---
title: "10 things we learned about prompting an AI for CAD"
description: "After thousands of generated parts, these are the patterns that separate a one-shot model from five rounds of fixes. Practical advice for describing 3D printed parts."
canonical: https://speakcad.com/blog/prompting-ai-cad-lessons
published: 2026-09-03
updated: 2026-09-23
publisher: SpeakCAD
---

# 10 things we learned about prompting an AI for CAD

> After thousands of generated parts, these are the patterns that separate a one-shot model from five rounds of fixes. Practical advice for describing 3D printed parts.

## 1. Lead with the mating dimension

The number that decides whether the part works is the one it has to fit: the shaft, the pipe, the PCB. Give it first and give it precisely. Everything else can be approximate.

## 2. Name the fit, not just the gap

"Press fit", "sliding", "snap", "free". The AI knows the clearance for each on an FDM printer. If you give a raw number instead, say whether it is the nominal or the finished size.

## 3. Say what it is for

"A hook" and "a hook for a 2 kg bag" produce different thicknesses and fillets. Purpose unlocks design rules.

## 4. One part per model

A model with a box, a lid and a bracket becomes hard to edit. Make separate models, or ask for "two parts side by side" only when they belong together.

## 5. Numbers in millimetres, decimals welcome

6.35 mm hex bits, 22.2 mm bearing pockets. The engine is exact; use it.

## 6. Photos need a scale

A photo shows shape, not size. Attach it, then write the two or three dimensions that matter.

## 7. Ask what was assumed

"What did you assume?" returns the defaults the AI chose. Correct the ones that matter before printing.

## 8. Edit with amounts

"2 mm wider" beats "wider". Quote the measured gap after a test print: "the lid is 0.3 mm too tight".

## 9. Ask for parameters you will tweak

"Make the height a parameter" costs nothing now and saves a rebuild later.

## 10. Trust the printability defaults, override when needed

Minimum walls, chamfers at the base, parts resting flat: these happen automatically. Say "single wall for vase mode" or "no chamfer" when you have a reason.

## Bonus: keep the conversation going

The chat is the design history. If print three failed for the same reason as print one, the AI can see it. Re-downloading after a small fix is free until the design substantially changes. Asking the AI for the fix is one request (2 credits); typing the number in the viewer is free, so iterate freely.

## 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