Designing for FDM — orientation, overhangs, supports

An FDM printer lays plastic in layers from the bottom up. Each layer is typically 0.2 mm tall — about the thickness of a sheet of paper. Every layer needs the previous layer to sit on, or it falls.

Your design choices ripple from this single constraint: orientation, overhangs, and supports are all answers to "how do I make sure every layer has something to land on?"

Whatever face is touching the build plate prints perfectly flat. Use this. A phone stand prints with its base on the plate; a name tag prints lying down, not standing up. The biggest, flattest face goes on the plate.

Rule of thumb: rotate parts in the slicer (not in the CAD tool) — your CAD origin stays sensible and only the slice rotation changes.

An overhang is any face that points down at the build plate. Under about 45° from vertical, each layer's edge sticks out only ~ 0.15 mm beyond the layer below — well within filament's tendency to droop. Above 45° (closer to horizontal), the new layer has nothing to grip and falls.

Design fix: chamfer or fillet sharp downward-facing edges to ≥ 45° (a 1 mm chamfer on a 90° overhang turns it into 45°). Or accept supports.

Bridges and tall overhangs need support material — temporary scaffolding the printer adds beneath them. You don't model supports in CAD; the slicer adds them automatically. OrcaSlicer's tree supports are very forgiving.

Design implication: if your part needs supports on a critical surface, plan to clean up that surface (a few seconds with a knife) or redesign so the supported area faces non-critical surfaces.

A horizontal cylinder hole (axis parallel to the plate) prints perfectly. A vertical hole prints fine but the bottom of the hole has a small "floor" sag — design with this in mind or print the part on its side.