Text-to-CAD prompting — engineered parts from a description

Text-to-CAD is the workflow when you need a part with dimensions — a bracket, an enclosure, a fixture, a mating jig. Unlike text-to-3D (which produces a fixed mesh), a CAD agent generates editable parametric code: you can say "wall thickness 3 mm" and get exactly 3 mm. This guide is the prompting structure that works for the PrintPal CAD Agent — and similar OpenSCAD- or Python-driven tools.

Why text-to-CAD prompts look different from text-to-3D prompts

Text-to-3D prompts are stylistic ("a chunky low-poly owl, cartoon style"). Text-to-CAD prompts are specification documents. The agent writes code, and the more precise your specification, the cleaner the code.

The mental model: pretend you're commissioning a part from a machinist who has never seen the application. Every dimension you forget to specify is a guess they're going to make — sometimes well, sometimes badly. Specify everything that matters, accept defaults for everything that doesn't.

Anatomy of a great text-to-CAD prompt

1. Part type and overall purpose

Start with the noun and the function. "Wall-mount bracket for a small wireless router". "Cable strain-relief grommet for a USB-C cable". "Replacement knob for a stove with a D-shaped 6 mm shaft". The function tells the agent which features are mandatory (e.g. "knob" implies a friction-fit interface) and which are optional.

2. Overall dimensions

Bounding box or critical exterior dimensions. Use real units; "L x W x H in mm" is the safe convention. Always specify mm explicitly — "30 x 60 x 12 mm" is unambiguous, "30 x 60 x 12" can be interpreted as inches in some contexts.

3. Features and their positions

For each feature (hole, slot, mounting boss, fillet, chamfer, recess) state:

• What: "through hole", "blind hole 5 mm deep", "slot 10 mm long", "fillet", "chamfer".
• Size: diameter, length, radius. Round to standard sizes (M3 = 3 mm; M4 = 4 mm).
• Where: "centred", "8 mm from the left edge and 5 mm from the top", "in a 60 mm-centre pattern".

4. Mating parts and clearances

If anything has to fit something else, state the mating reference and the clearance. FDM defaults:

• 0.2 mm clearance per side for sliding fits.
• 0.1 mm clearance for tight push-fits.
• 0.0 mm or -0.05 mm for press-fit threaded inserts.
• 0.3 mm clearance for textured-bed prints (the textured layer adds ~0.1 mm).

5. Print-friendly hints (optional but powerful)

• "Designed to print without supports" nudges the agent to chamfer overhangs at 45° instead of leaving them as bridges.
• "Flat bottom" guarantees a printable first layer.
• "With a 2 mm fillet on outside edges" reduces stress concentrations and produces nicer prints.
• "Wall thickness 2 mm minimum" avoids walls thinner than a typical 0.4 mm nozzle can print well.

Worked examples

Bracket

Good: "Wall-mount L-bracket. Horizontal arm 60 mm x 30 mm x 5 mm thick; vertical arm 40 mm x 30 mm x 5 mm thick. Two M4 countersunk through-holes on the vertical arm, 10 mm in from each end, centred vertically. Two M4 through-holes on the horizontal arm, 10 mm in from each end. 3 mm fillet on all external edges. Designed to print with the vertical arm flat on the bed."

Bad: "A bracket for mounting things on the wall." (No dimensions, no holes specified, no orientation — the agent will guess all of it.)

Box / enclosure

Good: "Snap-fit enclosure with a separate lid. Internal cavity 80 x 50 x 25 mm. Wall thickness 2 mm. Lid sits inside the body with a 0.3 mm clearance and snaps in via two 1 mm lugs on each long side. 1.5 mm fillet on outside edges. Flat bottom; lid prints separately."

Bad: "A box with a lid that closes." (No size, no clearance, no snap mechanism specified.)

Replacement knob

Good: "Replacement stove knob. 35 mm diameter outer cylinder, 18 mm tall. Knurled outer surface (6 mm tall knurl band on the upper third). Central hole sized for a D-shaped shaft: 6 mm diameter with a 4.5 mm flat on one side, 10 mm deep, 0.1 mm clearance. Pointer indicator: 2 mm-wide raised line running the full radius on the top face."

Mounting bracket for a specific device

Good: "Vesa-compatible monitor mount for a 75x75 mm pattern. Backplate 100 x 100 x 4 mm. Four M4 countersunk through-holes in a 75 mm square pattern, centred. One 10 mm-diameter cable pass-through hole below the bottom-right mounting hole. Two 5 mm slotted wall-mount holes on the back, 70 mm apart horizontally."

The iteration loop

Text-to-CAD shines on iteration because the underlying model is parametric — the agent can keep the existing code and just edit the part you mentioned. Patterns that work:

• "Make the wall 4 mm instead of 2 mm." Single-parameter edit. Re-renders in seconds.
• "Add four M3 mounting holes, 8 mm in from each corner." Additive feature; the rest stays intact.
• "Move the central hole 5 mm to the left." Position adjustment.
• "Add a 2 mm chamfer to the top edge." Surface refinement.
• "Make this 10% bigger overall." Uniform scale.
• "Hollow it out with 2 mm walls, open from the bottom." Cavity addition.
• "Add a label on the front: 'BAY 3', 6 mm tall, embossed 0.6 mm." Text features (text-to-CAD handles these reliably — text-to-3D does not).

How to specify dimensions the agent will respect

Common mistakes

• Leaving out a dimension and hoping the agent picks the right default. It usually doesn't — or it does this time, and not the next.
• Specifying dimensions in inches without saying so. The CAD Agent assumes mm. Add "in inches" or convert.
• Forgetting clearance on mating parts. A "5 mm hole for a 5 mm shaft" will be a press-fit at best. Use 5.2 mm for a sliding fit.
• Asking for features the agent has no reference for. "Make it look cool" or "make it ergonomic" don't map to geometry. Be concrete.
• Re-prompting from scratch instead of iterating. Once you have a base part you like, edit it. A fresh prompt produces a fresh part with new quirks to debug.
• Asking for organic shapes. Text-to-CAD is for geometric / engineered parts. For sculpture, use text-to-3D.

Hybrid workflows: AI mesh + AI CAD

The best results today often come from combining the two AI approaches:

1. Generate the artistic part with text-to-3D (a figurine, a sculpture, a custom shape).
2. Generate the functional part with text-to-CAD (a base, a stand, a mounting interface).
3. Assemble in the slicer by importing both STLs and positioning them together.

Examples: a custom miniature on a 25 mm round CAD-designed base; a sculpted bust on a magnetic phone holder; a 3D-scanned mug handle attached to a CAD-designed cylindrical mug body.

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