This project means extra funny work for the more daring of you readers ;-) indeed it's only a 3d model, and it has to be built.
I'm not very familiar with 3D printing, so I'm not sure about tolerance between moving parts. Anyway there is a fast explanation of my design and how to model it in 3DS Max. It's not a full modeling tutorial, because it would have been too long and complicated, but I'll show you the main phases and I'll attach the 3ds complete model in next step so you can see it in your 3D favorite software.
Step 1: Some sketches
I'm sorry about the bad condition of my sketches, but I had trashed them before thinking to post them in the instructable ;-)
I also made a big mistake in modeling, which I could have avoided if I had looked better at my drawings (yes I know that you're thinking there is not much information in them!).
The project consists in a flat safe-box with a combination lock. It's very flat, about 8 mm thick, and it's arranged with two inner compartments. You can keep inside a lot of folded banknotes or a ring, a necklace, anything fits in a 4 mm drawer.
Step 2: The working principle
The working principle of this safe is very simple. It consists in four disks inserted in the cover with a groove engraved in each disk, these grooves keep eight nail heads attached to the rib on the bottom container. These nail heads are released when the disks are in certain position, so that cover and container can mutually slide and release each other.
Step 3: The locking system
To build a better locking all around the sides of the cover, I decided to make it slide in the nails seats, so to unlock all the fastening on the sides.
In this way there are two movements to open the cover after setting the right combination of the disks:
Step 4: The disks
To make the rings the best way is to draw a shape of one ring section, then duplicate it four times, leaving a little space between them, and make them following a circle (with the loft function).
I closed the central disk since I didn't need the hole.
You notice that with this disposal of the side teeth of the rings, if you pull up the grooves, the "pyramid" will dismantle, but if I had bended the disks in the right direction, the teeth should have kept the pyramid joined. This was my big mistake and I'll fix teeth direction in last part of modeling.
Step 5: The nail grooves
Here you see as I built some shapes to enlarge the grooves in the positions corresponding to the unlocking rotation. Since I decided to paint (or attach) the letters on the finished product, you can "drill" these holes wherever you want around the circles. However the most appropriate way is to make them aligned, so to attack them handy over the rib between compartments.
Step 6: Releasing mechanism
When you have the mold ready you can copy it on every groove and subtract them from the disks. Each disk will have two holes, opposite in diameter, one external and the other inside the groove.
Step 7: Tolerances
The mold has to be a little larger than the nail head, so to let ot comes out with no restriction. You see in pictures the tolerance I left between parts, both for horizontal and vertical movements. These tolerances could be a bit shorter, depending by the precision of the 3D printer.
Step 8: Attach nails
When all the nails are in position inside the holes you can translate them into the locked position, reveal the box and merge them with the central rib.
Step 9: Side fastening system
To make a stronger locking system I decided to add ten grooves on the sides, where little cylinders could slide with the cover movement. These side spines would also keep the cover straight when sliding.
I decided to make an hole for the pivot, so to distinguish better a side of the box, and also to let you substitute it with a metal screw in case it breaks.
Step 10: Removing the interferences
Now you can unhide all the model parts and try to make them move. You'll notice some interferences between box and cover, so we have to smooth the long edge near the pivot.
Step 11: Types
As types you can use letters or numbers, or also any symbol. On the external disk I decided to put all the alphabet, and I chose some letters for the inner disks, so to compose many words. Look the final movie to discover the secret word made for this ible.
If you engrave the letters in the model, it will be more pretty, but you can't change the word. On the other hand, if you only add a gap where to glue an adhesive, you can change it when you wish.
Step 12: The notches
Add one or two little notches to mark the right alignment of the secret word letters. If you add two of them it will add some confusion to an occasional user, but you can remember which is the right side.
Step 13: Assembling helper
With a wedge and the boolean function of the program cut the side plates to make an easier entrance for the pivot. The slide will let the pivot to enter in his place with a little flexing of the plastic when the cover is opened, and will be a sturdy join when the cover is closed. The pivot will stay in place also with box opened, if no flexing is applied to the piece.
Step 14: Smoothing the edges
Let's refine the look of the box smoothing some external edges and corners, as you see in wireframe views.
Step 15: Types texture
To draw the texture for the disks you can save the top view of the model and keep it as reference to know the disks position. I didn't put a lot of effort in placing the text so it resulted crooked and some types exits from the area, but since they could be decals, this mistake gives reality to my rendering. You can use the same image as bump, mine is a little blurred, anyway you don't notice it in images.
Step 16: Suggested improvements
I've made a movie of the working gear:
Writing this instructable some improvements came to my mind, and you can add them to your own project: