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How to write an AI CAD prompt for a part that fits

By Partvo · Updated September 11, 2026

A useful AI CAD prompt explains what the part must do and what it must fit. A name such as “box” or “stand” leaves many reasonable but different designs open. Start with the function, then describe the geometry that determines whether the part works.

Separate the outside size from the fit

Write the units explicitly. Outside width, depth and height define the envelope. A board outline, mounting-hole spacing or frame thickness defines a mating feature. Label each measurement rather than expecting the generator to infer it from a photo.

Describe an opening as a path

If something slides in, say where it enters and what stops it. Specify whether the slot is open at the top, side or end. For a frame stand, identify the ground-contact foot, the surface supporting the frame and the angle reference. “75 degrees above the tabletop” is clearer than “at 75 degrees.”

Example brief to adapt

Create a noncritical indoor sensor enclosure. Outside size: 60 × 55 × 45 mm. Use a separate removable lid. I will provide the board dimensions, mounting-hole spacing and connector positions before finalising the interior. Keep the cable opening accessible with the lid fitted. Ask about missing fit dimensions rather than treating the outside size as usable internal space.

This is a prompt template, not the recorded input for the sample below. Replace the dimensions with your measurements. Do not invent mounting dimensions to make the brief look complete.

Make revisions specific

Ask for one change at a time when fit matters: “Keep the foot length; move the cable opening to the left wall.” If a new instruction overrides an earlier one, say so. Compare the new version with the previous version before downloading. A successful generation can still misunderstand an instruction.

Use a fit check before the full print

Clearance depends on your printer, material, orientation and fit goal. There is no single clearance that guarantees success. Print a small test section around the mating feature when practical, then update the design from the result.

Inspect our generated enclosure sample or use the design brief checklist. For export choices, see STL vs STEP.