When it comes to convex edges.....

[clayallison]

What do you like in a convex edge?

• Do you have a preference for the shape of the edge?
• Do you prefer more curvature close to the edge?
• Or do you prefer to round the shoulder?

I did a fun experiment (not very detailed or perfect) to play with the different shapes I could achieve. The blade is cut in cross section, then asymmetrically ground. I then made the long, narrow side convex. I rounded the edge and the shoulder and then photographed it at 50x. I'm hoping people will weigh in on it with their ideas. Please feel free to mark the image up and show the shape you'd most prefer.

 

[grizzled gizzard]

My convex edges vary considerably from knife to knife. I experiment a lot. I have a 1x42 Kalamazoo, so depending upon where I'm grinding, from right below the idler, to right above the plenum, from tight belt to slack belt (I had to put an "idler arm" on the back side of the belt to eliminate belt flutter on really slack belts), I've tried about everything I can think of.

Basically, I've found that I prefer less convexity. I have a few fixed blades that I keep highly convexed for looks, durability, and to see how sharp a knife can get with a lot of convexity.

On my EDCs that I grind, I tighten the belt up and grind them acutely with a wide bevel for max sharpness. My S30 Seb, which I carry more than anything, likes to be middle of the road. I don't use my pocketknife for hard duty, so I don't have to touch it up a lot, so I kinda forget what I'd done previously, so I do whatever feels right at the moment.

I don't take it real seriously. I often carry two knives, so one is always super sharp, and the other is more durable. It doesn't take much more effort to carry two knives, and more often than not, I carry a Carter necker for precision duty. That knife loves to be shaaaaarp, but it's a v-edge.

Guess that was a lot of words to say very little.

To answer your question, my hard use knife has more convexity to the edge, and the sharpy has a tall eased shoulder, kinda like you would expect.

That's a cool picture you put up there, and I love the cross-sectional view. I take it that's just some hunk of metal you were foolin around with, hm?

 

[clayallison]

I had a bunch of 154CM cut into 3 inch pieces which I use to experiment with different geometry. I like being able to look at the edge in cross section.

 

[Obsessed with Edges]

This is very timely, for me. I'm very curious to see where this thread goes, what opinions are shared.

I'm just beginning to 'experiment' with convexing a few of my blades (in an enthusiastic, but amateur sort of way ). I'm using sandpaper on top of my strop block (leather on wood). I've noticed some different things with different blades, and I'm curious about what's the 'best' type of convex edge (if there is such a thing).

I've consistently noticed how effective the convex shape is for cutting heavier materials (like cardboard and leather). I'm assuming this has much to do with the rounded shoulder of the convex, it just seems to 'glide' through the material being cut. However, I've also noticed a decrease in the shaving/hair-whittling capability, at least on the bigger, thicker blades, that seems to be easier to attain with a V-bevel edge. That being said, I'm still very impressed, so far, by the possibilities I see in convexing.

I have no doubt that I could still improve & refine my technique, with more practice. But, it has got me to thinking about finding ways to reduce the curvature at the very edge, either by maintaining a lower angle or perhaps by using a thinner/stiffer backing under the sandpaper, or even using the sandpaper on a hard backing, using the same edge-trailing technique.

So, as you can see, I'm very anxious to see what more of the real 'experts' here have to say. Hoping this will evolve into another definitive thread on the topic.

 

[technician1]

What do you like in a convex edge? strength, I like it on thick blades (.187 or >) just the edge.

Do you have a preference for the shape of the edge? not a high primary grind (to keep as much material on the blade) so, v grind with a convexed edge. "thick round shoulder."
Do you prefer more curvature close to the edge? thick curavture
Or do you prefer to round the shoulder? yes

 

[clayallison]

I just ordered a bunch more metal to cut into cross section so I'll start creating a variety of convex shapes. I'd like to create a way to test the durability that is very repeatable so that we can learn a little more about how each edge performs. I'm thinking of some kind of arms to which I can attach the blades and use a load sensor to help control the pressure. If anyone has ideas on a good testing method, plus jump on and share them.

 

[grizzled gizzard]

Great Clay! I'm excited about this. I've never seen anyone do cross sectional views before. I think that's a great idea, and one that I can perform as well. That will be a perfect way to view the actual shapes of the edges of all that goofing around I was talking about.

Thank you for sharing that, Clay!

 

[clayallison]

Great Clay! I'm excited about this. I've never seen anyone do cross sectional views before. I think that's a great idea, and one that I can perform as well. That will be a perfect way to view the actual shapes of the edges of all that goofing around I was talking about.

Thank you for sharing that, Clay!

Grizz, you're welcome. As soon as I get these new pieces in I'll start on them. The one in the photo is cut very roughly by hand. I'll ask the machine shop to really make the edges crisp on the new ones so it will be even easier to see the shape of the edge.

 

[clayallison]

I have heard of three benefits of convex edges. The first two have to do with rounding just behind the edge:

1. A convex edge has more metal supporting it like a Gothic arch supports a structure. The extra support helps keep the edge from rolling over.
2. The shoulders just behind the immediate edge act to spread the material being cut so that the pressure on the edge itself is lessened. I know that competition ax guys use a convex grind for that reason. The leading edge is keen and when it hits the wood, it initiates a cut and then is followed right away by the rounded shoulders that start spreading and stretching the opening, ensuring that as the leading edge continues to penetrate, it is subjected to much less stress.

The third benefit has to do with rounding off the shoulders of the bevel to reduce drag through the cut.

It would be great to hear opinions and additions to this as well as peoples' experience in cutting with these types of edges.

 

[grizzled gizzard]

OK, I'll start- Most of my convex edges are full convex, like the bottom example. I think they cut well, especially push cutting paper and cardboard. I think they loose their edge quickly, but there may be extenuating circumstances to that, as in, most of my edges like that are in S30, so it's likely to be a steel issue as well.

It's not at all unusual for me to put a full convex bevel on a blade, and then touch it up with ceramic rods. I do this because I have a rod holder that allows me to match the bevel right at the very edge. I find that very useful in helping determine exactly what my "edge of the edge" bevel is on something convex. It's like putting a precise custom microbevel on the edge for immediate touchups.

I really would like to get a few more of these units out to some of you guys who like to touch up with ceramics.

 

[killa_concept]

Generally when I convex a blade, I do a full convex without a noticeable shoulder. The level of curvature depends on what I plan to do with the blade, but usually I keep it relatively thin due to the sort of woods I work with (mostly softer evergreens here in the NW).

It would be great to hear opinions and additions to this as well as peoples' experience in cutting with these types of edges.

Funny you bring this up as I just finished establishing a somewhat odd edge on my 10" wetterlings hatchet a couple weeks ago. On one side, I did a total convex grind with no noticeable shoulder (like the bottom picture). On the other, I did a fairly flat grind with the shoulder knocked off (like the 3rd down). The hatchet is used for light chopping, small splitting chores and a good bit of basic wood shaping/carving. I found that this combination gave a fairly robust edge that would stand up to chopping/splitting while having a more V-ground edge on the side that meets the wood during carving allowed me to have the advantages generally associated with a scandi edge (namely, not as fidgety when push-cutting as the depth of the cut is easy to gauge).

 

[clayallison]

Thank you for that real world example! I am really curious to hear more like that, how the different shapes perform, what is helpful for which type of cutting.

 

[Noctis3880]

I also like a good convex edge pictured in Figure 4, which is like the one on my Lionsteel SR-1. The grind makes it easier to cut through thick objects with a lot of potential drag, like an oversized apple. I had an easier time cutting a big apple with my SR-1 than I did with a much smaller apple using my ZT 0551.

The concept also seems to play a big role in japanese katanas and their ability to cut most objects.

Very important with thick blades, but I can't imagine it would do much for say, a chef's knife.

Still, if nothing else I would want it because it makes touchups on strops much more forgiving.

 

[clayallison]

I also like a good convex edge pictured in Figure 4, which is like the one on my Lionsteel SR-1. The grind makes it easier to cut through thick objects with a lot of potential drag, like an oversized apple. I had an easier time cutting a big apple with my SR-1 than I did with a much smaller apple using my ZT 0551.

The concept also seems to play a big role in japanese katanas and their ability to cut most objects.

Very important with thick blades, but I can't imagine it would do much for say, a chef's knife.

Still, if nothing else I would want it because it makes touchups on strops much more forgiving.

I agree about the stropping being very forgiving. Lately I've been stropping at an angle more narrow than the final edge angle, allowing the leather to form around the bevel but keeping the pressure light so as not to round the edge. CBWX32 has been doing the exact opposite and stropping at a slightly wider angle and reporting great results. Do you have any opinions on either method?

 

[grizzled gizzard]

Of the two options, Cbw's is better, but you have to use Zero pressure.

Why not jut strop at the same angle?

 

[clayallison]

I like to come in at a more narrow angle because I feel like the leather conforms around the bevel. In that way, I take a little bit of the shoulder down without rounding over the edge. I'm going to have to do some real experimenting with CBW's method though and see what kind of results I get. How much wider do you think I'd want to go?

 

[KennyB]

I have heard of three benefits of convex edges. The first two have to do with rounding just behind the edge:

1. A convex edge has more metal supporting it like a Gothic arch supports a structure. The extra support helps keep the edge from rolling over.
2. The shoulders just behind the immediate edge act to spread the material being cut so that the pressure on the edge itself is lessened. I know that competition ax guys use a convex grind for that reason. The leading edge is keen and when it hits the wood, it initiates a cut and then is followed right away by the rounded shoulders that start spreading and stretching the opening, ensuring that as the leading edge continues to penetrate, it is subjected to much less stress.

The third benefit has to do with rounding off the shoulders of the bevel to reduce drag through the cut.

It would be great to hear opinions and additions to this as well as peoples' experience in cutting with these types of edges.

I see a lot of convexing behind the edge on people's knives after they've ground them by hand for a while, even if they're trying for a V grind. They seem to cut pretty well, but to me it doesn't really have any notable difference between a very flat V grind and a convexed V grind. If anything it just produces drag because the curvature is in all the wrong places.

Convexing at the shoulder is actually something I've heard a lot of people doing in kitchen work, but I haven't used edges like that myself. However, from what I hear they're very good for thin slicing because the material tends to kind of flake away from the blade instead of stick to it.

The third is what I do by creating three bevels on the benchstone and then rounding them off to a radius. Most of the knives I've done this to are kitchen knives or "light use" knives. I think they work better for cooking than V grinds mostly because I would have to make a V grind too acute to match the same cutting ability that the convex edge provides. I think that there's really quite a lot of edge strength afforded by it aswell--the people I live with aren't kind to my kitchen knives and they've probably chopped more porcelain plate and stainless steel than they deserve. It never really accounts more much more than a dent on the edges I put on them, because I can leave them quite thick in the middle, but because they're thin everywhere else they cut well even when they've been dented and blunted to hell.

 

[bladestamp]

I feel a clarification of "stropping" is needed. Are we talking about zero compounds, just clean leather pushing a burr back in line or teasing it off. Or are we thinking there is some (however light/microscopic) abrasive on the strop? If there is any abrasive, I don't see how a flatter angle cannot be dulling the edge, or at least reducing the angle.

My instinct tells me to strop a convex bevel, with abrasive, at just less than the final angle but with just enough pressure to form the leather to the exact final angle. No more, and obviously no less. The angle of strop depends on the convexity and the density of your leather, the level of abrasion desired and the part of the bevel you want to hit. All this has to be judged by the user and is part of the learning curve.

I believe there is a school of thought that after any sharpening, even .25µ stropping, a clean leather strop should be used to remove the burr. Then I guess a higher angle will not do any harm and would perhaps work quicker.

 

[Noctis3880]

I agree about the stropping being very forgiving. Lately I've been stropping at an angle more narrow than the final edge angle, allowing the leather to form around the bevel but keeping the pressure light so as not to round the edge. CBWX32 has been doing the exact opposite and stropping at a slightly wider angle and reporting great results. Do you have any opinions on either method?

To be honest, my stropping has almost always dulled the edge to some degree relative to the edge I get after coming off the slotted paper wheel.

It's doubtful that my pressure is the cause given that I'm pretty much lifting the knife when stropping. I just assume my angle is too wide because I like to raise it until I can physically see the very edge itself touch the leather/balsa wood.

Since my new SR-1 still has a nice factory edge, I'm trying to keep any personal sharpening of that edge to a bare minimum. But I get the feeling that next time I'll try to use a more narrow angle to be on the safe side. Though I'll make a point to use the sharpie trick to see where I'm currently stropping.

 

[Jason B.]

IMO the examples shown are not convex but a modification of convex to fit a modern blade grind. To me convex is what you find on japanese swords or a BRKT for example.

Putting a convex bevel or zero convex (convex that blends with the main grind) on some knives can greatly improve their cutting ability but one must know when making that change is doing little to nothing for actual performance.

For example: Often the ESEE fans want to convex their knives and while most of the ESEE line is well suited for that theres one that's not. The ESEE-3 is that blade, with a thickness of 1/8in then ground razor thin for cutting performance the benefit of convex is pushed aside by the existing grind. Same goes for other blade that are thin or ground for increased cutting ability like a hollow grind.

Convex is a art that is much more difficult to understand and master than most give credit for. Its become so commonly suggested that the understanding that YOU are creating the geometry has been put on the back burner in favor of "its easy and forgiving". The first mistake in this is that the common direction has become lay it low and use very light pressure. Light pressure and low angle will give you a very sharp edge but one that will also deform from minimal uses.

Convex is all about the shape and how you form it, it involves being able to read a blade and set a correct geometry so the edge is not so thin it easily becomes damaged or so thick it cuts poorly.

Clay, a lot of your thoughts are correct but realize there is so much more to it than just making a curved bevel.