Sharpness vs Cutting Ability

Larrin

Larrin

Knifemaker / Craftsman / Service Provider
Joined
Jan 17, 2004
Messages
4,999
New article on the definitions of sharpness and cutting ability, where I answer such questions as:

How is sharpness defined in the scientific literature?
Is sharpness better measured with its cutting behavior or by observing the edge?
Is a thinner edge automatically sharper than a thicker edge?

https://knifesteelnerds.com/2018/08/06/sharpness-vs-cutting-ability/
 
Interesting.

Especially this conclusion:
... I think the best definition for sharpness is the energy required for initiating a cut.
Click to expand...

I think a lot of people agree with you even if they don't know it. What else are they demonstrating by cutting the flimsiest, unsupported substrate as possible to show the sharpness of a knife? A phone book page tube for example. They're showing, the only way they can through a visual representation, how little force is required (through common understanding of the thin paper's flimsiness) to initiate a cut.
 
Interesting that the sharpness tests are all push cut based. The only real world use of push cutting I can think of at the moment are specialty cases like a razor or cases where you might design for toughness over sharpness (chopping tasks). Many of the things I can think of where folks are particularly concerned with sharpness (kitchen, game processing) are slicing tasks. As I think i understood, a slicing based sharpness test seems to have some complexities to designing a scientific test for. If there was a good test, one of the things that I’ve always been curious about if valid vs myth is the notion that lower grits make “sharper” slicers due to “micro serrations.”
 
lanternnate said:
Interesting that the sharpness tests are all push cut based. The only real world use of push cutting I can think of at the moment are specialty cases like a razor or cases where you might design for toughness over sharpness (chopping tasks). Many of the things I can think of where folks are particularly concerned with sharpness (kitchen, game processing) are slicing tasks. As I think i understood, a slicing based sharpness test seems to have some complexities to designing a scientific test for. If there was a good test, one of the things that I’ve always been curious about if valid vs myth is the notion that lower grits make “sharper” slicers due to “micro serrations.”
Click to expand...
Also rock chopping with a chef's knife.

Micro serrations are definitely a thing. Not a fan, but a knife at 220 + strop will definitely saw through some materials. Comparing the edge to an 8k + strop, the keener edge will outperform on most tasks, but for something like cutting up cardboard, the lower grit knife seems to do just about as well and takes no time at all to sharpen.
 
I usually explain this subject by asking ,"Which will cut a loaf of fresh baked bread best - A super sharp yanigi-ba or a serrated bread knife?" Then I flip the question, "Which will slice a fresh tuna better?" The answer is each edge has its own realm, and saying one or the other is sharper is specious.
Just as in Physics and other sciences - a knife is only sharp in its frame or reference.
 
Slicing isnt the same as sawing. I think sawing through a loaf of bread has a totally different cut initiation situation. Instead of having to overcome the surface tension along an entire edge-to-material line, you have a much smaller serrated tooth-to-material line. So it is the same physics, it is just applied differently. Also, slicing is perpendicular to material whereas sawing is parallel, so the force required to initiate a cut may be different.
 
That is exactly my point. It is determined by the task ( frame of reference). I use the most exaggerated ends of the spectrum in the example to show how large the difference can be.
Comparing a micro-serrated edge to a polished edge (eg) will depend on what it is slicing ... and if the edge is designed to slice the tst material.

I consider total sharpness an edge sharpened to it's smallest edge thickness possible without having grains fall out. This is only a few microns thick. This is an empirical edge, and would not survive use.
From the empirical edge, it is geometry that determines how well and long it cuts. I consider micro-serrations and even full serrations a part of the geometry.
 
But does a bread knife actually cut bread better than a yanagi ba?
 
I have a riddle for you. Explain scientifically why a saw is difficult to initiate a cut, where only a few teeth are making contact with the material, but then once you get a little notch, and more teeth are contacting the material, it gets easier to continue cutting?
 
That's simply pressure distribution and subsequent depth of cut. When you start a cut, very few teeth are engaged. All of the force down is focused on those few teeth, causing them to take extremely deep depth of cuts. When the kerf is opened up slightly, more teeth are engaged and the force more evenly distributed, with a shallower depth of cut per tooth.
 
Ah, so the total sum of the depth of all the teeth of a saw contacting a material is actually decreasing as the number of teeth increases. This makes sense. If there were infinitely many teeth contacting the material, each tooth would be supporting essentially 0 force, and so wouldnt initiate a cut at all. In theory.
 
No the opposite. It would initiate very easily (just not very quickly in relation to linear saw travel:depth). This is a phenomenon you can really only see in surface grinding as it's the only controlled process people do that can simulate "infinite" number of "teeth" but our common idea of grinding is lower grits grind material faster/easier. This is true to a certain hardness, where the material begins to reject a depth of cut greater than (X) regardless of grit size, and a finer grit wheel with more "teeth" will actually remove material faster, because if the depth of cut is a constant limited by the material resiliency, then more teeth per RPM at the same depth of cut moves more material than fewer teeth per RPM.

This is hard to realize with belts because in those finer grits they tend to lose grit too quickly, but with a solid wheel it's definitely noticeable.

Think of a file, and trying to break a sharp corner. If you engage the file on the sharp corner parallel to the file teeth, it's almost impossible to get the cut started. That's because the corner is engaging the entire tooth. One, that takes a lot of force to shear off that corner for the entire depth of the tooth, and two, even if you have the force to do it, there's no clearance left in that tooth for the material to go.

Now turn the file so the teeth are perpendicular to the sharp edge and lay it along the entire edge for the entire cutting length of the file. Zip, clean and easy as could be you've broken that edge.
 
I find that a sharp clean knife makes cuts in bread more cleanly but requires more force than a serrated blade. I don’t have a serrated bread knife. Instead I just made a long, thin bread knife with very thin geometry. Cuts beautifully
 
joedhiggins said:
I find that a sharp clean knife makes cuts in bread more cleanly but requires more force than a serrated blade. I don’t have a serrated bread knife. Instead I just made a long, thin bread knife with very thin geometry. Cuts beautifully
Click to expand...
In my kitchen we use knife I made from 1.5mm thick 1.2519 steel ....
we thought that that knife was a sharpest knife in the world /sharpen on 800 grit diamond stone / ....and it was sharp ......hair popping sharp....Then I sharpen that knife on 40 grit ceramic belt .........Now it bites if you just look at him . I never experienced that kind of easy cutting ........
 
Cutting food is like cutting other materials. Initiating a cut/fracture requires more work than propagating it.
 
Well Larrin Larrin , for me it is simple .....Coarse edge last longer then fine edge .Coarse edge will make push cut . Much fine edge will better do push cut of course but can t do well sawing ......
End here is my proof ....................:D
This hand wood saw will cut wood even when teeth are dull .Not as when teeth are sharp but will cut, right ? Now try to cut wood with spine of that saw , can you ? Of course no .The same is happening with edge on knife.Coarse edge have bigger teeth/ serration and is needed much more time for them to worn to zero .Very fine edge have very small teeth/serration and when they a gone you are trying to make cut with spine of saw :) Make some logic this ?
97nC99n.jpg
 
Last edited:
You must log in or register to reply here.
Back
Top