It's amusing to view such a comversation.
I started out by saying in essence with extra work (and tricks) tolerances greater then the lathe specs are obtainable. The skill of the operator also comes into play. I severely doubt someone can hold a toleramce of .0001 on a Unimat (any model). Judging by the U3 I had, I doubt anyone could hold .0001 repeatedly.
But not all lathes are equal. The typical engine lathe is not made to the same specifications as say a Hardinge HLV-H. Or a Levin. Much more is capable repeatedly. The old Rivetts were reknowned for their accuracy. There was an article on the net over 20 years ago describing how a guy modified a Rivett 608 to cut 100,000 threads per inch. Creating what was called a diffrection grating. I imagine you can still look that up.
Is stuff like that doable on a Chinese 7 x 12? Go ahead and try. I remember a guy saying years ago it's nice when you can make 2 "identical" parts on a lathe. But with those you couldn't make 1!!!. I am not knocking people's machines. It's just that there is a wide variation in quality. And that's just the way it is. A very well made machine is capable of higher tolerances - repeatedly. The cheaper machines require more work/tricks/skill to achieve better results.