if you find cnc routers cool, as i once did, i recommend you check out the world of machine tools- i.e. things that are used to make steel (and aluminum) parts. i would suggest starting by understanding how a manual lathe and vertical mill work (blondiehacks on youtube might be a good place if you are totally unfamiliar), and then seeing how they are driven with cnc is very cool. though that part is somewhat similar to cnc routers, but just much more demanding. and the precision is incredible if you arent familiar with the world of machining- even low grade hobby machines can hold tolerances of 1 thousandth of an inch easily across 18" of travel. its also a very interesting world because its been steadily evolving over the last 100 years, and metalworking for who knows how long before that. just be prepared to watch 1000's of hours of youtube videos, machinists like to make 45 minute+ videos for some reason.