A thousandth of an inch would do it? They couldn't give more margin-of-safety to a critical part like that?

A thousand of an inch isn't such a theoretical number. It's about 25 microns, and I've shimmed one of my back-focusing photography lenses for less than that much (about 10 microns, to be specific). This is something that they ought to be able to machine for, but depending on the context, it might not leave much room for error.

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gottorf 2 days ago | parent [-]

> A thousandth of an inch would do it? They couldn't give more margin-of-safety to a critical part like that?

If it's true, that's truly terrible design.

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privatelypublic 2 days ago | parent [-]

Its likely a misunderstanding and/or mischaracterization of "tolerance stacking."

A safe example is bike chain. If each one is 1 inch +- 0.01", if every single one is +0.01" then ten links will be long by a tenth of an inch. And might pass QC on the bike when pedaled by hand- but it'll fall off when somebodies full bodyweight and 100hrs of wear is out into it.

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whyever 2 days ago | parent [-]

That's not how errors add up, it's nonlinear. You have to take the sum of squares. So in your case, it wouldn't be 10 * 0.01 = 0.1, but sqrt(10 * 0.01^2) = 0.032, which is less than one third of a tenth.

	▲	privatelypublic 2 days ago \| parent [-]
		I provided a "worst case", not statistical, example. For those who want an example, calculator, and demo see: https://www.smlease.com/entries/tolerance/tolerance-stackup-... NB: using disks like the site does provides a clearer example.