I just had an image of a bunch of insect-like robots crawling over and through a giant metal block, chipping away at internal spaces with precision cutting tools to produce the desired shape, carrying away waste material, like an robotic ant colony digging a smooth steel nest.

they wouldn't have to carry anything, you can blow air into the cavity to get the material out. they could have wheels and drive up and down the cramped cavity with a drill on the end. in the end I think additive has won.

You can print voids then flow mill it? https://en.wikipedia.org/wiki/Abrasive_flow_machining