What Are The Advantages of a Convex Grind?

Mick_1KRR said:
I could not be more calm, im just honestly confused that people are trying to side step the point i was trying to make. You even just said yourself that i am correct with the example i'm making, it's pure fact and drawn for the eyes to see. I have said more than once that on a large scale, if the apex angle was to span for a long distance the V grind has more metal, but that's simply not applicable in real life on a real knife, edges are not long enough for that concept to even matter. I'm happy to just step back since it's clear some of us are sticking to one mathematical example about the very peak of the edge angle being followed in a straight line for an infinite distance. While a couple of us are basically talking about a small real world knife with a convex grind.
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Here is a real world diagram. I drew it in Autocad to scale: rectangular "stock" 1/4 inch wide by 2" tall:

Knife_Edges.jpg


The first one is a random convex surface. I don't know the exact edge angle, I didn't measure it. It doesn't matter because:

The second one is a V grind that perfectly matches the edge angle of the first one, whatever it is.

The third one shows them interposed. The black area is the area between the two edges. The convex edge is thinner.

For some reason that I can't understand you are assuming a full flat grind, as if that is the only grind that exists besides convex.
 
SpySmasher said:
Here is a real world diagram. I drew it in Autocad to scale: rectangular "stock" 1/4 inch wide by 2" tall:

Knife_Edges.jpg


The first one is a random convex surface. I don't know the exact edge angle, I didn't measure it. It doesn't matter because:

The second one is a V grind that perfectly matches the edge angle of the first one, whatever it is.

The third one shows them interposed. The black area is the area between the two edges. The convex edge is thinner.

For some reason that I can't understand you are assuming a full flat grind, as if that is the only grind that exists besides convex.
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The point i made was a couple posts back in my quick picture. Now the one you have drawn is almost a zero grind convex, not a normal convex with a shoulder that comes off the actual blade grind. Look at the size of the bevel you have drawn for the convex, it is about twice the size of the V edge thus making it look thinner behind the edge. Now the point i made was that the shoulder and the apex had to be the same distance apart. My example has nothing to do with a zero grind convex. When the shoulder and the apex are the same distance, the convex has more metal, if we change things like bevel width, zero grind, long distance - then everything can change. I understand what you're saying tho. I think there are too many variables for us all to settle on an agreeable point, i didn't intend for this to get into a long winded debate, i think we should all just relax and go buy a new knife.
 
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The shoulder does not make sense to keep as a fixed value. Shoulder position and visual bevel width are mere artifacts of a certain geometry being imposed on the base stock. If you keep that as your fixed value, then yes a convex will be thicker, but it will also have a thicker edge angle and you cannot convert an already ground knife from a convex to a flat without either reducing the blade width by wearing back far enough to produce that effect or else by reducing the edge angle. If you start with a V edge of a given angle you can convert it to a convex of equal edge angle by reducing the shoulder. This will also have the consequence of moving the shoulder back. However, the edge will remain at a consistent angle.
 
You are correct, I drew a zero ground convex but I can draw alternate convex grinds that would always have the same result if we base them on edge angle.

You are also correct that if we constrain other dimensions besides edge angle a convex grind can have more metal behind the edge than a V grind. That would mean that we cannot constrain edge angle. The two grinds would have different edge angles. However, if we only constrain edge angle, which was the original claim, a V grind will always have more metal behind it.

We are, of course, talking about basic grinds. In reality, a knifemaker has quite a lot of freedom in crafting subtle grinds that combine aspects of different basic grinds to get the performance we need. We all know there is no one ultimate grind. Everything comes with a little compromise.
 
SpySmasher said:
You are correct, I drew a zero ground convex but I can draw alternate convex grinds that would always have the same result if we base them on edge angle.

You are also correct that if we constrain other dimensions besides edge angle a convex grind can have more metal behind the edge than a V grind. That would mean that we cannot constrain edge angle. The two grinds would have different edge angles. However, if we only constrain edge angle, which was the original claim, a V grind will always have more metal behind it.

We are, of course, talking about basic grinds. In reality, a knifemaker has quite a lot of freedom in crafting subtle grinds that combine aspects of different basic grinds to get the performance we need. We all know there is no one ultimate grind. Everything comes with a little compromise.
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Yes this is correct, we are both arguing our points based on minor details that are making us seem like we are completely disagreeing with each other. There are variables which change both sides of the story.
 
Mick_1KRR said:
Are you both honestly trying to say the picture on the left has more metal between the shoulder and the apex of the edge?.
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No we are not. We are saying the one on the right has. But the two do not have the same edge angle. I can make a vee grind with a even wider edge angle than both and it will have more steel behind the edge than either of them. So what?

All you have demonstrated is that a more obtuse edge angle has more steel behind it than an acute one.
 
Mick_1KRR said:
The point i made was a couple posts back in my quick picture. Now the one you have drawn is almost a zero grind convex, not a normal convex with a shoulder that comes off the actual blade grind. Look at the size of the bevel you have drawn for the convex, it is about twice the size of the V edge thus making it look thinner behind the edge. Now the point i made was that the shoulder and the apex had to be the same distance apart. My example has nothing to do with a zero grind convex. When the shoulder and the apex are the same distance, the convex has more metal, if we change things like bevel width, zero grind, long distance - then everything can change. I understand what you're saying tho. I think there are too many variables for us all to settle on an agreeable point, i didn't intend for this to get into a long winded debate, i think we should all just relax and go buy a new knife.
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Here's a low quality diagram I made to illustrate the difference you're talking about:

GHxUlaY.jpg


Ex. 1 shows bevels with the same distance to the tip. In this case the convex is thicker. Ex. 2 shows the convex "bevel" starting twice as far back as the v-grind bevel, in which case the v-grind ends up thicker.

My apologies to everyone if this is a poor explanation!
 
brancron said:
Here's a low quality diagram I made to illustrate the difference you're talking about:

GHxUlaY.jpg


Ex. 1 shows bevels with the same distance to the tip. In this case the convex is thicker. Ex. 2 shows the convex "bevel" starting twice as far back as the v-grind bevel, in which case the v-grind ends up thicker.

My apologies to everyone if this is a poor explanation!
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Not a poor explanation, and yes i agree, if you pull back the convex bevel to match the angle of the V then it naturally becomes thinner, as the nature of the convex is to have that thicker curve. Achieving the zero grind like in the picture on the right is a much more difficult task for people to do by hand, many knife makers do these by design with the intention from the start to achieve a purpose made convex blade. I think the lay person will be more inclined to use some kind of belt system to achieve and edge similar to the one on the left. For instance the way CRK Sebenzas come from the factory, with a standard convex that quite honestly is a little too broad for my taste. Great for those who like to sharpen by hand on stones tho.
 
brancron said:
These threads always turn into geometry fights.
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Indeed. :)
We have one of these threads every few months it seems.
I prefer convex edge full flat blade because they are sexy, they cut well and they are easy to hand sharpen.
The only non convex edges I use are on my straight razors ;)
I do not give two :poop: about all of these diagrams
 
ridnovir said:
The only non convex edges I use are on my straight razors ;)
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You don't strop your razor before you shave??? :eek:

These days i convex back-bevel nearly all my knives (i.e. round-down the shoulder) but the apex of everything except my straight-razors gets its final edge off a flat hone, and that includes my axe and my lawnmower blade :D
Well, if I throw in the chainsaw as well, those edges are all hollow-ground, but that's the only implement cutting with a hollow-ground apex among my tools.
 
I have nothing useful to contribute. I just wanted to be included. :)

I did see something about cookies being handed out? Yes, please?

Edit: unless I have to do maths to get cookie. I don't like maths.
 
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Blues Bender said:
I don't want to go off on a tangent (rimshot), but you can test it yourself with a straight edge and a compass. ...

5782389D-852C-4E51-A6F8-AB7EB90C540B_zpsc0vzgq6t.jpg
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This is a very good pic which also reinforces what i said as well as your point also. As i said numerous times, at the very edge if you continued on a straight line the V grind will always be thicker. But, if you treat each of those left and right corners as knife edges, which one is actually realistic? One is a usable convex, and the other is basically a sharpened cube 90 degree angle for a knife edge. Now i've edited your picture to show what im talking about, and basically the only point i have been trying to make - If a normal, usable convex was put on a knife with a normal hollow grind or saber ground blade geometry (not a zero grind convex from the spine) then the convex will always contain more steel behind the edge, even tho it would be a chunky edge. I'm talking shoulder to apex being an even distance for the entire primary edge bevel. The mathematical formula has always been correct, but it hasn't been the focus of the example i have been trying to explain. Either way guys, i'ts been a good discussion, no chips on any shoulders, i enjoy this kind of thing, we're both correct, we're just making different points.
10ifgvn.png
 
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