What Are The Advantages of a Convex Grind?

redsquid2

redsquid2

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Knifemaker / Craftsman / Service Provider
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If you prefer a convex either sometimes or all the time, what do you do with it? Hunting? Whittling? Carving? What applications does it excel at? Where does it fall short? Does it hold an edge longer? I never had a knife that was completely convexed.
 
I could make this really long but will try nutshell on my own use. I prefer convex for kitchen knives and they also work really well for wood whittling. Reason being is that the curve of the bevel does not get wedged into whatever it's cutting as much as a V grind, as the apex cuts the surface of your material the curve of the convex then almost pushes the material out of the way a bit stopping the wedge effect. Personally on all folders i prefer a V grind for maintenance and touch ups, but for cutting food and wood there are slight advantages with a convex. As far as edge holding, well it's subjective and there are too many variables, in theory there is a bit more steel behind each edge for stability but it's not going to be a night and day difference. You wont get M390 edge holding from 8cr13 just by convexing obviously. You will notice more difference in edge performance when you start applying the correct edge geometry for the right tasks. Eg - 18 degrees or lower for food as opposed to say 22-25 degrees for hard cutting of tough material where there could be more impact to the apex. Here i've drawn a very artistic masterpiece on how convexes are less prone to wedge into cutting material, its zoomed in a lot of course, but you get the picture. The convex curve forces the material out a bit and has less surface area to create friction against the material. The same way bearings in knife pivots have less surface area and appear smoother. Round surface rubbing flat surface has a tiny contact patch.
11jm35d.jpg
 
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The thing about the edge angle is people are in fact talking about sharpening angle. If I sharpen a blade at 17° flat bevel (V) on a fine stone and then sharpen an (all)identical blade at 17° convex bevel on my belt sharpener, the convexed edge will have more steel behind the edge (let's say at 0.24" behind the edge). However, the edge angle will be wider, the bevel height will be higher and the width of the blade will be (very) slightly narrower. If we're talking strictly "edge angle", the V bevel has obviously more steel behind the edge. I have been moving over to apply convex grinds to several favourite knives and I can say it's "all advantages" : smoother cutting action, easier reprofiling and honing (with Ken Onion's WorkSharp...), definitely longer lasting edge. All good stuff in my book.
 
redsquid2 said:
If you prefer a convex either sometimes or all the time, what do you do with it? Hunting? Whittling? Carving? What applications does it excel at? Where does it fall short? Does it hold an edge longer? I never had a knife that was completely convexed.
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The one area I find that convex isn't desired is a shaving edge where you want it as thin as possible at the Apex.

For Food, Wood, Heavy Camp, Skinners, I prefer a Slight convex grind & edge.
 
marcinek said:
Not, in theory or reality, for equal edge angles. V grind has more steel behind the edge.

That said, I prefer a convex grind, due to their ease of maintenance/sharpening.
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No, that isnt entirely correct, because it's impossible to have the exact equal angle on both. You cannot equate a curved line with a straight one because the angle simply changes as you travel along the convex. The angle of the convex curve is different at its outward most point to its angle at the very edge. You can get them close in degrees, but the convex protrudes outwards more than a straight line. Its like saying a person with a curved bear belly has less fat storage than a flat stomach. A curve or bulge contains more steel near the apex, its quite simple.
 
Mick_1KRR said:
No, that isnt correct, because it's impossible to have the exact equal angle on both. You cannot equate a curved line with a straight one because the angle simply changes as you travel along the convex. The angle of the convex curve is different at its outward most point to its angle at the very edge. You can get them close in degrees, but the convex protrudes outwards more than a straight line. Its like saying a person with a curved bear belly has less fat storage than a flat stomach. A curve or bulge contains more steel near the apex, its quite simple.
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Sorry. Not true. Two curves (like the sides of a convex ground knife), meet at an angle that is easily defined.

Math. Look it up. :thumbsup:
 
marcinek said:
Sorry. Not true. Two curves (like the sides of a convex ground knife), meet at an angle that is easily defined.

Math. Look it up. :thumbsup:
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You're looking at the concept wrong when it comes to knives. Read what the guy above said. Sharpen a convex on a belt at the same angle as a V edge and you have more steel behind the edge on the convex. Simple. I'll draw you a picture of you cant grasp the concept.
 
marcinek said:
I looked it up for you.

CQrRsXd.gif


Sorry.
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You just completely proved my point. As i said above, the angle changes from the tip to the middle then to the very start of the convex bevel. Are you seriously trying to argue that those 2 geometrys are equal? You're basically saying that a 50 degree v grind is equal to a 20 degree convex bevause the tip of the convex has that initial steep angle. You're simply incorrect in real world cutting use. Look at the lower half of that picture and tell me which blade will have less resistance when pushing through material. You're clinging to a partial technicality in initial edge angle and ignoring the rest of the geometry. Because, the convex changes angle and the straight V does not hence its angle widening continuously with length.
 
Rhinoknives1 said:
The one area I find that convex isn't desired is a shaving edge where you want it as thin as possible at the Apex.
For Food, Wood, Heavy Camp, Skinners, I prefer a Slight convex grind & edge.
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That's right. However, if I convex a kitchen knife that came with a 15° V edge, I will convex it on the belt sharpener at 12 or 10° (in order to keep it more or less as thin behind the edge). These knives perform beautifully (because of the smooth transition ?) and I have the "feeling" the edge lasts still a bit longer. Wasn't bold enough (yet) to apply this to the classic razor...
 
The reason the edge lasts longer with a convex is at the very apex of the cutting edge it is initially quite a steep angle. But it then curves inwards to give a lesser geometry offering less resistance. Convex is a completely different concept in geometry to a V grind.
 
Mick_1KRR said:
You're looking at the concept wrong when it comes to knives. Read what the guy above said. Sharpen a convex on a belt at the same angle as a V edge and you have more steel behind the edge on the convex. Simple. I'll draw you a picture of you cant grasp the concept.
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I'll be waiting for that. Draw 2 angles that are the same. Draw the v grind in one and the convex in the other.
 
Backing 2005, Bill Moran told me that the problem with a lot of convex grinds is that they are WAY too convex, aka too thick.
 
Mick_1KRR said:
You're looking at the concept wrong when it comes to knives. Read what the guy above said. Sharpen a convex on a belt at the same angle as a V edge and you have more steel behind the edge on the convex. Simple. I'll draw you a picture of you cant grasp the concept.
Click to expand...
You're just proving you're confusing sharpening angle and apex angle. They will be the same if you work your blade with a steady angle on a flat stone : hold the blade at 17°, you'll get a 17° V bevel, all 17° up to the apex. Now, take the same blade to a belt sharpener, set the angle of the tool at 17° and proceed. Now measure the angle at the apex and I can tell you it will be more than 17°. By the way, your artistic picture shows it quite well : the convexed blade on the left has been sharpened at a way steeper angle than the V to acheive a more or less similar "apex angle". Geometry is real world stuff.
 
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Mick_1KRR said:
Are you seriously trying to argue that those 2 geometrys are equal? You're basically saying that a 50 degree v grind is equal to a 20 degree convex bevause the tip of the convex has that initial steep angle. You're simply incorrect in real world cutting use.
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The geometries are not equal but the edge angles are. You are simply misunderstanding mathematics. And, as much as would might want to ignore it, the real world behaves according to the laws of mathematics and geometry.

I'm sorry neither support your argument.
 
Mick_1KRR said:
You just completely proved my point. As i said above, the angle changes from the tip to the middle then to the very start of the convex bevel. Are you seriously trying to argue that those 2 geometrys are equal? You're basically saying that a 50 degree v grind is equal to a 20 degree convex bevause the tip of the convex has that initial steep angle. You're simply incorrect in real world cutting use. Look at the lower half of that picture and tell me which blade will have less resistance when pushing through material. You're clinging to a partial technicality in initial edge angle and ignoring the rest of the geometry. Because, the convex changes angle and the straight V does not hence its angle widening continuously with length.
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Actually he's right, and I don't think you're understanding what he's showing. Convexes will have a continuously decreasing angle the further back you get from the edge but at the apex will be identical in angle. The closer to the edge you get the greater the order of magnitude of influence specific geometry has on cutting performance. For equal edge angle, a convex will have superior cutting performance, but less lateral support to resist torque/side-loads. A 50° V edge would have to be compared to a convex with a 50° apex, but of the two the convex would overall be thinner. Convexes, not being limited to a linear geometry, can have more variation in them, though, and so some may be done thinner or thicker while retaining that same edge angle at the apex. When people convex their knives and make them tougher it's because they're not properly accounting for the deflection of their abrasive surface and so are inadvertently just thickening their edge angle. You could do a similar thing by just thickening the apex of a V grind. A straight V grind will thicken continuously, the angle is staying constant. That's kind of what makes it a V instead of a convex or hollow. :)
 
Mick, Marcinek is right. When comparing a V grind to a convex grind of equal edge angles, the V grind will have more metal behind the edge. The convex grind starts at that angle but quickly gets thinner while the V grind retains it's angle.
 
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