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.
Be specific about what, where, and how big. Start with the part type, then the bounding box, then features (holes, slots, fillets, mounting points) with their dimensions and positions. Always include a clearance allowance for mating parts (0.2 mm is the FDM default). When something isn't right, change one feature in plain English and re-render — don't re-prompt from scratch.
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.
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.
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.
For each feature (hole, slot, mounting boss, fillet, chamfer, recess) state:
If anything has to fit something else, state the mating reference and the clearance. FDM defaults:
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.)
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.)
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."
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."
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 thicker, add holes, change the size, and round the corners" in a single turn is asking for trouble — if anything goes wrong, you don't know which change broke it. Edit, render, verify, edit again.
The best results today often come from combining the two AI approaches:
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.