Let's say you needed to make a perfectly circular steel ring in an enormous size, like 25 feet in diameter. What production method do you use? Bueller, anyone?
You'd use what's called ring rolling, or more properly "seamless rolled ring forging." Check out the first 2.5 minutes of the video below to see it in action. It kind of reminds me of watching people make pizza crusts or use pottery wheels:
So what do you do with a big-ass part like that? Ring rolling is used to form parts for rocketship sections, pipes, turbines, and enormous wedding bands for matrimony-minded giants.