How to Build a Simple 3D Model in CAD

When designing a part to fit another, accuracy is everything. I used calipers for precision, but a simple ruler would’ve been enough for this project. The goal was to understand three things: Lens diameter — Both the viewing and photo lenses measure 29.9 mm. Convenient. To avoid the risk of scratching the brittle lenses by measuring center-to-center, I measured the outer rims (14.2 mm apart) and added the lens radii to get a center distance of 44.1 mm. Lens height — Using calipers again, I measured the depth at 6.8 mm. Metric or imperial—use whatever you’re comfortable with. Precision is universal.

I use FreeCAD—a powerful, open-source CAD tool. It's capable, and more importantly, accessible. (link at the bottom)

Use this guide to find feature tools used in the following steps.

I drew the first circle at the origin (the lower lens). Using origin provide a strong reference point and help constrain a sketch. The second circle (upper lens) snaps into place along the vertical axis. I set the bottom circle to 29.9 mm and used the “equal” constraint (E) to match the second one. Then I set the vertical distance between centers to 44.1 mm. Once both circles turned green, I knew the sketch was fully constrained—all positions and dimensions defined.

I then created new circles around each lens with a 4 mm larger diameter to provide a 2 mm wall thickness. Again, I used the "equal" constraint to keep both consistent. Since FreeCAD doesn’t allow the extrusion of two disconnected bodies, I added a bridge between the two shapes.

With the sketch bridged and closed, I extruded the cap body to 8 mm tall. This gives a snug fit over the 6.8 mm lens height, with added wall thickness for strength.

Switching to the top view (press 2), I selected the top face and opened a new sketch. Using the Reference Geometry tool, I locked into the original center points of the lenses. I sketched circles slightly larger than the lens size to account for printer tolerances. My Bambu Lab A1 printer performs best with an additional 0.25 mm of clearance, so I set the new diameter to 30.15 mm. After sketching, I used the Pocket feature to cut 6 mm deep holes, forming a recessed space for the lenses to sit in.

Should we have a little fun with the design? I switched to the bottom view (press 5) and started a new sketch on the bottom face. Eyes: Two concentric circles (8 mm outer, 7 mm inner), spaced 14 mm apart. Eyebrows: Rectangles, tilted and constrained horizontally. Mouth / Mustache: I sketched half a stylized mustache and mirrored it using the vertical axis. I didn’t fully constrain this sketch—a rare exception for me, but worth it for the visual expression. Using the Pocket tool again, I cut the design 0.8 mm deep, which creates roughly four visible layers when printed at 0.2 mm layer height.

With the part complete, I selected the Body and exported it as a STEP file. Although STL is standard, BambuStudio supports STEP files directly, which results in smoother surfaces.

The part printed cleanly. The material has a beautiful speckled color, and the filament lines align perfectly with the mustache design—almost like angry teeth. But more importantly, the cap fits. It’s snug enough to stay on, without putting pressure on the lenses. If I were to redo it, I might bevel the edge slightly more. But as it is, the cap works, protects the glass, and adds personality to my camera.