teach me about convex edges

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Waldamir

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Jun 30, 2012
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I've known of the existence for while but never really looked into. I'd like to learn more about it and see if it's worth throwing on some of my knives. thank you for any feedback and shared knowledge.
 
Ok basically the only benifits of a convex edge that I know of is edge retention. I personally only use it on my choppers.
The reason it has better edge retention is because there's more metal behind the edge.
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Here's a useful link about convexing a edge tohttp://bill-hay.com/Convex/Convex.htm

Hope it helps and cheers!
 
Take a straight beveled edge and grind off the shoulder. Transition this into the cutting bevel and the back bevel respectively. You now have a convex edge. Main advantages - turns thick bladed knives into OK cutters and thick bladed choppers into very good choppers - far better than the original axe/hatchet/thick bladed knife with a broad V bevel. Convex edges hold up much better for me when used on hatchets and machetes that will be seeing seasoned hardwood or heavy chopping chores - this also assumes the inclusive angle isn't too thin for the task. A convex edge sharpened to too acute an angle will fail at about the same rate as a V bevel ground to too acute an angle. On fully convexed edges (convexed from spine to apex) one can see a bit of slicing improvement even on knives with thin spines - however most smaller, thinner knives will exhibit very little difference in my experience.

If one is diligent one can more easily maintain the same cutting edge to back bevel geometry through the life of the blade, much like a Scandi grind. This can also be done with most other grinds as well, but requires one to specifically grind the back bevel as a separate step. Knives with only the cutting edge convexed need to be treated as a regular flat grind in this respect, so no advantage on this point IMHO.

Might be my imagination based on how I sharpen them (with a bench stone), but due to the cutting edge being part of an arc one can grind a fresh apex with a bit less metal removed than would be the case with a flat bevel. This is likely me discounting the time spent thinning the rest of the arc - since its not work on the cutting edge it can be done fast and dirty with very little effort spent carefully regulating pressure and angle.

You can do some of your own testing very easily by simply grinding off the shoulder of some cheap knives and seeing how they perform.
 
One advantage of a convex edge is the ease with which one can change directions when cutting. If you're boning chicken, convex is good. If you're slicing a boneless roast, the V edge is better. In fact for slicing, a single bevel is better than a double bevel.
 
I have a couple of knives now that came with convex edges. They are amazingly good cutters. My chefs' knife seems to glide through everything I throw at it, even though it doesn't seem to have a particularly sharp edge, but it could be that it is currently my only carbon steel kitchen knife (since I sold the other one).

I have to admit that I am fascinated by this, and I wish I had more objective data on the performance aspects of a convex edge - like two knives that are exactly the same except for a v-ground edge vs. a convex edge and then test them against each other on a variety of tasks. The physics of why a convex edge baffles me, because the actual edge is at a more obtuse angle than an equivalent v-ground edge. This seems like it should make a convex edge less proficient in cutting/slicing in practice. One possible reason that a large chefs' knife with a convex edge might perform well in the kitchen is because the curvature of the edge bevel would tend to direct moist foodstuffs away from the blade, reducing drag that provides resistance while cutting, while a v-ground edge would have the food dragging along the wnole cutting edge bevel until it breaks at the junction with the shoulder. To put it more simply - the elimination of a shoulder lessens drag when cutting.

Any one care to comment on this?
 
sharpandsafe said:
I have a couple of knives now that came with convex edges. They are amazingly good cutters. My chefs' knife seems to glide through everything I throw at it, even though it doesn't seem to have a particularly sharp edge, but it could be that it is currently my only carbon steel kitchen knife (since I sold the other one).

I have to admit that I am fascinated by this, and I wish I had more objective data on the performance aspects of a convex edge - like two knives that are exactly the same except for a v-ground edge vs. a convex edge and then test them against each other on a variety of tasks. The physics of why a convex edge baffles me, because the actual edge is at a more obtuse angle than an equivalent v-ground edge. This seems like it should make a convex edge less proficient in cutting/slicing in practice. One possible reason that a large chefs' knife with a convex edge might perform well in the kitchen is because the curvature of the edge bevel would tend to direct moist foodstuffs away from the blade, reducing drag that provides resistance while cutting, while a v-ground edge would have the food dragging along the wnole cutting edge bevel until it breaks at the junction with the shoulder. To put it more simply - the elimination of a shoulder lessens drag when cutting.

Any one care to comment on this?
Click to expand...

The bolded portion above is what it comes down to, for me. I've constantly sung the praises of 'smoothing or rounding' the shoulders of the bevel, while leaving the original V-edge intact. I do this by first re-bevelling to a fairly acute (30° or less) inclusive 'V'-edge, and then 'stropping' my bevels on wet/dry sandpaper, keeping the angle conservatively low, so the edge itself isn't significantly altered. This also ensures that the 'convex' edge angle won't be any more obtuse than the previous 'V' edge. In order for any edge to cut well, the very apex still needs to be crisp & sharp. Even the best-exectuted full convexes (all the way to the edge) will still essentially be a 'V' at the edge itself, when viewed under magnification. Just smoothing out the shoulders makes a HUGE difference in slicing, especially with thicker blades used in tough materials, like heavy carboard, wood, stiff plastics and thick leather. And polishing the 'shoulders' of the convex takes it up a couple more notches, to what sometimes might seem 'scary slick'.


David
 
Obsessed with Edges said:
I've constantly sung the praises of 'smoothing or rounding' the shoulders of the bevel, while leaving the original V-edge intact.
Click to expand...

Thanks, David.

This makes a lot of sense to me, but to be technical about it, a v-edge with rounded shoulders is just that and not a TRUE convex edge, though the distinction may be academic. One question that interests me is how a convex edge (produced from the get-go with no v-edge) might be a good slicing edge though it may be technically rather broad right at the edge. I don't know if MY knives with convex edges started out as v-edges and then got touched up or not. I do not know what the true angle of the cutting edge is (which, if it were a TRUE convex edge would be 2x the tangent of the convex arc where it intersects the arc of the opposite side).

If one starts out making a convex edge without first making a v-edge, for example, by doing the mousepad/sandpaper sharpening routine, and if they could hold the knife at a constant 15 degree angle to the sandpaper the whole time, they are going to end up with a cutting edge something greater than 30 degrees inclusive, depending on how firmly they are pressing down on the mousepad/sandpaper. Let's say that this routine yields a true cutting edge that is 40 degrees inclusive. I can IMAGINE a scenario where this 40 degree convex edge produced in this manner would out-slice a 30 degree v-edge. Of course, I don't think that there would be any way to evaluate this in a practical, real world way.

What do you think?
 
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sharpandsafe said:
Thanks, David.

This makes a lot of sense to me, but to be technical about it, a v-edge with rounded shoulders is just that and not a TRUE convex edge, though the distinction may be academic. One question that interests me is how a convex edge (produced from the get-go with no v-edge) might be a good slicing edge though it may be technically rather broad right at the edge. I don't know if MY knives with convex edges started out as v-edges and then got touched up or not. I do not know what the true angle of the cutting edge is (which, if it were a TRUE convex edge would be 2x the tangent of the convex arc where it intersects the arc of the opposite side).

If one starts out making a convex edge without first making a v-edge, for example, by doing the mousepad/sandpaper sharpening routine, and if they could hold the knife at a constant 15 degree angle to the sandpaper the whole time, they are going to end up with a cutting edge something greater than 30 degrees inclusive, depending on how firmly they are pressing down on the mousepad/sandpaper. Let's say that this routine yields a true cutting edge that is 40 degrees inclusive. I can IMAGINE a scenario where this 40 degree convex edge produced in this manner would out-slice a 30 degree v-edge. Of course, I don't think that there would be any way to evaluate this in a practical, real world way.

What do you think?
Click to expand...

This might sound like heresy to some, but I believe a 'true' convex (all the way to the cutting edge), in the strictest sense, is vastly overrated and prone to problems in maintaining or creating it. When the curvature extends all the way to the edge itself, there's too much opportunity for the edge (apex angle) to become wider, as you've described, or just simply rounded off. This is why I'm an advocate of convexing the bevels, without getting too close to the edge itself. When I 'convex' my edges, I think of it in terms of 'sneaking up' on the edge, without quite touching it (or just barely 'kissing' it, at feather-light pressure). Too many folks have tried and tried to stick to the pure definition of 'convex', by sanding all the way to the edge itself, using a too-soft backing with too much pressure and inconsistent angle, and have come away with what amounts to a dull edge as a result. All created by the handicaps thrown in (less than perfect angle control, too much pressure, too soft a backing, etc.) and by stubbornly trying to convex the entire bevel, as opposed to protecting a perfectly good V-edge and convexing what's behind it (I say this, because I went down that road myself, before realizing it's completely unnecessary, and almost always counter-productive).

A convex doesn't have to be obtuse, or even more than 30°. It's all about degrees of curvature. Some convexes can be very thin & subtle, but still convex. Any freehand-sharpened edge will eventually become convex. The angle used, the consistency with which the angle is maintained, pressure exerted and the firmness/hardness of the sharpening medium (mousepad, firm/thin leather, wood, stones) will be the biggest factors influencing how thin/acute the result will be. I have an old Buck 112 that I 'convexed' in the 'true' sense, using only a thin leather-on-oak strop block and sandpaper. Completely erased/removed the original V-edge in the process (which was obscenely thick), and the finished cutting edge is still quite 'thin', even on this thick-bladed knife. It's dependent on maintaining a lower angle, on the thin & firm backing. I've never liked using a mousepad, because it's difficult to avoid making the edge too wide. The backing is too soft, and will round off the edge, if pressure is just a little too heavy. A firmer backing, such as thin or very stiff leather on a hard backing (glass, hardwood) is still forgiving, but less prone to the problems created by softer backing. Similar results could be obtained by using a backing of varying thicknesses of stacked paper over glass (fewer layers = firmer, more layers = softer). I've used this method to thin the convex on a couple of Opinels, and it works very well. In fact, I did the heavier sanding & thinning over a more forgiving backing of more layers (for the sake of the convex above the edge), and then started decreasing the number of layers as I approached the cutting edge, and for the refining stages. In a nutshell, making the backing under the sandpaper firmer, the closer I get to the cutting edge. By the time I've gotten down to the fine-tuning of the edge itself, I might only be using one or two thickness of paper over the glass. This greatly reduces the chances of rounding the edge, as could happen with an errant pass or two over softer backing.


David
 
Obsessed with Edges said:
This might sound like heresy to some, but I believe a 'true' convex (all the way to the cutting edge), in the strictest sense, is vastly overrated and prone to problems in maintaining or creating it. When the curvature extends all the way to the edge itself, there's too much opportunity for the edge (apex angle) to become wider, as you've described, or just simply rounded off. This is why I'm an advocate of convexing the bevels, without getting too close to the edge itself. When I 'convex' my edges, I think of it in terms of 'sneaking up' on the edge, without quite touching it (or just barely 'kissing' it, at feather-light pressure). Too many folks have tried and tried to stick to the pure definition of 'convex', by sanding all the way to the edge itself, using a too-soft backing with too much pressure and inconsistent angle, and have come away with what amounts to a dull edge as a result. All created by the handicaps thrown in (less than perfect angle control, too much pressure, too soft a backing, etc.) and by stubbornly trying to convex the entire bevel, as opposed to protecting a perfectly good V-edge and convexing what's behind it (I say this, because I went down that road myself, before realizing it's completely unnecessary, and almost always counter-productive).

A convex doesn't have to be obtuse, or even more than 30°. It's all about degrees of curvature. Some convexes can be very thin & subtle, but still convex. Any freehand-sharpened edge will eventually become convex. The angle used, the consistency with which the angle is maintained, pressure exerted and the firmness/hardness of the sharpening medium (mousepad, firm/thin leather, wood, stones) will be the biggest factors influencing how thin/acute the result will be. I have an old Buck 112 that I 'convexed' in the 'true' sense, using only a thin leather-on-oak strop block and sandpaper. Completely erased/removed the original V-edge in the process (which was obscenely thick), and the finished cutting edge is still quite 'thin', even on this thick-bladed knife. It's dependent on maintaining a lower angle, on the thin & firm backing. I've never liked using a mousepad, because it's difficult to avoid making the edge too wide. The backing is too soft, and will round off the edge, if pressure is just a little too heavy. A firmer backing, such as thin or very stiff leather on a hard backing (glass, hardwood) is still forgiving, but less prone to the problems created by softer backing. Similar results could be obtained by using a backing of varying thicknesses of stacked paper over glass (fewer layers = firmer, more layers = softer). I've used this method to thin the convex on a couple of Opinels, and it works very well. In fact, I did the heavier sanding & thinning over a more forgiving backing of more layers (for the sake of the convex above the edge), and then started decreasing the number of layers as I approached the cutting edge, and for the refining stages. In a nutshell, making the backing under the sandpaper firmer, the closer I get to the cutting edge. By the time I've gotten down to the fine-tuning of the edge itself, I might only be using one or two thickness of paper over the glass. This greatly reduces the chances of rounding the edge, as could happen with an errant pass or two over softer backing.


David
Click to expand...


This post mirrors much of my own experience. At first I was taken entirely with the concept and had good results on some of my knives. Then my kit began to grow as I added varying densities of backers for knives with different spine thicknesses and blade widths etc. I also had issues with complete burr removal using strictly edge-trailing on sandpaper. One too many turned out with an overly broad apex, especially when maintaining them over time, so I set it aside for a while.
Came back to it and tried to use a stone instead, inspired by a conversion of an overbuilt sabre ground belt knife to a full convex using just a coarse bench stone. Using a stone worked very well tho with a few cosmetic challenges solved easily with some thought. Now I had complete control over the apex angle, and the shape of the arc as it transitions into the back bevel.
My convex edges turn out far more consistent and better cutters than ever. I can see no advantage to working a convex deliberately to maintain an arc all the way to the apex, that's not where the advantage lies and it makes long term maintenance more difficult than it needs to be.

Edit to add: The only tools I have that are full convex to the apex is by chance, not intent. My hatchets, axes, and especially machetes are maintained almost entirely with two grades of compound applied to a sheet or two of newspaper wrapped around a bench stone. I'm not being gentle, these are working edges and I'm fixing dings and chips as well as making them sharp again. Only because the steel is so soft and I'm only using them as hard choppers can I get away with this method to this extent. On a regular knife the edge would be overly broad and lack bite for draw cutting, even if it will easily shave arm hair (and most of the time facial stubble). Even then, if they get the point where they need to be reground on a stone I do them as a V bevel with reduced shoulder/backbevel.
 
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David, Heavy,

Thank you for your cogent arguments and some excellent food-for-thought. I hope the OP appreciates your posts as much as I. :thumbup:
 
Excellent discussion!

I wonder if I might posit that if we look at drag through various mediums, air, being the thinnest practical medium, could render useful comparisons about convex edges. The two areas don't seem to be directly related, but the best bullets with the least drag use a secant ogive profile to maintain velocity and reduce wind drift. There are cone-shaped bullet profiles, but they are almost never used for any long-range, low-drag applications. The bullet profiles correspond somewhat to edge geometry in this case.

The fact that the edge shoulder creates a more sudden directional change to the energy flow of resistance dictates that it demands more energy to traverse any given medium.

Maybe this would also tend to explain why, at least in my experience, flat ground blades seem to cut better/easier than other profiles.

Any thoughts?
 
Nailgunpilot said:
Excellent discussion!

I wonder if I might posit that if we look at drag through various mediums, air, being the thinnest practical medium, could render useful comparisons about convex edges. The two areas don't seem to be directly related, but the best bullets with the least drag use a secant ogive profile to maintain velocity and reduce wind drift. There are cone-shaped bullet profiles, but they are almost never used for any long-range, low-drag applications. The bullet profiles correspond somewhat to edge geometry in this case.

The fact that the edge shoulder creates a more sudden directional change to the energy flow of resistance dictates that it demands more energy to traverse any given medium.

Maybe this would also tend to explain why, at least in my experience, flat ground blades seem to cut better/easier than other profiles.

Any thoughts?
Click to expand...
Convex material (air/fluid/tomato/etc..) dynamic make sense however do depends on relative velocity between 2 interactants (i.e. blade & material). Slice/cut vs chop would make a big difference on the result, depends whether the objective is penetration or edge preservation.

For a blade with the same thickness, flat ground or zero-ground slice better than other grind because FG/ZG behind the edge is thinner. Again, how much improvement is depend on the blade height & material being cut. ZG/FG defers to big wedging thickness at spine, while other grind big wedging shoulder either at bevel shoulder or hollow grind shouder. Material properties stiffness/gives/etc influence the wedge height, stiction, fracture and so on, thereby translate to performance. Convex edge geometry is an additional variable complexity in this interaction - good thing that we use divide & conquer approach when trying to wrap our mind around this complex topic ;)
 
Here is a REALLY quick example of how I convex my knives. I normally do my convexing on a Water Stone.

Untitled_zps5c76cc60.png
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I like to convex my knives, if I am planing on doing any heavy chopping. They are just plain SHARP!
 
bcknives said:
Here is a REALLY quick example of how I convex my knives. I normally do my convexing on a Water Stone.

Untitled_zps5c76cc60.png
[/IMG]

I like to convex my knives, if I am planing on doing any heavy chopping. They are just plain SHARP!
Click to expand...

Sorry, about typing. I have a new keyboard, it's taking some getting used too!
 
Either type of edge can be a great cutter, but I prefer V edges because convex sharpening adds complexity and imprecision to hand sharpening.

But when it comes to comparing V edges to convex edges, the comparison has to be consistent. Any V edge -- other than a full flat grind -- can be improved by convexing it. And any convex edge can be improved by converting it to a V edge.

If you compare V edges to convex edges that start and end at the same three points -- the apex and the two points where the bevel turns into the blade -- and those points are exactly in the same place, the V edge will cut better because it will have a more acute angle. But if you convex that V edge by grinding off the shoulders, you push the two points where the bevel turns into the blade further from the edge, giving the convex edge an advantage. Likewise, if you convert a convex edge to a V edge, and also move back the shoulder points, the V edge will be better.

Ultimately, what makes the best cutter is the thinnest blade and the most acute edge that will still stand up to both the cutting medium and to the forces being applied.
 
Convex Edges and V-bevels can be made equally sharp.

Both can have the edge retention the user seeks if they are properly tuned to the medium being cut.

You tend to see V-bevels on most knives because they are much easier to produce.

The makers of Convex edges have to spend more time and money training a skilled staff to produce these edges, not to mention the added time in production, thus you only see them on more expensive products.


Personally, I feel it's the adjustability of the Convex edge is what makes it so special.


Convex edges can be zero ground or have blended convex micro bevels, most users can't tell the difference.

But it's the tuneability of the convex edge that really sets it apart.

It is easily tailored to the steel and heat treatment of the knife.

Because the complex curvature is easily adjusted, a skilled maker can make the edge of the edge of any given blade a savage slicer, or a bit thicker to take more abuse.

Many of the convex knife makers I know like to test each blade as they're setting the final edge (think slamming the edge into seasoned hardwood).


Granted, these adjustments can also be made on a V-bevel knife, but not without more serious re-grinding of the flat bevels.

If those V-bevels are done on automated equipment (like most), forget about any final adjustments.

These guided systems combined with some of the coarse aftermarket stones do give the end-user a good option.


Use what works for you, but don't be fooled into thinking V-bevels are somehow superior to Convex, it all really comes down to finding a trusted maker who can build a knife designed to handle your needs.




Big Mike
 
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People say they hold their edge longer as there is more behind the edge, I say they hold their edge for a similar amount of time but the gometry is making it cut with less resistance :)
 
Razorsharp-Travis said:
People say they hold their edge longer as there is more behind the edge, I say they hold their edge for a similar amount of time but the gometry is making it cut with less resistance :)
Click to expand...

Amount of pressure needed to complete a cut can make a big difference in edge retention. Sounds obvious, but it explains why tailoring the level of refinement to the task/type of cutting action can make a large improvement in the functional lifetime of an edge.
 
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