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Edge Retention vs Sharpness Retention
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Larrin
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Sorry I'm a bit late to this thread. I got a bit lost in the many definitions being discussed so here is an article on the definition of sharpness. Spoilers: there is no agreed upon definition. Sharpness vs Cutting Ability - Knife Steel Nerds
Perhaps the simplest definition of sharpness is using geometry, the more it looks like a pointy triangle the sharper it is. If the width of the tip of the edge (also called apex) is 0 you have infinite sharpness, and the wider it is the lower your sharpness is. That can also be defined by the radius of the apex.
But you can also define sharpness using cutting measurements where you take the energy to initiate a cut (as opposed to energy for a complete cut). There is a very strong correlation between the energy measured and the width of the edge but there are other complications apart from edge width, several of which are laid out in that article. And we also must remember that the energy for a slice is less than the energy required for a push cut, and so you must be aware of how the test is being performed and what you are testing for.
Cutting ability is the energy required for a complete cut, so it is a combination of sharpness (edge width) and edge geometry (such as edge angle and thickness behind the edge). You can have an edge that is incredibly sharp but have relatively poor cutting ability because of heavy geometry. You can also have an edge with good cutting ability but relatively poor sharpness, especially if the materials you are cutting are not sharpness limited (easily deformable materials would not be a good choice).
Then on to the discussion of sharpness retention vs edge retention I don't think those are very good terms to use. I believe what is being described is the difference between initial sharpness loss between different steels or knives. This is all under the same umbrella of "edge retention" but we are more discussing the shape of sharpness (or cutting ability) loss curve. Some have claimed that steels with fine carbide structures or higher toughness will maintain initial sharpness better, but steels with higher wear resistance (possible from coarser carbides) will maintain some medium amount of sharpness for longer. This is a complicated area without a lot of definitive answers. I can say that in a slicing edge retention test where the edge loses sharpness to wear there is no difference. I discussed that here: Which Steel Has the Best Edge Retention? Part 2 - Knife Steel Nerds
However push cutting can work a bit differently than slicing. For example, push cutting is helped by having more polished edges while in slicing you benefit from coarser edges. So perhaps push cutting can benefit from higher toughness and/or a finer microstructure. Or perhaps benefit from hardness more. This would likely require that the sharpness loss mechanism was deformation (like rolling, this is controlled by strength/hardness) or from micro-chipping (from poor toughness). I think pure wear behavior would likely still behave similarly to the CATRA slicing test.
Perhaps the simplest definition of sharpness is using geometry, the more it looks like a pointy triangle the sharper it is. If the width of the tip of the edge (also called apex) is 0 you have infinite sharpness, and the wider it is the lower your sharpness is. That can also be defined by the radius of the apex.
But you can also define sharpness using cutting measurements where you take the energy to initiate a cut (as opposed to energy for a complete cut). There is a very strong correlation between the energy measured and the width of the edge but there are other complications apart from edge width, several of which are laid out in that article. And we also must remember that the energy for a slice is less than the energy required for a push cut, and so you must be aware of how the test is being performed and what you are testing for.
Cutting ability is the energy required for a complete cut, so it is a combination of sharpness (edge width) and edge geometry (such as edge angle and thickness behind the edge). You can have an edge that is incredibly sharp but have relatively poor cutting ability because of heavy geometry. You can also have an edge with good cutting ability but relatively poor sharpness, especially if the materials you are cutting are not sharpness limited (easily deformable materials would not be a good choice).
Then on to the discussion of sharpness retention vs edge retention I don't think those are very good terms to use. I believe what is being described is the difference between initial sharpness loss between different steels or knives. This is all under the same umbrella of "edge retention" but we are more discussing the shape of sharpness (or cutting ability) loss curve. Some have claimed that steels with fine carbide structures or higher toughness will maintain initial sharpness better, but steels with higher wear resistance (possible from coarser carbides) will maintain some medium amount of sharpness for longer. This is a complicated area without a lot of definitive answers. I can say that in a slicing edge retention test where the edge loses sharpness to wear there is no difference. I discussed that here: Which Steel Has the Best Edge Retention? Part 2 - Knife Steel Nerds
However push cutting can work a bit differently than slicing. For example, push cutting is helped by having more polished edges while in slicing you benefit from coarser edges. So perhaps push cutting can benefit from higher toughness and/or a finer microstructure. Or perhaps benefit from hardness more. This would likely require that the sharpness loss mechanism was deformation (like rolling, this is controlled by strength/hardness) or from micro-chipping (from poor toughness). I think pure wear behavior would likely still behave similarly to the CATRA slicing test.
