Design guide · FDM
Design parts that print right the first time.
The numbers we check in every file before it prints. They are for our printer and materials. Where a value is a rule of thumb, the section says so.
Minimum wall thickness
A wall is built from extrusion lines about 0.45 mm wide. At 1.2 mm you get roughly three lines, which is the least that prints solid and repeatable.
Walls that carry load, take screws or get clamped should be 2 mm or more. Keep thickness even across the part; sudden changes cool unevenly and distort.
- Minimum wall
- 1.2 mm
- Loaded, bolted or clamped
- 2.0 mm or more
- Line width, 0.4 mm nozzle
- ≈ 0.45 mm
Do
Hold 1.2 mm everywhere, and 2 mm or more wherever the part is bolted, clamped or loaded.
Don't
Leave walls under 0.8 mm. They print as one or two lines, or the slicer drops them entirely.
Holes, vertical and horizontal
Holes that run up the Z axis print round but slightly small, typically 0.1–0.2 mm under on diameter. Each loop pulls inward as it cools.
Holes that run sideways are built as a stack of layers, so the top of the hole is an overhang. Past about 6 mm, a 45° teardrop or flat top lets it print without support inside.
If a hole has to be accurate, we print it slightly under and drill or ream it to size.
- Vertical holes
- 0.1–0.2 mm under on Ø
- Horizontal holes
- Teardrop top above ~6 mm Ø
- Smallest practical hole
- ≈ 2 mm Ø
- Precision holes
- Drilled or reamed after
Do
Model holes at nominal size and mark the ones that matter on the drawing. We compensate or ream those.
Don't
Leave a large round horizontal hole with no teardrop and expect a round top. It droops or needs support inside.
Overhangs and supports
Every layer needs something under it. Faces up to 45° from vertical print on their own. Flatter ones need support, which leaves a rougher surface where it comes off.
A flat span between two walls bridges well up to about 10 mm. Longer bridges sag. On bottom edges, a 45° chamfer prints cleaner than a fillet, which goes nearly flat where it meets the face.
- Self-supporting angle
- ≤ 45° from vertical
- Clean bridge span
- ≤ ~10 mm
- Downward-facing edges
- 45° chamfer, not fillet
Do
Chamfer bottom edges and let overhangs face up where you can. We suggest an orientation in the quote.
Don't
Put a cosmetic face on the underside of a supported overhang. Support marks show.
Clearances for fits
Printed parts are not perfectly round or perfectly sized, so mating parts need a gap. The values here are the gap on each side, between the two mating surfaces.
They are a starting point. For anything critical, we can print a small test coupon before the full part.
- Snug / press, small parts
- 0.1 mm per side
- Sliding fit
- 0.2 mm per side
- Moving assemblies
- 0.3–0.5 mm per side
Do
Put the clearance on the printed part and leave bought parts, such as bearings, pins and shafts, at nominal.
Don't
Model mating parts line-on-line. They will not go together without sanding.
Layer orientation and strength
FDM parts are anisotropic. Layers bond to each other less well than plastic bonds within a layer, so a part is weakest when it is pulled apart along Z.
How much weaker depends on material and settings, and fibre-filled materials are the most directional. We orient each part so the main load runs along the layers, and say so if that costs surface finish somewhere.
- Strength across layers
- Often half of in-plane, or less
- Weakest case
- Bending that opens layer lines
- Orientation
- Chosen per part, shown on quote
Do
Tell us how the part is loaded. One line is enough: this hook takes 5 kg straight down.
Don't
Design a thin clip or hook that can only print standing up, with the bending load across the layers.
Threads and heat-set inserts
Screws that go in and out more than a few times need heat-set brass inserts. They take full torque and do not wear the plastic.
Printed threads work at M6 and above for low-cycle use, like a cap. Smaller printed threads are too coarse to rely on. Self-tapping screws straight into plastic are fine for parts assembled once.
- Repeated assembly
- Heat-set brass insert
- Printed threads
- M6 and larger
- Insert hole depth
- Insert length + ~1 mm
- Boss outside diameter
- ≥ 2 × insert OD
| Thread | Insert OD | Hole Ø | Min. boss OD |
|---|---|---|---|
| M2 | 3.6 | 3.2 | 7.5 |
| M2.5 | 4.0 | 3.6 | 8.0 |
| M3 | 4.6 | 4.0–4.2 | 9.5 |
| M4 | 6.3 | 5.6 | 13.0 |
| M5 | 7.1 | 6.4 | 14.5 |
| M6 | 8.7 | 8.0 | 17.5 |
| M8 | 10.2 | 9.6 | 20.5 |
Typical values for common tapered brass inserts. Sizes vary by maker, and the maker's datasheet wins. Tell us the thread and we size the hole to the inserts we use.
Do
Leave a plain hole, tell us the thread, and we install the inserts.
Don't
Print M3 or M4 threads and expect them to survive repeated assembly.
Text and small features
Raised or engraved, each stroke of a letter needs to be at least 0.6 mm wide and at least 0.4 mm high or deep to read cleanly. Bold sans-serif fonts print best; serifs and hairlines disappear.
The same limits apply to other small features. Pins and posts under about 2 mm across snap easily.
- Stroke width
- ≥ 0.6 mm
- Raised height or engraved depth
- ≥ 0.4 mm
- Pins and posts
- ≥ 2 mm Ø
Do
Use a bold sans-serif at 5 mm tall or more for part numbers and labels.
Don't
Engrave fine text on a face that needs support. It fills with support marks.
Large flat parts and warping
ABS, ASA, PC and the nylons shrink as they cool. On long flat parts that pulls the corners up off the bed. Our enclosed chamber helps, but geometry matters more.
Round outside corners, keep thickness even, and stiffen large flat areas with ribs instead of solid plate. If the part does not need heat resistance, PLA or PETG warp far less.
- Plan-view corner radius
- 3–5 mm
- Warp-prone
- ABS, ASA, PC, nylons
- Low-warp
- PLA, PETG, PETG-CF
Do
Add 3–5 mm radii to outside corners and ribs under large flat faces.
Don't
Ask for tight flatness on a large, thin ABS or PC plate without talking to us first.
Tolerances: what to expect
These are the accuracies we hold without special handling. They cover most brackets, housings and fixtures.
When a feature needs tighter, tell us which one. We can tune the print, ream holes, or print a test piece and adjust before the run.
- Standard
- ±0.2 mm typical on features under 100 mm
- Tighter
- On request, per feature
| Feature | Expected | Example |
|---|---|---|
| Up to 100 mm | ±0.2 mm | 50.0 → 49.8–50.2 |
| Over 100 mm | ±0.2% | 200 mm → ±0.4 |
| Vertical holes, as printed | 0.1–0.2 mm under | Ø5.0 → 4.8–4.9 |
| Holes drilled or reamed | Per drawing | Mark on drawing |
Typical values for well-designed parts in rigid materials. Flexible materials, large flat parts and fibre-filled nylons can vary more; we flag those in the quote.
Do
Put tolerances only where they matter. Three critical dimensions get more attention than thirty.
Don't
Carry over a machining title-block tolerance, such as ±0.05 mm on everything, to a printed part.
File prep
Send STEP when you can. It carries exact geometry, so we can measure features and fix small issues without guessing.
STL works for printing if it is exported fine enough that curves stay round. Model in millimetres, one part per file, and add a PDF drawing for anything with critical dimensions, threads or inserts.
- Preferred
- STEP (.step, .stp)
- Also accepted
- STL
- STL chord deviation
- ≤ 0.01 mm
- STL angle
- ≤ 5°
- Units
- mm
Do
Include a PDF drawing that marks critical dimensions, threads and inserts.
Don't
Export STL at default coarse settings. Holes come out as polygons and fits suffer.
Rather we checked it?
Every quote comes with free DFM notes. If something here would trip your part up, we say so before it prints.