Designing for 3D printing

A printable part follows a small handful of rules: walls thick enough to survive, tolerances generous enough to fit, overhangs gentle enough not to droop, orientation chosen to avoid supports, and the right edge geometry for the right places. This guide collects the numbers that work in practice, plus the printability checks the PrintPal AI CAD agent runs after every render.

Wall thickness

Tolerances & clearances

For FDM, real-world parts are typically 0.1–0.3 mm bigger than nominal due to extrusion width and over-expansion. Plan for it:

Overhangs & bridges

The fix for an overhang is almost always one of:

• Add a chamfer on the underside (a 45° bevel printed bottom-up has no overhang).
• Reorient the part so the overhang faces up.
• Add a print-in-place support rib that you snip off after.

Chamfers vs fillets

OpenSCAD doesn't have a one-click chamfer or fillet — you build them with primitives. Which to pick:

Quick recipes:

// 45° chamfer along the bottom edge of a box
difference() {
  cube([40, 20, 10]);
  translate([0, 0, 0])
    rotate([45, 0, 0])
      translate([-1, -5, -5])
        cube([42, 5, 5]);
}

// Fillet a 2D corner with offset() before extruding
linear_extrude(height=5)
  offset(r=2) offset(delta=-2)
    square([40, 20]);   // rounded rectangle

Orientation

Most printability problems disappear with the right orientation. Heuristics:

• The largest flat face goes on the build plate.
• Threaded holes for M3/M4 inserts should face up (clean cylindrical bore) or be reamed after printing.
• Anisotropy: FDM parts are ~50% weaker in the Z direction. Orient so loads pull along the layers, not across them.
• Cosmetic faces should be top or sides — never the bottom (elephant's foot, brim marks).

Small-feature limits

Printability checks the agent runs

After every render, the PrintPal AI CAD agent runs:

• Manifold check — every edge shared by exactly two faces; no zero-area triangles.
• Watertight check — closed shell, no holes in the mesh that aren't intentional cavities.
• Min wall thickness — flags anything below 1.6 mm by default (configurable per session).
• Bbox vs brief — actual bounding box compared against the envelope it committed to in the design brief.
• Floating geometry — surfaces the slicer would treat as unsupported floors.

Failures appear as warn-colored tool cards in chat. You can address them with a one-line prompt ("increase wall to 2 mm") or let the agent self-correct on the next turn.

Material-specific rules of thumb

Related tools