what are the proven advantages of a convex edge

[Jason B.]

Wow! 4 pages and we still have some that think the world is flat.

 

[sodak]

I don't care for them as I find them harder to put on a knife correctly than a v-edge. YMMV. But I have nothing against them either. I try not to jump into a thread and criticize something that I don't like, it tends to be a thread - killer. I've done that a couple of times, then felt guilty for raining on someone's parade. Unless it's about Mora's and scandi grinds - I can't stand those! ~running, laughing, ducking for cover...

As an aside, is anyone old enough to remember when they were popularized by Bill Moran? They used to call them Moran edges back then. They go back a ways, that's for sure.

 

[Bushman5]

Wow! 4 pages and we still have some that think the world is flat.

yes it is! :thumbup:

 

[kgd]

I know mathematicians think proof is a good formulae, but the OP asked for evidence, and that is not the kind of thing that can be demonstrated with diagrams or as one poster suggested computer algorithms (which are just another visualization of mathematics and their inherent assumptions). Evidence, on the basis of the OP, is asking for empirical observation.

As pointed out, we switch geometries (i.e. changing from V-grind to convex or vice versa) always by removing metal, never by packing a little bit back on the edge. So sharpening qualifies as that. When you sharpen the knife, whether or not you change the edge geometry, you are removing metal and optimizing cutting ability above what it used to have.

I guess, you need two identical knives, lets say V-grinds to start. You then sharpen both, one converting to a convex and the other slimming out the V and trying to remove about the same amount of material. Any takers?

 

[Lucky Bob]

While I respect kgd's thrust towards real-life evidence, I had a thought more along my math/engineering tendencies. I'd like to run some computer simulations, but first I need some media to simulate.

So, what materials do you all think would best demonstrate the kinds of things we're talking about? So far, just based on things I've used my pocketknife for since I recently re-read this thread, I'm thinking cheese (like cheddar, which has some give, but nowhere near runny) or the fleshy part of an apple, but I'm sure there are better and more varied examples hiding in y'all's heads.

 

[Dog of War]

I don't care for them as I find them harder to put on a knife correctly than a v-edge. YMMV. But I have nothing against them either. I try not to jump into a thread and criticize something that I don't like, it tends to be a thread - killer. I've done that a couple of times, then felt guilty for raining on someone's parade. Unless it's about Mora's and scandi grinds - I can't stand those! ~running, laughing, ducking for cover...

As an aside, is anyone old enough to remember when they were popularized by Bill Moran? They used to call them Moran edges back then. They go back a ways, that's for sure.

You're never a thread-killer for me, sodak -- can't recall anything you've ever posted that I really disagreed with (of course, since you and I seem to be the only ones who've never had an S30V blade we liked, maybe it's some kind of spell from the evil blade gods we live under?)

One of the few collector pieces I own is a Moran, and I believe he called it an 'appleseed' grind. But I doubt he ascribed any of the magical properties to it that some have ... plus his blades were so downright handsome, a bevel just wouldn't have looked right.

 

[gunmike1]

DOW, Sodak has never had an S30V blade he liked, either, so he is in the club. I'm not in the S30V fan club, but I have my Manix which gets extremely sharp with excellent edge holding to make me see it can be a good performer when heat treated right.
As for convex vs flat, I freehand in my edges shooting for a v grind. Biomechanics causes a small amount of convexing, ans I always use microbevels which end up the slightest bit convex as well. So, I am a fan of thin backbeveled v grinds with microbevels, as that is what I am used to and what works for me, but it is still the slightest bit convexed. I definately have nothing against convex edges, I just have never sat down with a mousepad and sandpaper to try it as I love working my stones and am happy with the results. If I had a belt sander I might be more apt to try convexing, but I don't so I'll stick with my v edge with microbevels even if a computer simulation proves it isn't optimal. It works good enough for me and I'm stubborn about giving up my Glasstones.

Mike

 

[db]

you don't need a soft backing to produce a convex bevel. In fact useing stones is my prefered method for convex sharpening. I've turned many knives into nearly full convex grinds by just scrubbing back and forth on a coarse bench stone. It is much easier than most think.

 

[sodak]

you don't need a soft backing to produce a convex bevel. In fact useing stones is my prefered method for convex sharpening. I've turned many knives into nearly full convex grinds by just scrubbing back and forth on a coarse bench stone. It is much easier than most think.

I wish I could sit and watch you do it sometime. I've heard others say this also, it would be interesting to watch!

 

[Dog of War]

DOW, Sodak has never had an S30V blade he liked, either, so he is in the club. I'm not in the S30V fan club, but I have my Manix which gets extremely sharp with excellent edge holding to make me see it can be a good performer when heat treated right

Hi, Mike. I was trying to say (?) that Sodak and I seemed like the two charter, and maybe only, members of the club ... but welcome aboard! Next Club Beer Summit is at your place.

you don't need a soft backing to produce a convex bevel. In fact useing stones is my prefered method for convex sharpening. I've turned many knives into nearly full convex grinds by just scrubbing back and forth on a coarse bench stone. It is much easier than most think.

I agree. Especially a mousepad with paper is way too soft IMO, too easy to round the edge if you get too much pressure or increase the angle just a bit.

I believe it might have been Moran himself, or maybe it's Jerry Fisk, who said all they do is use a regular stone, and then just slightly elevate the angle every 5th stroke. Let the natural variation in your muscles/hand do the convexing. Also notice that Moran's blades that I've seen and handled had good geometry, decently thin for the purpose, so again it gets back to the fact that as the blade gets thinner and thinner and the edge angle more acute, the less difference you have between convex and V-grind.

I wish I could sit and watch you do it sometime. I've heard others say this also, it would be interesting to watch!

Give it a try. Or if you already have like described, the problem might be that you were trying to hard ... couple more beers should remedy that.

 

[cotdt]

I agree. Especially a mousepad with paper is way too soft IMO, too easy to round the edge if you get too much pressure or increase the angle just a bit.

While you can get any convex shape using stones, the mousepad technique gives you a microbevel automatically, what you call "rounding the edges". That's what gives edges their strength and still allow them to be thin. Using stones tend to give that "appleseed" convex shape and you have to put on the microbevel in an additional step.

 

[gunmike1]

DOW, I was still half sleeping when I posted and completely missed the fact you were quoting Sodak. My wife had shoulder surgery this morning and I got less sleep than usual and blew it there, but I will host our next convention and provide the beers if you like!

db made me think about just how much convexing I am putting on my edges with my forward/back technique when grinding in and polishing a bevel. I use some speed which induces some slop in the angle, then factor in biomechanics and who knows how far off I am even if my grind line is crisp and my bevel looks nice and flat. I use heel to tip strokes when deburring and sharpening my microbevels, and I think those are closer to flat than my backbevels, but who knows. I will have to use the 400x setting on the biobocam heading my way to check to see if I can gain any insight into how much curvature I put on my backbevels. I don't lift the spine at a to try to induce convexing, but I know the speed I use when employing that technique has to leave more convexing than my microbevels have. It's all trivial anyway, as long as they cut good and resharpen quickly I'm happy.

Mike

 

[kgd]

While I respect kgd's thrust towards real-life evidence, I had a thought more along my math/engineering tendencies. I'd like to run some computer simulations, but first I need some media to simulate.

So, what materials do you all think would best demonstrate the kinds of things we're talking about? So far, just based on things I've used my pocketknife for since I recently re-read this thread, I'm thinking cheese (like cheddar, which has some give, but nowhere near runny) or the fleshy part of an apple, but I'm sure there are better and more varied examples hiding in y'all's heads.

Hey LuckyBob, I'm a modeler by profession, except not physical, more chemical and physiological. Check my profile and website if you want to look up my publications. Anyhow, my bent is more for the empirical because this really begets the OP. In the end, the heart of this debate is practical advantage, not theoretical. My quibbles are simply that all computer models do is provide a reinforcement of theory and the assumptions behind them. They can be complex, but those big black boxes hold within them a host of assumptions that accumulate, not unlike the discourse of this thread.

The tenants of whether convex actually gives advantage over v-grind in the slice is one that should be tested with a variety of materials. My curiosity is rather peaked, even though I've fallen into the hype and have convexed most of my V-grinds. Part of me wonders if that actually made a difference. A cartoon and animation won't likely convince me, unless of course you are willing to provide a sensitivity analysis and discuss how material assumptions influence your conclusions

 

[Broos]

db, I'd like to meet the guy who can freehand a flat bevel! Mine aren't really convex off the stone, but they have at least 48 different flat surfaces about a sixteenth of a degree apart on them! Approx one for each sharpening stroke I made with the last stone. Technically isn't this a compound flat edge? They get much closer to convex if they get the 10 micron loaded strop treatment, though.

I think it is a tall order to prove that the shape of the bevel or blade makes a measurable difference in cutting ability. We (meaning some real smart guys) could theoretically prove that a thinner blade will cut better than a thick blade, and there are testers who have shown it thru testing. Same thing for sharper versus duller, or acute edge angle versus big fat edge angle.

Before you can start to prove anything you have to be able to specify the two blades/edges you wish to compare, and it is not an easy task to specify. The cross section of a convex edge is similar to an airfoil, and it can be quite difficult to even communicate the dimensions and geometry clearly.

I think you need to identify the baseline conditions for the basis of comparison before you can even ask the question of which is better. What two edges do you want to compare - do they have the same edge angle (the convex edge will always be thinner than the straight edge with this), or do they have equal bevel lengths and/or widths (the convex edge will always have a more obtuse edge angle here)? You can't compare them on an equal basis, so you have to decide where to start.

Given equal bevel thickness, You could prove that a 5 degree convexed edge will be stronger in compression than a straight edge. But it will also have more material in it, and have a less acute edge angle.

Given equal edge angle, you can prove that the straight edge would be stronger in compression. And it will have more material than the convex edge.

Once you can identify the two you want to compare, I think you could prove some things about them. But you will not be able to say one shape is better than the other shape.

Of course being stronger will not relate at all to how well it cuts if your test does not put the edge near compressive failure.

Basically I think blade/edge thickness & sharpness trumps shape.

 

[udtjim]

Well I can tell you from convexing four or five Mora's that the sure seem sharper to me than when they were Scandi's. NOW that could mean the convex cuts better, OR that I am just better at Convexing than doing other edger. They sure seem to hold the edge better or at least cut well longer after I convexed them.

I dearly love the Scandi too though.

Man, Sodak, you must be old.

ME TOO.

 

[Lucky Bob]

Hey LuckyBob, I'm a modeler by profession, except not physical, more chemical and physiological. Check my profile and website if you want to look up my publications. Anyhow, my bent is more for the empirical because this really begets the OP. In the end, the heart of this debate is practical advantage, not theoretical. My quibbles are simply that all computer models do is provide a reinforcement of theory and the assumptions behind them. They can be complex, but those big black boxes hold within them a host of assumptions that accumulate, not unlike the discourse of this thread.

The tenants of whether convex actually gives advantage over v-grind in the slice is one that should be tested with a variety of materials. My curiosity is rather peaked, even though I've fallen into the hype and have convexed most of my V-grinds. Part of me wonders if that actually made a difference. A cartoon and animation won't likely convince me, unless of course you are willing to provide a sensitivity analysis and discuss how material assumptions influence your conclusions

Honestly, I'm an engineering student, so it just piqued my curiosity as a "hey, I should be able to do that!" and has little left to do with the actual "proof" that the thread originally asked for. The problem is that I need (a) media(s) on which to base the model, and asking professors about cutting into cheese doesn't work out so well.

 

[Jason B.]

The proof that everyone keeps looking for is in the shape. Its works in very similar ways to the kershaw groove, less points of contact = less friction and the convex shape helps to push material away from the blade.


Having less points of contact means having less friction, not something you can easily argue about. When there is less friction between the blade and the meida being cut the cut will be smoother and easier. It would actually be easy to test if we had a wind tunnel, you would just measure the drag coefficient and the one with the least would be the best.

 

[matt321]

I think you need to identify the baseline conditions for the basis of comparison before you can even ask the question of which is better. What two edges do you want to compare - do they have the same edge angle (the convex edge will always be thinner than the straight edge with this), or do they have equal bevel lengths and/or widths (the convex edge will always have a more obtuse edge angle here)? You can't compare them on an equal basis, so you have to decide where to start.

That makes perfect sense. :thumbup:

 

[sodak]

Man, Sodak, you must be old.

ME TOO.

Ouch! Birds of a feather....

 

[theonew]

less points of contact = less friction and the convex shape helps to push material away from the blade.

That strikes me as being a very material dependent "advantage". Might be beneficial for cutting something like manila rope but would be a disadvantage in cutting carrots.