Next, we modeled the chess pieces: King, Queen, Rook, Knight, Bishop, and Pawn. They are about 40mm ± 15mm tall with 20mm diameter bases. We gave them similar profiles so the gripper can grab any of them reliably. Each piece has a 12mm hole at the bottom for a magnet.

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Here are some of our design for the chess piece:

KING	QUEEN	KNIGHT

PAWN	BISHOP	ROOK

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We used magnets to keep the pieces on the board and help them self-center. We needed 128 magnets total: 32 for the pieces, 64 for the squares, and 32 for the capture zone. We chose 12mm x 3mm neodymium magnets with a center hole so we could screw them in instead of using glue

Because neodymium is so strong, we had to find the right amount of plastic to put between the board magnets and the piece magnets. If they touch directly, they are too hard to pull apart. We needed enough grip to hold the pieces upside down during movement, but not so much that the robot couldn't lift them.

We built a test bench with plastic thicknesses ranging from 1mm to 5mm. We attached a ring to a piece and used a luggage scale to measure the force needed to pull it off. We also checked if the pieces stayed attached during abrupt movements or when held upside down

After conducting our tests, we obtained the following results:

(The infamous inverse square law is back at it again, see our log on the small hydraulic arm •_•  )

The tests showed that 3mm of plastic was the best balance. The pieces stayed on while upside down or moving, but the pull force was still manageable

With this decision, we updated the sphere design accordingly to incorporate the 3mm thickness.