The Convex Grind/Edge Myth

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Feb 17, 2011
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I was doing some doodling to help myself grasp these differences while trying to choose a new knife and thought they'd be worth sharing because I've seen a lot of posts on all different forums that provide some very misleading information. I thought I'd share what I noticed with you all.

I want to start off by saying that I am a big fan of convex edges and grinds. I am very impressed with Bark River knives, Fallkniven, and I love the edges on my knives that have come out convexed on their own just from sharpening by hand. There is no denying that a convex edge or full convex knife has OUTSTANDING cutting performance because it doesn't have a shoulder to wedge into the material you're cutting.

During this analyzation, I have chosen to compare convex grinds to Scandi grinds. I chose to do so because of how commonly these two grinds are compared in general. If you take a pencil and paper and draw a full flat grind with a V edge grind, you will find that drawing the curved lines to make it convex (at the edge) without changing the thickness of the blade or the edge angle will provide very similar results.

I am about to analyze the one HUGE, blatantly false myth about convexed edges and grinds: That they are more durable by their nature by providing more steel behind the edge. As you can see in the following, it is as far from the truth as you can possibly get:

convexscandi_zps0bcb97ca.jpg


Apologies for the bad drawing here. The black lines depict a Scandi ground blade. As you can see, the red lines depict roughly what you would get if you made a convex grind using the SAME thickness of blade at the SAME edge angle. As you can see, the convex grind leaves actually LESS metal behind the edge than the Scandi. Because of all of the information that states that a convex grind gives you more meat behind the edge, I also drew the blue curved lines. This depicts HOW you can achieve more steel behind the edge of the knife. The ONLY way to do so is to use a wider edge angle (or to use a thicker blade stock to begin with, but my sentence assumes we are using the same thickness).

This goes very much against the common notion held regarding a convex grind's durability. I am glad that I stumbled across this during my doodles because it opened up a lot more options to me in my purchasing decision. Initially I thought I MUST stick with a convex grind for durability. It simply isn't the case.

As a little bit extra, although it's a bit of an aside, here is what I've concluded about the differences between a Scandi and a Convex overall:

As earlier noted, I've found that the Convex ground blades I've used tend to meet less resistance for the most part. This will come down to edge geometry and blade thickness more than anything, but the lack of a shoulder does seem to provide me with a cut that meets less resistance from what I can tell. The difference isn't enormous, but is apparent.

What I have seen to be the main advantage of a Scandi grind is that you literally NEVER need to reprofile the entire blade to keep from losing cutting performance. Simply sharpen along the grind back to your zero edge and you will maintain the same geometry for the life of the blade. In this way, the Scandi is very predictable and an overall breeze to maintain. The convex, in comparison, can be maintained quite easily with a strop for a long time, but when it comes time to actually sharpen it then you must remove metal from the ENTIRE blade (thinning the blade a bit) in order to maintain the same edge angle. If you only sharpen a convex ground knife at the edge, it will come out with a wider angle when you're done.

Here's another crappy drawing to compare the sharpening so I can illustrate what I mean:

scandiconvex_zpsb5d351b4.jpg


Anyway, I hope this has helped some of you. It sort of bothered me that I had been under the wrong impression for the last couple of years simply because I was taking in what other people were saying without really looking at it. The convex grind is actually my personal choice for a large fixed blade knife with a thick stock because I know that I'll never thin the blade out significantly enough in my lifetime to weaken it, but for a somewhat smaller and/or thinner knife I see more merits to the Scandi (i.e. I think that may be why it is so popular for a Bushcrafter). That's just me. And lastly, I really don't think that the convex actually having LESS metal behind the edge really matters in this comparison. If you look at how far up the blade you have to be for the convex to start to show a difference in thickness, then you know that if you have a chip that deep into your edge that it reaches the shoulders (or where they would be if you have a convex) that you have a pretty serious problem and I doubt that it has anything to do with the grind. This just illustrates that the advantage of the convex is in cutting performance, NOT in strength. Everything that goes into a knife design is a compromise, and the convex is not the "have your cake and eat it too" grind.

Note that I am not a knife expert. If there is somehow something that I failed to mention or illustrate that anybody thinks should be mentioned, please let me know. This is simply what I found out from doing some doodling.

Also, I'm sure that I'm not the only person who has figured this out. But over the last couple of years I haven't ever seen anybody pointing it out, so I wanted to make sure more people had accurate information since I know I sure didn't ever since I first started hearing about convex ground knives.
 
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The subject has been discussed here at BF not long ago and the general consensus was about the same as Yours.

However, there are a few things about Scandi edges, that came out wrong in Your text.

A Scandi edge can be convex, hollow or flatground, just as a sabergrind or a full flat grind can be.
Basically a Scandigrind is a low sabergrind.

If You do a zeroedge on a Scandigrind or by all means, any knife, the edge will roll, chip or dent on harder materials.

At my work as a kitchen carpenter, I use my knives in many tough-to-cut materials.
I use the very same brands as You do, but also several Mora models and even a thin kitchen paringknife.
They all benefit from convexing the edges on a 1" x 30" beltsander, leatherhone and a diamond/ceramic stone.

A regular 0,98" spinethickness Mora needs a thicker convex cuttingedge and convex shoulders, to perform in my work.
The Mora Robust with 1/8" spine and very thick bevel, needs thinning of both the shoulders and cutting edge, before it reaches the high-performance I seek.

I have to say that the Robust benefits greatly, from this thinning and I really like the increase of performance! :thumbup:
Now it performs like my best Fällkniven and Bark River models.
Ok, they mostly have more alloyed steels and they handle abrasion better than Mora carbon steel.
The Rc 60 is a great aid though, for the Mora edgeholding so the difference isn't very big.

Conclusion:
Work with the geometry on Your existing knives and get a huge increase in performance!


Regards
mikael
 
Your argument is only relevant when convex grinds are compared against a flat saber grind, or scandi grind as you put it. Most true Scandi grinds have an very slight hollow that aids in sharpening. If you diagram a Full Flat grind with a micro bevel then the statement that there is more material behind the edge makes more sense. Ultimately the convex blade is a compromise between the cutting performance of a FFG and the strength of a Saber grind.
 
The subject has been discussed here at BF not long ago and the general consensus was about the same as Yours.

However, there are a few things about Scandi edges, that came out wrong in Your text.

A Scandi edge can be convex, hollow or flatground, just as a sabergrind or a full flat grind can be.
Basically a Scandigrind is a low sabergrind.

If You do a zeroedge on a Scandigrind or by all means, any knife, the edge will roll, chip or dent on harder materials.

At my work as a kitchen carpenter, I use my knives in many tough-to-cut materials.
I use the very same brands as You do, but also several Mora models and even a thin kitchen paringknife.
They all benefit from convexing the edges on a 1" x 30" beltsander, leatherhone and a diamond/ceramic stone.

A regular 0,98" spinethickness Mora needs a thicker convex cuttingedge and convex shoulders, to perform in my work.
The Mora Robust with 1/8" spine and very thick bevel, needs thinning of both the shoulders and cutting edge, before it reaches the high-performance I seek.

I have to say that the Robust benefits greatly, from this thinning and I really like the increase of performance! :thumbup:
Now it performs like my best Fällkniven and Bark River models.
Ok, they mostly have more alloyed steels and they handle abrasion better than Mora carbon steel.
The Rc 60 is a great aid though, for the Mora edgeholding so the difference isn't very big.

Conclusion:
Work with the geometry on Your existing knives and get a huge increase in performance!


Regards
mikael



Interesting. A Mora had a very thin grind, doesn't it? If all else is equal, edge angle and blade thickness, I would think that neither would be more or less prone to chipping. How certain are you that you didn't widen your edge angle by convexing and that that isn't the reason your edge is now more durable?

I wish I had two knives identical in dimension but in each different grind to compare in a way that was truly apples to apples.
 
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Your argument is only relevant when convex grinds are compared against a flat saber grind, or scandi grind as you put it. Most true Scandi grinds have an very slight hollow that aids in sharpening. If you diagram a Full Flat grind with a micro bevel then the statement that there is more material behind the edge makes more sense. Ultimately the convex blade is a compromise between the cutting performance of a FFG and the strength of a Saber grind.

On a full convex vs a full flat, absolutely true. But I've seen it claimed to be an advantage over the Sabre/Modern Scandi MANY times which is partly why I wanted to make this post.

Although it must also be noted that when you have a V edge on a full flat ground blade compared to the same angle convex edge, you once again are losing metal behind the edge. A lot of people also claim that by convexing their edge (removing metal) they are gaining more steel behind the edge. Again, only if they come out with a wider edge angle in the end.

I don't by any means want to claim that the convex grind is weaker than all others, I really love it. But the idea that it has more steel behind the edge is often brought up in contexts where the opposite is true, i.e. the convexing of the edge on a FFG blade or when compared to a Scandi.

Thanks for your reply! I hadn't actually compared the full convex to the FFG, just the FFG with a V edge vs a convex edge, so when reading my third paragraph I can see that it could have been worded better.
 
Interesting.

#1. A Mora had a very thin grind, doesn't it?

#2. If all else is equal, edge angle and blade thickness, I would think that neither would be more or less prone to chipping.

#3. How certain are you that you didn't widen your edge angle by convexing and that that isn't the reason your edge is now more durable?

#1. The basic 0,08" and 0,098" spinethickness models usually have very thin edges.
The newer models in 0,125" thickness comes with a secondary bevel.

#2. Sounds reasonable.

#3. I did widen the edgeangle on the thinner models and thinned it at the thicker Robust edge.
What this angle actually is, I don't know as I don't measure it.
The point is that the edgeholding vs cutting resistance blends out in a compromize between opposing properties.
Thus the increased cutting performance in the materials beeing cut.

That said I tend to premier a tougher edge with allaround cutting performance.
This edge doesn't really shine in any discipline, other than that it will always cut reasonably well.


Regards
Mikael
 
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I don't by any means want to claim that the convex grind is weaker than all others, I really love it. But the idea that it has more steel behind the edge is often brought up in contexts where the opposite is true, i.e. the convexing of the edge on a FFG blade or when compared to a Scandi.

I believe this is the consensus here at BF!
However, this is a theoretical discussion and in practical use, the convexed edge wins, as it's a great compromize between cutting resistance and edgeholding.
And a wellshaped convex edge is a joy to work with! :thumbup:


Regards
Mikael
 
I put convex edges on most heavy use knives that I sharpen in our shop and they do produce positive results. What I cannot do is guarantee that its the same convex edge I put on it the last time it came into the shop to be sharpened, no one can. A flat ground edge, I can guarantee that its the same edge.
 
For the first time in a long time a convex thread that is not a complete debacle.
 
On a full convex vs a full flat, absolutely true. But I've seen it claimed to be an advantage over the Sabre/Modern Scandi MANY times which is partly why I wanted to make this post.

Although it must also be noted that when you have a V edge on a full flat ground blade compared to the same angle convex edge, you once again are losing metal behind the edge. A lot of people also claim that by convexing their edge (removing metal) they are gaining more steel behind the edge. Again, only if they come out with a wider edge angle in the end.

I don't by any means want to claim that the convex grind is weaker than all others, I really love it. But the idea that it has more steel behind the edge is often brought up in contexts where the opposite is true, i.e. the convexing of the edge on a FFG blade or when compared to a Scandi.

Thanks for your reply! I hadn't actually compared the full convex to the FFG, just the FFG with a V edge vs a convex edge, so when reading my third paragraph I can see that it could have been worded better.

Sorry, having re-read my earlier post I didn't mean to sound like I was disagreeing with you, fundamentally you are spot on. I have often been puzzled by claims people make about convex edges having more material behind the edge and thus being stronger. Both as a collector and sharpening enthusiast I do like a good convex edge myself, a couple of my edc's currently are convex and I am a big fan of Barkies. For me though the one application where convex really shines is in high performance kitchen knives with a full convex grind.
 
Once more into the geometry-free zone, where math is myth and myth is math.

You can make a convex edge cut better than a V edge or you can make a V edge cut better than a convex edge, just by changing the geometry. One does not automatically cut better than the other because they have different names.

A convex edge does not have an angle on the edge, but two intersecting arcs.

An edge is defined by three points: the apex and the two shoulders, the points where the edge meets the blade.

When those three points are kept equal (your blue line and your black line in the OP), the V edge will be more acute and the convex edge will have more metal behind the apex, making it more durable.

However, the differences can be extremely slight or very large. If the arcs that define the convex edge are acute, there can be a very large difference between the performance of a V edge and a convex edge, with the V edge cutting much better and the convex edge being more durable. If the arcs that define the convex edge are slight -- think the arc of a circle the size of the earth, there will be virtually no difference between the two edges because the convex edge will be almost exactly a V edge to anyone without an electron microscope.
 
Once more into the geometry-free zone, where math is myth and myth is math.

You can make a convex edge cut better than a V edge or you can make a V edge cut better than a convex edge, just by changing the geometry. One does not automatically cut better than the other because they have different names.

A convex edge does not have an angle on the edge, but two intersecting arcs.

An edge is defined by three points: the apex and the two shoulders, the points where the edge meets the blade.

When those three points are kept equal (your blue line and your black line in the OP), the V edge will be more acute and the convex edge will have more metal behind the apex, making it more durable.

However, the differences can be extremely slight or very large. If the arcs that define the convex edge are acute, there can be a very large difference between the performance of a V edge and a convex edge, with the V edge cutting much better and the convex edge being more durable. If the arcs that define the convex edge are slight -- think the arc of a circle the size of the earth, there will be virtually no difference between the two edges because the convex edge will be almost exactly a V edge to anyone without an electron microscope.

Agree for the most part, but I'm not sure if I read the last paragraph correctly. It came across as a statement that you could have the same acute edge angle on a V edge and a Convex edge and that the Convex could have more support behind the edge. I don't see how that's possible without the edge widening unless you're talking about a v edge on a flat grind and a full convex grind, but that isn't apples to apples.

Did I misunderstand? I feel like I did lol

My drawing shows what happens when you keep the angle of the cutting edge and the blade thickness the same with both types of grind. If you take a FFG blade and convex it without widening the edge angle, it's the same type of result on a smaller level. You need to widen it to get more support behind the edge if that's the goal.
 
Agree for the most part, but I'm not sure if I read the last paragraph correctly. It came across as a statement that you could have the same acute edge angle on a V edge and a Convex edge and that the Convex could have more support behind the edge. I don't see how that's possible without the edge widening unless you're talking about a v edge on a flat grind and a full convex grind, but that isn't apples to apples.

Did I misunderstand? I feel like I did lol

My drawing shows what happens when you keep the angle of the cutting edge and the blade thickness the same with both types of grind. If you take a FFG blade and convex it without widening the edge angle, it's the same type of result on a smaller level. You need to widen it to get more support behind the edge if that's the goal.

At the same terminal apex angle, on blades with identical thickness and width, a convex will always have less steel behind the edge than FFG or sabre grind. Only a hollow grind can have less.
 
At the same terminal apex angle, on blades with identical thickness and width, a convex will always have less steel behind the edge than FFG or sabre grind. Only a hollow grind can have less.

A full convex will have more material behind the edge then a FFG with a beveled edge. Although in spirit I agree with you, by the time the more material aspect kicks in we are talking about a fair bit up the blade and is likely to have relatively little effect on edge stability/cutting performance with the exception of drag.
 
At the same terminal apex angle, on blades with identical thickness and width, a convex will always have less steel behind the edge than FFG or sabre grind. Only a hollow grind can have less.

First, you're mixing edge geometries with blade geometries. But lets go with your blade geometry: FFG vs. convex blade grinds. If the blade stock is the same width and the blade height is the same, the convex blade will always have more metal behind the apex than the FFG. On the FFG, you would create a blade/edge profile with two straight lines from each side of the spine to the apex. Connecting those same three points with a convex edge, the convex edge would be created with two arcs, both of which arc outside of the FFG. Edge geometry works the same way.

Draw any convex edge or blade grind you want. The edge or the blade will be bounded by three points: the apex and the two shoulders. If you connect those three points with two convex arcs, you'll have a convex edge/blade. If you connect those same three points with two straight lines, you'll have a V edge/FFG and all the metal within the FFG will be inside the arcs of the convex sides. That's what convex means.

Yes, you can put a convex edge inside a V edge, but you can't do it without changing the three points that define the boundaries of the edge.
 
Agree for the most part, but I'm not sure if I read the last paragraph correctly. It came across as a statement that you could have the same acute edge angle on a V edge and a Convex edge and that the Convex could have more support behind the edge. I don't see how that's possible without the edge widening unless you're talking about a v edge on a flat grind and a full convex grind, but that isn't apples to apples.

Did I misunderstand? I feel like I did lol

My drawing shows what happens when you keep the angle of the cutting edge and the blade thickness the same with both types of grind. If you take a FFG blade and convex it without widening the edge angle, it's the same type of result on a smaller level. You need to widen it to get more support behind the edge if that's the goal.

An angle is created by two straight lines that have a common vertex.

A convex edge is created by two intersecting arcs, so you don't have an angle. Look at the blue line in your OP. The angle changes constantly as it moves away from the apex. If you create a convex edge by hand, you'll have to continually change the angle of the blade, unlike a V edge where you keep the angle constant. There is not angle on a convex edge. So, first, you are not keeping the angles the same on your drawings of convex and V edges.

Secondly, it's not just the blade thickness that matters, it's the blade height and width that you have to keep the same. Imagine a FFG V edge that is .125 inches wide. How acute is the angle? Well, it depends. How tall is the blade. If the blade is two inches tall, the edge angle will be much more acute than if the blade is just one inch tall. The problem with your logic is that you are comparing a tall convex edge to a short V edge. You can make the same argument by comparing a tall V edge to a short V edge. Angles change, even at a constant blade thickness, depending on the height of the angle. So you have to keep both the edge/blade width and height equal to make fair comparisons.

The final point is that a convex edge, even at the same height and width, can vary a lot, depending on the radius of the two arcs. A very gentle arc (long radius) will look and function exactly as a V edge/blade. A very tight arc (short radius) will behave much differently.

Saying that you have created a convex edge means almost nothing, because that convex edge, even at the same height and width, can vary greatly depending on the defining arcs.
 
First, you're mixing edge geometries with blade geometries. But lets go with your blade geometry: FFG vs. convex blade grinds. If the blade stock is the same width and the blade height is the same, the convex blade will always have more metal behind the apex than the FFG. On the FFG, you would create a blade/edge profile with two straight lines from each side of the spine to the apex. Connecting those same three points with a convex edge, the convex edge would be created with two arcs, both of which arc outside of the FFG. Edge geometry works the same way.

Draw any convex edge or blade grind you want. The edge or the blade will be bounded by three points: the apex and the two shoulders. If you connect those three points with two convex arcs, you'll have a convex edge/blade. If you connect those same three points with two straight lines, you'll have a V edge/FFG and all the metal within the FFG will be inside the arcs of the convex sides. That's what convex means.

Yes, you can put a convex edge inside a V edge, but you can't do it without changing the three points that define the boundaries of the edge.

If the samples have the same terminal apex angle, the convex will always have less, it is inescapable. Only if the convex has a broader terminal apex will it have more mass behind the edge. Only if the FFG has a smaller terminal apex will it have less steel behind the edge.
 
I don't dispute anything that I've read above, but I lose interest...
and everything goes blooey.......(even though it's relevant...)
I just got my first convex-edged knife: a Fallkniven TK2 in SG2...and it will be fun to check it out.
p.s. Keeping it sharp is also important. I suspect that the WSKO, or a loaded strop (or fine sandpaper on a mouse-pad) will do the trick.
 
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