Convex Grind vs Hollow and Flat Grinds

M

mattking

Joined
Dec 9, 2013
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Okay so I'll start by saying I am no veteran to knives, I just started collecting maybe a year ago but I saw the Ken Onion Work Sharp and pulled the trigger. It says on the box that the convex grind is better than a hollow or flat grind and will last longer, I sharpened one of my knives and really couldn't tell if it was really that much better than the original hollow grind on it. Any info would be appreciated, like I said I'm no expert and just want to know what knives would be better fitted for the convex vs a hollow or flat grind.
 
Well, those that know more than I will be along in a bit, but here's my take: you have more metal behind the edge with a convex grind, which makes it stronger than a hollow grind. I don't know if it makes much difference in how sharp you can get the edge, but that edge strength does help when it comes to hard use. And for me, it's also the easiest edge to sharpen. I have a HF belt sander, and it transformed how I sharpen knives and machetes.
 
Thanks man! I'm just wondering if its worth it to take the time and convex all of my knives which is around 40-50, most are pocket knives only have one or two fixed blades. I'm new to this forum and looked this up on the internet and YouTube but couldn't really find much everyone pretty much said the same thing. Thanks for the reply man!
 
I remember an old joke about quality of Opel cars, saying that every car sooner or later becomes an Opel. That's pretty much describes my view on a convex grind.
And I remember how I struggled with the first blade that I've made myself (by stock removal of course): how difficult it was not to make it in convex.
So I kind of not view convex grind as superior in any way. It's like modifying scandi grind for people who are not good at free-hand sharpening so they would not be embarrassed with the mess... just kidding!
 
I can't tell whether you're talking about blade grinds or edge grinds or both. I'll guess edge.

The problem in comparing convex to V edges is that people don't define the edges with actual geometry. A V edge can have more or less metal behind the edge than a convex edge, depending on how they are ground.

But in general, when compared fairly -- same shoulder width and height -- a V edge will cut better because it has a more acute angle and a convex edge will be more robust because it has more metal behind the edge.
 
Twindog said:
I can't tell whether you're talking about blade grinds or edge grinds or both. I'll guess edge.

The problem in comparing convex to V edges is that people don't define the edges with actual geometry. A V edge can have more or less metal behind the edge than a convex edge, depending on how they are ground.

But in general, when compared fairly -- same shoulder width and height -- a V edge will cut better because it has a more acute angle and a convex edge will be more robust because it has more metal behind the edge.
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Excellent point about blade vs edge grind. I was just assuming he was referring to edge grinds. Now, for my money, the best for me personally is something like a Fallkniven F1, which has a full height convex. I can get that thing screaming sharp with my belts.
To the OP, I hear your pain about converting 40 or 50 knives to a convex. For me, it was not so many. I perfected my technique on some old kitchen knives I had lying around. Rounded many a tip on those babies, let me tell you! I don't really convex my folders, I use the Sharpmaker on those. Every once in a while, I inherit a project knife, with all kinds of edge damage, that is in need of a full reprofile. Then I go to the belt sander. I'm just too lazy for a full makeover by hand, I guess.
 
Twindog said:
But in general, when compared fairly -- same shoulder width and height -- a V edge will cut better because it has a more acute angle and a convex edge will be more robust because it has more metal behind the edge.
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Nailed it. Beautifully put. The convex will be more robust because it has a larger edge angle and more steel behind that edge. And it will not cut as well.

That said, I do believe convex edges and grinds are "sharper," because they are easier to maintain without some complicated jig doohickey. Even with a Sharpmaker, you are not creating a vee grind, you are creating a series of microbevels. You are trending toward Opel-ness/convexity, as Poez so deftly put it.

Convexing isn't sloppy idiot proof, it depends on sloppy idiots to work. That's the beauty part.

Now if you got 40-50 pocket knives, star out with a cheap one that you aren't crazy about. You are probably gonna scratch it, and it takes a while to get the hang of "knocking off the shoulder." But once you are done, maintenance is just a quick stropping after each use.
 
Twindog said:
But in general, when compared fairly -- same shoulder width and height -- a V edge will cut better because it has a more acute angle and a convex edge will be more robust because it has more metal behind the edge.
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At the same time, a convex grind is what helps a Japanese Katana slice through meat so easily. Hollow and flat ground bevels create friction along the whole edge after the initial slice, but a convex grind helps to throw the material away from the blade. An axe has tremendous strength due to its convex grind and an obtuse angle. Knives that have a convex bevel do not actually have a convex grind - it's usually a combination of a flat grind with a portion of the bevel convexed. It decreases resistance, and a lot of ABS smiths prefer it.

It's a pretty complex subject that doesn't have a straightforward answer. Depending on your cutting task, each grind has its advantages. I'm sure the same goes for the edge.
 
marcinek said:
Nailed it. Beautifully put. The convex will be more robust because it has a larger edge angle and more steel behind that edge. And it will not cut as well.
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I think most people with convex edges have less acute angles than something sharpened with more clearly defined straight bevels. The beauty to me of a convex edge is that the edge blends in with the rest of the blade grind, you don't have to worry about uneven grinds, or bigger edge grind on one side vs. the other. Depending on the grits you finish up with you may not even be able to see the edge grind because it will blend in. And it seems easier to strop a convex than a straight grind but I'm not a stropping expert.
 
A Justice said:
Knives that have a convex bevel do not actually have a convex grind - it's usually a combination of a flat grind with a portion of the bevel convexed. It decreases resistance, and a lot of ABS smiths prefer it.
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That part I'll agree with. Most of the "convex" knives out there are flat ground with a convex edge bevel. You don't see much "full convex" let alone appleseed grinds.

The rest...not so much. I don't see how if you have a vee grind and a convex grind at the same edge angle, the convex somehow "throws off" material more.

And I know the whole thing is gonna start again, so here's is somebody-from-here's very good picture (balck and green on the right) of a convex and a vee edge that have the same edge angle, that, hopefully, will stop it...

ConvertedConvex_zps454c6044.jpg
 
V or flat edge is sharper, but for less time than convex.

Me personally, have modified very wide convex bevels which are almost flat, but have a slight curve so metal doesn't want to roll. Took me years to perfect for work knives. They will stay shaving sharp for at least 40 work hours (or 1 week of work)
 
bdmicarta said:
I think most people with convex edges have less acute angles than something sharpened with more clearly defined straight bevels. The beauty to me of a convex edge is that the edge blends in with the rest of the blade grind, you don't have to worry about uneven grinds, or bigger edge grind on one side vs. the other. Depending on the grits you finish up with you may not even be able to see the edge grind because it will blend in. And it seems easier to strop a convex than a straight grind but I'm not a stropping expert.
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Agreed. And its easier to strop a convex because you're not a stropping expert. :thumbup:

I knock the shoulders off most of my flat ground vee edge knives. I love that flat ground convex edge configuration. But I know it doesn't imbue my knife with some magical powers that allow it to transcend the laws of geometry somehow! :)

Does my knife become "sharper" after I have done that? Yes. Because 1) I have given it a more acute edge angle, and 2) I find it easier to maintain.
 
Twindog said:
I can't tell whether you're talking about blade grinds or edge grinds or both. I'll guess edge.

The problem in comparing convex to V edges is that people don't define the edges with actual geometry. A V edge can have more or less metal behind the edge than a convex edge, depending on how they are ground.

But in general, when compared fairly -- same shoulder width and height -- a V edge will cut better because it has a more acute angle and a convex edge will be more robust because it has more metal behind the edge.
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I agree with this statement fully, but one must point out that at the apex, the convex edge will have a much greater angle than the V edge, hence the edge will not be as keen for cutting purposes (might be repeating what you said). If I develop a convex edge by sharpening equal width facets from 15, 16, 17 and 18 dps with 18 degrees reaching the apex on both sides, then strop the edge to remove the facets forming a radius convex geometry, this knife will have less metal behind the edge, and hence be weaker, than if I sharpen the knife in a V grind all the way to the apex at 18 dps. In this scenario, the convex edge will likely cut more efficiently than the V grind because of the reduced metal behind the edge, but it will be weaker. The scenario that you described above is if I convexed the edge as stated, then compared it to a V grind all the way to the apex at 15 dps. The latter would cut better and be weaker.

However, I think that the OP was talking about convex/hollow/flat grinds. In all of these grind scenarios, you get into thicker metal with repeated sharpening's of a knife, as you are moving the edge up the blade. In order to maintain or achieve suitable blade geometry for the life of the blade, this grind must be thinned down in order for proper edge geometry and cutting performance. If I would rank the three grind types from least to most with respect to the amount of metal that must be removed to maintain proper edge geometry and cutting efficiency, it would be hollow (least) followed by flat and a convex grind would require removing the most metal in order to maintain proper cutting. Thinning hollow ground knives and flat ground knives are relatively easy by simply placing the grinds flat on a stone and grinding it while maintaining the original grind angle. On a hollow ground knife, this will form two train track like grind patterns, one near the edge and one near the flat. With repeated sharpening and grindings, these tracks will continue to get wider until they converge in the middle, hence making a full flat grind. Thinning flat ground blades will remove metal across the entire grind, also very easy to do on a flat stone. Convex grinds, on the other hand, are nearly impossible to do on a flat stone. One would have to resort to taking the blade to a belt sander with a spongy backing behind the belt to effectively thin this blade while maintaining the intended geometry. A very careful and skilled hand would be required.

In use, a convex ground knife is less likely to stick in wood when performing chopping or splitting duties due to the radius of the grind (think axe), but the maintenance problems (for me at least) in maintaining proper edge geometry make me avoid this grind. Other than this, I see no benefit to a knife with a convex grind.

CRK will regrind the blades of his knives for free if the knife is sharpened to the point that the geometry is off. Personally, I cannot see myself grinding the sides of an $800 Hinderer each time I sharpen it, or even a CRK. But I use this method for all of my other knives at each sharpening. This is another reason why I do not like coated blades. I am going to be grinding the coating off at the first sharpening.
 
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The term "Better edge" has to be looked at for job the edge is intended for. For a straight razor you want a Vee or flat edge. On some hunting knives a few prefer a hollow edge for cutting meats because there is less metal behind the edge.

My favorite is the Convex because for the reasons that the #2 poster raindog listed but you have to look at the dimensions of the knife and the intended use. Also I agree with the statement by A Justice that a Convex ground Katana sword works by having the entire blade continue to separate meat for the entire height of the blade. Then there is the issue of the grind and the edge. You can have a convex edge on a hollow ground knife for instance. I generally put a con vexed edge on 95% of the knives that come into my shop and all of the custom knives that I make.
 
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The convex angle should be modified to what you are cutting. While most my work knives are convex, they have different angles for what they cut.
 
Wouldn't it be awesome to have access to wind tunnels?

I believe it's the same forces of wings and air and steel and material.
 
marcinek said:
That part I'll agree with. Most of the "convex" knives out there are flat ground with a convex edge bevel. You don't see much "full convex" let alone appleseed grinds.

The rest...not so much. I don't see how if you have a vee grind and a convex grind at the same edge angle, the convex somehow "throws off" material more.

And I know the whole thing is gonna start again, so here's is somebody-from-here's very good picture (balck and green on the right) of a convex and a vee edge that have the same edge angle, that, hopefully, will stop it...

ConvertedConvex_zps454c6044.jpg
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I saw a demonstration of Katana cutting, and the guy explained that one of the biggest reasons a Katana cuts better than a european sword is due to the convex grind on the Katana and the hollow grind on the european sword. It proved true in the demonstration.

I can't find a video of the demonstration (I think it was on the History channel, modern marvels top 10 wHere is a few quotes from bladesmiths corroborating the decreased resistance of a convex grind to a hollow grind. It's the smooth transition to the shoulder and/ or flats that make a convex grind work when cutting flesh.



Ed Caffrey said:
The real key to the quality of edge that the convex provides is just that....its shape. A smooth radius means much less cutting resistance than something like the semi-hollow grind you see on many blades. (many of these blades have very obtuse bevels between the "hollow" and the edge, creating much greater cutting resistance) Most of us have also "tweaked" our heat treatment to compliment the particular grind we apply, which is another key element in producing a quality edge.

I can't speak for other ABS Smiths, but I do not apply a full convex to my knives. About 80%-90% of the blade is flat ground to reduce weight, and the last 1/8" to 1/4" of the blade's edge is convexed (the amount of convex depends on the specific blade/knife) I believe that if you look closely at many of the ABS MS knives, you will find they are similar. They will all have some degree of edge bevels, but in most cases those bevels are also radisued rather than flat. It's the transition between an edge bevel and the main blade bevel that makes all the difference.

I personally think that not enough attention is paid to cutting resistance in a blade. A dull blade that is setup/ground to reduce cutting resistance (as in one with a convex edge) will seem to cut as well as a different type of edge configuration that is freshly sharpened. In all actuality, that is why the convex edge shines in the cutting competitions....its not that it stays sharper any longer, its just that the cutting resistance is so low that even after many abusive tasks it still keeps going.
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Todd Robbins said:
I have found that no other grind performs as well as a properly executed convex grind or a flat primary grind with a convex edge. A full hollow ground certainly cannot. Hollow grinds and thin flat grinds might excell at slicing THIN material (like straight razors are required to do), but they will always have more drag when slicing thick material, like meat or cheese, and they certainly aren't the best for chopping, as has been said before. The geometry of the convex grind forces the material away from the sides of the blade while slicing or chopping, reducing drag, unlike a flat or hollow grind. The only exceptions to convex geometry's superiority in comparative testing is in slicing thin material, when a convex grind is overly thick (like many are, unfortunately), or (possibly, I haven't tested this one) when a really deep hollow grind is done while leaving the edge thick and sharpening it with a convex profile, like Jerry Hossum does.



All of the above is what I have found to be true by testing all sorts of grinds on all sorts of knives IN MY SHOP AND HOME, and my not reflect your findings/feelings.
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First off I want to thank everybody for responding to my post, I am quite new to the forum and it feels good to have people reply with some great knowledge. The info i took away from it really helped me out and sorted a few things I wasn't so sure about, and also explains why when I went to do the paper test it didn't cut right through like the flat grind did. Thanks alot everyone look forward to talking to all of you again!
 
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