[analog31]

I think that both the Celsius and Fahrenheit scales are now defined in terms of Kelvin and a formula. Most if not all of the units of measure are no longer related to their history except for an agreement to make the new units match the old ones at some point.

[0cf8612b2e1e]

The US is technically metric in that the units are defined in terms of the metric equivalent. A US inch is exactly 2.54cm.

[mikewarot]

You can thank Carl Edvard Johansson[1] for that, the inventor of the gage block.[2]

He was the one who decided to make it a convenient value, according to Wikipedia[3]

  As a result of the definitions above, the U.S. inch was effectively defined as 25.4000508 mm (with a reference temperature of 68 degrees Fahrenheit) and the UK inch at 25.399977 mm (with a reference temperature of 62 degrees Fahrenheit). When Carl Edvard Johansson started manufacturing gauge blocks in inch sizes in 1912, Johansson's compromise was to manufacture gauge blocks with a nominal size of 25.4mm, with a reference temperature of 20 degrees Celsius, accurate to within a few parts per million of both official definitions. Because Johansson's blocks were so popular, his blocks became the de facto standard for manufacturers internationally, with other manufacturers of gauge blocks following Johansson's definition by producing blocks designed to be equivalent to his.

This makes it relatively easy to handle making metric threads on imperial machines, and vice versa, because you can use 50 / 127 gear tooth pairs to do the ratios.

[1] https://en.wikipedia.org/wiki/Carl_Edvard_Johansson

[2] https://en.wikipedia.org/wiki/Gauge_block

[3] https://en.wikipedia.org/wiki/Inch#History

[analog31]

An odd little factoid: A metalworking lathe will usually have a mechanism for cutting screw threads, with a multi-ratio gearbox for linking the rotation of the spindle to the motion of the carriage. For cutting metric threads, you have to change a pair of gears. So there will be a pair of gears with 50 and 127 teeth, which produces the 2.54 ratio. Then you have to look up the correct gearbox settings for each metric thread pitch in the manual. [0]

It's a big royal pain in the arse, and I've never had a machinist agree to turn metric threads on a lathe. They would rather wait a day and order a metric tap or die. Nowadays the CNC lathes just do it auto-magically.

Also, among old bicycles, there are some weird national thread standards that had metric diameters but inch thread pitches, because of machinery imported into countries like Italy and France during reconstruction after WWII.

[0] Edit: Just noticed duplicate comment preceding mine.