Manta ray automaton

A desk automaton that swims on the spot. A motor in the base turns a stack of printed gears, two cranks turn that into a beat, and the beat runs out along 7 hinged panels to wingtips 8.1 in apart.

8.1
in from tip to tip, and 6.3 in tall
37
printed parts, from the 44-tooth gears down to the staples
5.8 oz
of PETG on the plate
19.3
hours of printing
The manta ray automaton on a dark surface: a teal base box, a fan of coloured pushrods, and a segmented wing whose tips curve upward

Follow the drive.

One motor, 37 printed parts.

A gear stack in the base, two cranks above it, 8 rods and 7 hinged panels that make up the wing. Keep scrolling; the camera follows the drive out to the tips.

It starts in a box.

3.1 × 2.5 × 1.3 in, with a lid and a printed cap where the motor cable leaves. A 17-tooth gear on the motor output turns the 39-tooth wheel on the end of the drive shaft.

44 teeth, meshed into 44.

The far end of that shaft is a 13-tooth pinion driving one of two 46 mm gears. The pair mesh each other, so they turn opposite ways and both wings beat together. Module 1 the whole way through, on 44.5 mm centres.

Four pins on one crank.

Each gear carries a crank, and each crank carries four pins 10 mm apart along its axis. No two sit at the same radius and no two sit at the same angle.

One rod per pin.

8 rods, each 3.4 mm thick, staggered 10 mm fore and aft so they pass each other without touching. Every one ends in a printed staple that pins it to a panel.

7 panels, 8 hinges.

One body panel down the middle, then an inner, an outer and a tip on each side, 8.1 in from one tip to the other. Because the pins are spread around the crank, the panels never reach the top of the stroke together.

Nothing in it computes the wave.

It is drilled into the crank. Below are the four pins of the left crank, drawn at the radius and angle they were measured at, and the height each one reaches as the crank turns. Drag the slider to turn it.

The crank along its axis. Pin radius is to scale; the 10 mm of spacing along the shaft is not.
Each pin's height above the crank axis, through two turns. The four peaks never line up.
0°
Measured off the assembled left crank
Panel it drivesPin radiusWhere it sits on the crank
Body panel10.2 mmthe reference pin
Inner panel5.8 mm113° round from the body pin
Outer panel9.4 mma further 19°
Wing tip16.1 mma further 11°

The three wing pins are 19° and 11° apart, 30° from the innermost panel out to the tip, so no two of them top out at the same moment and the stroke arrives along the wing instead of all at once. The pin that drives the centre body panel sits 113° away from the nearest of the three, which is why the middle of the animal is doing something different from its wings for most of the turn.

Standing still, it is a printed animal.

Each panel carries its own relief, modelled into the top face rather than added to it, and the 7 of them read as one skin once they are hinged together. The two tips are panels in their own right, each with its own rod and hinge, which is why the outline keeps moving all the way out to the point.

The mechanism underneath is not hidden. The gears, the cranks and all 8 rods stay in view, which is most of the reason to have one on a desk: the thing that makes the motion is the thing you look at.

The automaton seen from above: seven coloured panels form a manta silhouette, each with a different cast relief pattern
From above, the whole outline is panels: teal and purple on the left wing, blue and green on the right, the clay-coloured body running down the middle with its tail.
The automaton on a pale wooden table in a living room, wingtips lifted
8.1 × 5.4 × 6.3 in overall, so it takes about as much table as a hardback lying on its side.
The automaton from behind: two large spoked gears meshed together above the base, with pushrods fanning up to the wing
From behind: the two 46 mm gears meshed to each other, and the rods opening out to the panels.

What you print

37
printed parts
5.8 oz
of PETG
19.3 h
on the printer
Overall
8.1 × 5.4 × 6.3 in
Wingspan
8.1 in, tip to tip
Base
3.1 × 2.5 × 1.3 in, with a lid and a cable cap
Weight
5.8 oz of PETG
Printing
19.3 h, 37 parts, longest 136 mm
Bed
every part fits 150 × 150 mm
Gears
17 into 39, 13 into 44, 44 into 44; module 1
Reduction
7.8 motor turns to one wing beat
Cranks
2, 4 pins each, 10 mm apart
Pin radius
5.8 to 16.1 mm
Pushrods
8 at 3.4 mm thick, into 8 printed staples
Wing
7 panels on 8 hinges

The two big gears run on 44.5 mm centres, 0.45 mm wider than their tooth counts strictly ask for. That gap is the clearance that lets a printed gear train turn straight off the plate without anyone filing a tooth.

Drive the crank and the wave starts in the middle and leaves at the tips. Nothing in there decides that. It was drilled in.