What Are The Advantages of a Convex Grind?

Mostly theoretical, IMHO. I have both and really can't tell if one cuts better than the other, blade geometry being roughly equal. An ax, however, needs a convex grind to chop efficiently without binding. (A headsman's ax would be another matter....o_O)
 
This thread hasn't gotten anywhere haha.. I see both sides. Don't know which ones right nor do I understand the math involved.. I do see how having a convex edge will remove more material if u go from a v to a convex but how about something like bark river knives that are made from blanks to be a convex grind? It does look like there is more steel behind the apex and also feels that way and I know the angle isn't too obtuse cause most the time they cut decent.. is it just the full convex grind that makes it seem that way or is there any more meat behind the apex? And this is for everyone who is saying convex has less steel cause I've seen a couple of them say both sides have been right.
 
The sharpening angle is the same as your edge angle. Also, these images were drawn using state of the art drafting software (AKA Word), so please excuse the eye pain.

Just before we start, going from a V edge at 20 degrees to a convex edge does not create a 20 degree convex edge. Alright, let's go.

A "V" edge is formed with solid, flat abrasives. If you hold the knife at 90 degrees, and angle your sharpening stones at 110 degrees and 70 degrees (left and right sides of the blade), and then sharpen, you will get a 20 degree edge (40 degrees inclusive). (See below image)

VTqY2gw.png



A convex edge is formed by a belt system (or some type of system with a tension or "give" to the abrasive surface). If you hold the knife at 90 degrees, and angle the belts at 110 degrees and 70 degrees (left and right sides of the blade), and then sharpen, you will get a 20 degree convex edge angle (40 degrees inclusive). (See below image)

H0bxRV2.png


So, take sharpening systems that produce a V edge and convex edge, angle them both at 20 degrees with respect to the blade, and here is what you get:

hwksTpP.png


As you can see, the convex edge clearly has more material behind the edge than the V edge, with the same edge angle.

I drew the pictures to scale, so feel free to compare the angles on your screen. The red dots represent the points on the original knife blank (also at the same 20 degree angle).

I welcome any corrections or alternate views as to why this could be wrong (provide some sort of proof though).

Also, this does not take into account convexity, because that does not matter. Technically, you could apply this to a hollow grind too, where a wheel would be used, and would contact the knife at the two red dots as well. Changing wheel diameter would change concavity, but hollow grinds are a bit different, so it wouldn't be completely accurate.



09G3N
 
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The angle does look more obtuse but it seems to make sense... so really you can't make a convex edge out of stones.. I'm confused as shit now cause I just seen last week the theory where convex had less material but now I'm thinking it's the opposite if your grinding from a blank.. I'm no math expert or knife expert but that's what makes sense to me.. and also if these types of grinds had less material behind the edge why would everyone making these types of knives say the complete opposite..
 
The apex angle is not the same in those pics. The convex edge has a more obtuse angle at the apex, which is why it has more metal behind the edge.

Ah, so you are thinking more along the lines of this:


5WXUhzi.png


Thanks for explaining. So with this interpretation, keeping the apex angle constant, changing the elasticity of the belt system would be directly correlated with changes to the blade stock thickness and convexity. In my interpretation, changing the elasticity of the belt system would be directly correlated to changes in the convexity and apex angle.

Apex angle, convexity, and blade stock thickness appear to be the three variables, and the depending on what you set the independent variable as determines the blade shape.
 
They don't have the same edge angle. The black part, the convex, has a more obtuse edge angle.

The belt leaves the apex of the edge at your red dot on the bottom right is a more obtuse edge because the belt deflects into the triangle formed by the belt pulleys. This causes the belt to meet the bottom right pulley at an angle more obtuse than the 70 degrees it would have met were it not deflected. To get the same edge angle you need to start lower, maybe 60, so the angle after deflection is 70.

A convex edge can be thicker or thinner than a v edge. It all depends on how it was ground and what person grinding wanted. I've used both interchangeably for a long time, and don't see a difference. I just do whatever is faster, lately slightly convex by free hand on stones. Sometimes I will use a DMT Aligned clamp and put on a v edge. Just depends on my mood and the knife.
 
The angle does look more obtuse but it seems to make sense... so really you can't make a convex edge out of stones.. I'm confused as shit now cause I just seen last week the theory where convex had less material but now I'm thinking it's the opposite if your grinding from a blank.. I'm no math expert or knife expert but that's what makes sense to me.. and also if these types of grinds had less material behind the edge why would everyone making these types of knives say the complete opposite..

No, you can very easily make a convex on stones. Just hold at your desired edge angle and then drop your spine as you work that region of the edge. That action ensures that your bevel angle is what you want at the apex, but rolling the spine lower blends back the shoulder.
 
Exactly. If you wanted, you could convert that thinner convex back to a v edge, and it would be thinner than it's preceding convex edge. It all depends on where you start and what you want.

My comments are related to convex edges. C9nvex primary grinds are different and we need to be specific when we discuss them, as I feel sure this losseness of terms has caused confusion.

Ah, so you are thinking more along the lines of this:


5WXUhzi.png


Thanks for explaining. So with this interpretation, keeping the apex angle constant, changing the elasticity of the belt system would be directly correlated with changes to the blade stock thickness and convexity. In my interpretation, changing the elasticity of the belt system would be directly correlated to changes in the convexity and apex angle.

Apex angle, convexity, and blade stock thickness appear to be the three variables, and the depending on what you set the independent variable as determines the blade shape.
 
No, you can very easily make a convex on stones. Just hold at your desired edge angle and then drop your spine as you work that region of the edge. That action ensures that your bevel angle is what you want at the apex, but rolling the spine lower blends back the shoulder.

I was wondering if anyone would say this. You are absolutely correct, but I gave the belt system as an example of a true convex edge because it does it naturally. With stones, you would be required to rotate your hand at a constant rotational speed in order to get a perfect convex grind, because you would be following an imaginary circle.

It is like sharpening on the sharpmaker vs using wicked edge. You can get a terrific edge on the sharpmaker, but (assuming inexperience) your hand never stays at 90 degrees exactly. It may be 90.1 degrees on one stroke, and 89.8 degrees on the next, for example. With experience, it may be more like 90.05 degrees on one stroke, and 90.02 degrees on the next, but you get the idea. With the Wicked edge, the tolerance is increased much much more, so it may be 90.0001 degrees on one stroke, and 90.0002 degrees on the next, or something like that.

You are absolutely right though, you can get convex on stones.
 
I wish I could draw a diagram for u guys to show u what I mean by not being able to get a convex on stones.. I'm not saying it's right it's just the way I'm seeing it right now in my head after reading this thread.. yes u can grind the shoulders off to make it convex it a sense but you still wouldn't get that nice curve you would from a belt all the way up to the shoulder. Am I right or wrong
 
I wish I could draw a diagram for u guys to show u what I mean by not being able to get a convex on stones.. I'm not saying it's right it's just the way I'm seeing it right now in my head after reading this thread.. yes u can grind the shoulders off to make it convex it a sense but you still wouldn't get that nice curve you would from a belt all the way up to the shoulder. Am I right or wrong

Check out this image:
https://1.bp.blogspot.com/-IW3nHNLnmmU/VtJbA1P5LeI/AAAAAAAAG5o/UzvCPNw7ik8/s1600/Bevel-vs-convex.jpg

This is from the work sharp system, it kind of shows how you go from a V edge to a convex edge, and you can imitate the action of the belt with your hand. Imagine the point of contact with the stone you are using following the belt line, if that helps. The belt will be MUCH faster though, because it is applying force over the entire convex surface, whereas the stone would only be touching a point at any time.
 
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 love a flat convex with a micro bevel for my typical light use EDC's. No bevel shoulders means minimal drag in the cut, so they are great slicers. The micro bevel is quick to sharpen. Probably half of my 30 Spydies are convexed like this.

The Sage and Stretch below are very thin, and almost border on zero grinds, so I have to treat them with common-sense. A 15 dps micro bevel gives enough strength to prevent chipping in my day-to-day uses. Love them!

_20160911_175712_zpsea3xfufl.jpg


_20161024_080729_zpsnlwrzimv.jpg


Convexed to thin the edge somewhat, not for light use, but to turn it into a knife because it was a pry bar.
DSC_0765_zps5py7g2jt.jpg


20150325_190500-small_zpsholgkaxj.jpg
 
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I heard an interview with a staff member on Bark River Knives, where he was talking about convex grinds having more carbides gathered in the edge, making them hold an edge longer because of that. I didn't entirely manage to wrap my head around exactly how he meant though and I didn't quite catch if he meant convex edges vs v-grind with a secondary bevel or convex edges vs zero grind. If we're talking convex edge vs zero grind, I'm sure it's a factor.
 
Ah, so you are thinking more along the lines of this:


5WXUhzi.png


Thanks for explaining. So with this interpretation, keeping the apex angle constant, changing the elasticity of the belt system would be directly correlated with changes to the blade stock thickness and convexity. In my interpretation, changing the elasticity of the belt system would be directly correlated to changes in the convexity and apex angle.

Apex angle, convexity, and blade stock thickness appear to be the three variables, and the depending on what you set the independent variable as determines the blade shape.

As stated multiple times, you must understand tangency to grasp the geometry. Look at the drawings I did a page or two back.
 
I heard an interview with a staff member on Bark River Knives, where he was talking about convex grinds having more carbides gathered in the edge, making them hold an edge longer because of that. I didn't entirely manage to wrap my head around exactly how he meant though and I didn't quite catch if he meant convex edges vs v-grind with a secondary bevel or convex edges vs zero grind. If we're talking convex edge vs zero grind, I'm sure it's a factor.

Really ? Nice fairy tale ! A convexed edge is good stuff by itself, it needs no BS stories to hype it up. The fact is, it's no big deal on a thin blade : V or convex, the thin blade will slice extremely well. On a thick blade, or an axe, the convex grind is better to avoid wedging in. For me, convex grind is the best because it's easy to set and hone with the WorkSharp. If i was still stuck with mechanical / manual sharpening devices, I would baby my V edges as ever.
 
This is irrefutably correct. Anyone who claims otherwise is defying logic.
The sharpening angle is the same as your edge angle. Also, these images were drawn using state of the art drafting software (AKA Word), so please excuse the eye pain.

Just before we start, going from a V edge at 20 degrees to a convex edge does not create a 20 degree convex edge. Alright, let's go.

A "V" edge is formed with solid, flat abrasives. If you hold the knife at 90 degrees, and angle your sharpening stones at 110 degrees and 70 degrees (left and right sides of the blade), and then sharpen, you will get a 20 degree edge (40 degrees inclusive). (See below image)

VTqY2gw.png



A convex edge is formed by a belt system (or some type of system with a tension or "give" to the abrasive surface). If you hold the knife at 90 degrees, and angle the belts at 110 degrees and 70 degrees (left and right sides of the blade), and then sharpen, you will get a 20 degree convex edge angle (40 degrees inclusive). (See below image)

H0bxRV2.png


So, take sharpening systems that produce a V edge and convex edge, angle them both at 20 degrees with respect to the blade, and here is what you get:

hwksTpP.png


As you can see, the convex edge clearly has more material behind the edge than the V edge, with the same edge angle.

I drew the pictures to scale, so feel free to compare the angles on your screen. The red dots represent the points on the original knife blank (also at the same 20 degree angle).

I welcome any corrections or alternate views as to why this could be wrong (provide some sort of proof though).

Also, this does not take into account convexity, because that does not matter. Technically, you could apply this to a hollow grind too, where a wheel would be used, and would contact the knife at the two red dots as well. Changing wheel diameter would change concavity, but hollow grinds are a bit different, so it wouldn't be completely accurate.



09G3N
 
But it's totally incorrect. That's not the same edge angle--it's the same angle of approach to the un-deformed abrasive surface, but the slack belt is then deflecting and creating a much more obtuse actual edge angle.
 
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