Mysterious Screw

[Smaug]

you will find lead screws and ball screws in machines such as lathes and mills.
they are both do there job very well as for which is better it depends on what you are doing i have been told that for mills and lathes if set up right ball screw tend to be more accurate but u can get real accurate lead screws as well.

[Dick Linn]

Lead screws can be made exceptionally accurate. Moore Tool developed jig boring machines that are very accurate and they used lead screws. Richard Moore has been recognized as the man who "gave the world’s industry an additional decimal place of accuracy!"

His son Wayne wrote the book "Foundations of Mechanical Accuracy".

I don't know what they use these days, but they have jig grinders that will position within 90 millionths of an inch in 18 inches of travel.