Puzzle Box (Unibox) - 3D Print

I fielded a few questions lately about a puzzle box I made 7 years ago. Since I've learned a lot since then, I thought I would incorporate improvements into a redesign. Mainly, I hid the pivot/lock points so you can't see the solution from the outside. I also added another twist to the solution based on some of my other designs. Although 3D printed, the same puzzle could be made using wood or metal. I've included a dimensioned drawing for those interested in that route.

All 3D print (STL) files are included at the end of this instructable. Besides printing, you will also need a 5mm x 2mm magnetic and a metal rod to complete the build.

This video shows how it works.

• 3D Print Material (PLA used)
• Magnet: 5mm diameter x 2mm thick
• Metal (magnetic) Rod: diameter range 0.220" to .0250" (.240" used) by 1" long
• Glue

The box is 4" long x 3" wide and 2 1/16" tall. The parts are color-coded for reference.

The main part is the box which covers five sides. On the right side, you will see the swivel point. This is a reverse counterbore hole to capture the rotating lid. The two notches along that hole were added to help you remove the top if needed. On the lower side, a cylinder was added to capture the metal rod. This is the first part of the locking mechanism. On the left wall (not shown in the picture), there is a hole that captures a dowel pin. This is the last part of the lock.

This lid covers the entire opening. The slot is sized to allow the lid to slide along the rotating pin. The thru-hole is for the locking pin. The two blind holes are used as alignment features for the stationary lid.

This part mounts to the slotted lid. The two protrusions fit into the holes on the slotted lid. The oval-shaped hole encases the locking tab.

This part rotates along the hole in the box. I wanted it to be locking so I added an annular snap-fit joint. I was worried that it wouldn't work so I added the slots in the box. These allow you to push an object inside to deflect the cantilevers which allows the lid to be removed.

The counterbore hole is sized for the locking rod and magnet. I've used these magnets on previous projects so I had the hole size worked already.

The oval-shaped hole on the side captures the tab from the fixed lid.

The Locking Pin (copper color) is glued into the base. The rounded end faces out.

The Locking Tab is glued into the Fixed Lid. Note the orientation during installation.

The Metal Rod (silver color) was cut down from a bolt in my shop. I suggest finding something in this size range and also being dense (heavy) helps. It must be able to fit through the hole and make a relatively tight fit for the rotating top. You also might want to round the edges so it slides easily through the hole. I placed mine in a drill press and used sandpaper to smooth it out.

The 5mm x 2mm Magnet was purchased. You can buy a pack of 25 pretty for very little money from Amazon or eBay.

Before assembling anything, I suggest you do a dry fit with the base and lids. Don't install the pin or tab at this point. For mine, I noticed a slight interference between the Fixed and Rotating Lids. To solve this, I used a file to remove about .001" from each mating surface.

Make sure the Metal Rod slides easily within the hole.

Add the Locking Pin to the box with the round end on the outside. This should be an interference fit - glue if necessary.

Align the holes and protrusions as shown. I used superglue for the attachment. Add the tab as shown (note orientation). This should be an interference fit - glue if necessary.

Add the magnet to the hole. Make sure to push it to the bottom to reduce the magnetic field for the rod. I used another bolt and hammer to press it into the PLA. It should be an interference fit but glue is another option.

This is pretty easy:

1. Add Metal Rod into the hole in Box
2. Place the Slotted Lid Assy on top of the Box
3. Slide Slotted Lid Assy forward to capture the Locking Pin
4. With the angle open, snap on the Rotating Lid
5. Close Rotating Lid (should go into locking tab)
6. Turn the box over to lock Metal Rod

At this point, the box should be locked and ready to be solved.

If you have made it this far, the solution should be obvious. The key is releasing the Rotating Lid. To drop the Rod, smack the puzzle on a flat surface. This will cause the Metal Rod to release from the magnet and drop into the hole. From there, rotate the Rotating Lid. This will provide enough room to slide the Slotted Lid back. Lastly, rotate both lids to get into the box.

I decide to print in one color but you might want to consider printing the lids in a different color.

I've attached the STL files for printing. Everything was printed without supports and 50% infill. You will need to supply the rod and magnet. I've also included a drawing for anyone interested in making a version from wood or metal. If you go that route, you will need to come up with a different way to lock the rotating pin into the box. A simple solution would be a cotter or equivalent pin with an opened hole.

I'm happy with how it turned out. It did require a slight amount of sanding at the angled mating surfaces on the lid. Also, the durability might not be good enough for kids. To fix this issue, I think I would come up with a more substantial rotating pin with a tighter fit. Please let me know if you have any suggestions.

Thanks for viewing!