Crocodile and Fish Automaton Box: A Cam Mechanism Designed in Rhino 8
🌏 繁體中文版：[學習3D 機械設計軟體] - 齒輪小魚機械盒：鱷魚咬魚的機構設計
This crank-operated automaton box was designed in Rhino 8: turn the handle on the side and the crocodile on the lid snaps its jaws while a small fish bobs up and down beside it. This post walks through the mechanism, the drawings, and the workflow for building this kind of piece in Rhino.
The mechanism: one shaft, two motions
A single horizontal shaft runs through the box, with the handle on one end. Turning the shaft drives two mechanisms at once:
| Motion | Mechanism | How it works |
|---|---|---|
| Fish bobs up and down | Cam | A cam on the shaft lifts the thin rod holding the fish; once the lobe passes, the fish drops back down |
| Crocodile bites | Crank + pin lever | The shaft swings a lever that pushes the upper jaw open and lets it close |
Making the box semi-transparent shows how the shaft, cam, and lever line up:
Rendering the sub-assemblies
For a mechanism like this, it helps to model each sub-assembly separately first, so dimensions and engagement can be checked in isolation.
A freshly built model is plain grey (a "clay" model). Assigning materials to each part (wood grain, plastic, paint), then setting up lights and camera angles, produces renders that look close to product photos, so the appearance can be settled before cutting a single prototype.
Orthographic drawings
Renders show what the piece looks like; engineering drawings are what you actually build from. The overall size is 200 × 120 × 80 mm (L × H × W).
Without redrawing anything, the same Rhino model produces all three kinds of files needed to go from design to fabrication:
- Renders: check the look and show the structure.
- Orthographic drawings: Rhino's
Make2Dcommand projects the 3D model into front, top, and side line drawings, which can then be dimensioned. - Fabrication files: flat part outlines export as DXF for laser cutting; solids export as STL for 3D printing.
Why Rhino 8 for this one
Rhino is built around NURBS surface modeling: curves and surfaces are defined mathematically, so even a strongly curved shape stays a true curve instead of being approximated by lots of small flat facets. That makes it especially good at styling and free-form surfaces. Compared with parametric CAD such as Inventor or SolidWorks, the split looks roughly like this:
| If you need to… | Better fit |
|---|---|
| Iterate on part dimensions, build assemblies, manage fits and tolerances, produce machining drawings | Parametric CAD: Inventor, SolidWorks |
| Sculpt surfaces, style the form, make photorealistic renders | Rhino |
I pick the tool by the job. The geared flapping-wing bird needed motion simulation, so it was done in Inventor. The fish box is a styled piece made of wooden plates and discs, where the look and the drawings matter most, so Rhino was the better fit.
Takeaways
The design exercise here is "one input, several outputs": a cam and a lever share the same shaft, so two motions happen together from one crank. Rhino's strength is that a single model yields renders, engineering drawings, and laser-cut files at the same time, so a design change doesn't have to be redrawn in three places. When a mechanism needs lots of tolerance work or motion simulation, Inventor is still my main tool.
This post is adapted from the Traditional Chinese original on Vic's Iteration Notes.
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