[quote quote=“razoredgeknives” post=19760][quote quote=“CliffStamp” post=19735]
As an aside, increasing the apex angle doesn’t lower the sharpness, it doesn’t do anything to the sharpness at all because sharpness isn’t a function of the apex angle. [/quote]
I don’t understand… unless you are meaning it “scientifically”. But for all intents and purposes when you lower the apex angle (as long as you have a true burr free apex) then it will feel much sharper when cutting through any given medium…no?[/quote]
I would not presume to speak for Cliff Stamp but I think I may understand what he means. If so it would be helpful to think of sharpness (rather than over-all cutting ability due to geometry) as experienced by cutting something infinitesimally thin as there only the very edge (apex) is contacting the material. For example a relatively obtuse and wide edge on styling shears can still “pop” hair because hair is quite thin. Anything else is geometry behind the edge, not sharpness of the edge itself.
By this definition we spend only a small fraction of our time on a knife sharpening it; the rest is spent modifying the geometry in preparation for sharpening.
If so it would be helpful to think of sharpness (rather than over-all cutting ability due to geometry) as experienced by cutting something infinitesimally thin as there only the very edge (apex) is contacting the material.[/quote]
Exactly right, and that is why this is sharp :
and this is not :
But everything is the same geometry wise except for the condition of the very apex.
The first picture is the apex at 15 dps off of a new fine india stone (edge is at 5 dps). The second picture is ~500 slices through 1/2" hemp later. The knife is much more dull (sharpness, < 1.5% of optimal) but the only thing which has changed is the apex itself.
[quote quote=“Mr Wizard” post=19801][quote quote=“razoredgeknives” post=19760][quote quote=“CliffStamp” post=19735]
As an aside, increasing the apex angle doesn’t lower the sharpness, it doesn’t do anything to the sharpness at all because sharpness isn’t a function of the apex angle. [/quote]
I don’t understand… unless you are meaning it “scientifically”. But for all intents and purposes when you lower the apex angle (as long as you have a true burr free apex) then it will feel much sharper when cutting through any given medium…no?[/quote]
I would not presume to speak for Cliff Stamp but I think I may understand what he means. If so it would be helpful to think of sharpness (rather than over-all cutting ability due to geometry) as experienced by cutting something infinitesimally thin as there only the very edge (apex) is contacting the material. For example a relatively obtuse and wide edge on styling shears can still “pop” hair because hair is quite thin. Anything else is geometry behind the edge, not sharpness of the edge itself.
By this definition we spend only a small fraction of our time on a knife sharpening it; the rest is spent modifying the geometry in preparation for sharpening.[/quote]
Thanks for posting man… I think this helped clear things up in my head as to the definitions. So, while many things increase cutting ability (i.e. removing sap off of the blade, lowering the primary grind, handle ergonomics, etc) grinding metal away to actually apex the edge is what sharpens it. The apex itself is what sharpness is referring to.
So for example (much like cliff’s example). If I have one knife that is dull (edge blunted) such as a japanese knife with an extremely thin edge, and then have a western knife with a truly sharp edge but a thicker shoulder width, then even though the japanese knife would technically be dull it would have higher cutting ability than the western knife whereas the western knife would be sharper… correct?
[quote quote=“razoredgeknives” post=19809]
So for example (much like cliff’s example). If I have one knife that is dull (edge blunted) such as a japanese knife with an extremely thin edge, and then have a western knife with a truly sharp edge but a thicker shoulder width, then even though the japanese knife would technically be dull it would have higher cutting ability than the western knife whereas the western knife would be sharper… correct?[/quote]
In my experience it’s not that simple. It depends on the material. I believe the western knife in the analogy would cut hair (shave) while the J knife would not. It would be able to push cut thin but tough cord cleanly while the J knife would not. On the other hand the J knife might cut cardboard more easily, depending on just how blunted “dull” is. And the J knife would be a much better choice as a cheese knife. Ben Dale (Edge Pro Inc.) had a demo where he easily cut a carrot with a putty knife to demonstrate that on some materials geometry is far more important than sharpness.
Think of cutting ability as that which measures how much force a blade requires to make a cut. It all gets pretty simple then as there are three basic forces :
-the force right against the apex, some materials are very hard to start a cut (sharpness)
-the force to push material apart, some materials are very rigid (blade cross section)
-frictional losses, some materials are very sticky (this is related to but not equal to the second term)
A carrot does not take much to start a cut, you can even just push your thumbnail into it, thus a blade doesn’t need to be very sharp to cut it well. But a carrot is very rigid and thus a thick blade will break the carrot and requires more force. A carrot in general isn’t very sticky and thus there are no frictional concerns.
A piece of rope is very hard to start a cut, you will not be able to push your thumbnail into it (unless you emit gamma radiation). A knife has to be decently sharp to cut rope well. However rope tends to fall apart much more so than a carrot and thus cross section isn’t as critical. Rope isn’t very sticky at all.
A wet potato also isn’t hard to start a cut, but is very rigid and also sticky (starch is used to make glues). Thus a very thin convex profile works very well (or a relief hollow ).
Ideally you want a knife which is sharp, doesn’t bind and is resistant to sticking to cut a broad spectrum of materials well. But specialty knives can ignore 1 or even 2 factors if they just cut very few materials.
–
As an aside, those properties of the materials are actually measured by the rupture pressure, Young’s modulus and coefficient of kinetic friction.
[quote quote=“razoredgeknives” post=19822][quote quote=“CliffStamp” post=19820]
Think of cutting ability as that which measures how much force a blade requires to make a cut. It all gets pretty simple then as there are three basic forces :
-the force right against the apex, some materials are very hard to start a cut (sharpness)
-the force to push material apart, some materials are very rigid (blade cross section)
-frictional losses, some materials are very sticky (this is related to but not equal to the second term)
[/quote]
This is perfect… simplifies it to my level =) Totally agree here.[/quote]
Yep, the clearest explanation of cutting ability I’ve ever read! Many thanks, Cliff!
