Impossible Cube

thingiverse

I can't remember where I saw a version of this puzzle, but the concept fascinated me. I was drawn to it because this object so misleads the thinking process when trying to figure out how to separate the two parts. But then the model formed in our minds is based on what we see. The brain completes the rest including hidden features. So using fusion360 I set off to design the object and test it. I made several versions of the lock inside the object, This version works best. This particular lock configuration disguises the orientation in which you need to hold and manipulate the object when unlocking it. It also makes it less likely that someone accidentally succeeds, this happened in the first configuration I designed. I also found that it is not easy for people to work out what the precise movement is even after a demonstration (performed theatrically). Now for what you need. This thing is made up of 7 pieces, 4 printed, and 3 you have to source (unless you happen to have them). You need the following 2 No. rare earth magnets 2mm in diameter 1 or 2 mm thick available from amazon; https://www.amazon.co.uk/Magnet-Expert%C2%AE-2mm-thick-Neodymium/dp/B007JTKHR6/ref=sr_1_3?ie=UTF8&qid=1542635206&sr=8-3&keywords=2mm+dia++magnet A pack of 50 2mm dia magnets cost less than £5.00. I suggest using 2 mm dia because larger diameter magnets maybe be too strong. 1 piece of 9 mm diameter steel bar. I used the un-threaded shank of a 180mm long M10 Coach Bolt. The shank diameter is just under 9mm (8.94 to be precise) which is spot on for this purpose. You only need a small piece 11-11.5 mm long which you can cut from the un-threaded section of the M10 Coach Bolt. You can buy these from Wickes, screwfix amazon ...etc A pack of 2 bolts costs £2.57 at Wickes, but make sure there is an unthreaded section, some bolts are fully threaded. The other 4 components are printed from the stl files. Assembly 1 The 2 large pieces that make the cube need to be a tight fit, this is by design so that it is not easy for someone to figure out what's in the inside. Having said this, the two pieces should slide against each other without much force. Depending on how accurate your printer is, you might need to carefully sand down the sides of the trapezoidal projections so that the cube assembles and disassembles easily. Be aware of the corners, they are sharp and can stab you. I found that out of my printer the pieces are somewhat tight, but after sliding them past each other repeatedly they ease off and the fit becomes just right. 2 Now you need to make sure that the steel pin you cut out of the Coach Bolt is about 11 mm long but not more than 11.5mm. it needs to slide lightly in and out of the the two smaller pieces (30mm_puzzle_1.stl and 30mm_puzzle_5.stl. The steel pin should fall out under its own weight or with a light tap. Again depending on the accuracy of your equipment you might have to do some light sanding to the inside of the holes to smooth them out. The holes are designed to be 9mm in diameter which gives just enough clearance for the 8.9mm diameter steel pin. It is very important that the steel pin does not protrude outside the face of the larger of the two small pieces when pulled in by the magnet. If it does stand proud, you need to file it down or cut a bit off to make it shorter. You can use a dremel or a hacksaw). Make sure the cut face of the steel pin is smooth and has no sharp edges that can catch and obstruct its movement. I chamfer or round off the edge with a dremel or a file. See photo. 3 You need to drill out the 2mm magnet recess on pieces 30mm_puzzle_1.stl and 30mm_puzzle5.stl with a 2mm drill bit. push a magnet in each and make sure it is flush with the inner surface. Glue the magnet in position with CA adhesive. Make sure it is cured before inserting the steel pin. 4 Now find the little triangle on one corner of the chamfer on the bottom of piece 30mm_puzzle_2.stl. 3 corners form a little line but the 4th forms a tiny triangle. The magnet inserted into piece 30mm_puzzle_1.stl should be nearest to the triangle. 5 Make sure the steel piece is inside 30mm_puzzle_1, push fit this into the base piece. There is no need to use adhesive. assemble the 2 other pieces in the same way. The magnets are there to hold the steel pin in place (locked or unlocked). Hold the cube in one hand and give it a sharp tap diagonally against the other hand should lock or unlock it. You will hear and feel a click as the steel pin detaches from one magnet and gets captured by the other. once it is unlocked, the two main parts can be slid against each other and separated. Use and demonstration I suggest that you hand over the assembled and locked cube to others and challenge them to separate it. The vast majority will be busy trying to push, pull or twist all over the pieces. Make sure nobody uses force pulling and twisting and break to puzzle. I just tell people you have one clue "do not to use excessive force". Now, about how to conceal the unlocking action from your audience after they give up. Hold the cube and pretend that you are going to tap it against the other hand along a face rather than the diagonal. Just before your hands meet twist the cube so that the force acts on the diagonal. It will be difficult for others to notice this. This is not the easiest project to make, but its worth it. Enjoy.