What is the largest lathe in the world?
According to The Romance of Modern Mechanism, by Archibald Williams, published in 1910, there was a lathe with the following dimensions:
A Gun Lathe. 154 feet long between centres, for boring and turning guns which, with their mountings, weigh 165 tons when complete. The makers are the Niles-Bement-Pond Co. of New York.
It's only mentioned in the caption of the photograph. In the body of the text, it mentions this lathe:
The Midvale Steel Company have in their shops at Hamilton, Ohio, a monster boring lathe which will take in a shaft 60 feet long, 30 inches in diameter, and bore a hole from one end to the other 14 inches in diameter.
Shin Nippon Koki claims
We have developed the largest rotor lathe SLR340×20M that can take in and machine 400 ton of workpice that is the largest in the world.
This one seems to be only twenty metres, which is about sixty feet. However, it can take 400 tons.
Another contender is mentioned at The World’s Largest Lathe is For Sale:
It’s been reported Waldrich Siegen of South Africa built the largest lathe in 1973 at 126 feet (38m), weighing 485.6 tons, the same weight as a jumbo jet aircraft, and capable of handling a workpiece up to 330 tons.
Here is a video of the Waldrich lathe.
So, perhaps the Shin Nippon Koki is the largest in terms of weight, but there are longer lathes.