I don't know if this is the best place, but it seems the best out of the exchanges. How do you attach a plastic (or metal) gear to the little pole/nub that sticks out of a dc hobby motor? I have come up with 3 ideas: gluing, it doesn't seem like it would hold well; soldering, maybe but what about plastic gears; and making the hole on the gear slightly smaller so there is enough friction to hold it well, the most likely solution. Has anyone tried these or any other?
a grub screw? - file one side of the motor shaft a little flat, drill and tap a hole in the hub of a metal gear, cut off the end of a bolt with the right thread, then cut a slot in one end so you can turn it with screwdriver
With really small hobby motors, the usual scheme is to go for an 'interference fit', which is basically the 3rd option you mentioned. Even when the shaft is completely round and smooth, this works b/c the power and torques are so low. You will find this done in inexpensive toys, for example.