I wasn't sure if epoxy would stick to the wheel but I repaired it anyway. But I'll bet the repaired tooth breaks soon - they always do.
Since everybody will tell me that I can't make gear wheels I thought I would see if I could. We have no indexing head or anything to make gears with, so I cobbled up a holder using the old gear wheel which has 21 teeth. If it had 16 teeth I could have made an indexing plate. But how do you space 21 holes around a circle?
Here's the broken gear wheel on the left top, and after I repaired it with epoxy resin, at bottom left. http://i54.tinypic.com/fviuth.jpg
On the right is the piece of plastic that I turned up on the lathe, and after I cut the teeth in it using the mill. I considered using brass but I found this black rod that seems to be some exotic very strong plastic. I couldn't find any suitable cutters so the teeth are the wrong shape. It doesn't seem to make a difference to the horrible piece of engineering that it's going into, the one that broke in the first place. The gears still go around OK.
Here's the setup in the mill: http://i51.tinypic.com/2uij18m.jpg
I discovered that the holder in the mill has to be very accurate. The hole in the middle of the gears is 7mm and I don't have any bolts that size and I didn't want to stop and make one. So there was too much slop. Now I think I need to buy an indexing wheel for next time!