In this instructable, I will show you how I designed and assembled a sturdy puzzle box to give away prizes at the Hannover Maker Faire.

After deciding to exhibit my projects at the Maker Faire in Hannover, I decided to ask Instructables staff members, if it would be possible to get a few promotional items to give away. What happened blew my mind not only did I get a whole bunch of stickers, but also an entire box filled to the brim with t-shirts. In order to make the giveaway fair, I decided to design and 3D print a puzzle box.

Here are the things that were important to me while designing it:

I wanted it to look like the Instructables robot (You can of course use another design)
It had to be sturdy
It had to be easy to open (max. two steps)
I wanted to be able to lock it by just pushing the lid shut.
The mechanism had to be hidden even when the box was open. So that the next person wouldn't be able to see it.
I wanted to reward persistence, so it should be possible to open it just by trying long enough.
I didn't want to use centrifugal force to open it, because I figured it would be the first thing that people would try (btw. I was right it was the first thing many tried)
I didn't want to use a ball maze because shaking the puzzle box would give away how the mechanism works.
I wanted to add some deceiving elements.

Thanks a lot to Ellen for sending me a picture of her with the solved box!

Supplies

Here is what you need to create your own puzzle box

Software:

Fusion 360 (the free version for personal use will work)

Supplies

Filament (I used steel filament for the discs, you don't have to, but the mechanism will work more reliably if you do. I am explaining more about this in step 6 of this instructable. Everything else is printed in PLA)
An about 28 mm long rod with a 4 mm diameter
Two 4x12x4 mm ball bearings (e.g. from amazon.com*)
A strong two-component glue (I used this one*)
Optional: red paint + paint brush (I used it to paint the inside of the eyes)

Tools

3D printer

* Affiliate link. I earn from qualifying purchases. Thanks a lot for the support!

Step 1: Learning From My Mistakes

Overall I am very happy with how the puzzle box performed. More than 200 people tried to solve it and it worked really well. 15 people managed to solve it without any hints and quite a lot more after I gave them one (or more) hint(s).

I only had one week to design and print the box (while working, having a three-year-old, and renovating a more than a hundred-year-old house), so there wasn't much time to test it. In this Instructable I have corrected two flaws my original design had.

I wanted to add a deception and decided to make the antennas moveable. Which sadly wasn't a great idea. They broke within the first hour. After they broke I glued them in place and then they held up great. So in the improved version, I decided to not make them moveable.
Another thing that happened (luckily towards the end) was that the cover got loose. As you can see in the second picture a small gap appeared. When pulled with some force it was possible to open the box without knowing the mechanism. So in the second version, I added an additional groove.

Step 2: How to Build Your Own Box

I designed the box in Fusion 360 and was happy with how great it worked. I have uploaded all the files you need to print your own box to this Instructable, but since you might need different tolerances (depending on the printer and layer height you are using (I used 0.16 mm)) I decided to quickly show you how I designed it. You can also follow this Instructable should you want to change the design.

The design of the box is pretty straightforward. You can adapt the size and design to your needs.

You can take a look at my design here: https://a360.co/3OZFYE7

I have also uploaded my Fusion file to this step to make your life easier. Feel free to change it however you would like.

Step 3: Getting Started With the Box

Since the box is symmetrical, you will only have to design half of it. I wanted it to be slightly wider than long. Therefore I went with 110 mm x 100 mm (the box had to be big enough to fit an Instructable pin or a small measuring tape), but you can make your box bigger or smaller.

I've extruded the bottom by 3 mm and created another sketch on top of it (as shown in the second picture). The 5 mm notch is going to house a wall that is going to hold the mechanism. You can adapt the 60 mm and 45 mm to any measurement you would like to use, but don't make it too small.

Extrude the sketch we just created by at least 45 mm and place another sketch on top of it. This one has to be extruded by 5 mm.

One last time you will have to add another sketch on top of your extrusion. Hit "P" on your keyboard to project the faces from the layers underneath onto your current layer and create an extra line (the blue one in the fifth picture of this step) that extends the line black line that is underneath the notch. Finally, extrude the faces as shown in the sixth picture by 5 mm.

Congrats, the box is already done (except for mirroring it and creating decorations).

Step 4: The Lid

Next, we are going to create the lid. Create a sketch on top of the extrusion we just created and offset the line I marked with blue in the first picture by 0.2 mm (this is the tolerance that worked well for me. You can of course adapt it to your needs). Next, extend the line as shown in the third picture and project the faces underneath onto your sketch.

Now extrude the whole face by 5 mm, as shown in the fourth picture. Make sure that you select "New Body" or "New Component" otherwise, your lid will be joined with your box.

Turn the box over and extrude the offset face we just created by 9.5 mm. Add another sketch to the inside of the extrusion we just created and copy the measurements from the eights picture of the step and extrude them by 5 mm. This part is preventing that the lid can be lifted above the mechanism. Add a 2 mm chamfer to make sure that the lid can be closed smoothly.

The lid is not yet finished you will have to cut a groove into it, where the discs are going to sit. We are going to do this at step 7 when we are designing the discs.

Step 5: Finishing the Box

Next, I mirrored the box as shown in the pictures and add the wall (as a new part).

As you can see in the pictures I once again used a tolerance of 0.2 mm.

Now select the wall we just created and add a sketch to it. To figure out where I had to cut the hole into the wall, I simply drew the individual parts and measurements as shown in the seventh picture.

I used a hole size of 4.5 mm, even though the diameter of the rod is only 4 mm. I wanted to make sure I had enough room to add the glue later on.

Once you figured out the position of the hole simply cut through the wall and through about half of the wall of the box, as shown in the last picture.

Step 6: Onto the Mechanism

Now it is finally time to create the mechanism. There are only two steps needed to open the box, but you have to be very careful not to undo the first step while trying to solve the second.

The trick is to find the right position to hold the box. Once the box is in the right position the lid can be moved easily.

There are two discs inside the box. They are sitting in a groove that is cut into the lid. Therefore unless they are turned to the right position the lid can't be moved. As you can see in the image above the discs have a cutout on the side and therefore a certain point of gravity.

I used steel filament to print them because they worked more reliably than their PLA counterparts. The steel filament discs weigh about 20 g each and the PLA ones about 7.2 g. This results in them moving faster and therefore making it easier to open the box when you know the correct position. I tried using PLA discs which resulted in me getting frustrated since from time to time the box wouldn't open even though I held it right.

The best way to open the box is to constantly move the lid back and forth. If the lid is pushed against the discs it won't work. Therefore you should constantly move it back slightly. This makes the second step tricker than the first because you have to be careful not to undo the first step.

Step 7: Designing the Discs

Now that we know how the mechanism works, let's go ahead and design it.

You can find all the measurements I used in the pictures. Simply create the sketch and extrude it.

Make sure that the cutout in the second disc is facing the other direction. Once you have created both discs, create the cutout in the lid as shown in the last picture of this step. I used a tolerance of 0.5 mm to make sure that the mechanism wouldn't get stuck.

Step 8: Distance Holders

To hold the discs apart at the right distance we will have to create small distance holders.

All of them have an inner diameter of 4.5 mm and an outer diameter of 7 mm. The lengths that worked best for me were 2.8 mm, 4.3 mm, and 2.5 mm. Yours may be a bit different depending on your printer.

I also created a fourth holder to push everything in place and make the assembly easier.

Step 9: The Slider

The last thing to make the mechanism work is the slider inside the box. You can find all the measurements attached to this step.

Step 10: "Decorating" the Box

Feel free to add whatever design you want to the box.

I decided to go with the Instructable robot. To add the face I simply drew it in Inkscape and imported it to Fusion360 as shown in the pictures.

Step 11: Assembling the Lid

Assembling the face is pretty straightforward.

I don't have a multicolor 3D printer. Therefore I decided to use paint for the red in eyes of the robot. I placed the eyes on the lid and traced the parts I wanted to paint with a pencil. Afterward, I painted them and glued the eyes and mouth back in place.

Step 12: Assembling the Box Part 1

To assemble the mechanism place the rod through the hole in the wall and start stacking the following parts onto the rod:

DistanceHolder01
One of the discs
DistanceHolder02
The other disc
DistanceHolder03

Step 13: Assembling the Box Part 2

Next, you will have to glue the wall in place. Apply glue into the small hole which is in the middle of the top side of the box as well as all the places where the wall is going to be. Be careful not to undo the stack of discs and distance holders you build in the last step and carefully slide the wall in place. There should be enough clearance to place the rod into the hole that is in the box.

Next, use "DistanceHolder04" to push everything in place and glue it to the wall. Once the glue is set, cut the top part of "DistanceHolder04" off so that it is flush with the wall (I designed it so that the part is easier to hold).

Finally, apply glue to the end of the rod that comes out of the wall.

Step 14: Assembling the Box Part 3

Next, I glued the antennas in place and assembled the slider as shown in the pictures.

Carefully turn the disc and slide the lid over them. Add glue to the slider, lift the lid over it and glue it in place.

Congratulations, you are done!

Step 15: Bonus Step

As a bonus, I have uploaded my robot design (which was heavily influenced by audreyObscura) to this step. The robots were holding small signs with QR codes that linked to my Instructables. I also designed them in Fusion 360.

You will need a 3 mm pole and two small o-rings to assemble them.