Cliff Stamp
BANNED
- Joined
- Oct 5, 1998
- Messages
- 17,562
Garretts :
Cutting ability of a knife can be quantified by the resistance of material to the passage of the blade. This resistance is caused by the material exerting forces on the knife as it is travelling through the material which is being cut. These forces fall under two basic categories; (a) those exerted against the edge, and (b) and those exerted against the body of the blade. The separation is necessary as the forces come from two different properties of the material and different materials can thus behave in very different ways in these two aspects. Cheese for example exerts very little force direct against an edge, but has a very high binding force on the sides of the blade. In comparison, hard shelled poly rope held under tension exerts a huge force against the edge, but induces very little binding or wedging action.
In more detail, as the edge of the knife meets the material being cut, the applied force has to rise until the induced pressure on the material being cut exceeds the ability of the material to withstand it. The material then ruptures [the materials resistance comes from a combination of its ductility and tensile strength] and starts to be cut. Since pressure is force divided by area, you can increase the pressure by simply applying more force, or sharpening the blade which decreases the area of the contact [in a number of ways]. The great thing about sharpening is that there is no drawback. It is a myth that sharper blades are weaker, take damage easier and go dull faster. So simply sharpen your blades to their maximum and enjoy greater cutting ability and durability.
If you are curious how sharpening can decrease the contact area and not get any weaker - after an an edge has been formed (the two bevels meet), sharpening can be separated into mainly two distinct processes. The first is alignment. Think of a saw blade that has the teeth stuck out in random directions. It will be horrible to cut with because all the teeth when pressed through the wood make different cuts. On a truly sharp blade all the teeth cut in the same line and thus progress is greatly speeded up. Since the teeth are all in the same line they are also much stronger. Make a cupping motion with your hand so all your finger tips are in line, now press them against something and see how they respond to the stress, they are a very strong unit. Now splay them all out from each other and again press. Now each finger can take the whole strain, and even worse it can induce severe bending and twisting loads. It is the same thing for the teeth on a knife edge.
The second thing that sharpening does is refine the size of the teeth, the edge will be more durable for impacts the greater the polish as the teeth get smaller, and the edge surface becomes more even. As the teeth get smaller, any force applied to them is at the end of a shorter arm, and thus produces less torque and is thus less likely to break off the teeth. Take a plastic comb and hold it by one end and and press down on the other. It is easy to break it off. Now hold it right by the same end you are pressing, without the leverage you can do nothing. The smaller teeth are thus much more resistant to being bent in the same way. The effect of the polish is also easy to demonstrate. Take a piece of 2x4 and saw and make a bunch of random deep cuts into the wood. Now compare how hard it is to break that 2x4 compared to one with no cuts. Make another one with more shallow cuts and do the same. You will note that there is a direct relationship between the frequency (amount) of cuts and their depth to the weakness of the board. A highly polished edge is analogous to the uncut 2x4.
Now back to cutting ability, the second component of the force that makes up the resistance you feel when cutting, comes from the material pressing on the sides of the blade. This force is essentially proportional to how far apart the material gets pushed in the cut. Since a thinner blade has to distort the material less, it is easier to cut with. Thus the solution is to make a blade really thin. The obvious question is then what is stopping you from making it really thin, like 1/64", or a tenth of a mm? When you cut with a blade, because you are not a machine, you will be twisting and torquing the blade all over the place, you might not notice it, but the the force in not uniform over the entire contact area and straight down from the spine. Even if you were a machine, the material you are cutting isn't going to be perfectly uniform and thus the changes in its consistency can cause the blade to experience all kinds of torques and twists. Thus a knife has to be thick enough to give it the necessary strength to be able to withstand the forces applied to it during a cut. So the optimal profile will be of a cross section that is just at this level, any more and you have strength which is of no use, but just lowers cutting ability.
Now there is a complication to this which can actually be fairly significant. Many materials can split during a cut, which can cause the material to fall away from the knife and thus reduce the force during a cut. Thus you can by reducing the blade thickness prevent this splitting and cause the material to actually wedge more tightly around the knife. Thus there is a level above the thinnest blade at which point the splitting can be great enough to overcome the increase in general resistance of the thicker blade. This is noticed on materials like woods and rigid vegetables. Materials like ropes, flesh, rubber, meats, fabrics, soft vegetation, etc., don't split, and thus on them you just make the knife as thin as possible with the requirement that the cross section have the necessary strength and stiffness as noted previously.
Japanese cutting tools in general have the cross section close to the absolute minimum and thus the cutting ability near optimal. They have low requirements for durability as they place a very high emphasis on skill and tool selection. They also have a very high standard for sharpness. Even a very cheap production Japanese kitchen knife will be at a sharpness level above most western customs and will surpass nearly all in cutting ability.
No. There are steels which have both a very high toughness at a decent hardness. The argument could be made though that since the gain in toughness is much faster than the loss of strength, it would be beneficial in general to drop the spine a couple of RC points as you would gain a lot of impact durability with the loss of very little overall lateral strength.
No, you can differentially harden stainless steels. They can be edge quenched in oil for a mild difference, spine drawn while the edge is cooled, and possibly differentiated by the use of plate quenching with plates of a different thickness with an insulative buffer and possibly preheated.
The stainless steels are very strong in general. However this strength comes at the sacrifice of a very low impact toughness and ductility which makes them in general unsuitable for swords and other high impact blades.
phwl :
[corrosion resistance testing]
Yes, however the primary purpose of the "review" was to actually to do durability work, so I have to wait until that is done before soaking them, as deep pitting would influence the breaking points. There should be enough left over however to provide information on surface damage and edge effects from corrosion.
Steelhed, thanks for the offer, I'll send you an email.
UltraSteele :
Yes, the D2 is ~100% ahead of the VG-10. It took about three times as much material to induce the same loss of edge aggression on the D2 Deerhunter as on the AUS-8A one.
-Cliff
Cutting ability of a knife can be quantified by the resistance of material to the passage of the blade. This resistance is caused by the material exerting forces on the knife as it is travelling through the material which is being cut. These forces fall under two basic categories; (a) those exerted against the edge, and (b) and those exerted against the body of the blade. The separation is necessary as the forces come from two different properties of the material and different materials can thus behave in very different ways in these two aspects. Cheese for example exerts very little force direct against an edge, but has a very high binding force on the sides of the blade. In comparison, hard shelled poly rope held under tension exerts a huge force against the edge, but induces very little binding or wedging action.
In more detail, as the edge of the knife meets the material being cut, the applied force has to rise until the induced pressure on the material being cut exceeds the ability of the material to withstand it. The material then ruptures [the materials resistance comes from a combination of its ductility and tensile strength] and starts to be cut. Since pressure is force divided by area, you can increase the pressure by simply applying more force, or sharpening the blade which decreases the area of the contact [in a number of ways]. The great thing about sharpening is that there is no drawback. It is a myth that sharper blades are weaker, take damage easier and go dull faster. So simply sharpen your blades to their maximum and enjoy greater cutting ability and durability.
If you are curious how sharpening can decrease the contact area and not get any weaker - after an an edge has been formed (the two bevels meet), sharpening can be separated into mainly two distinct processes. The first is alignment. Think of a saw blade that has the teeth stuck out in random directions. It will be horrible to cut with because all the teeth when pressed through the wood make different cuts. On a truly sharp blade all the teeth cut in the same line and thus progress is greatly speeded up. Since the teeth are all in the same line they are also much stronger. Make a cupping motion with your hand so all your finger tips are in line, now press them against something and see how they respond to the stress, they are a very strong unit. Now splay them all out from each other and again press. Now each finger can take the whole strain, and even worse it can induce severe bending and twisting loads. It is the same thing for the teeth on a knife edge.
The second thing that sharpening does is refine the size of the teeth, the edge will be more durable for impacts the greater the polish as the teeth get smaller, and the edge surface becomes more even. As the teeth get smaller, any force applied to them is at the end of a shorter arm, and thus produces less torque and is thus less likely to break off the teeth. Take a plastic comb and hold it by one end and and press down on the other. It is easy to break it off. Now hold it right by the same end you are pressing, without the leverage you can do nothing. The smaller teeth are thus much more resistant to being bent in the same way. The effect of the polish is also easy to demonstrate. Take a piece of 2x4 and saw and make a bunch of random deep cuts into the wood. Now compare how hard it is to break that 2x4 compared to one with no cuts. Make another one with more shallow cuts and do the same. You will note that there is a direct relationship between the frequency (amount) of cuts and their depth to the weakness of the board. A highly polished edge is analogous to the uncut 2x4.
Now back to cutting ability, the second component of the force that makes up the resistance you feel when cutting, comes from the material pressing on the sides of the blade. This force is essentially proportional to how far apart the material gets pushed in the cut. Since a thinner blade has to distort the material less, it is easier to cut with. Thus the solution is to make a blade really thin. The obvious question is then what is stopping you from making it really thin, like 1/64", or a tenth of a mm? When you cut with a blade, because you are not a machine, you will be twisting and torquing the blade all over the place, you might not notice it, but the the force in not uniform over the entire contact area and straight down from the spine. Even if you were a machine, the material you are cutting isn't going to be perfectly uniform and thus the changes in its consistency can cause the blade to experience all kinds of torques and twists. Thus a knife has to be thick enough to give it the necessary strength to be able to withstand the forces applied to it during a cut. So the optimal profile will be of a cross section that is just at this level, any more and you have strength which is of no use, but just lowers cutting ability.
Now there is a complication to this which can actually be fairly significant. Many materials can split during a cut, which can cause the material to fall away from the knife and thus reduce the force during a cut. Thus you can by reducing the blade thickness prevent this splitting and cause the material to actually wedge more tightly around the knife. Thus there is a level above the thinnest blade at which point the splitting can be great enough to overcome the increase in general resistance of the thicker blade. This is noticed on materials like woods and rigid vegetables. Materials like ropes, flesh, rubber, meats, fabrics, soft vegetation, etc., don't split, and thus on them you just make the knife as thin as possible with the requirement that the cross section have the necessary strength and stiffness as noted previously.
Japanese cutting tools in general have the cross section close to the absolute minimum and thus the cutting ability near optimal. They have low requirements for durability as they place a very high emphasis on skill and tool selection. They also have a very high standard for sharpness. Even a very cheap production Japanese kitchen knife will be at a sharpness level above most western customs and will surpass nearly all in cutting ability.
No. There are steels which have both a very high toughness at a decent hardness. The argument could be made though that since the gain in toughness is much faster than the loss of strength, it would be beneficial in general to drop the spine a couple of RC points as you would gain a lot of impact durability with the loss of very little overall lateral strength.
No, you can differentially harden stainless steels. They can be edge quenched in oil for a mild difference, spine drawn while the edge is cooled, and possibly differentiated by the use of plate quenching with plates of a different thickness with an insulative buffer and possibly preheated.
The stainless steels are very strong in general. However this strength comes at the sacrifice of a very low impact toughness and ductility which makes them in general unsuitable for swords and other high impact blades.
phwl :
[corrosion resistance testing]
Yes, however the primary purpose of the "review" was to actually to do durability work, so I have to wait until that is done before soaking them, as deep pitting would influence the breaking points. There should be enough left over however to provide information on surface damage and edge effects from corrosion.
Steelhed, thanks for the offer, I'll send you an email.
UltraSteele :
Yes, the D2 is ~100% ahead of the VG-10. It took about three times as much material to induce the same loss of edge aggression on the D2 Deerhunter as on the AUS-8A one.
-Cliff
..... I was unable to test the VG-10 & D-2............. 
....] or next season. 