Why is Convex bad?

i have a freind that makes swords and knives he says "choping folk convex, sliceing tomatoes with your v" his english iseht the best and some times i dont know what hes talking about
 
DocNightfall said:
Rather, the convex edge is much thinner behind the edge than a comparable V-grind.

It has a thinner and smoother cross-sectional geometry and thus can cut with less resistance/drag than a 15 degree V-grind, but at the cost of lateral blade strength and toughness.
Click to expand...

What I meant is that with the "heavy pressure convex" that rolls the last bit of the edge fro 8 to 15 deg, the convex edge is more obtuse <at the edge> than a V grind at 8 deg. The convex is more robust at the cost of acuteness. But, it is no longer comparable as it is more obtuse.
 
hlee said:
What I meant is that with the "heavy pressure convex" that rolls the last bit of the edge fro 8 to 15 deg, the convex edge is more obtuse <at the edge> than a V grind at 8 deg. The convex is more robust at the cost of acuteness. But, it is no longer comparable as it is more obtuse.
Click to expand...

Yes, I got what you meant, but I didn't understand why you were trying to compare two things that are not comparable.
 
I can't count how many times I have read "convex edges are more robust because they have more metal behind the edge than a comparable V-edge" (Right here on BladeForums- The Leading Edge in Knife Discussion) and it just drives me up the wall.
 
Let's see if this sums it up.

1. For the same final included edge angle, a convex profile has less metal behind the edge than a v-ground edge.

2. Adding a microbevel to a v-ground blade is the process of sharpening at a relatively more obtuse angle, to make the edge more resilient. The same can be done to a convex edge by changing the degree of curvature near the edge.

3. A convex edge can only be more 'robust' if it is of a more obtuse angle, whether or not it is a 'micro' of sorts, or being compared to one.
 
hlee said:
I can't count how many times I have read "convex edges are more robust because they have more metal behind the edge than a comparable V-edge" (Right here on BladeForums- The Leading Edge in Knife Discussion) and it just drives me up the wall.
Click to expand...

Well, why can't that statement be true? Depending on the arc of the convex edge, I can see it having a lot more metal behind it.
 
Slopes, curves, tangents- these are interrelated. A trig/calc class should clear up the confusion in why the slope of a curve can/does not deviate from the slope of a line tangent to the curve. If you do not understand how curves, tangents, and slopes relate to convex and V edges and their comparison then you probably can't be helped. Just keep your knives sharp and enjoy using them.
 
If you use the same sharpening angle for both V and convex the convex bevel will have a thicker edge. If you sharpen both to the same edge angle the convex will be thinner.
 
d.weglarz13 said:
Hey guys. I hear so much on here debating Convex vs Vgrind. Well, my questions are:
Why would someone want a V grind instead of convex? some say V is better?
I know its talked about a lot, but why would convex be better?
And, what do you guys prefer on what type of knives? (style and size)
I am most interested in utility, tactical, camping, combat kinds of knives. What kinds of grinds are best for what kind of slicing/chopping, etc.
Like I said, i know there are many mixed feelings, so please express any and all feelings. Thanks for the info.
Click to expand...

A lot of people have bad opinions of a convex edge quite plainly because it was a bad one. The are many factors that change the ability of a convex.
A lot of people mistake a convex edge on a flat ground knife for a full convex.
I have Fallkniven F1 and 3 busse's, the F1 cuts better than any of the others. It has a great full convex... 50 went into designing that knife which could be the reason.
A convex is much stronger than a flat ground knife because there is more steel on either side. A pictures explains a thousand words:

http://backyardbushman.com/?page_id=25

In my experience a good full convex cuts much more smoothly in wood at more natural angles because the wood has a tendency to peel away as you cut which lowers the amount pressure on the surface of the blade so it can slice with less effort. Why do you think axes have been convex for thousands of years[more strength, better edge retention and cuts through wood better] because when you go against the grain they actually pop out chunks of wood rather then getting stuck in it.

Also here is a useful page explaining the uses and dynamics etc of different knife grinds.

http://backyardbushman.com/?page_id=13

-Gabriel.
 
If 2 different knives designed for the same purpose have the 2 different grinds from that first link, then someone did something wrong. Who depends on what the purpose of the knife is.
 
NRaiano said:
http://backyardbushman.com/?page_id=25
Click to expand...
There's only one problem with that image... the convex edge has TWICE the final angle at the cutting edge. There's no way it will cut as as keenly, even if it is much more impact resistant. I used to have a schematic somewhere of what a convex and flat grind with equal terminal angles look like, but if you imagine only grinding the shoulder of the v-grind to convex so that the last infinitessimal bit near the edge remains untouched, you can plainly see that a convex grind will retain less material than a v-grind of equal final angle and stock thickness (and if you're not going to keep those equal, it's hardly a fair/scientific comparison.)
 
And that's where most get convex wrong, if your final edge is the same as it was when it was a V grind then you did it wrong.

Convex is a method of making the final edge angle more robust while streamlining the shape of the bevel.
 
knifenut1013 said:
And that's where most get convex wrong, if your final edge is the same as it was when it was a V grind then you did it wrong.

Convex is a method of making the final edge angle more robust while streamlining the shape of the bevel.
Click to expand...
Alright, I can sort of buy that... but then how do you validly compare cutting ability? You now have two different variables (actually, an infinite number of variables, given the variable curvature, but let's just lump that into one variable.)

I just had a thought, you might be able to make a sort of reverse argument... a convex bevel that cuts as well as a V-bevel will be tougher. That argument removes angle as a factor and replaces it with cutting ability as an experimental constant, curvature as an independent variable and toughness as a dependent variable.
 
This will sound like I keep repeating myself but convex does several things for mainly the same reason, and that reason is shape.

Because of the shape there is multiple points of contact or always a new contact point. This means as the material contacts the blade there is less friction because it can only contact one small point at a time. Cutting ability is improved again because of the shape as it allows the object being cut to glide over the surface, unlike a FFG blade with a V bevel where the object needs to pass the flats of the bevel then the shoulders of the bevel then the flat of the main grind. All places where material can stick and bind on the large flat surface area.

Think of it like this, take two similar blades one flat ground and one convex and stick them in a wind tunnel.

Done correctly convex is kinda like a micro bevel without the loss in cutting ability. When I sharpen convex at 10 degrees my final edge angle is near 20 yet still cuts like its at 10.
 
Actually, your wind tunnel analogy, although I've used it before, is invalid. Fluids and elastic solids react differently to shear stresses, by definition. However, I'm going to be doing one of my two senior theses next year for my BS in mechanical engineering on the effects of bevel on cutting ability, particularly the distribution of contact forces, so I'll be able to respond more quantitatively once that study is completed.
 
You must log in or register to reply here.
Back
Top