This is what we did on our CNC this year.
We choose the chain size.
Measured the chain spacings and such
Calculated how many teeth we wanted on the sprockets.
Got some plate aluminum.
We then figured out how far the teeth/holes needed to be apart. Calculus....
Made a G code program.
Once we got the code ready we took a drill bit slightly bigger than the chain "hooks" or whatever you call them. The machine then drilled a series of spaced holes all the way around the plate of aluminum. We then placed a mill bit in the machine and machined half of the holes away.
Here are some pics of our arm's gears:
http://www.tigertrons.com/Pictures/2...n/100_0596.JPG
http://www.tigertrons.com/Pictures/2...n/100_0710.JPG
http://www.tigertrons.com/Pictures/2...n/100_0716.JPG
These worked great for us. We also made some 50 tooth sprockets for #35 chain in our drive train