# Week 4: 3D Printing a Functional Rubik's Cube in One Piece

## Idea

This week, I printed a functional Rubik's cube from scratch and in one piece using a 3D printer. If you've ever been as intrigued by the inside of a Rubik's cube as I have, this project is for you! The mechanics behind this simple puzzle are simpe but fascinating. Usually, it's made of various parts that are injection molded, while a few screws and a central cross hold everything together. Using a 3D printer and a sophisticated mechanism of support structures on the inside that dissolve in a solution, I hope to create a cube that can be created in a single print and is functional as it comes out!

## Modeling the Rubik's Cube

As you know, I am a beginner in CAD modelling and have only started exploring it with this class. This project was a perfect opportunity for me to level up my modeling skills. I started by looking at the original patent by Erno Rubik from 1983 (US 4378116 A) to understand the dimensions and the mechanism. A few YouTube videos of people disassemblying their cubes were also greatly helpful (thanks to Mad Scientist, Jared Owen and of course What's Inside?).

I started by modeling the interior cross structure that supports the center pieces. The modeling was quite easy, although I should mention that I later got rid of the fixed cross and instead replaced it with the 6 axes just "sticking out" of a sphere in the middle of the cube, as this gave the cube more stability. I continued modeling the edge pieces (connecting 2 faces) and the corner pieces (connecting 3 faces) of the cube according to the dimensions in the patent. The first challenge was to give each cube their respective "shaft" (the piece that sticks out into the inside of the Rubik's cube and holds the piece in place). I first tried to model it with a ton of projections from the other pieces, hoping to get to a profile that would just fit the inside of the cube. After an hour of failed attempts of doing so, I realized there was a much simpler solution: an intersection between the sphere in the middle (that forms the center of the cube) and a larger version of the piece I wanted to create a shaft for that stuck into the center. In hindseigth, I think the combine tool is much more powerful than I initially thought and I'll likely use it more often.

Another challenge were the tollerances, which I had not had taken into consideration when I modeled the first version of my cube. Obviously, having the faces of the individual pieces be right next to each other (on each other) meant that the 3D printer would just print one big cube. I went back to the sketches and added tollerances everywhere that were parameterized and could be adjusted based on the results I got from some test prints. Again, I think this shows powerfully how important it is to get the basics of your model right. It's in the initial sketches where I tend to model something too quickly or overlook an important feature, so that later I have to adjust it all the way from the start through the entire timeline.

So far, I had only modeled the cube's spherical center (cross), one center, one edge, and one corner piece. Since all other pieces have the same geometry as these three pieces, I used the mirror function to complete the entire cube with ease. Also, this still allows me to edit all other features previously used to model the pieces of the cube in the timeline and propagate the changes throughout the entire cube.

## Experimenting with Screw Mechanisms & Tollerances

I printed a part of the center sphere with one center, one edge, and one corner piece as a test print to see if the pieces were able to move along the center sphere as intended. While experimenting with different tollerances, I noticed that the cube's maximum stability is achieved then all parts are "pressed" against the center sphere. In the original Rubik's cube, screws with springs press the center pieces to the core sphere, which also exerts pressure onto the shafts of the edge and corner pieces and holds them in place. In my cube, I needed a similar mechanism without the use of any screws (since I want) the cube to come out fully assembled.

My initial idea was to add small hook-like structures around the circular shafts that connected the core with the center pieces, such that each center piece can be pressed into the cube's core and it is fixed that way. I printed and experimented with multiple shapes and diameters on PLA printers with PLA support structures. Unfortunately, I could not get the system to work reliably enough with PLA, the material I wanted to use for my cube. The hooks would either not fit through the opening in the core sphere at all, or if they fit through, they would come out just as easily. Moreover, keeping the size of the Rubik's cube in mind, most of these hooks needed to be much smaller than the 3D printer's accuracy of around 0.1 mm, so they could not be printed effectively and reliably to fit through.

Although it would have been possible to print the cube on a more expensive printer with higher accuracy and smaller tollerances, I wanted my print to work even on low-budget printers. Therefore, I decided to abandon the implementation of a screw mechanism and instead experimented with the tollerances until I found one that the printer could just print but that also kept the parts nicely locked. This certainly is a trade-off and I would love to solve it in a future iteration while preserving the idea of a cube that's ready to use in one print.

## Printing with PLA & PVA on a Multi-Material 3D Printer

So far, the test prints of just a few pieces all came out great, but they also barely needed support structures. Unfortunately, to produce the entire cube functional and in one print, I needed a lot of support structures within the cube that are inaccessible once it is assembled. To solve this problem, I chose to go from a single to a multi-material 3D printer and use PLA as the material for the cube and PVA as the material for the support structures. PVA has a similar extrusion temperature as PLA and slowly dissolves in water at room temperature, so it is an ideal candidate for support structures in inaccessible places on our final print. I chose our Flashforge Max Pro 2 printer that has two extrusion nozzles for this print and loaded one with black PLA and the other with transparent PVA.

Unfortunately, printing with the printer, needed some heavy bug fixing until I could get the fine structures as I needed them. The PLA was easy to work with – it printed perfectly at 215°C with a bed temperature of 60°C, although the nozzle often oozed material while it was idle. I fixed this by decreasing the extrusion temperature to 205°C and increasing the retraction to 3.5mm inside the nozzle. The PVA, on the other hand, needed much more care. When I first ran it at a temperature of 215°C, it burnt inside the extrusion chambr and completely clogged up the nozzle. It was also oozing without end while idle. I decreased the temperature over multiple experiments down to just 165°C – much below the temperature range given by the vendor – and increased the retraction to 3.5mm inside the nozzle. Next, the PVA would not stick to the print bed, so I increased the bed temperature to 65°C and used masking tape on the print bed to get it to stick, both of which did not help. I also tried using a PVA glue stick to create a PVA surface on top of the masking tape, which did not help either. Last, I decreased the extrusion speed of the PVA down to a mere 25mm/s and set the cooling fan to be always on. In combination, these adjustments finally gave me a working result – the nozzle still oozed a bit every time it was idle, but at least so little that the wiping tower could absorb the oozed filament. Overall, using and adjusting the printer to use PLA and PVA in harmony was tedious, but it resembles the experience of many on the internet. At least I learned about every setting in the slicer at this point. Luckily, the final print of the Rubik's cube itself came out flawless in my case. I immersed the cube in water at room temperature over night to dissolve the PVA support structures and had a fully functional Rubik's cube by the next morning!

After an overnight wash in water at room temperature, the PVA support structures had fully dissolved. Most importantly, the cube actually twisted and worked as expected! To give the cube its iconic color palette (and make it a puzzle), I cut out some color tiles using some leftover vinyl on my beloved vinyl cutter. I simply took one of the faces of the cube, exported it to Illustrator, and added round edges and a small edge around the stickers, before cutting them. The feeling of sticking them onto the cube to finally finish my first single-print Rubik's cube was amazing. Obviously, there are still plenty of ways to take this project from here – I have put a print job in the queue with a much better printer at SEC that also prints with a soluable support material. Moreover, I want to further tweak the mechanism for the cube center pieces to exert pressure on the other pieces, making it more sturdy. Last, I think the ideal colored tiles should not be vinyl color tiles, but maybe 3D printed textures in differntly colored filament, such that the cube is more accessible and can also be solved blind-folded for an extra challenge!