Hardness and hardness - not the same

[John Hancock Sr]

I have been working on doing a video on abrasives and to this end I am working on a tutorial on abrasives. It is a very deep rabbit hole. I already have a web site dedicated to using the Dremel which talks about abrasives, but only in relation to Dremel bits.
My first full time job was at an engineering supply company back in the late 70s (and no, they did not sell Tormek!). I did learn a lot about abrasives at the time sine we sold a wide range of abrasives and received industry training on many of the goods we sold. I also did materials science at college as par of my electronics training. This put me in a good position to understand some of the science associated with sharpening. However, I have found that it is a far deeper hole.
One of the issues is the term hardness. Unfortunately there are two entirely different definitions of hardness and sometimes they appear to be used interchangeably. There is the engineering definition and the geological definition.
Rockwell hardness is the resistance to plastic deformation. In short they push a ball into the surface with a given force and see how deep the hole is. This is typically used with respect to metals.  On the other hand MOH hardness is the materials resistance to scratching. A material with a higher Rockwell hardness is likely to have a higher MOH hardness but they are not the same thing, nor are they equivalent.
The reason that this is important is that Rockwell hardness does not determine which sharpening material to use, but MOH hardness does. Sharpening is a scratching process, not a plastic deformation process (although plastic deformation plays a part, especially when forming a burr). So you need a material with a higher MOH hardness to sharpen.
Whilst most knives and many tools come with a Rockwell hardness seldom, if ever do they specify MOH hardness. You can get MOH hardness test kits where you use the tool to scratch the material to find its MOH hardness these are not that common.
My practice is to see how well I can sharpen with a particular wheel or stone then change it if necessary.
The rule of thumb is, if the tool or knife is too soft then it will clog higher hardness wheels such as diamond or CBN. if the tool or knife is too hard then it will glaze the lower hardness wheels such as aluminium oxide.

I suspect that the Tormek Diamond wheels may be engineered in such a way they they do not clog with low carbon steels but I do not *know* this. Maybe a question for Tormek themselves.

[Sir Amwell]

I think I understand this John. Interesting.
Would this explain why some so called 'super steels' are easier to sharpen than others even though the Rockwell score is higher?
A cheap and reliable MOH tester may therefore be more useful to the knife sharpener than knowing the Rockwell hardness?

[John Hancock Sr]

If you do a search on Amazon or eBay for hardness test set you get a lot of Rockwell hardness test blocks and a lot of water hardness test kits and a few MOHs hardness test sets. The cheapest is over AU$100 but if you were a professional then this may well be worth the investment.

[tgbto]

John, this is such an enlightening post !

I didn't know about Mohs hardness before reading it, and I've spent yesterday night reading/watching stuff related to it.

A quick newbie question though : if I understand the principle correctly, wouldn't just the MOHS hardness level 9 tester be sufficient to choose between CBN/Diamond or SG ? If I'm not mistaken, level 9 means "scratched by Al2O3 but not by topaz". So in short : before sharpening, get your corundum tester, if it scratches then use SG, if not use CBN/diamond ?

Better yet : could we not try and scratch the surface of the knife with the edge of the SG wheel, observe, and choose the wheel accordingly ? It seems the SG is precisely Al2O3...

[John Hancock Sr]

So in short : before sharpening, get your corundum tester, if it scratches then use SG, if not use CBN/diamond ?

Correct

Better yet : could we not try and scratch the surface of the knife with the edge of the SG wheel, observe, and choose the wheel accordingly

Also correct.

Yu can actually feel it. If the knife is harder it simply slips across the surface. If the grit is harder then it grabs.

You have to test the actual edge steel. Some knives are laminated so it is only the very edge that is hard. Same with Japanese chisels and some plane irons.