Summary

I wanted to implement a lock mechanism which is fully printable and relatively secure. The http://en.wikipedia.org/wiki/Lever_tumbler_lock seemed like the best bet.

See it in action: http://youtu.be/Fzuhta2IQZ4

The model has three tumblers with 6 bitting positions for each. It's keyed [0, 3, 5].

Note! The derived Fully Printable Padlock http://www.thingiverse.com/thing:27205 contains an improved version of this mechanism.

Instructions

Print the plate. I used ABS and 0.20 mm layer height with 200% W/T value. The plate is scaled to 150% of what the SCAD file produces, and if your printer is not well tuned, you may want to increase it even more. It's quite tiny even at that scale.

Clean up the parts. The key bitting, the bolt pin and the top surfaces of the bolt and the tumblers needed some light sanding to make them smooth, but otherwise the mechanism worked almost straight away. The key bitting is the most critical part, since if it moves the levers incorrectly, the lock won't operate.

Assemble by inserting first the bolt and then the tumblers in the correct order. The bottommost one is the one nearest the key. Check that the whole mechanism works smoothly and if not, sand or otherwise fix the binding parts. This is a technology demo right now, so there's no cover or other mounting hardware yet.

To make the lock more secure, you can just modify the bitting array in the SCAD file and you get more tumblers and the height is adjusted automatically. However, most other parameters are not so easy to adjust since the key has to operate the tumblers and the bolt in the correct order both ways. Also, the number of bitting positions per tumbler (6) is probably the maximum for this size of mechanism. Increasing that would most likely require increasing the size of the tumblers, the key and the bolt.

Lever Tumbler Lock