nautilus paper-knife

A paper knife with the blade hidden inside the “shell.”
When an envelope is passed through the slit on the side, a blade attached to the inner spiral enters under the flap, and opens the envelope. A 3D printer was used to achieve the complex shape of the exterior morphing into the blade, but because of the limited sharpness of the blade, repeated experiments were performed for the cutter’s right shape, angle, and distance from the slit.
The name of the paper knife “Nautilus” comes from the appearance like a spiral seashell of nautilus or ammonite.  

Masahiro Ohgami
Akihiro Yoshida