Text-to-CAD examples: 10 prompts and what they produced

I cleared a Saturday afternoon for this. The plan was straightforward: write ten text-to-CAD prompts, ranging from embarrassingly simple to deliberately ambitious, run each one through Zoo.dev, download the STEP files, import them into Fusion 360, and document exactly what happened. No cherry-picking. No re-rolling until I got a good result. One prompt, one generation, one honest assessment.

I set up a clean folder on my desktop, made a pot of coffee that I optimistically assumed would last the whole session, and started typing. By prompt six the coffee was gone and by prompt ten I had strong opinions about which kinds of geometry AI can handle and which kinds it should probably leave alone for a few more years.

For the principles behind why some of these worked and others didn't, the text-to-CAD prompt engineering post covers the theory. For a catalog of prompts I've tested repeatedly and know work well, see best prompts for text-to-CAD. This post is the raw experiment.

Example 1: simple rectangular plate#

Prompt: "Rectangular plate, 100mm x 60mm x 4mm. Four 5.5mm through-holes, one at each corner, hole centers 8mm from each edge."

Result: good. The plate came back at 99.7mm x 60.2mm x 4.0mm, which is close enough that I'd use it directly for a prototype. All four holes were present, 5.5mm diameter, positioned within about 0.5mm of where I asked. The geometry was a single clean solid body with no internal faces or weirdness. Import into Fusion took two seconds. I measured everything, shrugged, and moved on.

This is text-to-CAD at its best. A simple prismatic part with basic features and explicit dimensions. If all your parts looked like this, you'd save real time every day.

Fix time: zero. I'd use it as-is for a non-critical application.

Example 2: L-bracket with mounting holes#

Prompt: "L-shaped bracket, 3mm thick. Vertical leg 45mm tall, 35mm wide. Horizontal leg 55mm long, 35mm wide. 3mm fillet on the inside bend. Three 4.2mm through-holes on the vertical leg: centered horizontally, spaced 15mm apart starting 7.5mm from the top edge. Two 4.2mm through-holes on the horizontal leg: centered across the width, 15mm and 40mm from the bend."

Result: good with minor issues. The overall shape and thickness were correct. Leg dimensions were within a millimeter. The bend fillet appeared at approximately the right radius. The vertical leg holes were all present and close to their specified positions, though the spacing was 14.5mm instead of 15mm. The horizontal leg holes were both there but one was about 2mm off from where I asked.

This is typical L-bracket performance. The AI handles the basic shape well, gets hole diameters right, but hole positions drift a bit, especially when there are several of them. I fixed the two position errors in about ninety seconds by re-drilling.

Fix time: about two minutes.

Example 3: electronics enclosure with separate lid#

Prompt: "Rectangular open-top box, outer dimensions 90mm x 60mm x 35mm. Wall thickness 2.5mm, bottom thickness 2.5mm. Four M3 screw bosses at the inside corners, boss outer diameter 6mm, boss height 30mm from inside bottom, with 2.5mm holes centered on each boss."

Note: I deliberately asked for just the box, not the lid. I've learned from painful experience that asking for "a box with a lid" produces fused garbage. I planned to generate the lid as a separate prompt.

Result: partial success. The box outer dimensions were close: 89.5mm x 60.3mm x 34.8mm. Wall thickness measured 2.5mm on three walls and about 2.2mm on the fourth, which is the kind of inconsistency that tells you the AI generated each wall somewhat independently rather than shelling a solid block. The bosses were the problem. Two appeared at roughly the right positions. The other two were missing entirely. The two that did appear had the correct outer diameter but the holes were 3mm instead of 2.5mm.

I've seen this pattern before. When features need to be precisely located at internal corners of a shell, the AI has trouble resolving the containment relationship. "Inside corners" is a harder concept than "5mm from the edge" because it requires understanding the interior geometry, not just the exterior.

The lid prompt (generated separately as "Flat lid, 90mm x 60mm x 2.5mm, with four 3.2mm through-holes on a pattern matching the box boss positions") produced a plate that was dimensionally correct but the hole pattern didn't match the bosses, because the bosses were in the wrong places to begin with. This is the cascading failure mode of multi-part text-to-CAD: if part A is wrong, part B built to match part A is wrong in a different way.

Fix time: about eight minutes, mostly rebuilding the bosses. At that point, I questioned whether I should have just modeled the box from scratch.

Example 4: spur gear#

Prompt: "Spur gear, module 2, 20 teeth, 14.5 degree pressure angle, 20mm face width, 10mm bore, keyway 3mm wide by 1.5mm deep."

Result: failed. What came back was a cylinder with bumps around the outside that resembled teeth in the way a child's drawing resembles a photograph. The tooth profile was not an involute curve. The root circles, tip circles, and pitch circles bore no relationship to the specified module. The bore was present and roughly 10mm. The keyway was a rectangular cut that was approximately correct in width but positioned off-center.

I expected this. Gears require precise mathematical curves that text-to-CAD tools don't generate. The involute tooth profile is defined by equations, not by description, and "module 2, 20 teeth" is a specification that needs to be computed, not interpreted. This is a case where a dedicated gear generator (even a free one like the GearGenerator add-in for Fusion 360) produces a perfect result in seconds, and text-to-CAD produces decoration.

Fix time: infinite. You can't fix this output. Start over with a real gear generator.

Example 5: mounting bracket with offset holes#

Prompt: "Flat rectangular bracket, 80mm x 40mm x 3mm. Four 4.2mm through-holes: two on the left half at positions (10, 10) and (10, 30) from the bottom-left corner, two on the right half at positions (70, 10) and (70, 30) from the bottom-left corner. 1mm chamfer on all edges."

Result: good with offsets. The plate dimensions were correct. All four holes were present and 4.2mm diameter. The positions were off by about 1-2mm each, which is consistent with what I've seen on other hole-position tests. The interesting part was the coordinate-style positioning. Using (x, y) coordinates from a corner seemed to work about as well as "mm from each edge" descriptions. The chamfers appeared on most edges but not all. Two edges on the bottom face were missed.

This was a deliberate test of coordinate-style prompting. The result suggests the AI can parse coordinates, but with the same positional drift that affects other reference styles. No magic bullet for positioning accuracy.

Fix time: about three minutes for hole positions and missing chamfers.

Example 6: pipe fitting adapter#

Prompt: "Cylindrical pipe adapter. One end: outer diameter 25mm, inner diameter 20mm, 15mm long. Other end: outer diameter 32mm, inner diameter 26mm, 15mm long. Transition section between the two ends: 10mm long, smooth taper on outer diameter, stepped inner diameter changing at the midpoint of the transition."

Result: poor. The AI produced a vaguely cylindrical shape with two different diameters, but the transition section was a mess. Instead of a smooth taper on the outside with a stepped bore on the inside, I got something that looked like two cylinders crudely joined with a fillet that was trying to do both jobs at once. The inner bore was continuous rather than stepped. The outer taper existed but wasn't smooth in the way I'd call "machineable."

Pipe fittings involve features that reference each other across a transition, and the AI struggled with the two different things happening (taper outside, step inside) in the same region. The outer and inner profiles need to be generated with different operations that share the same reference axis, and the AI apparently doesn't decompose the problem that way.

Fix time: longer than modeling from scratch. I scrapped it after five minutes of trying to salvage the transition section.

Example 7: phone stand#

Prompt: "Phone stand. Base plate 80mm x 60mm x 5mm. Angled support rising from one long edge, 60mm wide, 3mm thick, angled at 70 degrees from horizontal, 100mm long along the angle. 5mm lip at the bottom of the angled support, perpendicular to the support surface, to hold the phone. 3mm fillet where the angled support meets the base."

Result: decent. This was the prompt where I expected the AI to struggle with the angle, and it surprised me. The base plate was correct. The angled support appeared at something close to 70 degrees (I measured approximately 68 degrees). It was the right width and roughly the right length. The lip at the bottom was present, which I wasn't confident about, though it was 4mm instead of 5mm. The fillet at the base joint was there and approximately correct.

The overall shape would work as a phone stand. It wouldn't win any design awards, but if I printed it in PLA it would hold a phone. The angle being 2 degrees off doesn't matter for this application. The lip being 1mm short doesn't matter. This is the kind of part where "close enough" is genuinely close enough.

Fix time: one minute to adjust the lip height. I'd use the rest as-is for 3D printing.

Example 8: heat sink#

Prompt: "Rectangular heat sink base, 40mm x 40mm x 3mm. Nine rectangular fins on the top surface, each 40mm long, 1mm thick, 15mm tall, evenly spaced across the 40mm width."

Result: partial. The base plate was correct. Fins appeared on top, which was good. But only seven fins were generated instead of nine, and the spacing wasn't even. The fins that did appear were approximately 1mm thick and approximately 15mm tall, with two of them visibly shorter than the others. The fin thickness varied between about 0.8mm and 1.2mm.

Heat sinks test the AI's ability to generate repeated thin features, and the result tells you that repetition at fine dimensions is unreliable. The AI seems to lose count or lose dimensional consistency after about six or seven repeated features. For a heat sink you'd actually use, you'd want a fin pattern that's precise enough to calculate thermal resistance, and this isn't it.

Fix time: about seven minutes to delete the uneven fins and recreate them as a proper rectangular pattern. At that point I was basically re-modeling everything above the base plate.

Example 9: simple hinge#

Prompt: "Two-piece hinge. Leaf one: 40mm x 30mm x 2mm flat plate with two cylindrical knuckles on one long edge, each knuckle 6mm outer diameter, 3mm inner diameter (for hinge pin), 8mm long, positioned 5mm from each end of the edge. Leaf two: 40mm x 30mm x 2mm flat plate with one cylindrical knuckle centered on the same long edge, 6mm outer diameter, 3mm inner diameter, 14mm long, positioned to interleave with leaf one's knuckles."

Result: failed. I knew this was ambitious. A hinge requires two parts that mate together with precise interleaving geometry, and I asked for it in a single prompt despite my own advice about one part per prompt. I wanted to see what would happen.

What happened was a single solid body that was vaguely hinge-shaped, with knuckle-like cylinders on one edge that were fused to the plate rather than being separate interleaving pieces. There were two plates, but they were joined at the hinge line as one body. The cylinders existed but didn't have bores. The whole thing was art, not engineering.

Even generating the leaves separately would be tricky because the interleaving geometry requires precise positional coordination between two parts. This is the kind of thing that's easy to model in CAD (sketch, revolve, pattern) but hard to describe in text because the relationship between the two parts is geometric, not verbal.

Fix time: not attempted. This is a from-scratch job.

Example 10: cable clip#

Prompt: "C-shaped cable clip for 6mm cable. Outer diameter 10mm, inner diameter 6.5mm, wall thickness 1.75mm, opening gap 4mm at the top. Flat mounting tab extending 8mm below the clip, 10mm wide, 2mm thick, with a 3.5mm through-hole centered on the tab, 4mm from the bottom edge."

Result: good. The C-profile appeared with the correct proportions. Inner diameter measured 6.5mm. Outer diameter measured about 10.2mm, close enough. The gap was approximately 4mm. The mounting tab was present, correct width, correct thickness, with the hole in the right place. The transition from the curved clip to the flat tab was clean.

Cable clips are one of text-to-CAD's success stories. The geometry is simple, the features are well-defined, and there aren't many things to get wrong. I've printed clips from text-to-CAD output several times and they work fine. The tolerances don't need to be tight for cable management, and the worst that happens if the gap is slightly off is you flex the clip a little more when snapping the cable in.

Fix time: zero. Exported directly for 3D printing.

What the results say#

Out of ten prompts, three produced output I'd use with zero or minimal fixes (plate, cable clip, phone stand). Two produced output that needed moderate fixes but were faster than starting from scratch (L-bracket, mounting bracket). Two produced output that required enough fixing to question the time savings (enclosure, heat sink). Three produced output that was unusable (gear, pipe fitting, hinge).

The pattern is clear, and it's the same pattern I see every time I use these tools. Simple prismatic geometry with holes, chamfers, and fillets works well. Basic brackets and plates are reliable. Features that involve precise mathematical curves (gears), multi-body relationships (hinges), complex transitions (pipe fittings), or many repeated thin elements (heat sink fins) don't work.

If your parts live in the bracket-plate-clip universe, text-to-CAD is a genuine time saver. If your parts live in the gear-fitting-assembly universe, save yourself the trouble and model them in CAD directly. Knowing which universe your part lives in before you start typing is the most important skill in text-to-CAD, and it has nothing to do with prompt engineering.

The text-to-CAD guide covers the broader technology and where it's headed. The best text-to-CAD tools post compares the platforms if you want to try this yourself. And the best prompts for text-to-CAD post collects the prompts I've found most reliable for the categories that actually work.

My Saturday gave me seven STEP files worth keeping, three worth deleting, and one cold pot of coffee. By the standards of experimental engineering, that's a pretty good ratio. By the standards of text-to-CAD, it's about what you should expect. The tools are useful for the things they're good at and hopeless at the things they're not, and that line between useful and hopeless is drawn exactly where simple geometry ends and real complexity begins.