This is a modification (remix) of the secret box I designed a long time ago.

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Update 2024-12-31 I modified the top slightly. I noticed that the force of the neodymium magnet can be so strong that you have to hit the box on the table too much. Now the steel ball will not necessarily contact the magnet, depending how deep you push in the magnet. The magnet should be a tight fit, but do not forget to glue it with a drop of superglue.

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The challenge is how to open the box, The confusion is in the contradictory dove tails. Further the locking mechanism is not visible.

I use it when I give a “money-gift”. It is nicer when the receiver has something to unwrap, and also needs some effort to find out how to get hold of the money.

The compartment is now much bigger, and I fine tuned the tolerances. The better the tolerances between the parts, the more difficult ist to find out how the dove tails are oriented. This also means you need a well tuned printer to print this design.

You need to add a ⅜" steel ball (9.6 mm) and a round 6X2 mm neodynium magnet. The magnet does have a tight fit. You might have to add some glue.

I printed and tested it with PLA. 
PETG will likely also work, 
I did not print it in PETG so I do not know if that influences the tolerances. When the tolerances are to tight just scale to 101 or 102 %.

To open the box you have to hit the box vertically, lid up, with the right force, on a sturdy surface. Don't hit it to hard because the ball will bounce up and lock again.

As always, when you like the model give it some hearts. When you print it, post a few pictures and give feedback.

PS: The line in the picture is that I had the filament stuck a bit during printing.

PPSS: When you have difficulties opening the lock, print the version with the “deep hole”. There the chance that the ball bounces up is less  ;-)