Why convex edges are awesome--it's not why you think!

A.L. said:
I thought this was obvious.. :confused:
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Yeah I did too, but a lot of folks don't reach that conclusion because they just swallow the absolutist myth of "convex edges have more material supporting the edge, so they're tougher" without ever thinking about it for themselves. Figured it was about time to clear up that misconception.
 
about 90% of the knives i get in for sharpening get a convex edge. some guys never heard of a convex edge and once i tell them the advantages to a convex edge and send them the link to the vid unit made, they usually decide on the convex edge. bullets are another good example of a convex edge :D
 
Great thread, thanks for starting it!!

A lot of this is over my head, so I hope I don't look too stupid with my question.

If I take a knive that's been sharpened on an edge pro or similar & grind down the shoulders (the area between the black & green lines on your sketch), would I get the benefits of a convex edge? Even though the edge is 2 or 3 flat planes, would it perform the same as convex? Would I get the same edge durability?

Thanks,
Allen
 
To my understanding you should receive very similar performance. :) Like Richard said, though, a regular cpnvex can be.touched up in one step.
 
Once you go convex, you ll never go back to the V! They seem to hold longer and are easier to touch up? Would that be correct my edge guru's??
 
The edge will wear at the same rate as before. As far as being able to touch them up easier that's a matter of personal experience, technique, and comfort. :)
 
The drawings are misleading because they show the convex being taller -- meaning it gets the advantage of being sharpened over a longer distance. The convex is being sharpened from the dots, making it more of a convex blade than a convex edge. The V edge is sharpened only from the edge shoulder. This is an unfair comparison.

Start the V edge from the dots, like you do the convex edge, and suddenly the V edge is a much more aggressive cutter than the convex edge.

One of the problems with a convex edge is that it is freehanded and therefore less precise, varying quite a bit from person to person. Some will end up with drawing 1, some with drawing 2, others with a variety of other shapes and distances.
 
Twindog said:
The drawings are misleading because they show the convex being taller -- meaning it gets the advantage of being sharpened over a longer distance. The convex is being sharpened from the dots, making it more of a convex blade than a convex edge. The V edge is sharpened only from the edge shoulder. This is an unfair comparison.

Start the V edge from the dots, like you do the convex edge, and suddenly the V edge is a much more aggressive cutter than the convex edge.

One of the problems with a convex edge is that it is freehanded and therefore less precise, varying quite a bit from person to person. Some will end up with drawing 1, some with drawing 2, others with a variety of other shapes and distances.
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Not misleading at all. The convex MUST reach higher on the vlade in order to maintain the same edge angle while still fitting within the frame provided by the original geometry. No disrespect but I think you missed the point of what I was demonstrating. :o
 
No, I get your point. But to make a fair comparison for edges, the V edge and convex edge have to be the same height and have the same shoulder width, otherwise you're comparing apples to oranges and you can make either edge perform better. A taller V edge will outperform a shorter convex edge and vice versa.

I don't care where you start to sharpen the convex edge, just start the V edge from that same point and the V edge will always outperform the convex edge. The convex edge, at the same height, will travel outside the V edge, giving it a more obtuse angle.
 
Twindog said:
No, I get your point. But to make a fair comparison for edges, the V edge and convex edge have to be the same height and have the same shoulder width, otherwise you're comparing apples to oranges and you can make either edge perform better. A taller V edge will outperform a shorter convex edge and vice versa.

I don't care where you start to sharpen the convex edge, just start the V edge from that same point and the V edge will always outperform the convex edge. The convex edge, at the same height, will travel outside the V edge, giving it a more obtuse angle.
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For a fair comparison, the thickness has to be the same behind the edges. Going from convex to V with respect to creating a new angle from the same point that the convex starts will result in a thinner amount of steel behind the edge, and require more metal to be removed. Going from V to convex on the other hand, just removes the shoulders where the V starts, but maintaining the same thickness behind the edge.
 
When the shoulder width and edge height are held constant to ensure a fair comparison, the V edge has a more acute angle. When connecting two points, the V edge has the shortest distance. The convex edge travels in an arc, outside the V edge, that connects the same two points. If you don't hold the edge height and shoulder constant, you can make either edge appear better.

DSC01911.jpg
 
You ARE missing my point. ;) My point was that if you have two edges of equal edge angle the convex will have a lower sectional volume. The convex in you image has a broader edge angle. Converting from convex to linear while maintaining edge height WILL result in a better cutter, but will have less supporting material immediately at the edge and a narrower edge angle. The edge angle is the variable we're holding constant here, and is the only variable the it makes sense to do so with other than, perhaps, spine width.
 
I am acknowledging your point, but all you are doing is changing the conditions to give the convex edge the advantage. You can make either edge look better or worse if you change the conditions for one but not the other. If you made the V edge height taller, it would perform much better.

What you're doing is changing the edge height to give the convex edge the advantage. I can do the same thing with the V edge and give it the advantage.

I weigh 150 pounds and am not a wresting terror. But if you bring Hulk Hogan down to 150, I'll whoop his butt because he'll be a wasted shell of himself. Take me up to his weight, and he'd whoop me. You can prove anything by changing conditions.
 
Twindog said:
When the shoulder width and edge height are held constant to ensure a fair comparison, the V edge has a more acute angle. When connecting two points, the V edge has the shortest distance. The convex edge travels in an arc, outside the V edge, that connects the same two points. If you don't hold the edge height and shoulder constant, you can make either edge appear better.

DSC01911.jpg
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I understand both view points - it's cold 'outside' :p.

In this instance/example, the V angle is 60* and the convex angle is greater than 90* inclusive. Which is over 50% biased in favor of V edge. Hence, Orange & Pamelo comparison.
 
Show me, please, how a linear edge of equal edge angle to the original diagram can provide lowered sectional volume. This is a simple matter of geometry. :o The advantage I'm specifically detailing that a convex has over a linear edge is that it's able to have a reduced sectional volume whilst maintaining the same edge angle as the linear edge it began as.

This refutes the commonly touted claim that convex edges a in any way inherently stronger than a linear edge bevel if converted from the latter to the former. However, it asserts that there are, indeed, legitimate reasons why a convex edge can be appealing. It's just not for the reason folks commonly cite, which is based on false precepts.
 
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I'll try one more time: Yes, you are correct that if you make a convex edge taller enough than a V edge, you can give the two edges have similar angles and the convex edge can cut better through some media. That is your point, correct?

But I can give the V edge the same advantage by making the V edge taller than the convex edge.

You converted a V edge to a convex edge by giving the convex edge a much taller edge, allowing it to achieve a more acute angle. Now convert your second convex edge to a V edge and the advantage switches to the V edge. And if you give the V edge a taller edge, it will greatly improve cutting by having a much more acute edge.

When you hold the edge height and shoulder constant, the V edge will be more acute.

What people don't get is that it is very difficult to know what you're getting with a convex edge because most people who sharpen convex edges do so freehand.

Switch the comparison by converting two identical blades: one into a full flat grind (V edge) and the other into a full convex grind blade. The blades become the edge, one V edge the other convex. The full flat grind blade will be more acute.

Now try to convert the full flat grind edge into a full convex blade. You can do so only by making the edge more obtuse.
 
Right--I get what you're saying but that's totally separate from the point I was making. What I'm saying is that it's not so much the fact that it's a convex bevel that makes it cut better so much as that convexing removes material from the cheeks of the blade, which thins the material behind the edge without reducing the edge angle (and thus its ability to resist deformation.) At the end of the day, thinner cuts easier. And convexing is just one way to do that.

This would produce the same effect.
thinnedcheekbevels.jpg
 
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While I'm sure there will be some benefit to using a convex edge, those benefits will be imperceivable to anyone who isn't a robot.

Angle and thickness are all that practically matters, any edge shape can be ground appropriately for any job, whether that be a straight razor or the jaws of life.

(This applies to primary bevels as well.)
 
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