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

Joshua J. said:
While I'm sure there will be some benefit to using a convex edge, those benefits will be imperceivable to anyone who isn't a robot.
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Respectfully disagree. No reason to convex if the performance benefits aren't quantifiable/perceivable. Oh well, proper v or convex edge both will perform mighty fine. Sharpening knuts - agree to disagree.
 
bluntcut said:
Respectfully disagree. No reason to convex if the performance benefits aren't quantifiable/perceivable. Oh well, proper v or convex edge both will perform mighty fine. Sharpening knuts - agree to disagree.
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I agree with him to a certain extent in that edge angle and thickness do play the LARGEST role, but there are at least some noticeable differences. Ultimately thinner geometry usually results in increased cutting performance so long as it isn't reduced to the point of failure under the forces of the required task.
 
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FortyTwoBlades said:
I agree with him to a certain extent in that edge angle and thickness do play the LARGEST role, but there are at least some noticeable differences. Ultimately thinner geometry usually results in increased cutting performance so long as it isn't reduced to the point of failure under the forces of the required task.
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:thumbup: agree.
 
bluntcut said:
Respectfully disagree. No reason to convex if the performance benefits aren't quantifiable/perceivable. Oh well, proper v or convex edge both will perform mighty fine. Sharpening knuts - agree to disagree.
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Was recently backpacking with a brand new Bday present - Fiskars X7. My older Fiskars is nearly a full convex but I decided (in the interest of time) to slowly convex the new one as I touched it up. In a year or three it'll be a full convex like its older brother who's chopping abilities are legendary in my circle. I did touch up the factory edge as these are notorious for weak/wire edges out of the box, but kept the factory V bevel and angles. I was impressed at how well it split American beech rounds - some nearly two ft long, and could still chop with ease as well. It occurred to me that if you knew in advance what the material characteristics were of what you were chopping, you could design the perfect V edge with shoulder corners and back bevel thickness maximized for wedging/chopping/slicing action etc. Convex gives a greater margin of error in most cases but still not a silver bullet - all depends on what you're working on, and if the back bevel is sufficiently thinned out the argument becomes a lot less clear. Love my convex but also love my FFGs and Scandis and ....

Edit to add:
I work in the graphic arts, the blades on our paper cutters are D2 with two flat bevels into the cutting edge - they last a looong time.
 
HeavyHanded said:
Was recently backpacking with a brand new Bday present - Fiskars X7. My older Fiskars is nearly a full convex but I decided (in the interest of time) to slowly convex the new one as I touched it up. In a year or three it'll be a full convex like its older brother who's chopping abilities are legendary in my circle. I did touch up the factory edge as these are notorious for weak/wire edges out of the box, but kept the factory V bevel and angles. I was impressed at how well it split American beech rounds - some nearly two ft long, and could still chop with ease as well. It occurred to me that if you knew in advance what the material characteristics were of what you were chopping, you could design the perfect V edge with shoulder corners and back bevel thickness maximized for wedging/chopping/slicing action etc. Convex gives a greater margin of error in most cases but still not a silver bullet - all depends on what you're working on, and if the back bevel is sufficiently thinned out the argument becomes a lot less clear. Love my convex but also love my FFGs and Scandis and ....

Edit to add:
I work in the graphic arts, the blades on our paper cutters are D2 with two flat bevels into the cutting edge - they last a looong time.
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Very true, nicely put points :thumbup:. Use best geometry per tasks. A sub/torpedo can't get effective get on-plane :D
 
As soon as you start to sharpen at multiple angles on your V edge there really is no difference between the two, you could think of it as a "digital convex" (as previously mentioned).

Really this debate is more about belt sanders than edge geometry.
 
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Maniacal Pete said:
Thinner geometry dulls faster, too.
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I think that depends on the edge's method of failure.
If the edge fails because of wear, I'd say thinner dulls slower.
If the edge fails because it's too thin and it's bending or chipping, it could be dulling faster.


Great thread, OP. :D
 
Joshua J. said:
As soon as you start to sharpen at multiple angles on your V edge there really is no difference between the two, you could think of it as a "digital convex" (as previously mentioned).

Really this debate is more about belt sanders than edge geometry.
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No it's really not--it's about how the material removed from the blade's shoulder can provide an improvement in cutting ability, and that's the true primary advantage of a convex edge rather than what's commonly stated by many in the community at large. This advantage can be provided in other ways than using a belt sander, or even creating a continuously convex bevel. :)
 
FTB, I think everybody got your point and it makes sense. The problem is that your making generality of a particular case. Every single convexed V edge is not done the.way you describe. I even think that 90% of the conversions are done with the flat bevel as a guide, and then ppls are right saying that THEIR convex is stronger. Is even add that if the previousV geometry was on the thick side their.convex is.stronger AND cuts better because the very edge is probably more obtuse but.the transition is.smoother ....

So imho twindog is right both edges can be better or not depending on how its done. But most people do it the other way.
 
Personally, I think of a convex edge as a v-edge with the shoulders removed.
I can only see a convex edge as an improvement. Of course, there's more than 1 way to form a convex edge.

It's nice to have options.
 
pwet said:
FTB, I think everybody got your point and it makes sense. The problem is that your making generality of a particular case. Every single convexed V edge is not done the.way you describe. I even think that 90% of the conversions are done with the flat bevel as a guide, and then ppls are right saying that THEIR convex is stronger. Is even add that if the previousV geometry was on the thick side their.convex is.stronger AND cuts better because the very edge is probably more obtuse but.the transition is.smoother ....

So imho twindog is right both edges can be better or not depending on how its done. But most people do it the other way.
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Again, it's not really a generality so much as a pure matter of geometry as pertaining to edge angle. This isn't a thread saying that convex edges are better than linear edges--it's just explaining the difference. A stronger convex is naturally going to also be a more obtuse one and the same applies for a linear edge--a more broad edge angle on a linear edge bevel will be stronger than a thinner one, but convexing simply allows you to slightly reduce the shoulder without thinning the edge angle and thus making it more effective during the cut without weakening the very edge itself, although the overall lateral strength will be slightly reduced.

I don't quite understand your "conversion with the flat bevel as a guide" comment. Do you have the ability to draw a diagram? I can't think of a way that the bevel conversion would make the blade BOTH stronger AND cut better. It's a tradeoff. A convex bevel of equal edge angle to a linear bevel is going to be slightly weaker laterally.

I'm not saying that TwinDog is wrong--just that he's arguing a point that's outside the scope of what I was discussing. :)

Nullity said:
Personally, I think of a convex edge as a v-edge with the shoulders removed.
I can only see a convex edge as an improvement. Of course, there's more than 1 way to form a convex edge.

It's nice to have options.
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^Bingo.
 
When I say using the bevel as a guide, I mean convexing both the apex and shoulders at the same time.just following the existing bevel.

Imho when doing this on a blade with a poor edge geometry ( read thick edge angle on thick stock) it'll improve cutting a bit because you dropped the shoulders a bit, making a smoother transition, but the edge will actually be more supported too becausethe actual edge angle is increased ....

Every edge is a tradeoff, but imho a convex in itself is a better one because you can slightly increase edge angle and get it stronger without really feeling it because in the same time you create a thinner geometry above. Hard to explain in English, sorry.
 
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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FortyTwoBlades, I blame you for getting me interested in machetes. Most of what I've learned is from your posts. At the local surplus store I found a couple of Tramontinas, a Gavilan de Incolma and a Columbian Corona. I've got a Craftsman 2x42 belt sander and was wondering about the proper technique for getting a good convex edge. There's a slack space above the platen and I hold the machete so the spine just clears the belt. The edge looks nice but I think it might be too acute for the 1 to 2 inch woody branches I'm cutting. Should I use the platen to set the angle and then the slack to remove the shoulders? Thanks.
 
This thread reminds me of the discussions we had 30 years ago about "secondary micro bevels". Both sides of the argument were well thought out and well presented by many an expert. I could see both sides myself.

I would like to think of myself as a long time convex edge man since I didn't get a guided system and start using it until about 20 years ago. Then and now, only off and on. If anyone thinks convexing is new, simply ask anyone that freehand sharpens their knives on a stone or rod and examine their cutting edges to see how long convex edges have been around. Probably as long as sharpening.

Good posts on this thread and I for one enjoy the intelligent, polite interchange of ideas.

Robert
 
btrous said:
FortyTwoBlades, I blame you for getting me interested in machetes. Most of what I've learned is from your posts. At the local surplus store I found a couple of Tramontinas, a Gavilan de Incolma and a Columbian Corona. I've got a Craftsman 2x42 belt sander and was wondering about the proper technique for getting a good convex edge. There's a slack space above the platen and I hold the machete so the spine just clears the belt. The edge looks nice but I think it might be too acute for the 1 to 2 inch woody branches I'm cutting. Should I use the platen to set the angle and then the slack to remove the shoulders? Thanks.
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If the edges aren't dinging or rolling it's not too acute. I keep my edges at around a 30 degree edge angle. :) Probably the "best" method would be the 2-stage process of setting the angle then rounding the shoulders but with a machete it's not like you actually need it that precise. Doing it in one step works fine.
 
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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