Step 1: Watch the Video
I made a full tutorial on creating these gears. I recommend watching it, as it goes a little faster than reading this instructable, and goes into further detail.
Step 2: Templates
Step 3: Cutting Foam
Step 4: Casting
Make sure while doing this, that you have all the necessary safety equipment at all times. This includes: face shield, fire proof gloves, long handled pliers/tongs and a fire extinguisher.
Step 5: Filing/Finishing
After I pulled my gears out of the mold, they looked very rough and uneven. Since these gears aren't going to be held or touched very much, I won't need to polish them as much as my Aluminium Handled Knife, but I still want them to look better than coming straight out of the molds. I first clamped the gears in the vise and cut off the bit of aluminium that filled the chimney. For the first gear, I used a normal hacksaw, but it took a very long time so for the second round, I used a Sawzall with a metal cutting blade. Then I flipped the gears in the vise and used an angle grinder to grind the faces down to a more silvery finish. After that, I used a drill to cut out the holes that had been filled in the casting process. Finally, I used an assortment of metal files to fine tune the teeth and sprockets of the gear. In the end, I could have spent a lot more time on them but I didn't have a use for them, so for now, they will stay rough.
When I cast the gears, the larger one didn't fully take shape and ended up not having several teeth. To solve this problem, I used IFixit, a kind of putty-like epoxy, and sculpted the teeth by hand. It actually turned out very well and worked better than I had hoped.