Cardboard Mini Combo Safe

Locks and safes are very common in this modern world, when almost everything valuable needs to be kept secure. Obviously, cardboard isn't the best option as a holder for a very expensive item, but it can be cool to have. In this tutorial I will show both how a combination lock works and detail the steps I went through to make this cardboard "prototype".

The supplies for this project are definitely on the cheap side:

-cardboard boxes (double corrugated is better for more strength)

-standard printer paper

-elmer's glue

-hot glue sticks

-pencil

-cereal box

-q-tips OR toothpicks

Optional Supplies:

-super glue (only needed if you want good durability)

-resin (I didn't do this because of time and budget constraints)

Tools:

-box knife

-hot glue gun

-pencil/pen/sharpie

Important note: If you want to make this out of wood, it would be be exactly the same process. Just swapping the cardboard with wood would be a great choice if you want extra strength. The only reason I chose cardboard was because I am on a budget and don't have much choice. You could also coat the cardboard in a few layers of resin to make it as hard as a rock.

The outer layer is pretty easy to make. I started by cutting two 5''x5'' squares of cardboard. I then cut two 4.5''x5'' pieces and hot glued all four together. Try to be as neat as possible for a decent aesthetic later on. It is pretty self explanatory how these all go together.

Next, cut four 4 1/4''x5'' squares. Later on, these will be lined upon the inside to double layer the whole box. However, I wouldn't do it yet because on one square you still need to cut a small section out that the bolt will rest in.

The standard design for combination locks revolve around discs. Basically, there will be three small circles, the discs, on a cylinder that are able to freely spin. On each disc there is a tooth that will meet up with the other disc when the dial is spun a few times. Each disc also has a notch in it. Only when all three notches are lined up can the lever fall into the notches and pull the bolt out. This video explains the whole mechanism in detail, although for my safe there are a few modifications.

You'll want to start out by making the cylinder that the discs rotate upon. This is done by taking paper and wrapping it around a pencil with Elmer's glue. Don't glue it to the pencil, but glue the paper so that it is a decently hard cylinder that the pencil can spin spin freely inside of. Next, cut out three circles from cardboard that are a bit over an inch in diameter. The measurements don't matter two much in this combination mechanism, just make sure it will fit in the final box. Punch holes in two of the circles that will allow them to slide onto the paper tube. You want a little bit of friction but not too much or it will hinder the ability for the discs to rotate. On the third disc, punch a hole the size of a pencil, which should be slightly smaller than the paper tube.

Now add some teeth to the outer rim of each disc. Make sure it extrudes no more than 1/4''. Use the photos above for reference. In the end, when you rotate one disc, it should meet the teeth of the other discs until all three spin. Note that the middle disc should have two teeth, one on each side. Cut a notch into each disc with random placement so the combination will end up random.

Now take some thin cardboard and cut it into strips 1/4'' wide and long enough to be wrapped around the paper tube. These are the sections that separate the discs seen in the pictures above. Slide two discs into place along the tube, 1/4'' apart, and glue the cardboard strips in between the discs to separate them and keep them in place. The third disc needs to be glued to a section of the pencil. The pencil should extend long enough to go through the wall in order to attach the main dial. In my reference pictures, the pencil is green.

If done correctly, spinning the end of the pencil, which is attached to one disc, will eventually spin every disc as the teeth meet up. You can now try lining up the notches.

This part is slightly difficult to get right. It takes a lot of trial and error to get the right shape. It is sort of like a more slanted 'L'. Two of these shapes should be cut out of cardboard and spaced about 1/4'' apart. You will need two q-tips/toothpicks. One will go through the end, shown in the second picture. This will be able to rest in the notches when they are lined up. However, when the notches are not lined up, the lever won't be able to move. The other q-tip will be poked through somewhere near the center of the lever piece and this will be what connects to the handle on the other end. Once it looks nearly like it does in the reference photos, it should be done.

The front of the inner box should be a 4''x4" piece of cardboard. The bottom is going to be 4'' wide and however much you want long. I didn't put a back on my box, so it was 3 3/4'', but your opinion on this might differ. You'll want to build the walls up later, but for now just glue the front and bottom together. This will make assembling the locking mechanism easier. Punch a hole the size of a pencil in the center on the front. This is where the dial will be. You will also need to cut a 4''x 3 3/4'' that will be the second wall containing the lock. The photo above shows the walls with the lock inside. You can also see how the lever will be inside the walls. Punch a hole in the front aligning with the q-tip that the lever will turn upon. You can now also glue the cylinder with the discs to the back wall containing the lock mechanism. Remember, it should be directly in the center of the front. The pencil should stick through the hole in the front and should be able to turn.

The bolt should be as wide as the section containing the mechanism is, and 3 1/2'' long. It needs a section at the end that the lever will rest in. When the lever turns, it should push the bolt out and in from the right side of the wall. When pulled in, it must be flush with the wall as shown in the third picture.

Now you can glue together the locking mechanism and set it in place. The q-tip connected to the lever, as well as the pencil connected to the disc shaft, should stick out the front. Be sure to lay the bolt in before gluing everything together. Now you can build up the walls until it looks like the third photo. These don't need direct measurements, you can easily just cut it to fit. Also, if you remember the 4 1/4''x5'' pieces of cardboard you cut in the beginning, you'll need those now. Line these along the inside of the outer shell of the box. One of these however will need a hole that the bolt will stick through so cut that out first. Find where that hole will be by sticking the inner box with the lock in the outer shell and marking where the bolt is. Now everything should be done and fit perfectly.

The dial can be made from a circle of cardboard. Just make sure it isn't too big so it doesn't cover up the q-tip from the lever. I double layered my circle so it was thicker and lined it with another strip of cardboard. On the edge I marked each number with a line and I numbered the lines on the front of the dial. I made a mistake with mine and instead of measuring out 9 equally spaced marks I only did 8. That is why my dial is numbered 1-8 instead of 1-9. Indent the back of the dial and then super glue it onto the pencil that is sticking out. Make sure you do this when all of the notches are lined up on the inside so you don't accidentally make the combination in between the numbers. Next, cut a shape of your preference out of cardboard and glue it to the q-tip. Make sure it is able to rotate slightly without the dial interfering. Now you can slide the inner box into the outer shell and lock it.

Congratulations! Your cardboard safe is now completed!