I've seen plenty of metal printed parts, they do not come out with a very good finish, which is fine for some pieces, but for a cog you don't want rough surfaces. If you printed a cog you're going to need machining just to get a decent finish, so why not just machine it in the first place? If you left it unmachined it's going to act like a grinding stone on your chain, and I can't see that lasting all that long before a failure ...