Repelling Puzzle Box

You can do a lot of cool things with magnets. I build a fairly simple puzzle box which uses hidden magnets as a tool to open the box. The box is 3D printed as is the locking mechanism.

Spoiler: By looking at the construction below you will of course realize the solution to opening the box. So build it for your family or friends to solve.

Four 10mm by 3mm magnets

3D Printer and filament

CA Glue

The mechanism works like this: A nut connects two threaded bolts. One bolt is affixed to the bottom of a round box, while the second bolt is attached to the top lid. The nut has magnets glued to two outside faces. It can be turned from the outside of the box by using more magnets! When you turn the nut it moves down, eventually releasing the top bolt; and the lid comes free. For the design I tried to hide the outside magnets as much as possible, thus I used a second round box to fit over the first. This box is the ‘tool’ or ‘key’ to open the interior box, and it contains the two hidden magnets.

3D print all parts: upper bolt, lower bolt, nut, interior box, lid, outside box, and sleeve. The sleeve is used to hide the outer magnets, see below. IMPORTANT: When 3D printing the threaded bolts you need to scale them down (94% worked for me) in both the x and y direction, but NOT the z direction (bolt axis). This creates a loose fit and allows for easy turning of the nut by the magnets. You may have to adjust this scaling through trial and error. All other parts are printed at 100% scale.

Glue the lower bolt into the recess of the interior box. Glue the upper bolt into the recess of the lid. The bolts have a cross-shaped key/recess on their mating faces. This is crucial to properly align the threads on the bolts, otherwise the nut would get stuck.

Glue two 10mm magnets into the nut. Both magnets should have the South Pole facing outwards.

Glue the other two magnets into the outside box. Here both magnets should have the South Pole facing towards the inside of the box. I purposefully choose to have these magnets repelling the magnets on the nut; this way they are not as easy to feel or detect when using the puzzle. That is why I called it the 'Repelling' Puzzle Box.

Lastly, insert and glue the sleeve into the outer box to fully hide the magnets.

Screw the nut onto the lower bolt in the interior box. Line-up the lid/bolt with the interior box/bolt, this is where the cross-shaped key on the bolts is essential. Then use the magnets in the outer box to turn the nut upwards and connect the upper and lower bolts. You will turn the outer box counterclockwise in relation to the interior box. The interior box lid is now locked.

To unlock, partially lift the interior box and turn the outer box clockwise relative to the inner box. After a few turns the lid will release.