Cliff Stamp
BANNED
- Joined
- Oct 5, 1998
- Messages
- 17,562
By self-sharpening I mean a blade that actually gets sharper as you cut with it and thus the cutting ability increases. I noticed this off an on for a number of years without paying much attention to it. When cutting with knives for a long time it is difficult to judgeby feel the cutting ability as compared to what it was five minutes ago, when the changes are so small, maybe it was that the material just got a little easier to cut. When I started doing cutting tests awhile ago and measuring the sharpness of various blades and how it degraded during cutting, I didn't notice this self-sharpening effect so I figured that what I had saw in the past was just some kind of fluke. However recently I noticed it again and finally looked at it in enough detail to figure out what was going on.
First some background. Quite awhile ago (more than a few years) on rec.knives Alvin Johnson made a comment about one of the many serious drawbacks of stainless steels was that if you honed them at a very fine angle the edge blunted quickly when compared to carbon steels. Before you get ready to jump all over this with chants of the superior nature of ATS-34 and the like, this is in reference to sharpening angles of 5-10 degrees included. This is much finer than even the more acute production knives (Spyderco, Buck). I never payed much attention to that comment at the time, since my angles were much steeper and I saw no loss in sharpness of even the worst stainless (coarse grained) as compared to carbon. Sometime later (a few years) I was reading some posts by Jeff Clark on edge finish quality and how there was again a large difference in the stainless steels, he said more than that, that is just the relevant part here.
As of late (the last few years), I have been steadily bringing the edge profiles on my knives down (and adjusting the primary grind, but that is another story). During this process I noted time and time again that what Alvin and Jeff has said was indeed the case. The coarse grained steels can't take as fine polish at very acute angles as the finer grained carbon steels. This doesn't mean you can't get a stainless blade sharp, they will shave very easily for example. But they won't get as sharp as the carbon steels. If you check them under magnification you can see the edge tends to break apart during sharpening and you really can't polish it beyond a certain leve. I did some checking on the woodworking group and this had been noticed there before as well. Considering the size of the carbides in some of these steels (order of 50 microns or 0.05 mm), this isn't that surprising. Ref :
http://www.bladeforums.com/forums/showthread.php?s=&postid=1446725
Ok, on to the self-sharpening effect. When I started repeating the above mentioned cutting testing on the really acute edged knives I noticed that the blades would frequently increase in cutting ability during a run. You would start off with a very sharp blade, it would dull a little, then get sharper and sort of then oscillate getting duller then sharper. This of course slowed the degradation quite a bit, and radically enhanced the edge life of the blade. As an extreme case, with a 62 RC D2 blade with an edge of ~5 degrees per side, after doing ~1000 slices through 3/8" hemp, a further ~1000 slices did nothing to effect the edge aggression (measured in a few ways subjectively as well as quantitatively).
The explanation becomes clear when the edge is checked under magnification. You see the edge degrades in an irregular manner leaving "teeth" which are sharp and thus act as mini-saws. As these teeth wear and round out, new ones are produced as the edge gets damaged in other places and new teeth are revealed. Some times the rate of which new ones come in, is slower that the old ones wear so overall the blades still blunt, but at a reduced rate, however other times the reverse happens and overall the blade gets sharper. This random nature is why the blade can get blunt, then sharper, then blunt. In regards to the D2 knife, since it is such a coarse grained steel (30-50 microns), the edge breaks apart in a very aggressive manner and pretty much presents an all carbide edge to the material which induces pretty much zero edge blunting from that point on (as compared to normal steel blades).
How come you only see it in very acute edges? Well basically the depth of the teeth that are created are inversely related to the the angle of the edge (not exactly but close enough). This the more acute the edge the greater the size of the teeth, and essentially the more coarse the finish and thus the more aggressive the slicing action. Since the edges on most production knives are 5-10 times the angles we are talking about here, the teeth produced on them in the above fashion have far less aggression (it is the difference between a very fine hone and a very coarse one), and thus you never notice any self-sharpening effect. The same thing is in general happening, however the teeth are just too small to make any significant effect in the cutting ability.
To be clear these types of blades (such low edge angles) are only suitable for cutting ropes, foods, wood, fabrics etc., the edge durability is rather low so prying or twisting the edge, even lightly isn't a good idea. Or consider for example hitting a staple in a cardboard box. With a fragile and hard steel this can induce a break to a steel thickness of about 0.005"-0.01". For a knife sharpened at the common "22 degrees per side", this means you will see a small chip, just barely visible. With a knife of the profile we are talking about here, the same damage could knock out a piece of the edge about 1/16 to 1/8" or so deep. That would require a major sharpening effort to repair. Thus typically I carry a number of "rough" use knives for such tasks like a SAK, Leatherman etc., for such work.
Also note this sharpening effect is only seen in regards to slicing performance. In regards to push cutting, all these "teeth" degrade the push cutting ability and you will just see the edges blunt. In fact, the edges that work very well for this slicing self-sharpening, blunt pretty much the quickest for push cutting (as Alvin noted). As with everything, a gain in one area pretty much means a direct loss somewhere else. Some materials also don't induce this type of self-sharpening. but a lot of common materials will. Aside from ropes, cardboard does it very readily also. Getting a bit specific, this effect only sets in during the latter stages of the blades edge life. For example with the D2 blade, this was at ~50% of its optimal rope cutting ability.
-Cliff
First some background. Quite awhile ago (more than a few years) on rec.knives Alvin Johnson made a comment about one of the many serious drawbacks of stainless steels was that if you honed them at a very fine angle the edge blunted quickly when compared to carbon steels. Before you get ready to jump all over this with chants of the superior nature of ATS-34 and the like, this is in reference to sharpening angles of 5-10 degrees included. This is much finer than even the more acute production knives (Spyderco, Buck). I never payed much attention to that comment at the time, since my angles were much steeper and I saw no loss in sharpness of even the worst stainless (coarse grained) as compared to carbon. Sometime later (a few years) I was reading some posts by Jeff Clark on edge finish quality and how there was again a large difference in the stainless steels, he said more than that, that is just the relevant part here.
As of late (the last few years), I have been steadily bringing the edge profiles on my knives down (and adjusting the primary grind, but that is another story). During this process I noted time and time again that what Alvin and Jeff has said was indeed the case. The coarse grained steels can't take as fine polish at very acute angles as the finer grained carbon steels. This doesn't mean you can't get a stainless blade sharp, they will shave very easily for example. But they won't get as sharp as the carbon steels. If you check them under magnification you can see the edge tends to break apart during sharpening and you really can't polish it beyond a certain leve. I did some checking on the woodworking group and this had been noticed there before as well. Considering the size of the carbides in some of these steels (order of 50 microns or 0.05 mm), this isn't that surprising. Ref :
http://www.bladeforums.com/forums/showthread.php?s=&postid=1446725
Ok, on to the self-sharpening effect. When I started repeating the above mentioned cutting testing on the really acute edged knives I noticed that the blades would frequently increase in cutting ability during a run. You would start off with a very sharp blade, it would dull a little, then get sharper and sort of then oscillate getting duller then sharper. This of course slowed the degradation quite a bit, and radically enhanced the edge life of the blade. As an extreme case, with a 62 RC D2 blade with an edge of ~5 degrees per side, after doing ~1000 slices through 3/8" hemp, a further ~1000 slices did nothing to effect the edge aggression (measured in a few ways subjectively as well as quantitatively).
The explanation becomes clear when the edge is checked under magnification. You see the edge degrades in an irregular manner leaving "teeth" which are sharp and thus act as mini-saws. As these teeth wear and round out, new ones are produced as the edge gets damaged in other places and new teeth are revealed. Some times the rate of which new ones come in, is slower that the old ones wear so overall the blades still blunt, but at a reduced rate, however other times the reverse happens and overall the blade gets sharper. This random nature is why the blade can get blunt, then sharper, then blunt. In regards to the D2 knife, since it is such a coarse grained steel (30-50 microns), the edge breaks apart in a very aggressive manner and pretty much presents an all carbide edge to the material which induces pretty much zero edge blunting from that point on (as compared to normal steel blades).
How come you only see it in very acute edges? Well basically the depth of the teeth that are created are inversely related to the the angle of the edge (not exactly but close enough). This the more acute the edge the greater the size of the teeth, and essentially the more coarse the finish and thus the more aggressive the slicing action. Since the edges on most production knives are 5-10 times the angles we are talking about here, the teeth produced on them in the above fashion have far less aggression (it is the difference between a very fine hone and a very coarse one), and thus you never notice any self-sharpening effect. The same thing is in general happening, however the teeth are just too small to make any significant effect in the cutting ability.
To be clear these types of blades (such low edge angles) are only suitable for cutting ropes, foods, wood, fabrics etc., the edge durability is rather low so prying or twisting the edge, even lightly isn't a good idea. Or consider for example hitting a staple in a cardboard box. With a fragile and hard steel this can induce a break to a steel thickness of about 0.005"-0.01". For a knife sharpened at the common "22 degrees per side", this means you will see a small chip, just barely visible. With a knife of the profile we are talking about here, the same damage could knock out a piece of the edge about 1/16 to 1/8" or so deep. That would require a major sharpening effort to repair. Thus typically I carry a number of "rough" use knives for such tasks like a SAK, Leatherman etc., for such work.
Also note this sharpening effect is only seen in regards to slicing performance. In regards to push cutting, all these "teeth" degrade the push cutting ability and you will just see the edges blunt. In fact, the edges that work very well for this slicing self-sharpening, blunt pretty much the quickest for push cutting (as Alvin noted). As with everything, a gain in one area pretty much means a direct loss somewhere else. Some materials also don't induce this type of self-sharpening. but a lot of common materials will. Aside from ropes, cardboard does it very readily also. Getting a bit specific, this effect only sets in during the latter stages of the blades edge life. For example with the D2 blade, this was at ~50% of its optimal rope cutting ability.
-Cliff
. A bext step would be to make it fully convex.
) that come and tell you "I prefer this steel because there 0.5% more vanadium in it, or this steel because there is perlimpinpin powder in it"... What counts is factual data.