Thin knives, with thick edges?

[Steel130]

Alright guys. So I got to talking to someone on youtube. And now I am curious. For example. You have two knives. One is 1/4" thick, and the other 1/8" thick. Both are flat ground. Assuming grind geometries are close. Now the 1/8" knife has a 50* inclusive edge and the 1/4" knife has a 30* inclusive edge. Both have edges sharpened to the same polish. I was always of the thought that even though the 1/8" blade has a thicker edge, that because of its thinness behine the edge, it would still slice through material easier than its thicker counterpart. So could someone enlighten me?

 

[Obsessed with Edges]

Most of the friction or 'ease' of cutting through thicker material is due to the thickness and shape of the steel behind the edge, not the edge itself. The edge obviously needs to be sharp enough to initiate the cut (which is even more important for some things, like slicing a tomato or a grape), but once the body of the blade starts making contact with the material, that's where the 'ease' of cutting (or lack thereof) really becomes apparent.

The most obvious comparison would be in putting edges with identical geometry and refinement on two blades of different thickness, such as putting a 30 degree edge on each of a 1/16" thick blade and a 1/4" blade. Cut into some thicker material, or even slice into the edge of a piece of paper, and the difference will be obvious once the thicker blade is deep into the cut. The thicker blade will behave more like a wedge, in trying to split the material. That adds a lot of resistance in a deep cut. A box cutter is a perfect example; the edge of the utility blade can get dull quickly, but the tool still works reasonably well, because the blade is so thin and therefore encounters much less resistance deeper into the cut. A thicker blade with an equally-dull edge would be stopped cold in the cardboard.

Sometimes, such as when slicing a tomato or a grape, even a very thin blade might not help. I recently attempted to use a very thin kitchen knife (maybe 1/16" or less in thickness) to slice a tomato, and the edge itself was so blunt, it wouldn't cut through the tomato's skin. Ended up using my pocketknife, which was a thicker blade, but very much sharper at the cutting edge.

Another way to greatly improve ease of cutting with a thicker blade, is to convex the edge. Just rounding or smoothing the shoulders of the bevel can vastly reduce the friction normally seen with a thicker, V-bevelled edge. The hard-edged shoulders on a thick V-bevel can really create a lot of drag in thicker and tougher material, like cardboard or leather.

 

[Steel130]

Most of the friction or 'ease' of cutting through thicker material is due to the thickness and shape of the steel behind the edge, not the edge itself. The edge obviously needs to be sharp enough to initiate the cut (which is even more important for some things, like slicing a tomato or a grape), but once the body of the blade starts making contact with the material, that's where the 'ease' of cutting (or lack thereof) really becomes apparent.

The most obvious comparison would be in putting edges with identical geometry and refinement on two blades of different thickness, such as putting a 30 degree edge on each of a 1/16" thick blade and a 1/4" blade. Cut into some thicker material, or even slice into the edge of a piece of paper, and the difference will be obvious once the thicker blade is deep into the cut. The thicker blade will behave more like a wedge, in trying to split the material. That adds a lot of resistance in a deep cut.

Sometimes, such as when slicing a tomato or a grape, even a very thin blade might not help. I recently attempted to use a very thin kitchen knife (maybe 1/16" or less in thickness) to slice a tomato, and the edge itself was so blunt, it wouldn't cut through the tomato's skin. Ended up using my pocketknife, which was a thicker blade, but very much sharper at the cutting edge.

Another way to greatly improve ease of cutting with a thicker blade, is to convex the edge. Just rounding or smoothing the shoulders of the bevel can vastly reduce the friction normally seen with a thicker, V-bevelled edge. The hard-edged shoulders on a thick V-bevel can really create a lot of drag in thicker and tougher material, like cardboard or leather.

Thank you very much for the reply. Makes sense with two knives of varying thickness having the same edge.

 

[Bill1170]

In the context of this thread, with regard to cutting ability as a function of blade geometry, I see 4 separate related issues. They are:

1) overall blade thickness (at the spine)
2) degree of sharpness (i.e.- microscopic width of the flat or mesa at the very apex)
3) acuteness of final bevel angle
4) thickness behind the edge, meaning at the point where the sharpening bevel meets the primary bevel

 

[KennyB]

In the context of this thread, with regard to cutting ability as a function of blade geometry, I see 4 separate related issues. They are:

1) overall blade thickness (at the spine)
2) degree of sharpness (i.e.- microscopic width of the flat or mesa at the very apex)
3) acuteness of final bevel angle
4) thickness behind the edge, meaning at the point where the sharpening bevel meets the primary bevel

Sounds good, but maybe a 5th... The angle of the relief grind on the knife itself. Two knives of the same spine thickness, and thickness behind the edge, could have two different thickness mid-way between this point given a different relief grind.

I also think one should consider the thickness of the blade at the mid-way point, since it's not all the time that you will cut into a material deep enough to submerge the entire width of the blade. So for example, if you're slicing 1/4" thick leather on a table top, there really might not be any difference at all between a half-inch wide blade that is .100" at the spine but has virtually no relief angle, and a .160" thick 1" wide blade which has a relief angle as such that it is under .100" for a good portion of the blade behind the edge. In terms of the leather, if it's under .100" thick for at least 1/4" up the blade, then it would cut just as good if not better than the thinner ground blade, and the rest of the blade's thickness doesn't need to be considered because you're not cutting that deep.

 

[Bill1170]

Sounds good, but maybe a 5th... The angle of the relief grind on the knife itself. Two knives of the same spine thickness, and thickness behind the edge, could have two different thickness mid-way between this point given a different relief grind.

I also think one should consider the thickness of the blade at the mid-way point, since it's not all the time that you will cut into a material deep enough to submerge the entire width of the blade. So for example, if you're slicing 1/4" thick leather on a table top, there really might not be any difference at all between a half-inch wide blade that is .100" at the spine but has virtually no relief angle, and a .160" thick 1" wide blade which has a relief angle as such that it is under .100" for a good portion of the blade behind the edge. In terms of the leather, if it's under .100" thick for at least 1/4" up the blade, then it would cut just as good if not better than the thinner ground blade, and the rest of the blade's thickness doesn't need to be considered because you're not cutting that deep.

You are correct about #5. This is a reason why hollow ground blades can perform so well at certain tasks like skinning. The fat spine give lateral rigidity and the very thin part by the edge slices through with minimal effort. It is all about choosing good geometry for the job at hand.