I am very proud to present the finished Double Circle Real5x5x5. I've been working on putting a 3x3x3 inside a 5x5x5 off and on since November 2006. This is my second such design and I finally have a puzzle that works as intended. The notion of a 3x3x3 inside a 5x5x5 was first published in a Cubism For Fun (CFF) article called Hidden Cubes by Tony Fisher in July 1989. The first simulator which would let you play with and solve this puzzle was the program CubixPlayer2 made by Per Kristen Fredlund and released in April 2004. To my knowledge this is the first physical/functional and stickered version of this puzzle. The stickering was completed July 4, 2014.
This is a Super Multi-5x5x5. Super means each piece has a unique position and orientation in the solved state. Multi means that all 5x5x5 or 125 cubies are present in this puzzle and solvable. This includes the 1x1x1 at the center and the 3x3x3 on the inside. Note most 5x5x5's only allow you to solve 98 of the 125 cubies.
The finished puzzle has an 80 mm edge length and it weighs in at 262 grams. This one was printed in White Strong & Flexible (the default material) and dyed black using Rit dye.
Stickers for this puzzle may be purchased here.
In addition to the 281 parts you will get from Shapeways you will also need 12 M3 Button-Head Socket Cap Screws. These screws should be 10mm long and it is necessary that you get the Button-Head that requires the 2mm allen wrench. Many M3 screws use a 2.5mm allen wrench and these will not work here as the allen wrench must fit through the outer screw hole in order to screw the inner 6 screws into the core.
These are the screws that I am using.
You can read more about this puzzle in the Twisty Puzzle forums here.