Text-to-CAD tutorial: from prompt to STEP file

I needed a mounting bracket for a small stepper motor on a test rig I was putting together. Nothing production-critical. Aluminum, 3mm stock, a few mounting holes, a couple of slots for adjustment. The kind of part that takes me about fifteen minutes to model from scratch in Fusion 360 on a good day, and about twenty-five minutes on a day when the timeline decides to misbehave. I figured this was the perfect candidate for a text-to-CAD walkthrough: simple enough to succeed, complex enough to show where the AI stumbles.

I did the whole thing from prompt to finished STEP file, and it took about fifteen minutes total. Five of those minutes were the AI doing its part. The other ten were me doing mine. That ratio tells you something honest about where this technology is right now.

If you've read the how to use text-to-CAD post, you know the general workflow. This is the specific version, with an actual part, actual mistakes, and the actual fixes.

The part#

Here's what I needed: a flat mounting bracket for a NEMA 17 stepper motor. The NEMA 17 has a standard bolt pattern of 31mm between hole centers, arranged in a square, with a central bore of 22mm for the motor boss. I wanted the bracket to be 60mm x 60mm, 3mm thick, with the four M3 mounting holes on the 31mm pattern centered on the plate, plus the 22mm central bore, plus two 5mm slots on opposite edges for mounting the bracket itself to the test rig with some lateral adjustment.

This is a part I've modeled dozens of times. The sketch takes about two minutes, the extrude is one click, and the holes are a rectangular pattern. If text-to-CAD can't handle this, it can't handle anything.

Writing the prompt#

I opened Zoo.dev, signed in with the free tier, and sat there for about thirty seconds thinking about how to phrase this. That thirty seconds matters more than people realize. A vague prompt produces vague geometry. A specific prompt produces geometry you can actually use. I've written more about this in text-to-CAD prompt engineering, but here's the short version: every dimension you leave out is a dimension the AI guesses, and it guesses wrong more often than you'd like.

Here's the prompt I used:

"Flat rectangular plate, 60mm x 60mm x 3mm. Central through-hole, 22mm diameter, centered on plate. Four M3 through-holes (3.2mm diameter) arranged in a square pattern, 31mm center-to-center, centered on the plate. Two 5mm wide slots on opposite edges (left and right), each 15mm long, centered vertically on the edge, 3mm from the plate edge to the near side of the slot."

That's specific. I gave dimensions in millimeters for everything. I named the features by type (through-hole, slot). I described the pattern and the positioning. I specified the slot relationship to the edge. I left nothing for the AI to infer that I could specify directly.

Generating the model#

I pasted the prompt and clicked generate. Zoo's server chewed on it for about twelve seconds, which I've learned is normal for a part of this complexity. The progress indicator does its thing and then a 3D preview appears in the browser.

The preview looked roughly right at first glance. A rectangular plate. A big hole in the middle. Smaller holes around it in what appeared to be a square pattern. I could see features on the edges that might be slots. Proportions seemed reasonable.

I downloaded the STEP file. This is always the moment of truth, because the browser preview and the actual geometry can tell different stories. A preview might look fine while hiding internal faces, bad topology, or dimensions that are close enough to fool your eyes but wrong enough to matter.

Importing into Fusion 360#

I opened Fusion 360, hit Insert > Insert Mesh/File, and selected the STEP file from my downloads folder. Fusion thought about it for a couple of seconds and then the body appeared in the viewport. One solid body in the browser tree. No feature history, obviously, since this was imported geometry, not modeled natively. Just a dumb solid I could measure and edit.

First thing I did was rotate it slowly and look at every face. The plate was there. The central hole was there. Four smaller holes were there. The edge features were there. No obviously missing features, no mystery surfaces, no garbage geometry visible in wireframe mode.

Second thing: measure.

What the AI got right#

The plate itself was very close. I measured it at 59.8mm x 60.1mm x 3.0mm. The thickness was dead on. The length and width were within a fraction of a millimeter, which for an imported starting point is fine. I could work with this.

The central bore measured 22.0mm, which was exactly what I asked for. I'll take that win.

The M3 mounting holes were 3.2mm diameter, which is correct for M3 clearance. They were arranged in a square pattern centered on the plate, which was correct.

What the AI got wrong#

The mounting hole pattern spacing. I asked for 31mm center-to-center. What I measured was closer to 30mm. A millimeter off on a bolt pattern means the motor doesn't mount, and there's no "close enough" when you're trying to align a stepper motor to a lead screw. This is the kind of error that looks invisible on screen and becomes very visible on the bench.

The slots. This is where things got more creative. I asked for two 5mm wide slots on opposite edges, each 15mm long, centered vertically, 3mm from the edge. What I got was two slots that were approximately 5mm wide, approximately 14mm long, and positioned about 4mm from the edge instead of 3mm. The vertical centering was close but not exact. Every dimension was in the right ballpark and wrong in the details.

One slot also had a slightly different end radius than the other, which is the kind of inconsistency that happens when the AI generates each feature somewhat independently rather than applying a proper pattern or mirror operation. A human would sketch one slot and mirror it. The AI apparently generated each one as a separate thought.

Fixing the model#

This is where the tutorial becomes a CAD tutorial, because fixing text-to-CAD output is just regular CAD work on an imported body.

Step one: fix the bolt pattern. I created a new sketch on the top face of the plate. Drew a construction point at the center (which I could snap to by selecting the circular edge of the central bore). From that center, I created a rectangular pattern of four points at 31mm spacing, centered. Then I used the Hole command to place 3.2mm through-holes at those four points. After confirming the new holes were correct, I went back and filled in the original incorrect holes. In Fusion, you can do this by sketching circles on the face at the old hole locations and using Extrude to add material. Or you can use Press/Pull on the cylindrical faces of the holes. Either way, about ninety seconds to fix.

Step two: fix the slots. I deleted both slots by sketching over them and extruding to fill, then created a new sketch on the top face. Drew one slot at the correct dimensions: 5mm wide, 15mm long, centered vertically on the left edge, with the near side 3mm from the edge. Extruded it as a cut through the plate. Then I mirrored that cut feature across the center plane to create the matching slot on the right edge. Both slots now identical, both correctly positioned. About two minutes of work.

Step three: verify everything. I measured all critical dimensions again. Plate 59.8 x 60.1 x 3.0, which I decided was close enough for a test rig bracket (on a production part, I'd fix those too). Central bore 22.0. Mounting holes 3.2mm on a 31.0mm pattern. Slots 5.0mm x 15.0mm, 3.0mm from edge, centered. All correct.

Step four: export. File > Export > STEP. Done.

Time breakdown#

Here's how the fifteen minutes broke down:

Writing the prompt: 1 minute. Generating in Zoo: about 15 seconds. Downloading and importing: 30 seconds. Inspection and measurement: 2 minutes. Fixing the bolt pattern: 90 seconds. Fixing the slots: 2 minutes. Final verification: 1 minute. Export: 15 seconds. Random pausing to sip coffee and think about whether this was faster than modeling from scratch: about 7 minutes spread throughout.

If I stripped out the coffee pauses and the second-guessing, the actual working time was about eight minutes. Modeling this part from scratch in Fusion would take me about twelve to fifteen minutes, assuming no timeline drama. So the text-to-CAD approach saved me maybe five minutes on this specific part, with the caveat that those five minutes were distributed in a strange way: the AI did the bulk modeling fast, and then I did targeted fixes that required knowing what was wrong and how to fix it.

You still need to know how to use CAD software. The AI doesn't remove that requirement. It just shifts the work from "build from scratch" to "inspect and fix." Whether that's faster depends on the part, the quality of the AI output, and how fast you are at both approaches.

What I'd do differently#

The prompt was decent but not perfect. If I were doing this again, I'd add a line specifying that the slot pattern should be symmetric about the center. The AI apparently didn't infer that, and the asymmetric slot positions cost me an extra minute of fixing.

I'd also explicitly state "all features through the full 3mm thickness" because on other parts I've seen the AI generate blind holes or partial cuts when I meant through-alls. Being redundantly specific costs nothing in the prompt and saves time in the cleanup.

For this specific part, I probably wouldn't bother with text-to-CAD next time. It's simple enough that modeling from scratch is almost as fast, and I'd have parametric history from the start instead of an imported body I can't roll back. Where text-to-CAD pays off is when I need to explore several variations quickly, or when the part is slightly more complex and the first-draft geometry saves more than a few minutes.

The text-to-CAD guide has a more honest assessment of where the technology helps and where it doesn't. For this tutorial, the takeaway is concrete: text-to-CAD gave me an 85% correct starting point in seconds, and I spent ten minutes getting it to 100%. That's the real ratio right now.

Trying the same part in CADAgent#

After the Zoo attempt, I wanted to see how CADAgent handled the same prompt. I had the Fusion 360 add-in installed already and my Anthropic API key configured.

I pasted the same prompt into the CADAgent panel and hit generate. Instead of downloading a STEP file, I watched Fusion 360 build the part in real time. A sketch appeared on the XY plane. A rectangle got drawn. An extrude operation fired. Then came the circles for the holes, then the extrude cuts, then the slots.

The result was different from Zoo's in an important way: it had a timeline. I could click on any operation in the feature history and edit it. The sketch dimensions were there, editable. The extrude depths were parameters I could change.

The bolt pattern came out closer to correct than Zoo's, measuring about 30.8mm instead of the 31mm I asked for. Still off, but closer. The slots were more consistent because CADAgent had the good sense to create the second one as a mirror of the first, which is what a human modeler would do.

The downside was speed. CADAgent took about forty-five seconds to generate, compared to Zoo's twelve. And one of the extrude operations threw a warning that I had to dismiss before the generation continued. Minor, but it breaks the flow.

For a part like this, CADAgent's parametric output is more useful than Zoo's dumb solid. I could fix the bolt pattern by editing the sketch dimension directly instead of filling and re-drilling holes. That's a thirty-second fix instead of a ninety-second fix, which adds up over a day of iterating on designs.

The honest takeaway#

This tutorial is about a specific part. The NEMA 17 bracket is simple, well-defined, and exactly the kind of geometry that text-to-CAD handles best. If your parts look like this, the tools will save you time. If your parts are more complex, if they involve sheet metal bends, organic surfaces, multi-body assemblies, or tight tolerances, your mileage will drop fast.

The process itself is straightforward. Write a specific prompt. Generate. Download. Import. Measure everything. Fix what's wrong. Export. The AI handles the first draft. You handle the quality. Right now, that division of labor saves me a few minutes per part on simple geometry and nothing on complex geometry. It's a tool, not a replacement for knowing what you're doing.

If you want to get better at the prompt-writing part, the text-to-CAD prompt engineering guide goes into more detail on what makes a good prompt. For understanding the STEP file side of things, including why STEP matters and how to handle import issues, that's covered separately.

I still model most parts from scratch. But for the bracket-and-plate class of geometry, a twelve-second first draft from Zoo beats staring at a blank sketch, even if I have to fix two holes and a slot afterward.