Physics of Sharpening Stones

[jrmysell]

I believe what I have read on here about the harder the metal the finer grit stones you need, and the softer metals, you need lower grit. Can somebody explain the physics of this to me? It seems like the harder it is, the coarser the grit you would need to actually remove metal. Or am I just over thinking this?

 

[Benuser]

Some soft carbons get extremely sharp with high grid stones. E.g. French or vintage Sheffield ones. Finely grained, they benefit from fine polishing. With soft stainless though you better keep a coarse edge.

 

[Asteger]

I think the idea is that harder steels on well-forged/treated knives are better capable of benefiting from finer stones, because they can take and hold a sharper edge. With softer steel, a very fine edge wouldn't last.

You'd need coarse stones for maintaining any kind of knife, but of course softer knives would require more frequent attention.

Also, there's the old issue of what level of fineness is necessary and practical for kitchen knives. Good Japanese gyuto can definitely be sharpened much finer than you'd want, but for most purposes many feel that a 5k or 6k edge is plenty, if that, and anything finer, while fun and interested to mess about with, is overkill and impractical because fine, fine edges don't last as long.

 

[psfred]

I think the only real generalization you can make is that softer steel will not benefit from very high grit stones in terms of actual use. The edge isn't going to last very long in any case, and a rougher "toothy" edge will work longer in the kitchen as well as being easily restored by a steel, unlike a hard Japanese style knife where edge failure is due to chipping and microchipping, not rolling over.

It can also be quite difficult to get the very fine grit stones to do much of anything at the actual edge on soft steels as it will not abrade well, it's soft enough to spring back rather than cut.

That said, one really has to see what happens (or find out from someone else!) for any particular combination of stones and knives. The Naniwa "green brick" is reputed to put and wicked and long lasting edge on German stainless, but doesn't do much for hard Japanese knives, for instance.

Peter

 

[MAS4T0]

When considering this from an engineering perspective, it's important to realise that it's the strength of the steel which determines the edge stability; not the hardness. The main reason why hardness is stated is because it's easily verifiable and in the case of steel it correlates well with strength.

It's also important to realise that abrasion resistance and hardness do not generally correlate, so long as the abrasive being used is sufficiently hard to actually abrade (i.e. the abrasive is harder than the steel). Some soft stainless steel is incredibly abrasion resistant compared to much harder steels.

I don't follow your logic, there is no reason why you would need a coarser grit to abrade a harder material; to abrade harder material you need a harder abrasive, not a coarser grit.

 

[Geo87]

to abrade harder material you need a harder abrasive, not a coarser grit.

In terms of abrasion resistant metal I discovered this the hard way . I tried to sharpen very abrasive resistant soft German stainless on naniwa super stones (soft stones )
Bad idea.... Lesson learnt.

 

[Lizzardborn]

In terms of abrasion resistant metal I discovered this the hard way . I tried to sharpen very abrasive resistant soft German stainless on naniwa super stones (soft stones )
Bad idea.... Lesson learnt.

Isn't how hard or soft a stone is a function of the binder and not the hardness of the abrasive itself?

 

[MAS4T0]

Isn't how hard or soft a stone is a function of the binder and not the hardness of the abrasive itself?

Yeah. The hardness of the stone itself isn't related to the hardness of the abrasive, it's down to the binder.

Abrasion resistant steels and soft stones are not an ideal combination, you'll build up a lot of mud and use up the stone quickly.

 

[Geo87]

Abrasion resistant steels and soft stones are not an ideal combination, you'll build up a lot of mud and use up the stone quickly.

In this particular case mud would not form at all even after 5 mins on a 400. Usually takes 30 seconds.
No bur would form either. It was as if no metal was being removed at all... I'm not sure about the science as to why this happened it's just first hand experience trial and error.
A norton oil stone (silicon carbide?) worked fine bur formed quickly.

 

[Keith Sinclair]

Just my experience that low grit whetstones do not work that well with Soft Stainless. Diamond plates better, Atoma 140 & 600. Then finish on a medium whetstone.