Cliff Stamp
BANNED
- Joined
- Oct 5, 1998
- Messages
- 17,562
For some time now I have been interested in how the structure of the steel will effect the level of finish it can take. However after trying quite a lot of different steels as well as other alloys I was never able to see a real difference in regards to the maximum level of edge polish. No matter what the blade material I could obtain a smooth push shaving sharpness. To be a bit more specific this is the ability to shave fine hair on a straight push, no slicing motion and without skin irritation from excessive force. At this level of edge finish the blade will push cut through photocopy paper and cut light thread at about 115 +/- 10 g of force.
Recently I have been taking the edge angles down on most of my knives to examine how it effects cutting performance across a broad range of tasks as well as to see just how much support an edge needs. It was while doing this that I finally began to see the edge finish being effected by the structure of the steel. In the limiting case for a light use knife I had reduced the edge angle of my Sub-Sniper from Lynn Griffith to under ten degrees per side. I noticed after polishing that the edge seemed to be more aggressive on cutting rope than I expected. When I checked it under a scope I saw the edge was not uniform, it was missing small micron sized pieces. At the time I made nothing much of it, just assuming it was sloppy sharpening.
Recently I went back to this knife and attempted to put a polish on it that produced an edge that was uniform under a 10x magnification, thus the edge "damage" had to be under 10 microns. I first tried a series of DMT diamond abrasives, finishing with a 1200 grit ultra-fine pad. The edge was very sharp and push shaved easily but was not smooth under magnification. I then tried stropping it on CrO loaded leather, which did improve it some, but did not completely polish the edge. I figured that it might be the nature of the abrasive that was damaging the edge so I switched to a series of Japanese waterstones. I ended up with a 4000 grit finish, however the edge finish was not uniform under magnification. The edge "tooth" should be just at the limit of resolution as the stone is about 9 microns, however I could see pieces missing from the edge that were much larger.
To check to make sure that what I was seeing was the effect of the grain size and not the abrasive I took out my CrO loaded leather and prepared for a long session of stropping. After fifty passes per side the edge was looking much more uniform. After another fifty passes per side the improvement continued. For reference the CrO buffing compound is very fine, Lee Valley has it listed as 0.5 micron so it is capable of a finish as fine as the finest grained steel. However after another fifty passes per side the edge on the Sub-Sniper started to degrade. Now I could see pieces missing that were about 30 +/- 10 microns in depth. This makes sense to me because that abrasive can not cut the carbides so all it will do is remove the surrounding metal and eventually the carbides will have no support and break away from the edge.
Now to clarify, the knife is by any means sharp. The edge is ground to 8.3 +/- 0.2 degrees per side and will push cut light thread only requiring 65 +/- 5 g of force, note from above this is almost twice as sharp as "shaving sharp". Not only will the knife shave very well, if you angle it slightly it will crop off fine hairs above the skin. It will also just cut clean through quarter inch poly under a 1000 g load with no slicing movement required, the weight is enough to pull the blade right through the cord. So simply put the knife is very sharp indeed - however the grain size is limiting the sharpness somewhat because of the "holes" left in the edge. How much effect can only be estimated when a finer grained steel is taken to the same angle.
Which brings up the last point. Why does angle make a difference? Well the more obtuse the edge angle the thicker the edge which means the greater the size of carbide it can support. As the edge gets ground thinner and thinner eventually it will become so thin that carbides can't actually fit in there any more and thus when the edge is finely honed, the carbides break out leaving holes and thus reduce the push cutting performance. You also lose a lot of wear resistance as the carbides are the primary source of this aspect of performance.
Note all of the above is only dealing with sharpness as it effects push cutting performance. Knives are also used to slice materials and this pretty much desires the exact opposite set of characteristics. The optimum push cutting edge is completely smooth, the optimum slicing edge has enough edge irregularities (i.e. "tooth") so enable large scale tearing of the material being cut. In general steels with large grains sizes and large carbides will slice better than fine grained ones with small carbides when both steels are at a high level of polish. However the slicing performance of any steel is dramatically effected by the edge finish, no steel at a high level of polish (CrO for example ), can come close to even the finest grained steel at a coarse polish (fine Diamond for example).
However there are of course other aspects to consider besides raw slicing performance. Edge retention for example, the edge life on the polished steel is controlled mainly by the carbides, the edge life on the coarsely ground fine grained steel is much shorter as it is controlled by the durability of the "teeth".
-Cliff
Recently I have been taking the edge angles down on most of my knives to examine how it effects cutting performance across a broad range of tasks as well as to see just how much support an edge needs. It was while doing this that I finally began to see the edge finish being effected by the structure of the steel. In the limiting case for a light use knife I had reduced the edge angle of my Sub-Sniper from Lynn Griffith to under ten degrees per side. I noticed after polishing that the edge seemed to be more aggressive on cutting rope than I expected. When I checked it under a scope I saw the edge was not uniform, it was missing small micron sized pieces. At the time I made nothing much of it, just assuming it was sloppy sharpening.
Recently I went back to this knife and attempted to put a polish on it that produced an edge that was uniform under a 10x magnification, thus the edge "damage" had to be under 10 microns. I first tried a series of DMT diamond abrasives, finishing with a 1200 grit ultra-fine pad. The edge was very sharp and push shaved easily but was not smooth under magnification. I then tried stropping it on CrO loaded leather, which did improve it some, but did not completely polish the edge. I figured that it might be the nature of the abrasive that was damaging the edge so I switched to a series of Japanese waterstones. I ended up with a 4000 grit finish, however the edge finish was not uniform under magnification. The edge "tooth" should be just at the limit of resolution as the stone is about 9 microns, however I could see pieces missing from the edge that were much larger.
To check to make sure that what I was seeing was the effect of the grain size and not the abrasive I took out my CrO loaded leather and prepared for a long session of stropping. After fifty passes per side the edge was looking much more uniform. After another fifty passes per side the improvement continued. For reference the CrO buffing compound is very fine, Lee Valley has it listed as 0.5 micron so it is capable of a finish as fine as the finest grained steel. However after another fifty passes per side the edge on the Sub-Sniper started to degrade. Now I could see pieces missing that were about 30 +/- 10 microns in depth. This makes sense to me because that abrasive can not cut the carbides so all it will do is remove the surrounding metal and eventually the carbides will have no support and break away from the edge.
Now to clarify, the knife is by any means sharp. The edge is ground to 8.3 +/- 0.2 degrees per side and will push cut light thread only requiring 65 +/- 5 g of force, note from above this is almost twice as sharp as "shaving sharp". Not only will the knife shave very well, if you angle it slightly it will crop off fine hairs above the skin. It will also just cut clean through quarter inch poly under a 1000 g load with no slicing movement required, the weight is enough to pull the blade right through the cord. So simply put the knife is very sharp indeed - however the grain size is limiting the sharpness somewhat because of the "holes" left in the edge. How much effect can only be estimated when a finer grained steel is taken to the same angle.
Which brings up the last point. Why does angle make a difference? Well the more obtuse the edge angle the thicker the edge which means the greater the size of carbide it can support. As the edge gets ground thinner and thinner eventually it will become so thin that carbides can't actually fit in there any more and thus when the edge is finely honed, the carbides break out leaving holes and thus reduce the push cutting performance. You also lose a lot of wear resistance as the carbides are the primary source of this aspect of performance.
Note all of the above is only dealing with sharpness as it effects push cutting performance. Knives are also used to slice materials and this pretty much desires the exact opposite set of characteristics. The optimum push cutting edge is completely smooth, the optimum slicing edge has enough edge irregularities (i.e. "tooth") so enable large scale tearing of the material being cut. In general steels with large grains sizes and large carbides will slice better than fine grained ones with small carbides when both steels are at a high level of polish. However the slicing performance of any steel is dramatically effected by the edge finish, no steel at a high level of polish (CrO for example ), can come close to even the finest grained steel at a coarse polish (fine Diamond for example).
However there are of course other aspects to consider besides raw slicing performance. Edge retention for example, the edge life on the polished steel is controlled mainly by the carbides, the edge life on the coarsely ground fine grained steel is much shorter as it is controlled by the durability of the "teeth".
-Cliff
