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Comment by tonetegeatinst | Hacker News Reader

I think diamonds are used in some lathes as cutting tools. So suprising thats not more common. I though diamonds strength and iirc its heat tolerance would be attractive to the folks who cut stuff.

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Diamonds are excellent for cutting some materials, e.g. ceramics or some non-ferrous alloys, but they are bad for cutting metals that are good at dissolving carbon, e.g. iron, cobalt and nickel alloys.

So for the iron alloys, which are the material most frequently processed by machining, diamond is not suitable. Other hard crystals, like alumina a.k.a. corundum, are much better for this purpose, even if they are less hard than diamond.

For those metals, cubic boron nitride works and has about half the Knoop hardness of diamond (and more than twice the hardness of aluminum oxide.)

c-BN is isoelectronic with diamond; h-BN is isoelectronic with graphite (but is an insulator).

They're not that heat tolerant. They're fantastic heat conductors, but they'll burn away into carbon dioxide (via carbon monoxide). The temperatures needed to do that are high, around 900C, but that's not that high. Angle grinder sparks can be hotter than 1000C, as can the edges of carbide tools.

Diamond turning is used to produce optical surfaces with lathes. Here's a recent example on youtube: https://www.youtube.com/watch?v=ZPTFFPLOzCw

The hardness is attractive but the poor heat resistance is a major problem for many uses. For a normal angle grinder you can use normal abrasive disc without paying much attention, but with a diamond-grit one you can easily burn the diamonds.

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