Tell me about convex edges

S

SGTCap

Joined
Oct 21, 2007
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I have seen several references to convex edges but I am not familar with them. Could someone tell me more about how they are done or maintained? What are the advantages to such an edge or the disadvantages. All my knives are flat ground and resharpened with a stone and strop. Am I missing out on something here?
 
I see its been covered and I apologize. I keep seeing reference to a video of this being done with a cheap belt grinder but have yet to find the video. Can anyone point me in that direction?
 
I hate to refer you to Wikpedia because they have incorrect information on many things. However, they do have the straight info on blade grinds.

http://en.wikipedia.org/wiki/Grind

Convex edges are hard to produce and time consuming compared to bevel grinds. That's why most manufacturers don't do them. They cut very well because as the blade goes through the material, it is actually pushing it away from each side instead of 'wedging' into it. Samurai swords cut well because of this convex design. In fact , I think most sword designs use convex edges.

Fallkniven and Bark River knives use convex edges. They are different to sharpen than bevel grinds. I still haven't gotten good at sharpening my Fallkniven because it is so different. It does cut very well though.
I wouldn't recomend a convex to a newbie for this reason. They are expensive to boot.

Hope this helps.
 
i use a slack belt and let the belt curve around the edge doing each side evenly until i get a burr. you have to use a light touch and keep it even while following the contour of the blade. then i remove the burr with my cardboard buffing wheel. i resharpened a knife tonight for a friend and he shaved a stripe up his left arm in one pass.
 
I am very experienced with the lansky kits because I have been using them since I started sharpening knives as a kid about 20years ago. I have no problem putting a razor edge on most knives with that and an old belt loaded with jewlers rough.

Last year I bought a Kershaw Outcast in D2 and I can not get an edge on that thing to save my life. I damn near wore out a set of stones trying to get that thing sharp and only came close once. Now it is more of an experiment in sharpening than anything else which is why I ask about these convex bevels. Is that something I can do to this blade?

I have never had a problem putting a good edge on a knife, even some of the cheap stuff people ask me to sharpen for them, but this thing has me whooped so I though I would try a different approach.
 
for a large blade such as this is it better to use a large stone/sheet of sandpaper and work the blade across it or is it better to hold the blade and sharpen with a block and a smaller piece of sandpaper? It seems like with a recurve like this it would be easier to hold the blade steady and move the stone/sanding block than the other way around.
 
sgtcap, d2 can be hard to sharpen sometimes. i have no problem getting knives sharp with what i use. i would sharpen your knife for you free of charge since it doesnt take me long. all i ask is for you to pay shipping both ways including insurance. if you have a few more i will do them too so you can see if you like a convex edge or not.
 
I appreciate the offer but I'm more interested in trying to learn this process than the results on this particular blade.

If I get desperate or give up in my attempt I may take you up on that offer though. ;)
 
yes i do and I would appreciate it tremendously. I swapped from my outcast to my SRK for now until I get the hang of this. I now have an edge hat will cut a sheet of printer paper but will not drop the first hair. So far I am not impressing myself.
 
**UPDATE** Success. My outcast now sports a convex edge that is very sharp. I didnt have the proper tools for the task so I impravised. Most of the work was done on 60grit emery cloth with a piece of cardboard and outdoor carpeting as a backer. Sound stupid but it worked. I did use my dremel and a sanding drum to thin out the tip area. Other than that everything was done by hand starting at 60grit and working to 2000. I'll get picks up as soon as I get a chance if anyone whats to see them. I also stripped the finish of the blade and paracord wrapped the handle. It feels much better now.
 
i've got one i re-profiled last year. you're the first idio. .i mean, brave soul i've run across that had enough fortitude to FINISH one besides me, and i'm reeeal stubborn.

don't it cut NICE when you get it done, though? :)

you must have bigger hands than me, bad as i hate that ugly rubber handle it fits me perfect. I used a DMT blue hone to grind with, to break the corner on the bevel and take out the toothy factory grind, and a belgian blue stone i have to polish it out to an arc.. short, real blocky 2x4 x 1.5 belgian double grit. not just a PITA, a few evenings of mindless purgatory where nothing obvious happened.. but it got there. eventually.

NOW it's a knife, though, my lord. don't whack at something you didn't intend to make two pieces out of. i saw a review of it somewhere, without the reprofiling, as mostly a chopper, and it wasn't so very much with that short bevel. with the convex edge, it's Jim Dandy. much much improved...

haha. 65$ knife and 150$ worth of work on it.. ;) i ain't the Only stubborn guy around here..
 
I convex all my knives, SAK's especially benefit from the tougher edge. I don't know if you are happy enough with your convex sharpening system but I use EVA foam about 8mm thick as backing over a plywood base.

richard_j that was a really nice offer:thumbup:
 
SGTCap said:
I have seen several references to convex edges but I am not familar with them. Could someone tell me more about how they are done or maintained? What are the advantages to such an edge or the disadvantages. All my knives are flat ground and resharpened with a stone and strop. Am I missing out on something here?
Click to expand...

Convex bevels are very quick and easy to apply using the belt grinders manufacturers normally use to sharpen their blades. That's primarily why they apply them. On the other hand, they are hard to apply by hand. Generally, I recommend the end user grind them away into a flat grind for easier maintenance. I do that to my convexed bevelled knives before ever using them.

I remain unconvinced that they cut better. In my experience they don't. Because of the curved nature of the bevels, the edge itself becomes more obtuse than a flat ground bevel. It might be accurate to say that they cut better than a flat grind that was ground to the same obtuse final angle but I don't think that's the point. I think the Wiki article would be more accurate if it stated it that way.
 
Knife Outlet said:
...I remain unconvinced that they cut better. In my experience they don't. Because of the curved nature of the bevels, the edge itself becomes more obtuse than a flat ground bevel. It might be accurate to say that they cut better than a flat grind that was ground to the same obtuse final angle but I don't think that's the point. I think the Wiki article would be more accurate if it stated it that way.
Click to expand...

From my reading about convex edges, the consensus seems to be that a convex edge is tougher (less prone to dulling/rolling) because there is more material supporting the edge. The conslusion is that a convex edge cuts better longer, not just that it cuts better. The geometry suggests that a convex edge would initially be less effective for chopping/push cutting than a flat bevel BUT it would withstand chopping/push cutting longer. I have not yet seen a quantitative comparison test so the above is theory that makes sense to me.

Anyone know if quality ceramic blades are flat or convex?
 
Zeasor said:
From my reading about convex edges, the consensus seems to be that a convex edge is tougher (less prone to dulling/rolling) because there is more material supporting the edge. The conslusion is that a convex edge cuts better longer, not just that it cuts better. The geometry suggests that a convex edge would initially be less effective for chopping/push cutting than a flat bevel BUT it would withstand chopping/push cutting longer. I have not yet seen a quantitative comparison test so the above is theory that makes sense to me.

Anyone know if quality ceramic blades are flat or convex?
Click to expand...

Yes, of course, since the convex bevel has a more obtuse angle right at the edge then it will be less prone to failure. It's even OK to say that it would cut better than a flat ground bevel that was ground to the same angle. But to say it cuts better as a general statement would not be true in my experience.

I'm trying to imagine how it would dull more slowly but I'm not coming up with anything. perhaps it does but how it does it doesn't seem logical to me.

At any rate, I still recommend end users apply a regular flat bevel to the knife. It will be easier and faster to maintain. Since edges are temporary things and are restored readily by sharpening, I think the ease of maintenance trumps the hardiness of the obtuse angle.
 
can't think of any reason for the edge to dull any more quickly, but because of the shape, it might take more force to push through material than a flat grind at the same level of sharpness.
 
Knife Outlet said:
...I'm trying to imagine how it would dull more slowly but I'm not coming up with anything. perhaps it does but how it does it doesn't seem logical to me....
Click to expand...

As I understand it, a smoothe, very sharp edge still consists of micro teeth. When these teeth hit something hard, they bend and fold. The metal is still there. Steeling and stroping re-align the bent teeth. That's why soft leather can "sharpen" a hard steel blade. The conventional convex edge has more steel (thicker) supporting the very end of each of the teeth, thus making them stronger, better able to withstand pressure before bending or folding over completely(dulling). Imagine "sharpness" as the number of atoms comprising the edgemost width of a cross section of the blade. Fewer atoms of width at the very edge = greater sharpness. A 20 deg. flat bevel and a short radius convex edge could both be X number of atoms wide at the very edge, thus being equally "sharp". Have you ever cut yourself on a piece of broken glass with a 45 deg. bevel (a right angle)? Ouch! The theory is that a conventional covex edge has more atoms (thickness) supporting the edge-most atoms to keep them in position (sharp) than does a conventional flat bevel thus making the convex edge more resistant to dulling. So much for my "obtuse" explanation:)
 
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