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Comment by stormdennis | Hacker News Reader

When I go to buy wood I buy 2" by 4" in 4.8 metre lengths. Was buying screws the other day and I was asking for 3 and a half inch and he was saying that's 90 or something that. I had only written everything in imperial because another guy there uses that. We seem to be stuck in a half and half place.

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Hah yes when I buy timber I ask for a four by two inch quote that is labeled as 50mm by 100mm planed all round and measures 45mm by 90mm. The length is usually correct.

Fasteners are categorized using a higher-level system because there are multiple, independent dimensions--thread pitch, thread width, shank width, unthread shank length, threaded shank length, head type, tip type, etc--at play, and the relationships are relatively arbitrary (AFAIU--maybe there's some sophisticated, deliberate geometry at play I'm not aware of). There's no such thing as a 3 1/2" screw--there are many different types of screws that are 3 1/2" long, and they each have a particular label. So you can't really directly compare the qualities of metric or American standard fastener sizing systems.

I must put you on to my carpenter friend then, seems he's wrong. Not in America btw.

If you do a particular type of work, such as building decks, then 3 1/2" will mean a very specific type of screw. But if you go into a good hardware store (Ace, a smaller, neighborhood chain, is often better than big-box Home Depot or Lowes in terms of variety) and tell the clerk you want a 3 1/2" long screw, things can get frustrating rather quickly unless you can provide some context. When you're working with wood, it's easier to fudge things, but when you're working with machine screws, details like pitch and shank profile often cannot be ignored.

> AFAIU--maybe there's some sophisticated, deliberate geometry at play I'm not aware of

Some earlier fastener standards were somewhat arbitrary, but mechanical engineers have calculated "optimal" values for these parameters, and e.g. the ISO metric screw thread parameters are based on such calculations.

Yeah, I live in Europe. When I go to a restaurant in North America, they have beer in 335,341,474 ml cans, draught beer in pints, 12oz, 16oz etc. Very confusing. If I ask the waiters, they also have have no cloue about what gets me the me most beer for my money. (I did check it all with units(1) now, but will probably have forgotten it next time I need it)

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