This week, the goal was to go from design to machinable wax to oomoo to (in my case) metal alloy object. Lecture notes here. I chose to create a star-shaped electrode in order to take a first step toward my final project.
Professionally built electrodes here and example image here:
Estimated timing: 30 mins to design, 30 mins to mill the wax mold, 20 mins to prepare the oomoo cast, less than an hour to let the oomoo dry, 1 hr to heat the oven to 425 F and less than 30 mins to cast it in metal.
Estimated cost: commercially from $8-$90 but I'll be making it for less than $2.
Instructions and guidelines on electrodes from: DIY passive electrode guide
Constraints on the design: 10 mm for the diameter of the cup (source: http://www.biopac.com/product/gold-cup-electrodes/?attribute_pa_size=au-cup-electrode-green-reuse) and inferred that a thicker electrode would probably impair the electrochemical performance (http://www.sciencedirect.com/science/article/pii/S0013468612005270). Some concern that the metal I'd be using (a bismuth based alloy) would not be conductive enough, but apparently it has a reasonable tin content, so I was convinced it was viable to use this as a proof of concept for my electrode.
Decided to make it a star, because, well, why not. Realized that if the circular design was playing some important role, I'd be losing it if I kept the same diameter as the original cup, so I made the distance between the tip of one arm and it's opposite longer than 10 mm. Used gray scale in the following manner: the black is 100%, the dark gray is 85%, and the registration circles are 50%.
Ultimately, my design in Illustrator looked like this.
I exported the design as a PNG and loaded it into fabmodules.org and had it talk to the Roland Mil. I cut machinable wax to create the first mold (used a rough cut and then a smooth cut at 80% overlap that worked well; first pass took about 6 minutes and smooth cut took about 8 mins), and it looked like this ultimately.
Now, the oomoo mold. I added talcum powder so the metal alloy would get into all the nooks and crannies more smoothly and easily. Rubber bands to keep it all together while the metal alloy is poured in. Warmed the metal alloy (it was a bismuth based alloy ingot)