Puzzle Box

Each of the outer rings turn at a slightly different rate so that they only align in one position. It will take many (14,950) turns to go full circle. Print the core and shell separately or together if you are confident in your calibration. Separate the ring gears with a sharp edge after printing (slicing at 0.2mm should preserve a one-layer gap making this easy). Turn the knob to free up the gears. I've added some "peek" holes to give you a fighting chance. A kind of rotary encoder can be made by marking the rings at significant locations, such as when a mark on the bottom visible ring aligns with one on the outer shell. I'm thinking of mapping out such locations and printing a scale which can be glued or transferred onto the rings. You could also keep it cryptic and just mark dots at the starting point (included in v2 with some other refinements). Here's a clip of the gears in action - 14,950 turns for a full cycle.