what are the proven advantages of a convex edge

[Rat Finkenstein]

It's good for choppers (slightly stronger & less prone to sticking) and easy to maintain, easy to learn, somewhat forgiving of technique. I have a small knife with a very thin convex edge and it cuts very well. But so do my non-convexed edges. All personal preference.

Sharpening freehand often results in a slightly convexed edge.

 

[theonew]

It's good for choppers (slightly stronger & less prone to sticking)

Less prone to sticking makes some sense but how is it stronger? For the same included angle a convex edge has less metal supporting it.

 

[jimmyh]

There is an awful lot of debate going on for something that is just simple geometry. I'm gonna attempt to say everything that needs to be said to end this, though I fully expect this to spark further debate

The more angles you throw on a V edge, the more closely you approximate a convex edge (the optimal solution is but one of many convex shapes). The question becomes "how many stages do I need to do before the difference is insignificant?" and relatedly "Is that too much of a PITA to do that many different bevels?"

Scandi grinds suck because for the final edge to hold up, the rest of the knife is too thick and has poor cutting ability. One V wont approximate the optimal solution very well. This doesn't make "convex edges be better". The thing is that it the same holds for crappily convexed edges as well.

You have to compare the geometries with all other relevant variables equal. In this case, it's a question of strength at any distance from the edge and the stress it is going to be subject to. Since the stress it will be subject to is a smooth function of distance (there aren't any magical distances), the optimal strength curve will be continuous (ie convex) as well.

Since multiple bevels reduces the amount of material removed at each step, it is actually more convenient than doing fewer bevels anyway (up to about as many grits you use). I typically have 4 distinct bevels on my knives (primary grind down to ~10mils, coarse diamond down to 50 microns, 600 grit down to the last 10-15 microns, then polished to the edge).

The difference between a geometry like that and the convex edge that follows the spline defined by the V geometry is very small. The biggest difference is gonna be on the corner of the second bevel which is only 10 mils wide anyway, so the difference is gonna be a few mils. I'd be amazed if someone was able to reliably detect the difference in cutting ability.

I don't know how to control the geometry of a convex edge that well, so I expect any convex edge I make to be farther from optimum than my V approximation, but others may have better skills than I here (how do you determine the geometry?). I suppose I could grind the series of Vs and then round the shoulders off, but that is strictly more effort than grinding the V edge, and I expect negligible improvement.

Clamp setups and microbevels make sharpening quite independent of hand skills, and it just takes a couple paragraphs to describe the technique needed to get to hair whittling edges in a few minutes.

Summary: The optimal edge will be convex, but it is easy to do a near perfect approximation with a series of V bevels, while it may be significantly harder to achieve a decent approximation with a convex edge.

There are other benifits of convex edges of course, but a better geometry just is not one in practice.

 

[Jason B.]

The reason a convex edge works is very simple, less points of contact give you less friction. The natural curving shape pushes material to the sides unlike a flat surface where the friction only increases from the point of contact. I learned once I got my DMT aligner that with the perfect grind, the thiner edge micro chipped more often and the points of damage were much larger than with a hand sharpened edge. Only the best of steels seem to do OK with a precision edge. A edge cut by a jig looks good, feels sharp and is usually faster than doing it by hand but that's about where the advantages stop.


Do a simple test, cut a piece of cardboard with a mora then cut it with a convex blade. Its very easy to tell the difference that way.

 

[huugh]

I find convex edges have lower friction and cuts more aggressive than flat edge. We can all agree that convex edges outperform flat edges at any given angle, simply due to physics. It glides better, the edge is thinner..

It is not.

Less prone to sticking makes some sense but how is it stronger? For the same included angle a convex edge has less metal supporting it.

Convex edge has more metal supporting it, it is always thicker than other types.


"Zero" grinds:

With the same edge bevel angle:

Required height of a grind to match the edge angle:

This shows that what people consider as "removing shoulders" from V grind is actually only need of a convex to MATCH the original V edge angle. However, if you protrude flat bevel as high as you would a convex one, you get a more acute angle.

 

[cotdt]

You're arguing over semantics, huugh. By convex, we are talking about the same angle at the very edge. In your diagrams the convex edge is at a more obtuse angle than the flat edge. Actually, an edge is always flat at the microscopic scale, convex is when you have a smooth transition to the secondary bevel.

 

[Rat Finkenstein]

You're arguing over semantics, huugh. By convex, we are talking about the same angle at the very edge. In your diagrams the convex edge is at a more obtuse angle than the flat edge. Actually, an edge is always flat at the microscopic scale, convex is when you have a smooth transition to the secondary bevel.

No, you simply cannot understand his simple diagram.

 

[Roger999]

IMO it's kind of hard to test a convex edge vs V edge because V edges are simple and straight forward, 1 angle only, with convex edges how hard you push into it determines how thin/wide your edge will be, there would be soo many variables in the angle of the convex edges that even if you do put up a test all the convex fans would be saying "oh you didn't do the convex correctly" "you pushed into the sandpaper too hard and made the angle too steep" or "you didn't push into the sandpaper enough so the edge was very thin".

 

[cotdt]

No, you simply cannot understand his simple diagram.

Looks like somebody doesn't know what a microbevel is. You should learn how to read diagrams before you post in the Maintenance, Tinkering, and Embellishment section.

 

[Kronos]

i think convex only appears sharper because you are removing more metal. if you start with a v edge, and then sharpen to convex, you must remove metal in order to obtain that convex edge. therefore, the knife will have improved cutting performance

 

[Knife Outlet]

I don't think there is any proven advantage to the convex edge other than the fact that it is really easy to apply with a belt grinder. That's really the reason it is seen so often. People invent advantages for it but they aren't real. They are just easy to apply with a belt grider and that's what knife factories use to sharpen edges. They are fussy to maintain. I always recommend an end user saber grind the edges for easier maintenance.

 

[Jason B.]

I don't think there is any proven advantage to the convex edge other than the fact that it is really easy to apply with a belt grinder. That's really the reason it is seen so often. People invent advantages for it but they aren't real. They are just easy to apply with a belt grider and that's what knife factories use to sharpen edges. They are fussy to maintain. I always recommend an end user saber grind the edges for easier maintenance.

Japanese sword makers proved it a long time ago.

 

[richard j]

i dont think a v edge would hold up as well as a convex or even a half convex like i put on my knives. check out the vid panch0 posted in my time important thread http://www.bladeforums.com/forums/showthread.php?t=578787&page=21 post #406 i dont think a v edge would have held up as well or cut as good.

 

[theonew]

Convex edge has more metal supporting it, it is always thicker than other types.

However, if you protrude flat bevel as high as you would a convex one, you get a more acute angle.

If you "protrude" the edge bevel of a flat grind you are narrowing the included edge angle. Here's an accurate diagram comparing exact same edge bevels that was made by a bladeforums member during a previous discussion:

 

[hardheart]

now if you have a 15 per side bevel with the ~20 per side micro (edge is not centered, 53 pixels to the left, 43 to the right, it's closer to 22/18 as drawn), you get this

 

[Knife Outlet]

Try cutting through cheese and you would notice a significant difference in the force required for the blade to tranverse through the cheese between convex and flat at the same edge angle. I've tried it.

How do you define same angle? It is pretty difficult to make a convex edge as acute as a saber ground edge. It is even harder to measure the true bevel angle of a convex ground edge. Generally, convex edges are more obtuse. You can get pretty much same result by saber grinding to a more obtuse angle.

 

[Knife Outlet]

Japanese sword makers proved it a long time ago.

Now that's a definitive statement. What exactly did they prove, how did they prove it and how do you explain the physics? Where is a comparative analysis of the various ways a Japanese sword could be ground and what measurable effect it had on blade performance? What the Japanese sword makers proved a long time ago is that they could make good swords. It has zero to do with convex edges.

 

[richard j]

knife outlet, its not difficult to make a convex edge as acute as a saber grind if you know what you are doing. i made myself a chopper that has a thin convex edge. you can see the knife at this link http://sites.google.com/site/richardjsknives/Home/more-knife-pics-and-vids/more-knife-pictures its the top knife. the other knife on that page that i'm chopping the tree with is a chisel grind with a half convex edge. its the same knife thats in the vid i posted a link to above.

 

[huugh]

If you "protrude" the edge bevel of a flat grind you are narrowing the included edge angle. Here's an accurate diagram comparing exact same edge bevels that was made by a bladeforums member during a previous discussion:

In the same previous discussion I also posted this:

You can notice I kept the same edge angle and only from the point where original author started convex (blue line) I pictured flat, drawn to the same height as his convex (red line).

And of course it is narrower. Ceteris paribus, convex is ALWAYS thicker than other grinds.

 

[theonew]

Huugh, I don't know about you, but when I sharpen a blade I choose my edge angle based upon the steel, the intended use and overall geometry of the knife. I don't think how wide I would like my edge bevel to be and let the edge angle follow that. For me at least the edge bevel is whatever it is based upon how acute an edge angle I think I can get away with using.