checklist we run through ourselves, written for someone sending us a model for the first time.

1. Export in millimetres, at real size

An STL has no units — it is just numbers. Slicers assume millimetres, so a part designed in inches or centimetres arrives 25× too small or 10× too big. Before exporting, check the model’s bounding box against a real dimension you know (a hole for an M3 screw should be about 3.2 mm across). If you can export 3MF or STEP instead of STL, do: 3MF carries units and colours, STEP keeps the exact geometry so we can adjust tolerances if needed.

2. Make it one solid, watertight body

A mesh needs to be closed — “watertight” or “manifold” — for a slicer to know what is inside and what is outside. Common problems: two overlapping shells instead of one merged body, missing faces, flipped normals, or a surface model with no thickness at all. Most CAD programs export solids correctly; sculpting and game tools sometimes don’t. Your slicer’s repair function or a free tool such as Microsoft 3D Builder or Meshmixer will show and fix most issues. If you are sending an assembly, export the parts as separate files so each can be oriented on its own.

3. Respect the minimum wall thickness

An FDM printer draws lines about 0.4 mm wide. Anything thinner than a single line disappears; anything thinner than about 1.2 mm (three lines) is fragile. Text, logos and fine ribs should be at least 0.8 mm wide and 0.4 mm tall to survive. Thin, tall features like spikes and antennae print better slightly thicker than they look right.

4. Design for holes, pins and fits

Printed holes come out slightly smaller than modelled and printed pins slightly larger, by roughly 0.1–0.3 mm depending on material. For parts that must fit together, leave 0.2 mm clearance for a snug fit and 0.4 mm for a free-moving one. For screws, model the hole at the screw’s nominal size plus 0.2–0.4 mm, or tell us the screw size and we’ll set it. Heat-set threaded inserts are an option for anything that will be screwed in more than a few times.

5. Think about orientation and overhangs

Layers are the weak direction: a printed part is strongest along its layers and weakest across them. If a hook, clip or lever will be loaded, tell us which way the load goes so we can orient the print accordingly. Surfaces that overhang more than about 45° need support material, which leaves marks; a small chamfer instead of a sharp overhang often removes the need for support entirely. Large flat bottoms print cleanly; a flat face pointing down usually gives the best-looking result.

6. Know the build volume

Our Voron 2.4 prints up to 350 × 350 × 340 mm in one piece. Bigger parts are split and joined with keys, dowels or dovetails — we do this for you, but it helps to know in advance whether a seam is acceptable and where it may go.

7. What to send with the file

A quote is calculated from filament used, print time and any design work at an hourly rate, so the things that change those numbers are the things we need to know:

• What the part does — one sentence is enough; it decides the material (see our filament guide)
• Quantity — batches are cheaper per piece
• Colour(s) — up to six in one part
• Finish — as printed, sanded, painted, smoothed
• Deadline, if there is one
• Critical dimensions or fits — anything that must mate with another part

Quick pre-flight checklist

1. Units are millimetres and the size is right
2. Single closed body per file, no overlapping shells
3. No wall thinner than 1.2 mm, no detail smaller than 0.8 mm
4. Holes and fits have clearance designed in
5. You know which direction the part will be loaded
6. It fits 350 × 350 × 340 mm, or you’re fine with a seam
7. Material, quantity, colour, finish and deadline are in the message