Some of this is covered in One Good Turn: The Natural History of the Screwdriver and the Screw, which I enjoyed more than The Perfectionists. https://www.amazon.com/gp/product/B00CVR143M/

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.

Fast-forward to 2016 to see how much the technology has changed:

https://www.nist.gov/news-events/news/2016/09/meet-bob-pmls-...

> Incidentally, this is the real reason manholes were round.

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)

But that works with equilateral polygons as well. How would a square lid fall into a smaller square hole?

Diagonally, because the hole is 1.4 times wider diagonally than the cover is on each side

Me dumb. The hole would have to be much smaller.

Diagonally. The length of a side is considerably shorter (1/sqrt(2) times shorter, actually) than the diagonal.

There's a lovely article from 1886, written by Henry Rowland, who made the first decent diffraction gratings. He wrote about his process of lapping the screw threads to take out minor local errors. His screw didn't have to do a lot of work, but it had to be damn uniform.

https://en.wikisource.org/wiki/Encyclop%C3%A6dia_Britannica,...

That is a marvelous article, thank you!

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.

Interestingly, even after that work, they ended up having to debug a really tricky problem. They sent their gratings to the premier astronomer at Harvard, who reported they weren't "perfect. Eventually this was debugged to a situation where a cam advanced the cutting tool after completing a cutting row, but only occurred when the cable connecting the motor to the tool had a slight bulge due to how the cable was cut and joined.

More fun: http://snl.mit.edu/pub/papers/WP/Nanoruler-White-Paper.pdf

If you are interested in the history of precision engineering. I can highly recommend the book titled "The Perfectionists: How Precision Engineers Created the Modern World".[1]

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...

If you're going to go down such a rabbit hole, you'll be well served by checking out the ostensible bible on the subject as well, "Foundations of Mechanical Accuracy" by Moore.

This book astounds me. The depths they went to achieve millionth of inch accuracy was crazy. That book is quite expensive and rare it seems. I have a copy but it was 100$+ dollars.

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.

without the wait https://archive.org/details/FoundationsOfMechanicalAccuracy/...

I saw this book precisely once at the bookstore and could never find it again.

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.

Why are people so obsessed with "the real reason" for round manhole covers?

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.

The reason people are obsessed, is that it was published in a book about software interviews, but the stated answers aren't always "right". For example, there are square manhole covers and the world doesn't fall apart. The reason isn't necessarily "because the geometry dictates this answer" but at times was probably expedient.

Even today modern precision work is cheaper for round things than other shapes. This is why you will not see internal combustion engines with square pistons / cylinders.

Worth noting that modern pistons have ovality!

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...

Interesting! My statement was based on what I read in the Museum of Retro Tech:

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.

Something I've discovered by accident is that the lathe sits idle in most machine shops these days. If you can design something that can be made on the lathe, you can often get it done with virtually no waiting time.