Another Cryptex (I first made out of wood before I was in possession of a printer). Special feature: you can change the code! (and better not forget it!)

I was searching for a tool to boost my pupils' motivation to do their math - and so, on every session
I "coded" the result of a bunch of exercises in the cryptex and hid some "reward" on the inside.

In the photos you see one of the prints - in the current version you have a diameter of 4cm and a length of 6 cm in the inside.

*Edit: Just found out that the idea is already out here :(
http://www.thingiverse.com/thing:586169

Printrbot

Simple Black

No

Yes

0.2mm

it depends...

Gears and numberwheels don't need any infill nor supports or rafts (at least I didn't, printing with PLA).

For the frame you want to have an infill of about 10% and for the part with the teeth you should consider supports to print the teeth right.