what are the proven advantages of a convex edge

[bigmark408]

Only when you folks have held a Kiku Matsuda will you know what a convexed grind/edge is.

 

[C.S. Graves]

It's good for choppers (slightly stronger & less prone to sticking) and easy to maintain, easy to learn, somewhat forgiving of technique. I have a small knife with a very thin convex edge and it cuts very well. But so do my non-convexed edges. All personal preference.

Took the words out of my mouth, Rat.

The advantages of convex are probably negligible for smaller knives seeing light use, but for high-impact applications like chopping, the convex-style geometry provides a good balance between penetration and strength. Heck, hatchets and axes are typically convex in profile (with exceptions for more specialized uses, like splitting).

 

[Bill DeShivs]

As I said-cheese knives and hatchets.

 

[David Martin]

OK,Bill.You sold me.DM

 

[jimmyh]

I think the discussion should be more about the optimum edge geometry (for a specific task), rather than worrying about whether the edges are rounded. I doubt there is much of a practical difference assuming the basic shape is the same.

I'm glad we're in agreement

Since you mentioned it, I started a thread on just that and put all my thoughts down.

 

[Roger999]

As I said-cheese knives and hatchets.

I put convex edges on my hatchets. I've noticed better edge retention even though I made the bevels higher than before, but I push into the belt pretty hard when sharpening to make sure it thins it out but puts on a wide inclusive angle aswell for better edge retention.

IMO convex edges work similar to micro-bevels, you sharpen at a lower angle than the factory to thin it out, and the curving of the belt/mousepad/leather puts a transition of bevel angles make the angle higher for strength, similar to how microbevels put more steel behind the edge, but the convex edges would usually have more edge retention but be less sharp than the micro bevel. You also don't have to thin out the knife every couple of sharpenings with a convex edge though.

 

[huugh]

I kinda thought Phillip was just being a little snarky, and this is what he meant about it being a good thing that a straight line from edge to spine can fit in the geometry.

You can do it with any grind, even chisel.

Which indeed doesn't have anything to do with what I was saying. I.e. demonstrating that convex, ceteris paribus, is always thicker than flat or hollow.

Nor can you show me any of your blade profiles that I can't draw a straight line from spine to edge inside your geometry. It is a good thing, though.

I showed there is infinite number of such straight lines that CAN'T BE DRAWN INSIDE the non-convex shape.

That is a correct concave geometry. What is the edge angle on that blade? Ideally it can approach exactly zero degrees (but now quite, in Euclidean space). Do you have an actual knife that matches that geometry? Do you have a radiused stone or wheel to achieve it? If you do, please tell me your secret.

?
It absolutely doesn't matter what edge angle is there. You can leave out the edge completly and instead make a horizontal line in the half of the height and it would still be non-convex and everything I said would still apply. (that's also for you GregY)

My straight razor looks similar, but is not the same. It is actually convex from edge bevel to spine. The ideal razor is perfectly flat from spine to edge (with a hollow in the middle for flexibility while retaining the weight and stiffness of the thick spine). This is not quite possible with actual sharpening equipment. The stone eats at the edge slightly faster than the rest of the bevel. The edge angle ends up just a bit greater than the ideal flat bevel.

You are contradicting yourself.

Here's what a real knife edge looks like.

This is a hollow grind with secondary edge bevel. On the right are two tangents that define the overall geometry from edge to spine. By integrating those lines, you develop a curve that defines in a single equation, the shape of the edge. That curve has convex geometry. You can easily prove that (using your own definition) by the fact that a straight line from spine to edge is inside part of the bevel.

First I didn't use "my" definition, it is general definition of convexity. Second, your conclusion about convexity of drawn shape is wrong. It is not convex.

Is a full concave (or flat) grind sharper than a convex? Of course it is.

That's what I'm saying, if we accept that thinner grind leads to sharper knife.

Is it useful for knives? No. Not until we find steels capable of supporting edge angles less than about 10-degrees inclusive (depending on the size and thickness of the blade).

Believe me, as soon as we invent a material capable of it, I'll be right on the incredibly sharp, super-thin, truly-concave blade geometry.

I didn't say anything about that and frankly I do not know why you think I did

 

[db]

Really some outstanding discussion. Recalling when this subject came up in years past on BFC, it certainly didn't seem that near as many people had given this so much thought.

Ultimately, it comes down to the fact that, if you were to take a V-grind edge, whether single or multi-bevel, and round off just the shoulders between the bevels, you would wind up with a thinner overall geometry that would cut better. But then, if you were to take that blade and thin out the convexing behind the very edge, again making it a single or multi-bevel, you'd have thinner geometry still, with better expected cutting performance. You could do this, convexing then flattening, back and forth, each time improving performance, until you exceeded the limits of the steel and the edge/blade became to weak to hold up.

Which is best?

I agree you can flatten a convex and thin it and then thin a flat by convexing. The real question is witch one do you stop at before the edge is too thin and week?

IMO it has almost nothing to do with whether it's convex or V-grind, and everything to do with geometry and cross-section of the blade.

I'll disagree with this a little I think that shape, thickness and finish are all important in how well a knife will cut. And really isn't being convex, flat, or hollow a description of the geometry?

 

[Dog of War]

I agree you can flatten a convex and thin it and then thin a flat by convexing. The real question is witch one do you stop at before the edge is too thin and week?

I suppose that would all depend on where you started, in terms of the blade/edge's original thickness, and how much you removed at each step.

Now, as a practical matter, I've found that a little bit of optimization goes a long ways. My "default" is to sharpen a new knife to around 12 degrees/side, maybe 10 deg. if I know the steel and I intend it for lighter-duty use, and finish with a microbevel of 17-20 deg./side. I do this whether it's a larger chopper or a small blade, and in most cases see a very noticeable improvement over the (typically) more obtuse factory edge.

Going thinner, however, down to 8 deg./side for the primary, I honestly don't notice any improvement in cutting performance, including slicing. And this usually will make the blade much more subject to damage. Soo-o-o-o....

I'll disagree with this a little I think that shape, thickness and finish are all important in how well a knife will cut. And really isn't being convex, flat, or hollow a description of the geometry?

Yeah, I didn't word that too carefully, should have just said cross-section.

Regarding finish ... IMO very important for the edge apex, otherwise not a big deal. I never polish primary or relief bevels beyond "medium grit" (about 240-300); If convexing, of course I go higher than that, but never beyond 600-800 grit since I always microbevel convex edges/blades. Can't comment however on really coarse finish, although I can see where if you left the primary/relief bevel(s) at like 90-150 grit finish you might notice some drag with some materials.

Finally, "shape" ... I think I know what you're getting at, convex or "smoothed" vs. V-bevel(s) with shoulder(s). And I believe much of the point hardheart, myself and others are making is that the thinner and more optimized the blade, the less difference you have between convex and V-bevel: you're basically approaching the theoretical limit of a perfectly thin, and perfectly flat blade. Luckily, since we don't have the perfect steel that can hold up at infinite thinness, we can still enjoy many of the benefits of thinning out our blades, and even when just modestly optimized as discussed, I just don't believe anyone is going to notice any difference whether the edge is convexed or V.

As some have pointed out, with fairly obtuse overall geometry, say, like many hatchets, those shoulders are going to hurt performance. And that's why I often convex hatchets ... along with the fact that convexing such edges can be done very quickly and easily, in basically one step if you're not horribly picky.

 

[db]

I personally agree with all of that DOW. As you start to get a thin/shallow convex there is very little if any noticable difference from a flat small edge bevel. However, it really depends on how the convex bevel is applied, and the thickness of the blade. I put on very wide and shallow convex edges and do notice a big difference from the thicker flat grinds with a shallow edge angle. I personally like and put on convex edges mainly because for me it is the easyest to sharpen and maintain more so than any difference in cutting ability.
edited..Forgot to comment on finish on the bevels and blade grind. I have noticed but it really depends on what is being cut. Sticky stuff will really hold on to coarser finished bevels I even notice it with tape and as that builds up I beleive I can tell. Could realy just be my imagination though. d

 

[cotdt]

Why don't we just run a computer simulation? The thinner the blade at the given point, the better it is at penetrating. The thicker, the stronger the edge. Hence, microbevels are a good compromise and convex edges are the optimal form of the microbevel. You get it automagically with the sandpaper/mousepad technique.

 

[C.S. Graves]

automagically

There's that word again!

 

[Dog of War]

There's that word again!

So long as nobody says something is a "teachable moment" ...

.

 

[GregY]

So long as nobody says something is a "teachable moment" ...

.

But it's intuitively obvious.

 

[Bushman5]

soooo just to throw more gas on the fire.....

all the convexed knives i have excel at chopping, BUT suck at making shavings.

whereas all my scandi grinds suck at chopping, and make tons of papery shavings.

but in the end it does'nt matter, WHY? because i carry at least 5 - 10 knives of all different grinds with me when i hike anyways.....

 

[ArtinNC]

While that's pretty impressive, I'm not so sure that it really proves anything about a convex edge. Further, it looked to me like the middle to middle upper part of the knife did the chopping, while the upper curved portion was used to shave at the end. Granted the video was really choppy and kept freezing in the player, so maybe the curved portion did touch the wood at some point.

I think that steel (1095 tempered to 63 - 65 RC) probably had a LOT to do with how tough the edge of that blade is.

Brian.

On Sept 1,09 I was at Richards J and I used his K2 knife to cut a piece of 4" elm into with that knife and I used the middle of the blade and I could shave hair on my arm with the middle of the blade after cutting the elm into. And I have never seen that done before. So I would say that his convex blade is a good one. And I don't plan on using any knife to chop wood in the future, I'll leave that to a axe. And I beleave his knife is made from 1075 steel.

 

[cotdt]

On Sept 1,09 I was at Richards J and I used his K2 knife to cut a piece of 4" elm into with that knife and I used the middle of the blade and I could shave hair on my arm with the middle of the blade after cutting the elm into. And I have never seen that done before. So I would say that his convex blade is a good one. And I don't plan on using any knife to chop wood in the future, I'll leave that to a axe. And I beleave his knife is made from 1075 steel.

Doesn't the paper wheel system produce concave edges? concavity = 1/convexity so they are opposites.

I agree 1075 makes a wonderful chopper though.

 

[Roger999]

Doesn't the paper wheel system produce concave edges? concavity = 1/convexity so they are opposites.

I agree 1075 makes a wonderful chopper though.

Richard starts off the convex edge with a belt sander and takes off the burr and polishes it with paper wheels, the fine wheel doesn't take off enough metal to make it concave.

 

[DocNightfall]

Given equal edge angles, the convexed blade will encounter less resistance. That's all.

Bevel shoulders are the devil.

Anyone who says convex edges are inferior is unfamiliar with the correct way to convex. You don't do it by convexing at the edge (thus increasing the edge angle). The proper way to do it is to convex behind the edge, eliminating bevel shoulders to shrink and smoothen the cross-sectional geometry.

 

[cotdt]

The advantages of convex are probably negligible for smaller knives seeing light use, but for high-impact applications like chopping, the convex-style geometry provides a good balance between penetration and strength. Heck, hatchets and axes are typically convex in profile (with exceptions for more specialized uses, like splitting).

Convex edges are not just for chopping. There's also knives for carving wood, which are small knives. With a convex grind there is no bevel shoulder to get in the way of your carving. That smooth transition makes a huge difference.

For carving, unlike most convex edges I don't put a microbevel, it's more like a flat edge with no bevel shoulder. Note that the sandpaper/mouse (and leather stropping) sharpening technique automagically puts on a convex microbevel.