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Comment by swighton | Hacker News Reader

What I find amazing is that one thousandth of an inch is not particularly precise when it comes to machining. You can easily make parts to that accuracy with common mills and lathes.

Also surprisingly (to me) is if you have a pin that goes into a hole that is 1-2 thousandths of an inch larger than the pin, it feels like a sloppy loose fit. If you want the pin to feel like a really nice smooth fit you need more like a couple TEN THOUSANDTHS of an inch clearance.

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Yea, I'm always blown away by the old master machinists I've worked with when they talk about getting to "50 millionths" (ie half a tenth of a thou) on manual lathe and measurable with a bench micrometer [1].

Also the auto body guys whose hands are like profilometers [2] and can just feel various surface quality [3]. This is actually where precision originally came from with the 3 plate lap where you're just using fingers and candle soot to create crazy high surface flatness [4].

[1] https://willrich.com/product/starrett-direct-reading-bench-m...

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

[3] https://en.wikipedia.org/wiki/Surface_roughness

[4] https://ericweinhoffer.com/blog/2017/7/30/the-whitworth-thre...

You should watch the documentary on the most famous Card Mechanic (Card Magician, but they like to go by 'mechanic') - Richard Turner.

He fan feel thousands of inches and can note the individual thickness of each card he touches.

He is also 100% blind, and he talks about how sensitive his touch is.

He is unbelievably remarkable

https://en.wikipedia.org/wiki/Richard_Turner_(magician)

the documentary he is in is called "Dealt"

https://www.imdb.com/title/tt3127902/

Very cool - thank you!

Or as a machinist might say, a few tenths.

You can do it on common equipment but I dunno if I’d call it “easy.” There’s a reason that +/- 0.05mm costs a lot less than 0.02mm.

This.

For the unindoctrinated, this concise Typical Tolerances of Manufacturing Processes reference[1] may be useful towards establishing pragmatic expectations for manual processes.

[1] https://mae.ufl.edu/designlab/Lab%20Assignments/EML2322L-Tol...

Which is why a more common unit in imperial machining is the "tenth", which is one ten thousandth of an inch.

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