My father's machine shop has a lathe that can handle 47" diameter turning stock. I know they have turned some things 18-20' long. If need be they can turn longer stuff. It is two phase electric not the normal three phase. Came out of The Naval Yard in Philly originally I was told. The motor is like a small car. Pre WWII and was moved on a flat car and lifted off and into the shop by a crane. Fun to work with big tools.
can you imagine the 1" steel scallops you could make segmenting on this lathe. It would be amazing. Now keeping your pen in the shirt pocket could be a trick.