Kinetic Koi — Swimming, Turning & Wagging in Motion

This project features a two mechanical koi fish that can "swim" around a pond and move their heads, tails, and fins.

The design is built around two circular boards connected by a turntable bearing. The lower board holds a fixed central gear, while the upper board rotates and carries two side gears that orbit the center. Each side gear controls the movement of a single koi fish.

The koi's head and tail motions are driven by levers connected to the rotating side gears, while the fin movement is triggered by the turning of the koi’s head.

Larger side gears produce bigger movements, though the result also depends on the positioning of the motion-control levers. The size ratio between the center gear and the side gears is equally critical—a larger center gear relative to the side gears results in more frequent but smaller koi movements per full rotation.

Feel free to experiment with gear sizes and lever positions to achieve the desired motion effect.

Material

1. Filament for 3D printer
2. One 4-Inch Lazy Susan Turntable Bearings, 5/16” Thick
3. Two M3x120mm or M3 x 150mm Fully Threaded Rods
4. M3 screws of length 15mm, 25mm, 35mm & M3 nuts
5. Assorted Round Spacers
6. Foam board/Wooden Boards
7. Clear plastic sheets (No need to buy! I upcycled clear salad box)
8. Clear tape and double-sided tape
9. Calligraphy rice paper
10. 10mm open jump rings

Optional (for decoration):

1. Spray paint
2. Epoxy Resin & color
3. Watercolor
4. Glitter glue

Tools

1. 3D printer & Filament

Print the following gears and accessories. We used clear filaments for this project.

1 x KoiCenterGear

1 x KoiCenterPlateGear (remixed from https://www.thingiverse.com/thing:50851)

2 x KoiGears

2 x KoiHeads

2 x KoiSupportRipple (remixed from https://www.thingiverse.com/thing:2542801)

For the top board, we used foam board. Cut out a donut shape with an inner radius of 40 mm and an outer radius of 215 mm, as shown.

Most compasses will not be large enough to draw the outer circle. We used a pin and string to draw it instead.

Use two screws to attach the turntable gear to the printed center plate, as shown.

Mount the side-motion gear by securing its center to both the printed center plate and the turntable gear.

Attach one end of the long lever to a side hole along the edge of the side-motion gear, using a spacer as shown. Repeat.

Insert the rod into the ripple-support structure.

Carefully adjust both the lever alignment and the placement of the ripple-support structure on the top board to ensure the side-motion gears move smoothly around the center gear.

Once we finalize the gear position, uninstall the gears so we can decorate the boards.

Spray paint both the top and bottom board in silver. This provides a better reflective base and results in more water-like effects.

Then, epoxy pour the boards with royal blue, blue, lake green, and green (or any other cool colors you have!).

Tip: When we adjusted the gear position earlier, we might have created extra holes and scratches on the surface of the top board. This is the time to fix those holes - apply masking tape before spraying paint.

Cut out a circular board with a radius of 370 mm.

Attach this bottom board to the turntable, which is already connected to the top board (see illustration).

Mount the center gears to the bottom board as shown. Add spacers as needed to ensure the center gear and side gears are aligned at the same level.

We tape 10mm-wide clear plastic strips to form the koi fish structure, as shown in the illustration.

Cut and tape plastic strips to make koi body (front), koi body 2 (back), koi head, and koi tail as shown.

Note: The grey line in each koi body marks an additional support strip.

For the koi head side: use one strip, split it lengthwise down the middle while leaving one end uncut. Loop the split ends back to the intact end to form the head shape.

Drill holes as shown for part connections.

Assembly:

1. Mount the koi head onto the ripple-support rod.
2. Secure the sides of the koi head to the 3D-printed koi head piece with rings.
3. Attach the koi body (front) to the next rod.
4. Fix koi body 2 (back) to the supporting lever with a screw.
5. Connect the koi head to the front body, then join the two body parts together with rings

To create a flapping motion, the fins are mounted to the koi body (front) with an extension strip that extends into the koi head. As the koi turns its head, this little extra strip shifts and causes the fins to flip naturally.

Tip: The fins are attached later during the decoration stage, but the motion mechanism is explained here for reference.

Note: To create a colorful koi fish, you can paint the entire sheet of rice paper with watercolor before assembly—this is the easier option if you want a koi in a different overall color. If you only want selective highlights, you can always paint the koi after construction.

Fold the rice paper and cut out small circles as shown; these will serve as scales.

Tape the scales onto the body, starting from the tail and working forward.

Draw and cut head pieces according to your koi head structure, then mount them as shown.

Form the tail and fins using supporting plastic strips, then decorate as illustrated.

(Note: Fin motion is implemented here for inner fins of both koi fish)

Add final details such as gills, eyes, and mouth as desired.