Convex or V edge ?

Ankerson,

You wrote:
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I really don't think you are understanding exactly what you are doing and why those edges are stronger.

If the edge is stronger in the end then the angle is steeper than the V edge, not the other way around, you can't remove metal and make it stronger, it's just not going to happen in the real world.
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I completely agree with you in your first two paragraphs above. In this regard, I haven't said anything to suggest the opposite. I am however, talking about the efficiency of the convex.

By the way, it's aerodynamics + hydrodynamics and it's a shame that you can't see the analogy. I won't go off topic with it.

You seem to have this idea that convex knives are stronger, though. They are and they aren't. Since the angle can never be the exact same as a v-edge, a convex is either stronger or weaker (thicker or thinner), but never sharper if around the same apex as the v-edge line. The two can never coincide on the same geometry, so the convex is either thicker or thinner at the edge, but never the same. Honestly, where would you measure the angle of a convex as a comparison to a v-edge anyway? At the tip of the edge? A few millimeters behind it? Where..? You can't put a straight line on a curved one and call them the same, so measuring a convex against a comparable v-edge is an approximation at best.
 
stoffi said:
Ankerson,

You wrote:
_____________________________
I really don't think you are understanding exactly what you are doing and why those edges are stronger.

If the edge is stronger in the end then the angle is steeper than the V edge, not the other way around, you can't remove metal and make it stronger, it's just not going to happen in the real world.
_____________________________

I completely agree with you in your first two paragraphs above. In this regard, I haven't said anything to suggest the opposite. I am however, talking about the efficiency of the convex.

By the way, it's aerodynamics + hydrodynamics and it's a shame that you can't see the analogy. I won't go off topic with it.

You seem to have this idea that convex knives are stronger, though. They are and they aren't. Since the angle can never be the exact same as a v-edge, a convex is either stronger or weaker (thicker or thinner), but never sharper if around the same apex as the v-edge line. The two can never coincide on the same geometry, so the convex is either thicker or thinner at the edge, but never the same. Honestly, where would you measure the angle of a convex as a comparison to a v-edge anyway? At the tip of the edge? A few millimeters behind it? Where..? You can't put a straight line on a curved one and call them the same, so measuring a convex against a comparable v-edge is an approximation at best.
Click to expand...

When convexing an edge one of two things will happen.

The edge will get thinner and weaker.

The edge will get thicker as in a steeper angle and stronger.

Aerodynamics has nothing to do with knife blades at all period, they are two completely different topics and don't relate to each other in any way.

The pressure applied isn't even close to the same nor is it applied in the same way.

Just way too many variables to even try and make a relationship between the two.

That's keeping it simple.

It really doesn't take a PHD to figure it out, it's not really that hard. :)

Somethings just don't need complicated explanations that those big brains make. ;)

A tear drop shape is very aerodynamic, but it wouldn't make a very good knife blade. ;)
 
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parbajtor said:
Good diagram but you forgot to factor in the way stress builds up in a structure. With no shoulders on a convex, there is nowhere for stress to build at a single point, as the stress is distributed more evenly across the materials, so the overall effect is of a more durable edge. It's why round tube is stronger than RHS for the same wall thickness and similar cross sectional area and also how arches get their strength.
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Yup, you are right. But, you're talking about stress from one direction. Stresses can come from all directions. For instance, bending things with a thin blade.

Ankerson,

all I can say is: take you're regular v-edge. Carve some nice chunks out of wood. Then knock off the shoulders, leaving the edge as it was... and then carve with it again.
 
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stoffi said:
Ankerson,

all I can say is: take you're regular v-edge. Carve some nice chunks out of wood. Then knock off the shoulders, leaving the edge as it was... and then carve with it again.
Click to expand...

I have many times over the years and I can't tell the difference. ;)
 
I think you're operating under a serious misconception that a convex edge isn't going to v-out at the end. It will, even if you machine it, because we're talking about the edge. If you're talking about a knife made by a human being, you can forget about mathematically perfect convexes. Just like you can forget about mathematically perfect V-edges. At the very edge, which is what matters for cutting ability, both edges are V-shaped, and have a particular angle.

The speed boat analogy is not well thought out. The boat has to be curved because if it had a very sharp v-edge, like on a knife, guess what. It would sink. The curvature provides more surface area against the water. What makes the boat faster and reduces drag is that they have a sharper edge on them, kinda like a knife. But you can only take that so far. If, by your logic, the curvature somehow made the boat faster, a wide curved boat would be faster than those speedboats which have a hull much closer to a v-edge. But, they don't. The analogy has all kinds of holes in it.

So, two points.

1. The edge angle has to be the same for a reasonable comparison. In both theory and practice, you will have exactly the same edge angle. A convex edge isn't curved the entire length of the edge anyways, not by a long shot. If you're going to take it to the microscopic level, perhaps you should take a look at a convex edge on a knife and see what it really looks like. It's not going to look like your pictures.

2. You keep claiming that somehow, the shoulders are going to inhibit cutting. I haven't seen any evidence to support that. The only thing that convex really has going for it is the strength of steel behind the edge. If you want a really good cutting GRIND, you'd be more advised to go hollow-ground. Both flat ground and convex place the entirety of the grind against the material you're cutting, upping the frictional force. The presence of a shoulder doesn't matter; if anything, on a deeper cut, the v-edge will have LESS material against the edge because the shoulders will force the material away, and it will only make contact along the shoulder, as opposed to along the entire length of the grind. In either case, the frictional force of the actual EDGE, as opposed to the grind, will be pretty negligible.
 
stoffi said:
That's a shame ;)
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I have done the testing and I saw no noticeable difference in cutting force that was measurable in my testing.

That's keeping the edges around the same angle.

Now yeah if the edge is thinned out then I saw a noticeable difference in cutting force, but no more than it would be than if I thinned out the edge and keeping the V.

Thickness behind the edge is the difference here and always will be, it's not that the convex cuts better, what that really is and really means is that thinner blades cut better than thicker ones, that same thing can be done by making the V edge thinner.

Now if it's strength that we are talking about then the edge has gotten thicker. ;)

There is nothing new here to be learned really, the basics still do and always will apply.

The Convex edge is a marketing tactic to sell knives, nothing more than cool-aid and people buy into it blindly.....

So it's Hype, Urban legend and just plain BS in the end and it never was more than that.


The basics that will never change:

Thinner blades cut better.

The thinner the steel is behind the edge the better it will cut.

The lower the edge angle the better it will cut.

The thicker the blade it is the stronger it is

The thicker the steel is behind the edge the stronger it will be.

The Steeper the edge angle is the stronger the edge will be.


So in the end if someone or a Company is really pushing Convex edges for other than choppers then I would really start to question what it is they are trying to sell and the products they are selling. ;)

That main question is:

What is that Convex edge really compensating for? Heat Treat Issues and tempering issues? Low Hardness? Other issues?
 
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stoffi said:
all I can say is: take you're regular v-edge. Carve some nice chunks out of wood. Then knock off the shoulders, leaving the edge as it was... and then carve with it again.
Click to expand...

You're still running under the same misconception. If you convex an existing v-edge, you're thinning out the edge. That says nothing about the cutting ability as compared to a v-edge of the same angle. If they both have the same cutting edge, they're going to cut the same, regardless of the presence of shoulders. I could take a convex edge, flatten it out to a v-edge, and guess what? It would cut better. Doesn't prove anything.
 
Ankerson said:
The Convex edge is a marketing tactic to sell knives, nothing more than cool-aid and people buy into it blindly.....

So it's Hype, Urban legend and just plain BS in the end and it never was more than that.
Click to expand...

A-ha! Now I know why you are being so difficult! ; )

Honestly, the convex edge is probably the oldest blade and edge grind there is. It's nothing new, but yes I'll agree that it has become more popular in recent years and stirred the pot among knifenuts.

Much of what you're saying is common sense dude, but heat treat issues n' stuff has nothing to do with this, if we're talking about the exact same slab of steel and as close to the same edge angle as possible.

Crimsonfalcon007,

A convex edge doesn't v-out at the end as dramatically as you say. The only part which could possibly look like one is at the very edge apex where it cuts you. If you convex a regular v-edge with a 3mm edge bevel that edge bevel will be convex.

The only reason a boat floats is because it displaces its weight in water, which incidentally also has to do with atmospheric pressure. A knifes edge as a keel won't make it sink. You could infuse a hair-poppin' sharp blade along the keel of a boat and even make the keel deeper and more like a blade and it still wouldn't sink (look at sailboats!), as long as the whole boat displaced its weight in water. It's not about surface tension. But, that's not what I meant. I meant going forward. All fast boats have a convex type of knife edge at the front, whether it be a cigarette or catamaran. They don't however, look like a perfect v-edge. Nor does a bullet. It's just an analogy. I only meant that such a curve helps in guiding away the stream of water (or wood in this case), just like a bullets shape or the nose of an airplane helps it be more slippery through the medium which in it must travel.

The shoulders are hardly a part of the actual cutting edge, so it's not "thinning the edge" in my example of knocking off the shoulders, because the actual edge is untouched and still the same. One can do it on a stone with just one angle of sharpening. I'd be thinning behind the edge, not the actual edge. It's just an example of what I'm trying to say, which is that shoulders create drag.

Hollowgrinds and flat grinds have more drag than convex grinds. This is my point.
 
Watch this:

[video=youtube;cFTV4ZgeLiY]http://www.youtube.com/watch?v=cFTV4ZgeLiY[/video]
 
you cannot honestly be trying to relate wood to water and air. and you can "knock off the shoulders" with a relief grind that is still flat. The convexity again is not the issue, the thickness behind the edge is. Drag has nothing to do with sharpness/the radius of the edge apex. There is this constant jumping between trying to discuss what is going on at the sharp bit and what is happening behind it.
 
Hardheart,

Regarding "knocking off shoulders", the sharp bit is the same. Nothing is changing it. I found the clip above just now and it explains it quite well. Have you watched it, or were you writing your post while I posted it? Check it out.

Relating water, air, wood, dirt... they're all mediums. It's an example.

You say I'm jumping around. I'm not. I've constantly been trying to say that what happens behind the cutting edge, if comparing as close to the same angle as possible on different edge grinds, makes a difference.
 
hmm, just watched the YT vid you linked. Glad I already made the point that a relief grind to improve cutting can be flat. Kyley being Kyley with the inflammatory and unfounded remarks, no news there.

At least it agrees with what everyone but you has been saying - you can convex an edge and match the edge angle of a v-bevel. I'm not even sure why you think that isn't how it works.
 
Now we're back to my original statement. The knocking-off-shoulders on a v-edge is another.

My original statement was that if you convex an ENTIRE edge, it will be convex all the way to the apex, but the difference to a COMPARABLE v-edge would be so exceedingly small (at the apex), that they would be practically the same; but, not after the edge apex.

My other statement concerns knocking off shoulders on an EXISTING v-edge. Here, as in the video clip above, the v-EDGE is untouched (not convex). But, what happens behind it is changed. Not the apex of the edge, or even the first millimeters behind it.
 
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hardheart said:
hmm, just watched the YT vid you linked. Glad I already made the point that a relief grind to improve cutting can be flat. Kyley being Kyley with the inflammatory and unfounded remarks, no news there.

At least it agrees with what everyone but you has been saying - you can convex an edge and match the edge angle of a v-bevel. I'm not even sure why you think that isn't how it works.
Click to expand...


Just had to do that myself on my S90V Para 2, it was starting to cut like a brick at .035" behind the edge (I use it a lot) so I made a 16 degree inclusive relief and removed about .010".


DSC_3389.JPG
 
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