FDM design rules
Everything numeric this course uses, on one page. Built to be printed and stuck above the printer. The published values are starting points for a 0.4 mm nozzle in PLA — the table at the bottom is where your own measured numbers go, and those override everything above.
Geometry the printer can actually make
| Overhang | Up to 45° from vertical, unsupported | At 45° each layer still has about half the layer below it underneath |
|---|---|---|
| Bridge | Under 5 mm spans cleanly | Filament stretched across the gap stays taut long enough to cool |
| Bottom edges | Chamfer 0.3–0.6 mm, never fillet | A chamfer is self-supporting; a fillet leaves the solid horizontally |
| Downward faces | Replace flat roofs with two 45° faces | A ridge prints without support where a flat ceiling cannot |
| Horizontal holes | Teardrop the top, or accept an oval | The top of a horizontal circle is a 90° overhang and sags |
Minimum sizes (0.4 mm nozzle)
| Wall thickness | 0.8 mm absolute minimum, 1.6–2.4 mm structural | Whole multiples of extrusion width fill with complete perimeters |
|---|---|---|
| Vertical pin | 5 mm diameter, or add a base fillet | Thin towers wobble under the nozzle and never cool in time |
| Vertical hole | 1 mm minimum, 2 mm to be comfortable | Smaller circles are swallowed by the extrusion width |
| Horizontal hole | 2 mm minimum | Sagging at the top eats a fixed amount regardless of size |
| Embossed detail | Wider than 0.9 mm; engrave instead where possible | A raised feature narrower than one bead cannot be printed at all |
| Heights that must mate | Whole multiples of layer height | Otherwise the slicer rounds them and the lip lands off-target |
Fits — published starting points
| Press fit | 0.05–0.10 mm per side | Assembles with force and does not come apart again |
|---|---|---|
| Slip fit | 0.20–0.30 mm per side | Pushes together by hand, holds position, separates when wanted |
| Free fit | 0.40–0.50 mm per side | Rotates and slides after the plastic has relaxed |
| Hole diameter | Add 0.2–0.4 mm to nominal | Printed holes come out undersized as a matter of course |
| Clearance on a diameter | Twice the per-side value | The gap exists on both sides of the part |
My printer — measured
Filled in from the test coupon. Reprint the coupon and redo this table whenever the material, nozzle, layer height or slicer profile changes.
| Press fit (tightest that goes in) | ______________ mm |
|---|---|
| Slip fit (hand pressure) | ______________ mm |
| Free fit (spins freely) | ______________ mm |
| Hole compensation (nominal minus measured) | ______________ mm |
| Extrusion width | ______________ mm |
| Layer height | ______________ mm |
| Printer, material, profile | ________________________________ |
Where these numbers come from
- Protolabs Network — overhangs, bridging, pin diameter, base chamfers
- Hydra Research — wall and feature minimums as multiples of extrusion width
- Layer X — hole compensation and clearance starting points
- 3DPut — the press, slip and free fit ranges
Full links, with what each source is good for, on the resources page. Where sources disagree, the more conservative value is printed here.