Kevin R. Cashen
Super Moderator
You ever notice how each knife user has their own taste in sharpening stones? Not just types or brands, mind you, such as your uncle swears by natural Arkansas, but your Granddad wants a Norton, but how some prefer widely differing grits. Some folks insist in progressively finer stones, followed up with a strop, and others are good with a couple of strokes on a single carburundum block. If one guy is polishing down to less than a micron while the other is cleaning deer like crazy with visible grit marks on the edge, how can they both be “sharp?”
Sharp is a funny concept that really cannot be completely separated from opinion. One of my biggest pet peeves is hearing some “expert” on television utter the words “this sword was as sharp as a razor.” Is that his “expert” opinion, because if it is, he really doesn’t understand much about swords or razors. But the term “razor sharp” has been widely misused for so long that most of us seem to have forgotten that there really are such things as straight razors, and even our best hunting knives will never have an edge that resembles it; and it would be a terrible knife if it did.
This is just one small example of how totally subjective the very concept of “sharp” can be. But when one approaches the topic with more objective observations and measurements, there are still variables to confound pinpointing a single definition of sharp. To start with, one can identify at least two types of cutting action. In order to help students visualize these concepts, when I teach, I describe the two types of cutting action with the analogy of the saw and the chisel. Both are “sharp” and both cut quite effectively, but do it in very different ways.
The chisel separates the cut material like a wedge in a straight-line pushing cut, the more streamlined and polished its edge geometry, the less resistance there will be. In very fine cuts it shaves, but in heavier cuts it cleaves and displaces masses of material as a wedge.
The saw severs the fibers of the material by drawing its many smaller sharp points, or “teeth”, across them. The more of these teeth per area, the finer the cut. The fewer in number, and larger, these teeth, the more aggressive the cut, tearing its way through more than slicing.
If you try to push a saw blade into wood, like a chisel, the fibers will simply collect between the teeth and bind up. But if you try to draw cut, or saw with the edge of the chisel, its polished edge will simply slide back and forth over the wood in a soapy fashion with only the slightest of penetration. So, which one is sharp? Used correctly, they both are, used incorrectly neither are.
For this reason, the fine polished edge of a straight razor, or fine chef’s knife, will often have a smooth feel to the edge when touched, leading some to think that they are not sharp. But applying pressure directly into that edge, in its preferred mode of cut, would be ill advised if you didn’t want to unpleasantly gain a whole new understanding of “sharp.”
On the other hand, the “bite” of a sharp hunting knife blade is what many would more immediately recognize as sharp. That toothy, microscopic, grab of every ridge of your finger pad, bringing the short hairs of your body to attention, is a classic feel of what many would call sharp. It is an edge designed to sever fibers like a microscopic saw. But if you were to try to shave with it you would find your skin left burning as it savagely raked its way over the surface, pulling hairs as much as cutting them.
These two distinctly different edges lead to much confusion about “sharp.” I have found that edges with greater hardness tend to polish, more than deform, on a stone, lending itself much more to the chisel type cut. Edges with lower Rockwell levels will much more readily engage the abrasive to form those toothy rakers conducive to the slashing draw cut. I have found hunting knives left at 62.5HRC, or higher, will bring a frown to potential customers brows, due their lack of tooth, despite how well they may shave hair or slice a vegetable.
This is the affect at play when the old hunter who, using a rather coarse India, puts and edge on a blade that looks like it was done with a 36-grit belt, lays back impressive sheets of skin and flesh from large game in long sweeping cuts. It is simply a different kind of sharp. This edge can also give the impression of longevity, just like a serrated edge continues cutting despite being much less than sharp between the teeth points.
Now, we once again come full circle to how steel selection can work with heat treatment for these edges. Steel is not a material monolithic in nature, its properties as a mixture are often quite prominent. For this reason, one must not make the mistake of confusing abrasion resistance with hardness (strength) as they can be quite independent of each other.
We think of steel of a mixture of iron and carbon, but what it actually is a mixture of very low carbon metal and carbide. The metal matrix is capable of having vastly different levels of hardness than the carbide particles suspended within it, which the metal itself could never hope to equal. Think of glass chunks mixed in playdoh. You can dry out the playdoh and make it hard, but never as hard as the glass chunks it holds. Our you can leave it soft, but the glass chunk will still be hard.
If you make a playdoh knife, and leave it soft, you can still cut many soft things with a sawing motion as the glass does its thing, but you will get tearing and deformation of the soft edge areas between. And, of course, if you try to hit something harder than playdoh the whole thing will just fold over on you. If you dry it out, and harden the playdoh, you can now cut many more things with it, and the glass will still do its thing, and with less loss of playdoh, so you can cut for much longer.
Steels with added carbide forming elements, like Cr, V, or W, will increase this effect of abrasion resistance, independent from hardness. With these steels one can play what I call “carbide games” creating an edge that will cut quite aggressively, like dull serration, on soft fibrous materials (this is why cutting notebook paper is really not impressive). You just don’t ever want to go at something more solid with it. However, if you want a finer edge, you must get those carbides finer. They may make nice coarse rakers, but they won’t make a really fine push cutting blade if they are big. A large carbide D-2 razor would be like a garden rake on your face.
So, with a hunting knife, loaded up with W carbides, you can give the impression of edge holding sharp, while still drawing it back far enough in the temper for it to fold up on a 2x4. But you can also refine carbides and leave the steel quite hard and get the best of both worlds in a finer cutting blade.
Sharp is a funny concept that really cannot be completely separated from opinion. One of my biggest pet peeves is hearing some “expert” on television utter the words “this sword was as sharp as a razor.” Is that his “expert” opinion, because if it is, he really doesn’t understand much about swords or razors. But the term “razor sharp” has been widely misused for so long that most of us seem to have forgotten that there really are such things as straight razors, and even our best hunting knives will never have an edge that resembles it; and it would be a terrible knife if it did.
This is just one small example of how totally subjective the very concept of “sharp” can be. But when one approaches the topic with more objective observations and measurements, there are still variables to confound pinpointing a single definition of sharp. To start with, one can identify at least two types of cutting action. In order to help students visualize these concepts, when I teach, I describe the two types of cutting action with the analogy of the saw and the chisel. Both are “sharp” and both cut quite effectively, but do it in very different ways.
The chisel separates the cut material like a wedge in a straight-line pushing cut, the more streamlined and polished its edge geometry, the less resistance there will be. In very fine cuts it shaves, but in heavier cuts it cleaves and displaces masses of material as a wedge.
The saw severs the fibers of the material by drawing its many smaller sharp points, or “teeth”, across them. The more of these teeth per area, the finer the cut. The fewer in number, and larger, these teeth, the more aggressive the cut, tearing its way through more than slicing.
If you try to push a saw blade into wood, like a chisel, the fibers will simply collect between the teeth and bind up. But if you try to draw cut, or saw with the edge of the chisel, its polished edge will simply slide back and forth over the wood in a soapy fashion with only the slightest of penetration. So, which one is sharp? Used correctly, they both are, used incorrectly neither are.
For this reason, the fine polished edge of a straight razor, or fine chef’s knife, will often have a smooth feel to the edge when touched, leading some to think that they are not sharp. But applying pressure directly into that edge, in its preferred mode of cut, would be ill advised if you didn’t want to unpleasantly gain a whole new understanding of “sharp.”
On the other hand, the “bite” of a sharp hunting knife blade is what many would more immediately recognize as sharp. That toothy, microscopic, grab of every ridge of your finger pad, bringing the short hairs of your body to attention, is a classic feel of what many would call sharp. It is an edge designed to sever fibers like a microscopic saw. But if you were to try to shave with it you would find your skin left burning as it savagely raked its way over the surface, pulling hairs as much as cutting them.
These two distinctly different edges lead to much confusion about “sharp.” I have found that edges with greater hardness tend to polish, more than deform, on a stone, lending itself much more to the chisel type cut. Edges with lower Rockwell levels will much more readily engage the abrasive to form those toothy rakers conducive to the slashing draw cut. I have found hunting knives left at 62.5HRC, or higher, will bring a frown to potential customers brows, due their lack of tooth, despite how well they may shave hair or slice a vegetable.
This is the affect at play when the old hunter who, using a rather coarse India, puts and edge on a blade that looks like it was done with a 36-grit belt, lays back impressive sheets of skin and flesh from large game in long sweeping cuts. It is simply a different kind of sharp. This edge can also give the impression of longevity, just like a serrated edge continues cutting despite being much less than sharp between the teeth points.
Now, we once again come full circle to how steel selection can work with heat treatment for these edges. Steel is not a material monolithic in nature, its properties as a mixture are often quite prominent. For this reason, one must not make the mistake of confusing abrasion resistance with hardness (strength) as they can be quite independent of each other.
We think of steel of a mixture of iron and carbon, but what it actually is a mixture of very low carbon metal and carbide. The metal matrix is capable of having vastly different levels of hardness than the carbide particles suspended within it, which the metal itself could never hope to equal. Think of glass chunks mixed in playdoh. You can dry out the playdoh and make it hard, but never as hard as the glass chunks it holds. Our you can leave it soft, but the glass chunk will still be hard.
If you make a playdoh knife, and leave it soft, you can still cut many soft things with a sawing motion as the glass does its thing, but you will get tearing and deformation of the soft edge areas between. And, of course, if you try to hit something harder than playdoh the whole thing will just fold over on you. If you dry it out, and harden the playdoh, you can now cut many more things with it, and the glass will still do its thing, and with less loss of playdoh, so you can cut for much longer.
Steels with added carbide forming elements, like Cr, V, or W, will increase this effect of abrasion resistance, independent from hardness. With these steels one can play what I call “carbide games” creating an edge that will cut quite aggressively, like dull serration, on soft fibrous materials (this is why cutting notebook paper is really not impressive). You just don’t ever want to go at something more solid with it. However, if you want a finer edge, you must get those carbides finer. They may make nice coarse rakers, but they won’t make a really fine push cutting blade if they are big. A large carbide D-2 razor would be like a garden rake on your face.
So, with a hunting knife, loaded up with W carbides, you can give the impression of edge holding sharp, while still drawing it back far enough in the temper for it to fold up on a 2x4. But you can also refine carbides and leave the steel quite hard and get the best of both worlds in a finer cutting blade.
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