In the 1970's, Stewart Coffin created a burr puzzle using 12 hexagonal rods, he called it “Hex Sticks” (made commercially for a few years under the name Hectix).  Here I have replaced the rods with threaded cylinders.  It screws together into the same shape. It is not easy to assemble, but is simple to take apart (just start unscrewing).

There are 10 possible piece types and I like to print each type in a different color.  The basic (easiest) puzzle uses 1 yellow (code 0), 1 orange (code 1), 1 black (code 5), 3 green (code 2), 3 red (code 7) and 3 blue (code 11) rods (second photo). The hardest puzzle (first photo) uses one piece of each color, plus an extra green and red. Each piece is labeled with its code, so you could print them all one color if you like.

The code in binary tells you which of the 4 notches are present.  For example 6=0110, indicating that the middle two notches are present.  You might confuse piece 6 and 9, but you can tell from the notching as 9=1001. Note that both basic and hard puzzles have exactly 24 notches (no unused notches).

I recommend printing the basic version first.  It is hard enough to assemble (a pdf solution is included). The pieces are printed vertically, they do not require supports but are are only attached to the build plate by a small circle.  I use a wide brim (7 mm) with a separation gap of 0.1 mm to keep them from falling over.  The brim is removed after printing. I recommend a layer height of 0.15 mm or smaller to resolve the threads.

The pieces are about 7 cm long and 17 mm in diameter. The assembled puzzle fits neatly in a 3" cube. The included (optional) rhombic dodecahedron fits inside the puzzle. If there is interest, I can add a solution file for the hard puzzle.