This puzzle is based off the wooden version I posted last month. The solution is the same but I've reduced the size and added features that hide the potential solution. Also, with 3D printing, you can guarantee that the parts fit and the puzzle will open and lock easily. I've included the STL files at the end along with drawings at each step in case you want to model/modify the design.

As an added feature, I was able to convince my son to animate the CAD files in Blender. Besides showing how it works, I think it looks pretty cool.

This video shows the construction and solution.

Supplies

Tools:

3D Printer
Sand Paper

Materials:

Filament: PLA used - 2 colors
Glue/Epoxy
Magnet: 5mm Diameter x 2mm Thick (x4)
Steel Ball: 3/8" Diameter (x2)
BB: .18" Diameter (x3) - optional

Step 1: Animation

This quick animation shows how it goes together and has slices through the model to show how it works.

Step 2: Base

The main part of the puzzle is the base. It started as a 2.5" square by 3" long block. I chamfered the ends and added rounds at the chamfer lines. The large hole is where the dowel will go through the puzzle. The two side holes house the steel balls when the puzzle is in the unlocked position. The small hole in the top is a decoy but does give you a hint on how to open it.

Step 3: Dowel

The dowel is 1" in diameter and 3" long. It has two counterbore holes in it at the locations shown. The bottom small hole will house the magnet while the large hole accepts the ball. Note that when dropping the ball into the dowel, approximately 1/2 of the ball should stick out. This is the lock for the puzzle. The slot along the top is for alignment - I will explain later.

Step 4: Dowel Assembly

Two 5mm diameter magnets are glued into the holes. Note that the hole is deeper than the 2mm thickness of the magnet. You will need to determine the correct depth to insert magnet into the hole. For me, I wanted to just have just enough force to easily break the joint (have ball release) with a whack on my hand. Unfortunately, this is a trial and error process. I suggest trying to the smaller pieces which are shown in the next step.

Step 5: Strips

You will need to print out two versions of the side strips. The first one is just a simple extruded rectangle that fills the gap in the base. The other one is similar but also contains the extruded portion for the magnet retention. Print two of each part. Add the magnets using the same process as the dowel.

Step 6: Plug

Nothing special here. Just a plug for the top hole. Note that the hole/plug is a perfect/interference fit. Depending on your printer, you might need to enlarge the hole slightly in the base. Use sandpaper if needed.

Step 7: Assembly

First, do fit checks on the parts. The large dowel should fit through the center hole. I made this a tight fit (.007" larger hole) so it wouldn't easily spin while locking the puzzle. Next, make sure the 3/8" diameter steel balls easily drop through the side holes in the base. Do this without the Dowel inserted into the base. If they bind, sand or drill out the hole.

Place/align Dowel back into Block. Use the alignment slot as a guide. Since the slot is through the center, you will need to look into the base to determine the correct orientation.

Drop balls back into the holes. You should hear a click if the balls make contact with the magnets. The Dowel is now locked in the base. For removal, smack the puzzle against a surface. Repeat for other side.

Next, fit check the strips on the side. Everything should line up correctly. Glue strips to the base.

The last step is to drop some noise makers into the top hole. I dropped three BBs into the hole. Glue in the plug to seal the hole.

Step 8: Locking/Unlocking Puzzle

Although I've already discussed it, the solution involves smacking the block against the two correct faces as shown in the video. This allows the balls to drop into the unlocked position. The 1/4 Plug on the one end references the sides where the ball will be recessed (unlocked). The BBs rolling back and forth in that hole are just a decoy.

Once locked with both balls, it should be a challenge for anyone to figure it out. With two locks, you don't know when you have reached half the solution. In addition, you might inadvertently set it back to lock with more smacks. Knowing the correct sides to hit is the key to solving it.

Step 9: Pictures

Here is the completed puzzle. This the first 3D print where I've actually glued pieces together. This process also allowed me to use contrasting colors which I feel helped out with the aesthetics.

Step 10: More Pictures

Here are some pictures of the project. Sorry for the bright flash :)

Step 11: STL Files

Attached are the STL files for printing. I printed the Base and Dowel with an infill density of 50%. The strips and plug were printed at 100%. Hopefully you can use my drawing files to modify the design if something doesn't fit on your print.

Please let me know if something isn't clear or you have questions.

Thanks for viewing!