Based on my small sample of friends: no, it's really not.
It used to be, but these days there's too many other options for cheap and strong materials, and places and methods to get parts made. There's definitely ways to position yourself to make good money, especially if you have connections and experience, but it's certainly not as simple as "set up a shop and make a good living".
If you can get contract jobs for aerospace, petroleum etc. then yes. For example, short order prototyping in aerospace and repairs in petroleum. Kind of like getting a programming related job, it takes some networking and reputation. Custom work for cars/motorcycles can also be good with networking and reputation. Other than that, it's basically being the cheap mechanic that can fabricate new parts instead of buying them. Which is not good in the good-vs-evil sense, but not so lucrative.
It used to be, but these days there's too many other options for cheap and strong materials, and places and methods to get parts made. There's definitely ways to position yourself to make good money, especially if you have connections and experience, but it's certainly not as simple as "set up a shop and make a good living".