In looking at this question:
@Pᴀᴜʟsᴛᴇʀ2 gave an excellent answer that got me thinking. First, lets include the same picture from that answer so we're on the same page:
When reconditioning the crankshaft, it is spun on a lathe to either remove enough material from the journals (both main and crankpin journals) to make then round again, or just to polish them. [Refer to the diagram] The crankshaft is designed to be balanced when spun along it's central axis, which is the center of the circle made by the main journals. So machining the main journals is just a question of the usual machining techniques, and polishing is as described in the referenced answer.
But, what about when trying to machine or polish the crankpin journals? These are not concentric with the main journals and there doesn't seem to be any way to mount the crankshaft so that it is spun along the central axis of each crankpin journal. How are the crankpin journals machined or polished?