I didn't have any particular design in mind, so I designed it as I built it. Being so small at only 4" square, there wasn't much room for many varied tracks for the ball bearings, so I eventually settled on the design of the letters USB as the tracks. The ball bearings are about 4mm taken from an old dead bicycle hub, it probably work better with more ball bearings, but I haven't got any yet. The Marble machine has a USB port on the front which which can be handy,
- Plywood 12mm and 18mm, about a foot square of each should plenty.
- Ball bearings, around 4mm
- An old VCR read head, with bearings in good order
- A small DC motor from a CD drive or similar
- 6 - 8 diodes, any will do
- Small slide switch S.P.S.T fine
- Wood screws 1"
- Epoxy (5 minute cheap stuff fine)
- Thin clear plastic, from drinks bottle or craft store
- USB cable with male connector
- USB cable with female connector
Warning: I am not an expert in electronics (in case you didn't notice from my dire soldering), if you blow your USB port up, your computer, your house or your cat, it's not my fault! Proceed with caution!
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Step 1: The Base
The base is made from two pieces of 12mm ply. The bottom piece has a cutout for the electronics. Both pieces are 100mm square. I made the cutout for the bottom piece about 1cm from the edge all the way around, the exact shape and size doesn't matter. When you've cut the bottom piece you need to drill two holes in either side so that you can screw the two pieces together. You also need to carve three grooves for the USB and motor wires. I used a 6mm veiner for this.
Step 2: The VCR Head and Back Plate
The wooden wheel spins on an old VCR read head, I dismantled it down to just the bearings and aluminium wheel. The shaft is fitted into a piece of wood at the back of the marble machine. The VCR bearing centre is 40mm from the base bottom edge of the board. The motor hole is 25mm and positioned with it's centre about 20mm in from the corner. The total width of this piece is 100mm, the curves on the top were just added as decoration. The motor shaft is a tight fit in it's hole, I had to hammer it in like a nail.
When you've made this piece, you can screw it to the base with two 25mm wood screws, make sure you countersink them. The motor is epoxied in place with 5 minute epoxy, make sure that the motor is set straight as the pulley will work it's way loose if it isn't. You can add the rubber band for the motor drive at the stage.
Step 3: The Spinning Wheel
The wheel is made from 3/4 inch ply wood about 82mm diameter. So that it holds the ball bearings, the holes are drilled at a 30 degree angle towards the outside of the wheel. The holes are just a little bigger than the ball bearings. I made a paper template to mark the position of the holes then set my drill press table to about 30 degrees. The back of the wheel has a 1 1/4" hole drilled in it - this to fit over the raised part of the VCR head.
When you've finished the wheel, you can epoxy it to the VCR head, make sure it's centred. It's best to leave it flat while the epoxy dries. I also sanded a gentle curve using a Dremal, on the leading edge of the holes to help them pick the ball bearings up easier.
Step 4: The Wheel Cover
So that the ball bearings don't fall out 3/4 of the way around, there's a cover the holds them in place until they reach the top of the wheel. There they fall out into a channel and roll onto the USB sign. The channel is carved using a 6mm Veiner (a U shaped carving tool). Test this when you've carved it. I sanded the edge that meets the wheel with a Dremel to give it a bit of an angle to help the ball bearings fall out of the wheel.
Step 5: The Letters: U.S.B
The letters are marked and carved, then they are cut out using a coping saw (or a scroll saw if you have one). I carved the channels using the 6mm veiner, make sure that the ball bearings fit as you carve them as it's almost impossible to carve them once they are cut out!. The letters have little blocks added on so that you can glue them together, don't forget these. Don't forget to drill the holes in the B before you cut it out, it's really hard to hold something so small and drill it!
Step 6: Glue the Letters Onto the Wheel Cover
I super-glued the three letters together and then glued them onto the wheel cover. The underside of the wheel cover is angled, so that the letters will be angled down helping the ball bearings flow.
Step 7: Screw the Wheel Cover in Place
The wheel cover needs to sit very close to the spinning wheel so that the bearings don't fall out. Hold in place to test, then mark and drill a hole for the screw. Then drill a pilot hole in the ply-wood, so that it doesn't split.
Step 8: The Bowl
The bowl sites at the bottom of the machine, so that all of the ball bearings roll back towards the wheel. I measured the space left over between the wheel and the front of the machine, this is the size of the bowl. Make sure that you mark a notch in the right hand side for the wheel cover, I forgot to do this, fortunately cutting it after carving didn't cause a problem. I carved the bowl using a 12mm Veiner to get the shape and a small almost flat gouge to smooth it out. Finally I sanded it smooth. It took me two tries to get this right, as the bowl needs to sit slightly above the holes in the wheel, so that the ball bearings just roll into them.
Step 9: Test the Machine
Hold the bowl in place, and connect a 1.5V battery to the motor. It's a good idea to test it in a deep sided container as the ball bearings will probably start flying in every direction. Fill the bowl with a few ball bearings, and it should start flowing around. If you happen to have done an outstanding job of carving the U.S.B it should work perfectly, but for the rest of us mere mortals it'll probably need some more work! I had to add little pieces of plastic to the curves of the S as the ball bearings tended to fly off the curves. When you've got the it working OK, you can glue the bowl to the base using, super-glue
Step 10: Electronics and Motor Speed Control
The two USB cables are connected together as if there were no break in the cable, solder them colour to colour. Then solder an extra two wires for the motor power.
Underneath the machine there is a simple circuit to slow the motor down. To do this I used 6 diodes in series, each diode drops about 0.6V. I found the number of diodes purely by trial and error, you'll need to do the same, I started with about 8 diodes and removed them until the motor would run slowly. Originally I tried resistors to slow the motor down, but I couldn't get this to work effectively, as the resistance required to slow the motor down limited the current too much so that the motor wouldn't start itself. The diodes don't limit the current but drop the voltage enough for it to run at a nice slow speed. There's almost certainly a much better way to do this, but I don't have the electronics know how to build it. I've included a diagram showing all the parts.
The motor is connected in series with a slide switch. I cut a slot for this in the side of the ply wood using a small wood chisel. It is glued in place, with super-glue.
Step 11: Glue the Electronics in Place
I epoxied all the electronics in place underneath the machine. They must be fixed in place, if they short out it's possibly bye bye PC/USB ports! When you've epoxied everything in place,
you can screw the bottom piece of wood on. I fitted a piece of thin plastic over the base to protect the electronics.
Step 12: Add a Plastic Guard and It's Finished
The ball bearings sometimes roll out of the bowl, so I added a plastic guard around the bowl to keep the bearings in. It's made from thin plastic that I bough from a craft store, about the same thickness as a plastic drinks bottle.
That's it finished, I hope you enjoy being mesmerized by this as much I am, as well as never fumbling around the back of your PC for the USB port again (yes I know people with PC's made after the War usually have USB ports on the front of the tower!).
Runner Up in the