Why convex edges are awesome--it's not why you think!

Joshua J. said:
It's kind of funny. I agree with the OP entirely, with the caveat that both edges start with the same geometry. I've been having fun grinding my blades to a zero edge for many years now, so it all seems kind of silly to me. Perspective is everything.
When I see this thread, in the back of my head I instinctually think "why would anyone want to do that when they can grind off 75% of the blade and get a distal taper full flat grind" and how absurd it would be to say that any blade modified to a convex edge could possibly create less drag in use than a knife with no edge bevel at all.

What I find people really want to say when they get excited about convex edges is "thin is sharp and belts are fast" not "this one specific geometry is superior" (Scandi grind people exhibit the same behaviour, just replace "belts" with "sharpening flat on the stone"). What I usually end up reading is something equivalent to saying that the Sharpmaker is better than benchstones because of the geometry it produces. Everyone talks about the geometry, and that's where most of the misinformation is (there's usually no caveat), but what actually makes people excited is that the inherent attributes of the belt sander creates an incredibly convenient sharpening solution.
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YES. Exactly this. :D Personally I don't go so far as to bring my knives to a zero edge because it's too delicate for a lot of the work I do, but I do like my blades thin, both overall and in edge angle.
 
FortyTwoBlades said:
Gotcha'. Personally I think that cutting performance would likely still drop because the edge angle would be increased even further than it was prior under the circumstances you described. In order for the region behind the edge to provide its advantage the edge itself has to displace more material initially due to the steeper and shorter "ramp" at the edge. Anyone handy with that kind of math? It's been a long time since I last took a physics class. :D

Edit to add: If I understand you correctly, you mean like this?

obtuseconvexthinnedcheeks.jpg
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Kind of. Your curve los way extreme compared to what I get when convexing on my hard backed leather, so the angle here is extreme too but you got the idea.

My point here is that too much emphasis is out on the an glee of the very apex. I really think that the geometry of the first inch or so behind the edge is much more important in the cutting performances. Got this idea the first time I used a very thin Japanese kitchen knife at work. Used it all day long and was amazed by edge retention until I had to cut for tomatoes. The knife was actually butter knife dull but it was so much of an improvement over my whustoff and zwilling stuff that I didn't notice. The knife was a suisin inox honyaki.
 
I think that your point is valid, but only when cutting certain materials. :)
 
I think what pwet said was (from flat v): put a micro bevel & round the shoulder. It'll be a curve, but with different curvature along the edges. Steeper near the edge & shoulder, and more flat at the center (where most of the v plane used to be)

PS: very good diagram making things clear!

Edit to add:
This below is also good to explain the effort to thin the bevel, but not yet fully apexing the edge, resulting in often asked question 'why it's not sharp?'
FortyTwoBlades said:
thinnedcheekbevels.jpg
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It doesn't make any conceptual difference to me. If a chip is minor I just sharpen it out. If it's really bad I'll sometimes sharpen the chip like a single serration.
 
But do you agree that a chip in a v-grind blade will sharpen out easier than a chip in an convex edge like the ones in your diagram? Sharpen it like a single serration? What do you mean by this? Like, make the interior edges of the chip beveled and sharp?
 
somber said:
But do you agree that a chip in a v-grind blade will sharpen out easier than a chip in an convex edge like the ones in your diagram?
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Technically it would sharpen out easiest in a linear edge--yes. But that's presuming that you care about maintaining the "true" nature of the convex. Personally after I thin and convert an edge on my belt sander I just sharpen it like a linear edge anyhow. Remember--the primary benefit isn't the curvature. It's the general reduction in necessary material displacement.

Sharpen it like a single serration? What do you mean by this? Like, make the interior edges of the chip beveled and sharp?
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Yup! It's very rare that I have to do that though. Helps save the life of the tool in severe cases. And I mean SEVERE to the point where grinding past it would result in significant blade loss. Sharpen it up and it minimizes the snagging effect when you hit that region during a slice. As you sharpen over time you'll gradually minimize it. I've done that to a few beater knives that people were going to throw away. Fixed 'em up like that and gifted them to folks.
 
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i think removing just the chip from a convex edge would be a lot easier since there is actually less area to work down. it would also be a lot easier to blend in the area where the chip was removed.
 
richard j said:
i think removing just the chip from a convex edge would be a lot easier since there is actually less area to work down. it would also be a lot easier to blend in the area where the chip was removed.
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Try it out. Take a lighter and try an edge flex test until the edge breaks on v-grind and then try it on a convex edge and fix them both, after the results are assessed (hopefully unbiased), you can give us your report! I'm sure everyone would benefit from that experiment.
 
Or just put dings in both blades by chopping a nail. :D

I agree--that would be an interesting test.
 
A roll from a nail isn't nearly as bad as a massive chip from an edge flex test. A lighter will give you a big enough chip that the answer will be definitive.
 
very hard test to take objectively for a lot of reasons, too many variables.

what do you consider a convex equivalent to a V edge for this test? same final angle? about same bevel height ?

depending the damage taken will change a lot, the former will take more damage than the V because same angle and less metal to support behind, the later will take less damage than the V because to accomodate the convex in the same height the angle will be steeper and the ease of repairing will vary too because there will be more or less surface to abrade. and there are all the inbetweens.

to be fair you'll have to use the same abrasives, ie sandpaper on hard backing for the V edge and on softer backing for the convex, but that's unfair to the V edge because there is a wider aray of stones that could work way faster than sandpaper ... but again when using sandpaper the softer backing will make it cut less aggressively than the hard backing ...

anyway without using any power tool i can't see how convex could be easier to repair in real life. easier to maintain, no problem but when it comes to big repairs, i doubt. maybe it's because it's not the method i'm the most proficient with ...
 
Honestly I personally think it's a moot point ultimately, but short of using power tools to do the work a linear bevel ought to take less time due to smaller area of contact against abrasives. In terms of comparing edges I still think it makes the most sense to keep edge angle constant rather than the height of the grind.
 
Another thing to consider I think is that stropping a blade polishes the steel.... polish makes a steel slippery in fact ( I think that until the medium your cutting...... can break-down the polish..... it can't make any in-roads into the knife edge.
 
somber, i put a small chip in k II a few years ago when i hit a piece of baling wire. the chip was about 1/32" deep x 1/16" wide. i was able to remove the chip real quick on my belt sander. i did not have to do the entire edge. i did however decide to thin the entire edge down a little after i removed the chip to make the knife chop better.
 
I learned something. Hadn't really considered it both ways and I confess I perceived it to be your first example in your diagram. I like pictures..
 
Great discussion. I am a noob and these types of discussions are helping me learn more and more. Thanks!
 
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