Here is a clever way to make gears that comes to us by way of LumberJocks.com. The method involves cutting a circular blank for the gear, then wrapping it with a section of car engine timing belt. The toothed belt conforms to the outside of the circle creating gear teeth. The belt is secured to the wood circle with strong a adhesive.
It seems like the tricky part might be in creating gears of different size to achieve certain gear ratios. It seems like a cardboard circle could be used to test how the belt will sit and if the teeth are spaced appropriately at the seam. Once the proper size circle is established with cardboard, the blank could then be cut in wood. Once you have the right sizes for several different gears, keep a record or a template and the job will be easy the next time around.
Here is the original full blog post with additional pictures on these gears made from wood circles and engine timing belt.
[ Thanks Charles! ]