- Joined
- Mar 8, 2008
- Messages
- 25,893
I'd like to preface this by stating that these are my observations only. Feel free to disagree--in fact I encourage it! If there's a fault with my reasoning I'd like to correct it. 
This is a long post, so here we go!
If there's one thing that everyone likes, it seems to be convex edges. Users often state how impressed they are with their performance. However, the oft-touted belief is that a convex edge has more material supporting the blade, and thus has better edge retention than a conventional "V" edge. This makes sense at a cursory glance because the shortest distance between two points is a straight line, and so a convex would mean there was more material and a concave would have less, right?
Sort of. A convex edge traversing those two points (the apex of the bevel and where the bevel transitions into the primary grind or blade stock) would have more material supporting the edge...but would also have a broader effective edge angle meaning it would have to displace more material during the cut and at a more rapid pace. This is really the dynamic of the old broad vs. thin edge angle tradeoff, and you can't add material back onto the blade anyway so the only way to increase the supporting material is by diminishing the blade height through sharpening so we push the red lines back into the thicker region of the blade. You can see that that would start removing a lot of blade pretty quick, and at the cost of cutting performance as well. What we really need to be looking at is a "V" edge being converted to a convex of equal effective edge angle. What you get is something like this:
So you can see that what you're really getting is a reduction of the transitional shoulder, giving you a thinner geometry but maintaining the same edge angle and without significantly reducing the material supporting the edge where it typically needs it most. This makes for a smoother cut (increasing controllability due to the more gradual shift in geometry) but also reduces the amount of material that must be displaced by the blade as it passes through the cutting medium. This means that your cuts require less energy, yet the edge is still almost as well-supported as a conventional "V" edge.
Given the actual realistic benefit provided by a convex edge, this is why I tell folks to not worry too much about maintaining a perfect convex edge when out in the field. If you're a perfectionist you can always restore it to "true" when you get back to the house. Besides--freehand sharpening results in slight variation in angle from stroke to stroke so you end up with a very slight convex form anyhow.
Most tests in which folks have been impressed with the edge-holding abilities of convex edges have been batoning their knife through a bunch of wood and then still been able to shave their arm at the end. Impressive, right? Well once the edge goes in the edge rarely actually touches the wood! The parts of the bevel either side of the edge and the bevel of the primary grind take the brunt of the action since the wood splits ahead of the edge itself. The convex edge doesn't significantly play a role in this effect other than perhaps allowing the split to start a little easier. Cross-grain batoning would be another matter, but little comparative testing has been done between two otherwise identical knives of equal edge angle with "V" and convex edges to provide substantive evidence for any sort of improvement in edge retention. If there is one it's simply that for equal amounts of force applied to each blade the thinner convex one will be able to cut deeper, and thus cut more material for equal energy expenditure, but the actual sharpness of the edge (or the thickness of the apex on the terminal "edge" bevel) will be affected about the same since it has the same edge angle and degree of wear resistance.
Discuss!
This is a long post, so here we go! If there's one thing that everyone likes, it seems to be convex edges. Users often state how impressed they are with their performance. However, the oft-touted belief is that a convex edge has more material supporting the blade, and thus has better edge retention than a conventional "V" edge. This makes sense at a cursory glance because the shortest distance between two points is a straight line, and so a convex would mean there was more material and a concave would have less, right?
Sort of. A convex edge traversing those two points (the apex of the bevel and where the bevel transitions into the primary grind or blade stock) would have more material supporting the edge...but would also have a broader effective edge angle meaning it would have to displace more material during the cut and at a more rapid pace. This is really the dynamic of the old broad vs. thin edge angle tradeoff, and you can't add material back onto the blade anyway so the only way to increase the supporting material is by diminishing the blade height through sharpening so we push the red lines back into the thicker region of the blade. You can see that that would start removing a lot of blade pretty quick, and at the cost of cutting performance as well. What we really need to be looking at is a "V" edge being converted to a convex of equal effective edge angle. What you get is something like this:
So you can see that what you're really getting is a reduction of the transitional shoulder, giving you a thinner geometry but maintaining the same edge angle and without significantly reducing the material supporting the edge where it typically needs it most. This makes for a smoother cut (increasing controllability due to the more gradual shift in geometry) but also reduces the amount of material that must be displaced by the blade as it passes through the cutting medium. This means that your cuts require less energy, yet the edge is still almost as well-supported as a conventional "V" edge.
Given the actual realistic benefit provided by a convex edge, this is why I tell folks to not worry too much about maintaining a perfect convex edge when out in the field. If you're a perfectionist you can always restore it to "true" when you get back to the house. Besides--freehand sharpening results in slight variation in angle from stroke to stroke so you end up with a very slight convex form anyhow.
Most tests in which folks have been impressed with the edge-holding abilities of convex edges have been batoning their knife through a bunch of wood and then still been able to shave their arm at the end. Impressive, right? Well once the edge goes in the edge rarely actually touches the wood! The parts of the bevel either side of the edge and the bevel of the primary grind take the brunt of the action since the wood splits ahead of the edge itself. The convex edge doesn't significantly play a role in this effect other than perhaps allowing the split to start a little easier. Cross-grain batoning would be another matter, but little comparative testing has been done between two otherwise identical knives of equal edge angle with "V" and convex edges to provide substantive evidence for any sort of improvement in edge retention. If there is one it's simply that for equal amounts of force applied to each blade the thinner convex one will be able to cut deeper, and thus cut more material for equal energy expenditure, but the actual sharpness of the edge (or the thickness of the apex on the terminal "edge" bevel) will be affected about the same since it has the same edge angle and degree of wear resistance.
Discuss!
