Why is Convex bad?

Lucky Bob said:
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.
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Yes, but for those without a mechanical engineering degree it makes it a little easier to understand.

I think its cool you will be doing a paper on the subject but remember its been in use for a very long time on one of the most iconic cutting weapons known to man, and for good reason.

Unfortunately convex has become a bit of a side show freek here at BF, being suggested to everyone and their brother with little understanding of when or where it should be used.
 
knifenut1013 said:
Yes, but for those without a mechanical engineering degree it makes it a little easier to understand.

I think its cool you will be doing a paper on the subject but remember its been in use for a very long time on one of the most iconic cutting weapons known to man, and for good reason.

Unfortunately convex has become a bit of a side show freek here at BF, being suggested to everyone and their brother with little understanding of when or where it should be used.
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I've noticed that you keep saying that every chance you get. Can you please put a up a sticky FAQ on the subject? Thanks
 
bradefolums said:
I've noticed that you keep saying that every chance you get. Can you please put a up a sticky FAQ on the subject? Thanks
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It is what it is, what sticky FAQ are you referring to? or what is your question? :confused: :confused:
 
I think the convex offers strength but the rounding doesn't help when cutting into soft materials so it is should be best for chopping stuff. Tho a thin convex edge would make for a good slice, it would be difficult to put a thin convex edge on some blades.

The bevel edge lacks the rounds so it slices into material easily but chopping with a thin bevel can chip or dent the blade edge. A bevel that has thicker angle would be better at chopping but not ideal like the convex.
 
knifenut1013 said:
It is what it is, what sticky FAQ are you referring to? or what is your question? :confused: :confused:
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Sorry.
Can you make an easy to understand post (pics where needed) explaining when or where convex should be used?
Then, get an admin, to make the post a sticky item at the top of this forum, like
Sticky: Shop safety tips
Sticky: Steel FAQ


Thanks
 
The only real advantage I see a convex edge having is strength, and not because there's more material or something, as someone has already pointed out that most of the time there's supposed to be less.

There is even a certain amount of "relief" a convex edge facilitates that I think can be matched with micro bevels.

However, geometrically speaking, the only real difference is that the rounded "arch" shape is stronger than the straight shape. I could go into reasons why, but just trust me; it's why arched bridges are stronger than straight ones.

I know this mostly because of my time spent in the machine trade though. Parts that had to handle a lot of stress were generally made with round corners instead of straight ones. I use to think this was because it allowed a little bit more material to remain there, but when I got into die making I found out that the transfer of energy along a straight-line incurs more stress on corners than if it transfered along a radius. Basically it's the difference between energy in a straight line wanting to keep traveling in that direction, and energy traveling in an arc being effected by centrifugal force.

In the end, the real difference was that dies with "hard corners" generally chipped or cracked under the stresses more versus the ones with "rounded corners".

Now if you consider this and then apply it to the "corners" of where your edge meets your blade, then I see that as the one and only feature of a convex edge you can't match with a V grind. Even when making the angle more obtuse, or adding angles, you're still just compensating; the convex shape literally changes the way energy transfer is handled.

However, this is talking strictly about the energy transfer from impact. Is it a coincidence that most of the applications I've seen touted as suited to convex edges are impact tasks? Chopping being the main thing that comes to mind.


To me it's important to look at the Katan rather than anything else when considering this. Could the Katana be made just as sharp with a V grind? Probably. Could it be made just as strong by making it wide enough, adding on a secondary angle? Yeah, sure.

Is having huge bevels, and multiple bevels, better than learning how to use the shape that's going to give you extra strength all on its own and learning how to refine that? No way.

Once you start getting into debates about relief, cutting friction, etc. you're just basically debating which edge acts like an edge better. There's not too much debate on the geometry there. However, when looking at the actual differences the geometry plays, I think it's less about what the convex blade has to offer and more about what the V grind lacks.

That is relief. Relief is very important to cutting because without it, the material being cut is going to travel along the edge face, and the deeper you get into the material the more pressure, until eventually it will bind. Typically blades with a V edge are ground with a relief already to help prevent this, but someone can add a "microbevel" past that to help as well. However, the V grind itself has no relief.

So what's the advantage to the V grind? I have no idea, and I think that's what everyone should be asking instead of what the advantage to the convex is.

My best guess would be precision. I don't really recall seeing a lot of tool bits or chisel with convex edges on them.

Anyway, that's just what I see in the difference in how they perform. After a while though I don't see much difference between them since they're both just sharp edges.


I prefer V grinds because they're easier to refine for me. I can get a convex edge hair popping sharp off of a 1K stone, but I can't get it as smooth and consistent as I could if it was a V grind.
 
KennyB said:
The only real advantage I see a convex edge having is strength, and not because there's more material or something, as someone has already pointed out that most of the time there's supposed to be less.

There is even a certain amount of "relief" a convex edge facilitates that I think can be matched with micro bevels.

However, geometrically speaking, the only real difference is that the rounded "arch" shape is stronger than the straight shape. I could go into reasons why, but just trust me; it's why arched bridges are stronger than straight ones.

I know this mostly because of my time spent in the machine trade though. Parts that had to handle a lot of stress were generally made with round corners instead of straight ones. I use to think this was because it allowed a little bit more material to remain there, but when I got into die making I found out that the transfer of energy along a straight-line incurs more stress on corners than if it transfered along a radius. Basically it's the difference between energy in a straight line wanting to keep traveling in that direction, and energy traveling in an arc being effected by centrifugal force.

In the end, the real difference was that dies with "hard corners" generally chipped or cracked under the stresses more versus the ones with "rounded corners".

Now if you consider this and then apply it to the "corners" of where your edge meets your blade, then I see that as the one and only feature of a convex edge you can't match with a V grind. Even when making the angle more obtuse, or adding angles, you're still just compensating; the convex shape literally changes the way energy transfer is handled.
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Just FYI, from a mechanical engineering point of view, that only applies to inside corners (ie where rounding the corner means adding more material). The reason that rounding inside corners helps is that otherwise the imaginary "lines of stress" get focused around sharp corners/notches, and rounding that corner reduces that stress concentration. Generally speaking, there are very few situations in which you can make something stronger by removing material.

Also, the bolded portion in the quote? Total technical gobbledegook.
 
Lucky Bob said:
Just FYI, from a mechanical engineering point of view, that only applies to inside corners (ie where rounding the corner means adding more material). The reason that rounding inside corners helps is that otherwise the imaginary "lines of stress" get focused around sharp corners/notches, and rounding that corner reduces that stress concentration. Generally speaking, there are very few situations in which you can make something stronger by removing material.

Also, the bolded portion in the quote? Total technical gobbledegook.
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Eh, I can't really defend it beacuse I didn't really understand all of it, but my friend's argument did seem convincing. I failed to mention this was in a university machine shop where there were several mechanical engineering students. I got exposed to a lot of theory. Theory which I cannot really substantiate at all.

In an essence though, since when you strike an object it returns energy into the object being used to strike it, that energy travels through the material ( apparently ) and is effected by things like that. To elaborate, my friend basically said that if the energy is traveling through the part, and it is traveling along the angle of the surface, it will want to keep traveling in a straight line and have more of a tendency to chip out if the energy is too high, whereas if the energy was traveling along an arc it would be in centrifugal form and it would want to come back in on itself.

It seemed like a very convincing argument, but I wasn't there for mechanical engineering, I was just on a basic machining apprenticeship.

No idea that only applied to inside corners though. Any idea why that wouldn't apply to outside corners?
 
KennyB, I don't think your analogy regarding arched bridges relates to convex edges. Bridges are built to withstand the forces of gravity, wind, temperature differences, erosion and stuff... and actually the strongest geometrical structure (gravity) is a pyramid. However, a sphere is the strongest geometrical shape, which of course is round.
 
KennyB said:
It seemed like a very convincing argument, but I wasn't there for mechanical engineering, I was just on a basic machining apprenticeship.
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Well, I am far from convinced. If someone tries to explain how something works and mentions centrifugal force I tend to assume that they are lacking somewhat in their understanding of physics due to the fact that centrifugal force does not exist.

Honestly I really don't believe that the difference between V grind & convex grind amounts to stuff all for 99% of cutting tasks. A sharp V grind will cut effectively and a sharp convex grind will also. The angle will determine strength & cutting performance more than whether it is convex or V.
 
I'm with Thom that thin & sharp cuts. Thinner will cut better every time.

If you start the comparison of flat versus convex with both edges being the same thickness at equal distances from the edge, then flat will cut better every time, because the convex edge will be fatter every time.

By the same token, if I take any v-grind edge, and convex it by sharpening, than every time it will cut better than before, because it is now thinner.

Then again, if I take that convexed edge, and make it even thinner by flat grinding all the convexity out of it, it will cut better every time.

Unfortunately that 7 degree zero edge folded over like an accordian by cutting a fingernail, and I had to convex a microbevel on it. ;)

How can two blade grinds and edges of dfferent shapes be compared? How can the starting basis for any comparison be described? I think at some point to define an edge you have to quantify thickness and distance from the edge for X many points.

It would be hard to agree on a proper starting point of an "equal" comparison.

I'll be looking forward to seeing your paper, Lucky Bob. Good luck at school!
 
Broos said:
I'm with Thom that thin & sharp cuts. Thinner will cut better every time.

If you start the comparison of flat versus convex with both edges being the same thickness at equal distances from the edge, then flat will cut better every time, because the convex edge will be fatter every time.

By the same token, if I take any v-grind edge, and convex it by sharpening, than every time it will cut better than before, because it is now thinner.

Then again, if I take that convexed edge, and make it even thinner by flat grinding all the convexity out of it, it will cut better every time.

Unfortunately that 7 degree zero edge folded over like an accordian by cutting a fingernail, and I had to convex a microbevel on it. ;)

How can two blade grinds and edges of dfferent shapes be compared? How can the starting basis for any comparison be described? I think at some point to define an edge you have to quantify thickness and distance from the edge for X many points.

It would be hard to agree on a proper starting point of an "equal" comparison.

I'll be looking forward to seeing your paper, Lucky Bob. Good luck at school!
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:thumbup:
 
Gadgetaholic said:
Well, I am far from convinced. If someone tries to explain how something works and mentions centrifugal force I tend to assume that they are lacking somewhat in their understanding of physics due to the fact that centrifugal force does not exist.

Honestly I really don't believe that the difference between V grind & convex grind amounts to stuff all for 99% of cutting tasks. A sharp V grind will cut effectively and a sharp convex grind will also. The angle will determine strength & cutting performance more than whether it is convex or V.
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Hmm, maybe he meant centripetal force then? Anyway, I'm not trying to convince anyone of a theory I barely understand, just sharing it since I'll learn by others like you correcting me.

So go easy on me, my only physics knowledge is from the Discovery channel! :D
 
Glad to see this thread coming along with so much interesting discussion and not a bunch of ego-driven chest thumping.

Myself, I'm totally sold on convex. It's easy and I can make hair popping edges using it. And that's more than good enough for me.
 
Broos said:
Unfortunately that 7 degree zero edge folded over like an accordian by cutting a fingernail, and I had to convex a microbevel on it. ;)
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My 10 degree zero edge (20 included) glided through a 20 oz. soda bottle like a breeze. What was left of the edge was cracked and all folded to one side.

sircantaloupe said:
Myself, I'm totally sold on convex. It's easy and I can make hair popping edges using it. And that's more than good enough for me.
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And that's the heart of what matters.
 
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