Week 7(Final): Tabletop Marble Machine

For the final project, the basis of the project was entirely up to us students, I knew I wanted to do something mechanical since the beginning of the semester since I was working on another project requiring some mechnical parts we didn't have which led me to think "man I wish I could just print out the parts I need." Since I wanted to make sure that it was doable by the deadline (it still wasn't) I kept the idea pretty simple but because the design was largely based on guess work for how the balls would react/bounce off hitting the wood there was a number of design flaws as reality was quite different.

CNC, plywood, hot glue, wood glue, metal wire, solder, marbles (steel balls)

You can see the evolution of my design in my notebook. At first I came up with a very simple design (which served as the first portion of the final product). The strange lines that almost looks like a signature in the first image was a more complex design with multiple routes for the ball to travel. The idea was for it to have a pachinko-pinball kind of peg system so that it could travel down any one of 3 routes but it seemed a bit too hard to accomplish in time. After getting the foundation down with the first couple of gears I knew I wanted, I used a gear generator to make the rest which gave me the measurements to design the rest as it is.

Luckily there are a number of sites that can export .dxf or .svg files which can be directly used for the CNC (I used geargenerator.com). In my case however, I needed them to move the 3 cylinders that would push the marbles around so there was a bit of editing to do afterward. I figured using multiples of 4 would be ideal so the first (medium) gear is 16 teeth (about 8cm diameter), the smaller one is half that at 8 teeth (4cm radius), and the larger 24 teeth (12cm radius). Originally I thought of keeping all the gears the same ratio but it seemed somewhat uninteresting so I added variety to change the rpm of each main portion.

I still wasn't sure if 3D printing or using the CNC was a better idea since my design required some diagonal cuts which the CNC couldn't handle, but at the same time 3D printing would be somewhat overkill considering the physical size of the parts, which would make it take days just to finish printing. I went ahead and made both 2D and 3D model representations in case (I also forgot for a while that the CNC doesn't take .stl's like the 3D printers).

The diagonal cuts I mentioned earlier I ended up using other machinery to edit (sander, drill press, etc). For the first wheel which would carry the marbles from below to above needed bores facing the center of the circle, which meant that as the marble went in below, it wouldn't drop out as it would be inclined downward toward the backboard, and when it got to the top it would then slope outward to drop the ball. Where I could I'd use wood glue as it's much stronger but since it doesn't work well as a filler for spaces (such as pegs into the peg holders and gears) I would use hot glue in those cases.

The main problem I didn't foresee was how hard creating the bridges would be. Since I was just going by hand/pliers to bend the tough wire, I could never get it to be the exact shape I wanted which caused quite a lot of stability/consistency problems. If I were to go back and redo the project I'd minimize that part, and probably consider making the bends and turns into 3D printed or otherwise fabricated parts so the only wire necessary would be straight lines.

The wheels passing the marbles also required some minor edits to prevent bouncing or sliding out, for example a cover for the first wheel to really make sure that the marbles only go in/out at certain points, the second a small inner tap to prevent the ball from falling into the gears and for the third a lower RPM, as it turned out to be quite hard to feed while moving so fast (3 times the speed of the other 2 cylinders).