Magic Octagon Illusion Coin: Two-Colour Arrow Flip Trick with Flush Inlay

What it is: a two-colour Magic Octagon illusion coin of my own design, styled as a road sign from the Bureau of Wrong Directions. One face has a big arrow, the other says DETOUR over a sideways arrow, both inside a hazard-stripe border. The two arrows are always 90 degrees apart, yet turn the coin 45 degrees before you flip it and the back arrow points the same way as the front, or the opposite way.

Key points

• Two colours, flush inlay: the black art is a separate part sunk 0.8 mm into the yellow body, so both faces are flat and smooth. No paint, no colour change by layer.

• Needs a multi-material printer: the colour changes within the same layers (both faces), so a single-nozzle filament swap at a layer height will not work. Any AMS, MMU, toolchanger (Snapmaker U1, Prusa XL) or IDEX printer is fine.

• No supports, prints flat; 3 mm outer brim recommended.

• Size: 50 mm across the flats, 4.8 mm thick, 0.6 mm chamfer on both edges.

• Smallest feature 1.0 mm: every black stroke and yellow gap is at least 1.0 mm (checked on cross-sections of the model), so a 0.4 mm nozzle resolves the text and stripes.

How to perform the trick

1. Hold the coin by its top and bottom points with the front arrow pointing up. Spin it over left to right: the back arrow points left.

2. Spin it back, turn it 45 degrees counter-clockwise (arrow now up-left) and ask where the back arrow will point. Most people say down-left. Pinch the new top and bottom points and spin: it points up-left, the same way as the front.

3. Start again, turn it 45 degrees clockwise (arrow up-right) and spin: the back points down-left, the opposite way.

Why it works: a flip about the vertical axis mirrors the back face. Rotating the coin before the flip turns the front arrow one way and the flipped back arrow the opposite way, so a 45 degree rotation shifts the angle between them by 90 degrees (90 becomes 0 or 180). People expect it to stay at 90.

Parts list

• 1 x coin body, yellow PLA (magic_octagon_bureau_body_yellow_x1.stl)

• 1 x inlay, black PLA (magic_octagon_bureau_inlay_black_x1.stl)

• Nothing else: no glue, no paint, no hardware

Print settings

• PLA, 0.4 mm nozzle, 0.2 mm layers (the inlay is exactly 4 layers deep on each face)

• 2 walls, 5 top and 3 bottom layers, 15 % infill

• No supports, 3 mm outer brim (gap 0.3 mm)

• My print: Snapmaker U1, yellow PLA body and black PLA inlay, about 27 min and 7.1 g

Files

• magic_octagon_bureau_two_colour_x1.3mf: body and inlay as one object with two parts, body on filament 1 and inlay on filament 2. Sliced as is in Snapmaker Orca (the U1 slicer); other OrcaSlicer-based slicers read the same part settings. Pick your two colours. If your slicer drops the part colours, use the two STLs.

• magic_octagon_bureau_body_yellow_x1.stl and magic_octagon_bureau_inlay_black_x1.stl: the same two parts as separate STLs in print position. Import both together and answer Yes when the slicer asks to load them as one multi-part object, then give each part its own filament.

• No STEP: the art is generated as a mesh (Python, manifold3d), so there is no clean solid to export.

The trick: it performs the classic Magic Octagon arrow-flip puzzle.

AI disclosure

Designed by me with AI-assisted CAD scripts (Python generators written with an AI coding assistant), printed and tested on my own printer. The first photos are real prints; the last two images are CAD renders labelled RENDER. No AI image or mesh generator was used.