A screw cutting lathe requires a good lead screw as its reference. Maudslay invented a "screw origination machine"[1], a clever special-purpose device for making a more accurate screw than the one you have. It only cuts soft metal, so the screw it makes is used in a second lathe to make one in hard steel. This is another of the key steps in bootstrapping to precision.
Early precision work was limited to flat things and round things. Take a look at a steam locomotive from around 1900. Every surface that has to be held to a tight tolerance is flat or round. That's because the precision tools of the era were the planer, lathe, and drill. The general purpose milling machine came later.
Incidentally, this is the real reason manholes were round. In the great era of city sewer-building, you could make round metal manhole covers easily, by casting and a quick lathe pass. The ring into which they fit could also be easily finished on a lathe. Making an iron rectangular frame and a lid to fit it would have been a much harder and more expensive job.
Circa mid-1960s, this defined precision: https://www.amazon.com/Foundations-mechanical-accuracy-Wayne... (at this level of precision, you need a room-within-a-room climate control, because thermal expansion of objects). That leads down the rathole of "jig borers" for making precisely located holes of precise radius and diamond turning machines which are nice for cutting optics.
https://www.nist.gov/news-events/news/2016/09/meet-bob-pmls-...
This doesn't pass the sniff test to me. A manhole lid fits loosely in its frame, and is simply cast. The round shape is used ecause that way the lid cannot fall into the hole (frame is a slightly narrower diameter than the lid is in any orientation)
https://en.wikisource.org/wiki/Encyclop%C3%A6dia_Britannica,...
This sentence under Mounting of Screws rings true today:
> The principle which should be adopted is that no workmanship is perfect; the design must make up for its imperfections.
More fun: http://snl.mit.edu/pub/papers/WP/Nanoruler-White-Paper.pdf
It does into detail on how precision manufacturing was created from the beginnings of the industrial revolution until today. The book has some funny anocdotes from history, and despite being quite long i found it easy to read.
[1] https://www.goodreads.com/book/show/35068671-the-perfectioni...
There are a couple guys on YouTube doing very nice demonstrations of high-accuracy operations. One is Ox Tool Company. Interesting grinding operations etc.
Really smart stuff. These machinists of the 1930 1940 1950s were just as clever as John Carmack or whoever your modern nerd heroes are.
Thanks for posting this link since now I may actually be able to get hold of the book. Previously I couldn't remember the name even.
Manholes are round, or were, so that they wouldn't cave in. Everything in a Victorian sewer is round. The passages are too.
You have a round hole in the ground. You need to put a cover on it. Why would you put a square cover? It's going to be smaller than the hole and restrict access or larger than the hole and waste materials.
That's plenty of reason. There doesn't have to be some aspect of machining or a brain-teaser about square covers falling in when on edge or anything else.
We put square covers on round ducts and vice versa for a bunch of reasons in HVAC today. Aesthetics, ease of making dampers, flow... it's a pain in the neck, but we do it, because something is forcing us to. You need a reason to put a square cover on a round hole.
See:
https://en.wikipedia.org/wiki/Piston
And a this one I found via DuckDuckGo
https://blog.wiseco.com/pistons-arent-round-profile-and-oval...
http://www.douglas-self.com/MUSEUM/museum.htm
Many designs have been tried over the years, but suffer from oil leaks, low compression or high manufacturing costs.
"""If, for instance, having made a small lathe with a pantograph, we find its lead screw irregular – more irregular than the large-scale one – we could lap the lead screw against breakable nuts that you can reverse in the usual way back and forth until this lead screw is, at its scale, as accurate as our original lead screws, at our scale.
We can make flats by rubbing unflat surfaces in triplicates together – in three pairs – and the flats then become flatter than the thing you started with. Thus, it is not impossible to improve precision on a small scale by the correct operations. So, when we build this stuff, it is necessary at each step to improve the accuracy of the equipment by working for awhile down there, making accurate lead screws, Johansen blocks, and all the other materials which we use in accurate machine work at the higher level. We have to stop at each level and manufacture all the stuff to go to the next level – a very long and very difficult program. Perhaps you can figure a better way than that to get down to small scale more rapidly."""
Many ideas contributed to the modern process of 5nm lithography, but having flat surface plates whose precision is determined by the correct application of operations rather than an external standard has turned out to be one of the most profound.
Now, of course, you can buy a granite surface place with accuracy exceeding almost anything available 100 years ago, for $30. And there are youtube videos of flat surface aficinados who scrape their own.