I got this idea after reading the following article. Could I make the impossible dovetail joint even more impossible? After a little thought I came up with an idea could increase the challenge a little. I then decided that more fun could be had by turning it into a box in which I could place some money which could then be sent to someone as part of a birthday or Christmas present.

Materials used:

Two different coloured 3D printing filaments
12mm acrylic
Epoxy glue

Tools:

3D Printer
Files: flat and round
4.5mm drill
CO2 laser cutter (not essential)

Software:

FreeCAD

Step 1: Box Dimensions

According to the diagrams, the box is square with diagonals of 140mm. The depth of each half is 45mm with the hollowed out part being 30mm deep.

After printing however the true dimensions turned out to be the following:

Diagonals = 139mm
Depth = 44.5mm
Hollow = 29.4

With the dovetail sizes being out by about 3-5mm. Not sure why this happened. I think that it may have had something to do with material shrinkage or distortion.

Whatever the reason, the overall appearance and how it all fitted together was fine.

Step 2: Printing Box

While everything printed out just fine, the two small dovetails on the one half eventually broke off. I therefore simply glued them onto the matching half in the position that they would normally be in when the two halves are slid together. While a better solution would be to change the drawing so that all dovetail pieces were complete, this worked just fine and did not detract from the overall effect.

A 4.5mm drill was using to widen the holes just a little.

All the interlocking edges where filed down to ensure that they two pieces would slide easily together.

In the original design, the dovetail joints were made up of sharp triangles. This however resulted in the 3D printed plastic edges being razor sharp, leaving me with many bloodied fingers when trying to open or close the box (I nearly called this Instructable the "Bloody Puzzle Box". I therefore rounded a number of the edges (as seen in the photos and final diagrams).

Step 3: The Lock

The pins were created by cutting out 4mm pins from 12mm acrylic. Because of the laser cutting process, a 4mm cut results in a pin that is about 3.8mm in diameter. This said I found that I still needed to sand one side of each pin so that they could move freely in and out of the holes.

In my initial design the slot was 15mm deep but this would result in the pin falling over when the box was orientated with the slot at the bottom. I therefore changed things so that the slot was only 5mm with the matching hole then being 15mm.

The attached diagrams explain the dimensions in greater detail as well as show how to open the box.

The basic principle to open the box is to orientate the box so that one pin can fall into the slot. This will then allow you to move one half of the box so that, when turned upside down, the second pin is unable fall out of its locking hole, allowing you to be able to continue sliding the two halves so as to open the box (but as already stated, the attached pictures will probably make more sense).

Lock & Solution.pdf
Download

Step 4: Final Comments

This is the first puzzle box that I have ever made and it can definitely be improved on. For starters the design could be changed so that the pins do not fall completely out of their holes once the box is unlocked. Also the challenge has a major flaw. If the recipient receives the box orientated in such that the first pin is already in the first unlock position the box can land up being opened without the person realising that there is a challenge. A simple approach would be to include some symbol or text on one side such that when the person reads or looks at it, the box will be orientated in the fully locked position (if this does not make sense then take a look at the solution in order to understand properly what I am saying).

In short this is a very simply puzzle that could do with some more development. When I get some time I will definitely develop it further.