Convex grinds

[REK Knives]

Thanks but it is not at all the point I am trying to make. The point I am making is that, for a vee grind, and a convex grind that have the same edge angle (the tangents at the point where the edges meet)...the vee grind has definitively more material behind the edge.

If you are not comparing the same edge angle, at you are saying is that a knife with a more obtuse edge angle has more steel behing the edge than a knife with a more acute edge angle.

I think where people get confused is that you theoretically can increase the convex apex angle to slightly above the same knife with a v apex angle and still have the same cutting ability due to the convex grind not having as much meat at the shoulders. So this is where the thought comes that the convex grind has more meat behind the apex. it does not... unless you increase the apex angle.

 

[marcinek]

We could do that, but people REALLY aren't to like THAT.

Look at my original picture and reflect the blue curve across the red tangent. Now you have a hollow grind with a 0 degree edge angle.

That isn't reality obviously. But...those red lines show the edge angle, right? See how the convex is always inside the red lines, the vee at the red lines? And the vee grind has more material behind the edge? Where is the hollow? Outside.

Coarse, but green/hollow/concave; red/flat; blue/convex...with the same edge angle.

 

[marcinek]

So this is where the thought comes that the convex grind has more meat behind the apex. it does not... unless you increase the apex angle.

Nailed it. And when you increase that apex angle....you have to increase the apex angle of the vee grind you are comparing it to. And then that vee has more meat behind the apex.

Again it is like saying my 35 degree inclusive vee grind has more meat behind the edge than your 22 degree inclusive vee grind. And if I said that I'd be called a moron with cries of "Uh doyyy." Not that I'm not called that already.

 

[marcinek]

I know there are members out there who took, understood, and remember calc. Admit it...it's ok.... Come out of the closet.

Derivatives? Slope of the tangent to a curve at a point? I know you're out there...

 

[stabman]

And if I said that I'd be called a moron with cries of "Uh doyyy." Not that I'm not called that already.

Forget about convex and V-grinds.
I can't believe someone in 2015 just used "Uh Doyyy".

 

[marcinek]

Forget about convex and V-grinds.
I can't believe someone in 2015 just used "Uh Doyyy".

People say it to me all the time. Though I think the youngsters say "derp."

 

[stabman]

People say it to me all the time. Though I think the youngsters say "derp."

Derp is probably passé now as well...
So hard to keep up with the times.

So sure, go for it...use "Uh doyyy"...you're just bringing it back!

 

[Twindog]

Actually a v grind of comparable angle has MORE steel behind the edge. The "convex has more steel thing" is nonsense. I have a pretty strong suspicion where it originates. Sorry. Geometry doesn't lie.

As far as maintenance...I find they are EASIER to maintain. It takes advantage of not being able to hand hold an exact edge eangle consistently, and can be made/maintained with a few bucks worth of simple materials. Some wet/dry sandpaper, a piece of leather, and a flat surface is really all you need.

So much misinformation. Both of those convex edges are how convexing works -- along with many other convex edges that can range from a convex edge virtually identical to a V edge or something entirely different. "How it actually works" is just one example. "How people are told conveying an edge works" is an equally valid example.

Convex edges do not have angles -- that's just geometry. The arcs are defined by a radius length of the circle of which defines the arc. When a convex edge has the same edge width and height, the only valid way to compare the general category of V edges to the general category of convex edges, the V edge is going to be more acute and the convex edge will be more robust because it has more metal behind the apex. Marcinek didn't show an example of a fair comparison, but it's easy to do. Just draw a straight line from the edge shoulder of a convex edge to the apex. The V edge will be more acute.

It is not easier to sharpen a convex edge, if you even know what kind of convex edge you have, which 99.9 percent of the people who use them do not. "Some wet/dry sandpaper, a piece of leather, and a flat surface" may be all you need for a convex edge, but you need only the sandpaper and a flat surface for the V edge. In reality, how simple or difficult these edges are to sharpen depends on your techniques and equipment. One type of edge does not have an inherent advantage.

I use the Wicked Edge to sharpen my Bark River knives. The so-called Bark River convex edge is almost instantly converted to a V edge because there is hardly any convex edge there. I can take any convex edge and convert it to a V edge that is either more robust or more acute than the original convex edge.

People seem to think that there is something magical or mysterious about a convex edge, but it's not better or worse than a V edge if the geometry of the two edges are similar.

 

[JJaggar]

all the geometry is great but what I'm really wanting to know is why a convex edge instead of a V edge? does it hold an edge better or worse than other grinds? any info on convex blades and bark rivers convex blades other than the geometry behind them is what I'm really interested in hearing about.

 

[bdmicarta]

I know there are members out there who took, understood, and remember calc. Admit it...it's ok.... Come out of the closet.

Derivatives? Slope of the tangent to a curve at a point? I know you're out there...

OK, fine...

Take the first derivative of the curve, at the point where the 2 curves intersect, and that is the edge angle.


I suspect that many knives with convex edges end up with fairly large angles after they have been sharpened a few times. OTOH I have looked at new Randall and Blackjack blades and the factory edge looks like it has a fairly small angle.

A knife with convex edge that has been sharpened a few times will start to require a lot of work to maintain its convex edge at the same angle. OTOH a hollow ground edge is intended to stay relatively thin behind the edge as you sharpen the blade. Chris Reeve talks about this in one of his videos. He uses hollow ground blades for this purpose and puts a lot of thought into the exact shape of the grind for this to work.

 

[Twindog]

all the geometry is great but what I'm really wanting to know is why a convex edge instead of a V edge? does it hold an edge better or worse than other grinds? any info on convex blades and bark rivers convex blades other than the geometry behind them is what I'm really interested in hearing about.

The convex edge vs the V edge is not important. You can use and maintain either. How well the convex edge -- or the V edge -- performs depends on how acute you make the edge. If you want an edge that is easy to maintain at a precise geometry -- such as a 30-degree inclusive angle -- go with a V edge.

The Bark River Ghost 2 is a karambit, and it is that blade geometry that is going to be more challenging. You can't really use a flat stone on a hawkbill because you'll be sharpening with just the edges of the stone. You can use a round stone. Wicked Edge has rounded stones for hawkbills. You can use the Sharpmaker's edges.

I haven't tried to sharpen a karambit on a belt. It seems like it could be tricky because you'd be making contact with just the outside edges of the belt. But maybe someone who does this can give you better advice.

On my hawkbill (Spyderco Superhawk), I use the Sharpmaker rods freehand. It works OK, but I lose the precision that I get with the Wicked Edge.

 

[REK Knives]

Convex edges do not have angles -- that's just geometry.

...The V edge will be more acute....

...but it's not better or worse than a V edge if the geometry of the two edges are similar.

No offense man, but this is simply incorrect. Whether v, concave, or convex the apex most certainly DOES have an angle. This is the point that Marcinek was trying to make... that it all depends on the apex angle if you want to compare apples w/ apples.

You say that the V edge will be more acute and you are talking about edge "geometry"... but more acute what?... more acute ANGLES is what you are talking about

I agree that it shouldn't be labeled as Marcinek did, I would rather label it as "Done incorrectly" and "done correctly" if matching the existing angles was the goal, but i got his point.

all the geometry is great but what I'm really wanting to know is why a convex edge instead of a V edge? does it hold an edge better or worse than other grinds? any info on convex blades and bark rivers convex blades other than the geometry behind them is what I'm really interested in hearing about.

To my knowledge no in-depth testing has revealed that it holds it's edge longer. It all depends on the geometry of your blade and final apex angle, along with sharpening/deburr method, etc. The advantage is that, for a given edge apex angle (say 15 dps) a convex edge will cut better because it will have less metal behind the apex (i.e. the "shoulders" are gone).

 

[REK Knives]

The convex edge vs the V edge is not important. You can use and maintain either. How well the convex edge -- or the V edge -- performs depends on how acute you make the edge. If you want an edge that is easy to maintain at a precise geometry -- such as a 30-degree inclusive angle -- go with a V edge.

The Bark River Ghost 2 is a karambit, and it is that blade geometry that is going to be more challenging. You can't really use a flat stone on a hawkbill because you'll be sharpening with just the edges of the stone. You can use a round stone. Wicked Edge has rounded stones for hawkbills. You can use the Sharpmaker's edges.

I haven't tried to sharpen a karambit on a belt. It seems like it could be tricky because you'd be making contact with just the outside edges of the belt. But maybe someone who does this can give you better advice.

On my hawkbill (Spyderco Superhawk), I use the Sharpmaker rods freehand. It works OK, but I lose the precision that I get with the Wicked Edge.

You can use your Wicked Edge diamond stock stones up through 1k on the most severe recurves w/ no issues fyi. They hold up great.

Belt works fantastic on a hawkbill... you just have to drift your belt off of the platen slightly and know how to work the blade on the belt. You can round/change the tip quickly if you aren't careful though.

 

[hank_rearden]

Actually a v grind of comparable angle has MORE steel behind the edge. The "convex has more steel thing" is nonsense. I have a pretty strong suspicion where it originates. Sorry. Geometry doesn't lie.

As far as maintenance...I find they are EASIER to maintain. It takes advantage of not being able to hand hold an exact edge eangle consistently, and can be made/maintained with a few bucks worth of simple materials. Some wet/dry sandpaper, a piece of leather, and a flat surface is really all you need.

actually, both pictures are exactly the same and the convex grind in each will outperform its respective flat grind. the key is the changing angle. beyond .003 mm of edge penetration, all you have is friction. so the convex gives less of that going into the medium.

two straight bevels will give the same result as a convex assuming there is some precision followed in its construction.

maintenance-wise, a straight bevel is easier to work by a long shot.

 

[Twindog]

No offense man, but this is simply incorrect. Whether v, concave, or convex the apex most certainly DOES have an angle. This is the point that Marcinek was trying to make... that it all depends on the apex angle if you want to compare apples w/ apples.

You say that the V edge will be more acute and you are talking about edge "geometry"... but more acute what?... more acute ANGLES is what you are talking about

I agree that it shouldn't be labeled as Marcinek did, I would rather label it as "Done incorrectly" and "done correctly" if matching the existing angles was the goal, but i got his point.



To my knowledge no in-depth testing has revealed that it holds it's edge longer. It all depends on the geometry of your blade and final apex angle, along with sharpening/deburr method, etc. The advantage is that, for a given edge apex angle (say 15 dps) a convex edge will cut better because it will have less metal behind the apex (i.e. the "shoulders" are gone).

No offense taken, but an angle is the amount of turn between two straight lines that share a vertex. Convex edges have no straight lines. They are formed by arcs. In a pure convex edge, those arcs are defined by the radius of the arc's corresponding circle. With convex knife edges, you're mostly dealing with a hybrid of arcs.

A convex edge defined by a short radius -- say the radius of a circle the size of a BB -- is very obtuse. A convex edge defined by a long radius, say the distance of the earth to the sun, shows no difference between a V edge, if both edges have the same edge height and width.

Convex edges defined by a short radius become very obtuse near the apex. Long-radius convex edges have a relatively constant acuteness from the edge shoulder to the apex and are comparable to V edges of similar acuteness.

Probably most factory knives come with a convex edge because they are sharpened on a belt, but those edges can be quite different and most are basically V edges for practical purposes.

The differences between most convex edges that people use and the V edges that most people use are insignificant.

 

[harkamus]

No offense man, but this is simply incorrect. Whether v, concave, or convex the apex most certainly DOES have an angle. This is the point that Marcinek was trying to make... that it all depends on the apex angle if you want to compare apples w/ apples.

To give another example, the only time a convex edge wouldn't have an angle created by two straight lines is if you had the arc taken from a circle and had that as an edge. Anything shy of a circle and you'll end up with some kind of angle at the apex created by two lines. A circle is 360 degrees. You take the arc of a circle and it will still have the same curvature. As soon as this circle becomes an "edge" it will have some angle created by two lines. In an extreme example, you might have a very terrible convex edge of 170 degrees, close to a straight line, which would be analogous to the edge a circle creates if you were to try and "draw a circle" with straight lines only.

It's been a long time since I've studied programming and things like that, but if I'm not mistaken, computer graphics are limited because they cannot render a perfect circle; they render polygons. What looks like a circle is just a bunch of lines connected to each other in such a way that the appearance is a circle. That's why modern games these days have anti-aliasing.

 

[Rhinoknives1]

Actually a v grind of comparable angle has MORE steel behind the edge. The "convex has more steel thing" is nonsense. I have a pretty strong suspicion where it originates. Sorry. Geometry doesn't lie.

As far as maintenance...I find they are EASIER to maintain. It takes advantage of not being able to hand hold an exact edge eangle consistently, and can be made/maintained with a few bucks worth of simple materials. Some wet/dry sandpaper, a piece of leather, and a flat surface is really all you need.

The other cool thing that I prefer about a convex Grind and a Convex edge is that it makes it easier to to cut in a circular motion which is very handy in the kitchen and can be so in the field as well.

If you want to shave with this edge on a daily basis, go for a Vee edge.

 

[PURPLEDC]

I read this thread and I seriously think my head is about to explode. Unfortunately I was horrible with math and I dont think I will ever understand edge geometry. Especially when people throw in hollow grinds. Someone said they are incredibly common. Yes hollow grinds for the primary grind of the blade but I thought the majority of actual edges were flat ground edges regardless of the blade grind. Sometimes when reading these conversations I cant make out which people are talking about.

 

[Krissig12]

But aaaaah ...........ummmmmm ahhhhhh............. weel any of theeeeeese doohickeys make my knive sherp????

 

[marcinek]

actually, both pictures are exactly the same and the convex grind in each will outperform its respective flat grind. the key is the changing angle. beyond .003 mm of edge penetration, all you have is friction. so the convex gives less of that going into the medium.

Agreed. It will outperform the vee. Because there is less steel thickness behind the edge.