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I learned in high school science that everything has a hardness index, that metal has a relatively low hardness compared to many kinds of stones, and that something cannot be scratched by a substance that is less hard.

Then why does the disc on an angle grinder wear down? Why can't it be made of a substance that can stand up to metal?

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  • $\begingroup$ Soft things wear hard things - a rubber seal can put a groove in a steel shaft... $\endgroup$ – Solar Mike Jun 23 '18 at 12:29
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Scratching is hardly the only thing that destroys objects. For example: Glass is harder than cast iron, a cast iron anvil thus can not scratch a glass pane. However if you drop the anvil on the pane it is very likely to shatter.

Now the individual particles on the grinding wheel are being hit repeatedly by a impulse force. This causes the ginding grains to fracture. More generally it causes material fatigue after enough cycles. Heat can also cause problems to the surface and certainly makes fatigue worse.

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  • $\begingroup$ The "cutting" part of grinding wheels is not usually made of metal, so the "metal fatigue" comment doesn't apply, but the general point the answer makes is correct. $\endgroup$ – alephzero Jun 23 '18 at 13:54
  • $\begingroup$ @alephzero To be fair, the answer uses the phrase "material fatigue". I have no idea whether fatigue is a mechanism of wear in this context. $\endgroup$ – Eric S Jun 23 '18 at 14:08
  • $\begingroup$ Sorry, my bad, I misread "material" as "metal". $\endgroup$ – alephzero Jun 23 '18 at 15:11
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as a traditional grinding wheel is used, the abrasive particles get shed in the process of removing material. this exposes fresh and sharp abrasive particles so the wheel cuts like new throughout its life.

(an exception: when grinding gummy materials like aluminum, the wheel will fill with galled-up metal which has to be removed by the user with a wheel-dressing tool. this also removes some of the wheel itself, but in the end, fresh cutting surfaces are exposed.)

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    $\begingroup$ The wheels are basically sand bonded together. The bond is chosen so that the wheel exposes fresh grit at the desired rate for the production process. Grinding is a very expensive way to form metal, so cost is crucial. Sometimes, using a bunch of cheep wheels is the way to go. Sometimes it isn't. There are literally thousands of different grinding wheel specifications to choose from. Not everyone wants to spend $1200 on a bench grinder to sharpen garden tools once a year. But the fellow sharpening your barber's shears and scissors probably does. $\endgroup$ – Phil Sweet Jun 23 '18 at 19:23

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