Tempering for different kinds of “sharp.”

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.
 
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A few years back I asked Kevin if a skinning knife I made would work ok...I had cut it out of flat ground O1 perpendicular to the grain of the material....then started second guessing myself...He said it would work well...maybe even be ideal....

It was....everyone that has tried it loved it...even though I have a good edge on it the "tooth" of the material cuts deer amazingly well...both skinning and boning.
 
A few years back I asked Kevin if a skinning knife I made would work ok...I had cut it out of flat ground O1 perpendicular to the grain of the material....then started second guessing myself...He said it would work well...maybe even be ideal....

It was....everyone that has tried it loved it...even though I have a good edge on it the "tooth" of the material cuts deer amazingly well...both skinning and boning.

just be aware that there are always tradeoffs. That same blade would have noticeably less impact toughness along the length due to that orientation. But does that really matter in a fine cutting little hunter?
 
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I think for the way I use it...it is an advantage...it never goes out in the field. We field dress to get the "coyote snack" out of the deer and do our skinning and cutting in a buddy's quonset.
It's a 1/16 thick blade almost ulu shaped...super comfy handle...can use it for hours. (I have not hunted for two years due to health issues...hoping to this season!)
 
The Damascus cutting effect.

What I have already described also sheds some light on a phenomenon that has long purported in the custom knife arena. The claims that Damascus steel cuts differently, or longer, have been around as long as the material itself. It is all to easy to chalk this up to simply more myth and tall tails surrounding Damascus, but my testing and examination leads me to say that would be a hasty mistake.

I have long noticed different behavior in the cutting behavior of Damascus blades over singular alloy ones. Back around the year 2000, when the ABS first started holding cutting competitions, I was approached to see if I would be interested in competing. It seemed that there had been some murmuring of the public that nobody would use a Damascus blade in competition because the material could actually hold up like a singular steel blade. I fully accepted the challenge and was the first to win a cutting competition with a Damascus blade, I also never competed with anything but Damascus.

The competition experience gave me the opportunity for comparing the performance of Damascus and non-damascus edges, side by side, under very similar and extreme circumstances. Cutting everything, from rope to aluminum cans, I knew I wasn’t imagining things, Damascus steel seemed to cut certain things more aggressively and showed a different wear after repeated cuts.

I later did several laboratory type studies, involving carefully measured edge wear in use, followed by microscopic evaluation. Tim Zowada and I presented the findings of one of these studies at the Ashokan seminar in NY around that time. The data was fairly conclusive, a Damascus steel edge is capable of some unique mechanism in cutting and wears in use differently than a mono-steel blade.

The non-uniform strength and wear resistance properties of the edge results in damascus steel having a slightly more aggressive cut on soft fibrous materials such as rope, paper or flesh. When sharpening, the weld zones, as well as the differently alloyed layers, wear at differing rates, thus more readily establishing micro-serrations. Mixes with greater discrepancies in hardness showed this effect to a greater degree and as they wore in use they continued to become more “toothy” rather than smooth, thus allowing them to cut those same soft, fibrous, materials longer than their mono-steel counterparts.

But this did not necessarily equate to a superior edge, since this effect was only sustained on those specific mediums that were cut, harder materials would tend to smear the teeth over and smooth the edge. This did not have to be the case, however. Steel mixes, that were tested, which consisted of equally hard layers, but with differing levels of abrasion resistance, could be sharpened to a very fine and polished edge and would continue to wear in a toothy fashion even on hard materials. This was knowledge that I was quick to exploit in cutting competition blades, allowing even a fairly weak armed cutter, such as myself, to win quite often.

One annoying quirk of this effect manifested itself when my example encouraged others to begin competing with Damascus blades. When cutting aluminum cans there would often be a tense, and dramatic, pause when the judges inspected the edges of the damascus blades. What first felt like an edge defect invariably turned out to be almost microscopic globs of aluminum welded to the edge. This would only occur with Damascus edges.

In testing a wide variety of damascus types I found that one could give the appearance of a super sharp blade with material choices that would be less than stable on the edge. Pure nickel mixes and welded cable were two examples. These materials looked as ragged as sharks teeth off the stone, and would not produce a fine polished edge. But if all you were ever going to cut was paper or rope, particularly in a draw cut, they would cut like crazy. However, if you whittled some wood, or anything of greater hardness, they would rapidly dull. On the other hand, a mix like 1084/15n20 could take a very fine edge and would cut very well across a wide variety of mediums, actually increasing its aggressiveness slightly as it wore, until it eventually went dull.

The take away from all of this was that the concept of different types of edge sharpness was very evident with Damascus steel, and that some of the tall tales of the cutting ability of this material were not as far-fetched as we may think. It can cut different, and while damascus doesn’t hold its edge longer than other steel, it does go dull in a different way, going into a micro-serration mode before finally losing its advantage.
 
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