IC Puzzle Keychain

This is an IC puzzle created as a 3D-printed giveaway for the University of Kentucky Department of Electrical and Computer Engineering, which is why it has the UK logo on the top and ECE on the bottom.

Where did this puzzle design come from? A polished cast metal version of this type of puzzle called “Magic Wands” is widely available from online sellers. A 3D model matching those was created by Joseph Escalona as CPU Puzzle, and he states that this puzzle was originally machined out of brass by Rocky Chiaro in 2004 as the AB-L puzzle. It is probably written up in Chiaro's 2020 book “Puzzle Sculpture,” but I haven't been able to confirm this. In any case, my design was created from scratch for 3D printing but does employ the same puzzle logic.

The design here is an improved version of my original. It is intended to be printed as nine pieces which, when assembled, produce a 36x36x5.5mm completed puzzle. The assembled puzzle can be used as a keychain by threading a standard 1" key ring through the hole in the corner. Making a bunch on my Bambu X1 Carbon and printing the pins separately from the IC bodies to minimize the number of filament color changes, the print time per puzzle is around 16 minutes – about half the time taken for my 48mm version.

There are 8 “pins” passing through the IC. Unlike the “wands” in the other puzzles, these pins are shaped so that they can be printed flat on the printer bed.  This shape also makes the pins significantly easier to keep oriented correctly when assembling the puzzle because they can't spin, and they even look much more like the pins of a typical IC. The pins of an IC would not stick far out of the IC body, so neither do these. Another subtle difference is that one of the wands has no cuts, which means that it can fall out unless held in place by another mechanism. My design instead places a flexible bump on that rod to cause sufficient friction to hold it in place – a simple example of a conformant material structure taking advantage of the ability of plastic to flex a little. Additionally, if a key ring is inserted, it will lock the part in place. I prefer metallic silk PLA for the pins, with copper and silver both looking very nice. All the pins are in an easily printed orientation in ukece*_1.stl. Generally, they don't need any cleanup after printing, and will smoothly fit into the IC body with only the conformant lock causing significant friction. Unlike the original version, in this version, each pin is unique because one of the two with the 3-cut pattern has a hole for the key ring.

The 9th piece is the IC body, which can be printed in two colors so the UK logo and ECE text on the other side stand out from the body. The University of Kentucky's colors are Blue and White, so I generally prefer those two colors of PLA for the logos and body. The body is ukece*_2.stl and the logos are ukece*_3.stl. The IC body design isn't particularly clever and has some symmetries that allow for more than one solution. There are internal spans in the IC body, but they are all short enough to be handled without support on every printer I've ever used. The biggest potential printing problem is that even a slight bit of warping can make the pins not fit, so make sure your print sticks firmly to the bed while printing, perhaps using glue and/or a brim.

Oh yeah. How does one solve the puzzle? I've made a short video showing how to assemble the puzzle. It is not a trivial puzzle to assemble but turns out to be fairly quick for most computer engineers to solve. The pins get inserted in order from least constraining to most constraining, and you have to keep track of what constraints remain because the IC housing hides the pin slots from view. Taking it apart is pretty easy once you've discovered which pin doesn't have any interlocking cuts.