Re: Ceramic or 1000 grit stone for convex bevels???
- This topic has 39 replies, 10 voices, and was last updated 02/17/2013 at 9:42 am by
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I just want to clarify my earlier posts on convex edges – stated C in V, or V in C or V intersect C. For me, most of the time, convex means ‘Practical convex’.
For ‘Extreme’, we are looking at nearly double or half the bevel angle. Even in the cases of micro-bevel – e.g. inclusive * 15/25 (67%); 20/30(50%); 30/40(33%) – we’re far from doubling the angle. Stropped edges are mostly convex micro-bevel anyway 😀
Hey guys,
I intended to write a summary of this thread, since I found it interesting and I have learned a lot from you. But then I found myself jumping to my own conclusions. I guess blood is thicker than water… I’ll post it anyway and please shoot! The reason for being here (other than having fun) is that I want to learn, so I can stand a good deal.
This thread started off with the question by Ken who wondered what the advantage is of a convex edge over a double-beveled or multi-beveled edge. I then generalized this question into what the use is of a convex edge anyway.
Lots of claims are made about convex edges. Some in this thread, some elsewhere. And some claims were supported in this thread, others debunked. What did I learn?
A convex edge is millennia old
This point was most prominently made by Leo, who claimed that even in the bronze age people made blades with convex edges. Now when you think about this, this could be true. Our ancestors didn’t have a WEPS, but we all know you don’t need a WEPS to create a convex edge. A mousepad and some sandpaper is enough. Uhm… but hey, they didn’t have mousepads in the bronze age! Not even sandpaper…
😆 OK, it’s true. There’s lots of empirical evidence that is beautiful, too.
A convex edge is marketing blurb
This point, made by Curtis (with a smiley), was appealing to me. I’m a naturally inquiring guy and I only believe things I have seen with my own eyes or at least understand with my own mind. Perhaps that’s one of the reasons why the sometimes traditional world of knife sharpening appeals to my investigative senses.
A convex edge is stronger than a single bevel
This claim is often followed by “because it has a better geometry†or “because it has more material behind itâ€. However, this claim says nothing. That’s because it omits the angles we are comparing. As Tom wrote, “angles are everythingâ€.
Yes, a convex edge that varies between 17 and 22 degrees is stronger than a straight edge of 17 degrees. However, it is not stronger than a straight edge of 22 degrees.
Yet the idea that a convex edge is stronger is appealing. This is Tom’s pencil point theory. We all know that by rounding the point of a pencil, the probability that it breaks off is much reduced.
This is similar to what we do when we put a micro-bevel on straight primary bevel. We still benefit a lot from the steeper primary bevel, but we make it less fragile by putting on a blunter micro-bevel.
In these cases we don’t only make the edge stronger. We sacrifice a bit of sharpness in favour of strength. We are trying to optimize the balance sharpness-strength.
As Phil said it, “it [convexing the edge] makes sense, but more so in a case where you are doing something that will make the knife steel fail at your chosen angleâ€.
A convex edge dulls faster
This point is made more often. The idea is that a convex edge is thicker right behind the edge, so as the edge wears, it gets thicker faster too.
But again, as Bluntcut, pointed out, this doesn’t say much about convex versus V-edge, but more about angles (“not so much about V or C, IMOâ€). A convex edge that varies between 17 and 22 degrees dulls faster than a straight edge of 22 degrees. However, it doesn’t dull faster than straight edge of 17 degrees.
A convex edge doesn’t wedge
Well, at least not as much as a straight edge, so goes the claim. One of the most annoying features of an axe is that it may wedge. This is the reason everybody prefers a convex edge on their axe.
Wedging is also a big annoyance for cooks when cutting harder materials like carrots or onions. In the world of high-end kitchen knives, blades should have such a profile that they are thin behind the edge. Some cooks go as far as recommending thinning out a knife every time it is sharpened.
Indeed, here a convex edge has an advantage over a straight edge. The convex edge acts as a wedge: there is simply a smaller part of the edge in contact with the material being cut, so there is less friction or stiction. This is illustrated by a picture I found on the Net (here).
Phil brought up the same argument for entire blades. “For instance a full flat grind would have more of the main part of the blade in contact with the material at one time versus a hollow grind or a convex grind. If you are cutting something where a major portion of the blade is engaged with the material, the sides of the blade will contribute more friction or resistance to cutting to the overall effort required than the bevel.â€
Also Ken supported this. “With a typical FFG knife cutting a firm moist substance, I get a lot of resistance from (what in the stropping threads we’ve termed stiction) the material.â€
Even though a convex edge may have an advantage over a straight edge here, does it also have an advantage over a multi-beveled edge?
A convex edge cut doesn’t cut (much) better than a multi-beveled edge
Bluntcut made the point that a convex edge is more aerodynamic than a multi-beveled edge. However, Phil wrote that “those designs are done using fluid dynamics as the basis. We typically are not cutting fluids nor gasses.†This was supported by a statement by Jay Fischer:
I read a comment once where the writer had claimed the convex grind or rolled edge has less friction because it only contacts on a tangential point. This would be true if only the material being cut has no give, no movement, no springiness to it. Also, as that material is cut, it would just open up, not pinch, but contact rigidly at one single point. But just what material would that be?
Does the fact that a convex edge is more aerodynamic than a multi-beveled edge make it a better cutting edge? Probably not much. As Bluntcut wrote: “In WE world – bevel angle precision & repeatability – convex doesn’t add much if your blade is thin behind the edge.â€
Conclusion
My conclusion is that some of the claims made in the knife world about convex edges are not so much true or false, but instead rather meaningless. A convex edge is not inherently stronger than a straight edge. Convex edges allow for different trade-offs between angle steepness and strength than straight edges.
This different trade-off between angle steepness and strength is similar to what we achieve when putting a micro-bevel on a straight edge. We make the edge stronger, but at the cost of a little cutting efficiency.
Similarly, a convex edge does not inherently dull faster than a straight edge. It all depends on the angles.
However, a convex edge does have an advantage over a straight edge in that it doesn’t wedge as much. There is a smaller part of the edge in contact with the material being cut, so there is less friction.
It seems that this advantage is most prominent when cutting hard, rigid materials. Perhaps that is the reason everyone prefers a convex edge on their axe. And cooks never complain that a knife is wedging when cutting tomatoes. But they may complain when cutting potatoes.
Does a convex edge have an advantage over a proper double-beveled or multi-beveled edge? Probably not: convex doesn’t add much if your blade is thin behind the edge.
The fact that we even ask this latter question shows we are from the Wicked Edge world. In the rest of the world nobody is able to create multi-beveled edges where the bevel angles are 1 or 2 degrees apart. If they try do so so, they end up with convex edges. Even if they try to create a straight edge, it will be somewhat convex. It has been that way since the bronze age.
Molecule Polishing: my blog about sharpening with the Wicked Edge
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