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Tusi Couple Desk Toy: Eight Beads on Straight Slots Trace a Rolling Circle

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A 3D-printable desk toy recreates the Tusi couple, a geometric device first described by the Persian astronomer Nasir al-Din al-Tusi in 1247. Eight beads sit in straight slots on a dial, and pushing one causes all of them to move so their positions form a circle that appears to roll. The model is generated in Python using manifold3d, with the bead motion derived from a closed-form formula for a disc rolling inside a larger circle. The author describes checking part overlaps at multiple angles to catch collisions before printing, and the design is licensed for printing and sale.

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