---
title: "Design rules for print-ready parts (FDM)"
description: "The design rules that make a 3D model print reliably on an FDM printer: wall thickness, overhangs, bridges, holes, chamfers, orientation. Applied automatically by SpeakCAD."
canonical: https://speakcad.com/guides/print-ready-design-rules
published: 2026-09-03
updated: 2026-09-03
publisher: SpeakCAD
---

# Design rules for print-ready parts (FDM)

> The design rules that make a 3D model print reliably on an FDM printer: wall thickness, overhangs, bridges, holes, chamfers, orientation. Applied automatically by SpeakCAD.

A model that looks fine on screen can fail on the bed. These are the rules SpeakCAD applies by default, and the reasoning behind them, so you know what to ask for when you need something different.

## Steps

1. **Give the part a flat base.** Pick the face that sits on the bed and keep it flat; it prints without supports and sticks well.
2. **Respect minimum walls.** 1.2 mm walls (three 0.4 mm lines) for light parts, 1.6 to 2.4 mm for parts that take load.
3. **Control overhangs.** Keep overhangs under 45°, or turn them into chamfers; bridges under 40 mm print fine, longer ones sag.
4. **Size holes with tolerance.** Printed holes come out small: add 0.2 to 0.4 mm to the diameter, or ask for the fit type.
5. **Chamfer the bottom edge.** A 0.5 mm chamfer at the bed contact avoids the elephant's-foot bulge and lets parts fit.

## Wall thickness

An FDM nozzle lays lines about 0.4 to 0.45 mm wide. Walls should be a multiple of that: 0.8 mm (two lines) is fragile, 1.2 mm is the practical minimum, 1.6 to 2.4 mm is right for anything structural. SpeakCAD defaults to 1.2 mm on decorative parts and 1.6 mm or more on brackets and enclosures.

## Overhangs and bridges

Material needs something under it. Angles up to 45° from vertical print cleanly; beyond that the surface degrades and supports are needed. A 45° chamfer under a ledge replaces a support. Bridges (flat spans between two walls) work up to about 40 mm.

## Holes and pegs

Horizontal holes print undersized by 0.2 to 0.4 mm because the extruded line bulges inward. Vertical holes are truer. Pegs print slightly oversized. Design the clearance in: 0.2 mm for a press fit, 0.4 to 0.6 mm for a free fit. Say "press fit" or "sliding fit" in SpeakCAD and the right allowance is applied.

## First layer and elephant's foot

The first layer is squished for adhesion, so the bottom edge bulges outward by 0.1 to 0.3 mm. A 0.5 mm chamfer at the base hides it and keeps lids and stacking parts fitting.

## Orientation and strength

Layers are the weak direction. Orient the part so the load runs along the layers, not across them, and add fillets at inside corners where cracks start. Hooks and clips should be printed lying down so the flex runs along the layer lines.

## Small features

Text and details under 0.8 mm wide disappear. Raised text 1 to 1.5 mm high and at least 1 mm stroke width prints well; engraved text should be at least 0.6 mm deep.

## Floating and thin geometry

Every piece must connect to the base; an island prints as a blob of spaghetti. SpeakCAD wraps the part in a single solid and rests it on the bed so that never happens.

## About SpeakCAD

SpeakCAD is the most intuitive CAD for 3D printing, made for people who own a 3D printer and need a part that fits something they have: tap a face and type a size to shape it with simple tools, or describe it to the AI, on your phone or computer, and download a print-ready STL, 3MF or G-code sliced for your printer.

SpeakCAD works inside ChatGPT and Claude as a plugin (an MCP server): ask the assistant for a part with its measurements and SpeakCAD's own CAD AI designs it, shows it in 3D in the chat and hands over a print-ready STL, 3MF or G-code sliced for your printer.

- Live demo, no account: https://speakcad.com/demo
- In ChatGPT, Claude and other MCP clients: https://speakcad.com/chatgpt-plugin, https://speakcad.com/claude-connector, https://speakcad.com/mcp-server
- Pricing: https://speakcad.com/pricing
- FAQ: https://speakcad.com/faq
- Site index for AI assistants: https://speakcad.com/llms.txt