Is All Hand Sharpening, Convex Sharpening? Murray Carter Says It Is

[Twindog]

I think Murray knows what he's talking about. There is no way a human can create a perfectly flat edge freehand. IMHO I believe a slightly convex edge will be stronger but not sharper than a flat edge. A "convex" edge will be sharper but not very strong. Great for shaving but not for cutting up cardboard.

Conversely, there is no way a human can freehand a perfect convex edge. So does that make it a flat edge? Just because freehanding does not create a perfect V edge does not automatically make it a convex edge.

I don't doubt Murray's expertise on sharpening, but I suspect he's just running out of topics to talk about.

A V edge can be sharper (more acute) than a convex edge, or vice versa. A V edge can be stronger than a convex edge, or vice versa.

 

[somber]

I don't doubt Murray's expertise on sharpening, but I suspect he's just running out of topics to talk about.

...video in question was posted Feb. 2011.

 

[Obsessed with Edges]

Conversely, there is no way a human can freehand a perfect convex edge. So does that make it a flat edge? Just because freehanding does not create a perfect V edge does not automatically make it a convex edge.

I don't doubt Murray's expertise on sharpening, but I suspect he's just running out of topics to talk about.

A V edge can be sharper (more acute) than a convex edge, or vice versa. A V edge can be stronger than a convex edge, or vice versa.

The convex needn't be 'perfect' to actually be convex. Only the outward curvature of the bevel defines the convex shape; quite literally in fact (from Random House Webster's Unabridged Dictionary):

"con·vex (adj. kon veksÆ, kÃn-; n. konÆveks), adj.
1. having a surface that is curved or rounded outward. Cf. concave (def. 1)."

There may or may not be such a thing as a 'perfect' convex (in a mathematical sense), but that doesn't seem pertinent anyway, in this context. Done freehand, as with grinding a 'flat' bevel, neither is going to be 'perfect' anyway, except perhaps in the eyes of the one making it. But there's no requirement for perfection here. If at least a tiny bit of outward curvature can be seen or proven, regardless of whether it's deliberate or not, it's convex.


David

 

[KennyB]

The convex needn't be 'perfect' to actually be convex. Only the outward curvature of the bevel defines the convex shape; quite literally in fact (from Random House Webster's Unabridged Dictionary):


There may or may not be such a thing as a 'perfect' convex (in a mathematical sense), but that doesn't seem pertinent anyway, in this context. Done freehand, as with grinding a 'flat' bevel, neither is going to be 'perfect' anyway, except perhaps in the eyes of the one making it. But there's no requirement for perfection here. If at least a tiny bit of outward curvature can be seen or proven, regardless of whether it's deliberate or not, it's convex.


David

Yeah, but I guess what I was getting at with my long-winded post is that there's a differnece between convex that's just rounded by happen-stance, and one that deliberately rounded to form a specific shape.

 

[Obsessed with Edges]

Yeah, but I guess what I was getting at with my long-winded post is that there's a differnece between convex that's just rounded by happen-stance, and one that deliberately rounded to form a specific shape.

I get that. But I think that was beside the original point asked in the OP (and/or commented on by MC in the video), in the sense that the 'convex' is going to happen anyway, whether it's by happenstance or deliberate. Either way, it's still convex, and will always be present to some degree in freehand sharpening. Some edges will look a lot more 'flat' than 'convex', but won't actually be truly flat and will have at least some miniscule rounding of the bevels. Therefore they will be convex, in the literal sense.


David

 

[NRA]

We labor so hard, building elaborate jigs to sharpen straight bevels on knife blades, and in the end, a slightly convex hand sharpened blade is just as sharp (sharper maybe) and stronger than the flat bevel. In the final analysis , what we have is a James Steward made for TV movie, Fools Parade.

I am not surprised to find out I have come full circle. Seems all of life is abundant with ending up where one starts.

 

[Obsessed with Edges]

We labor so hard, building elaborate jigs to sharpen straight bevels on knife blades, and in the end, a slightly convex hand sharpened blade is just as sharp (sharper maybe) and stronger than the flat bevel. In the final analysis , what we have is a James Steward made for TV movie, Fools Parade.

I am not surprised to find out I have come full circle. Seems all of life is abundant with ending up where one starts.

This is why I've fully embraced the convex. Why fight it, after all? I drank the Kool-Aid & have been fully assimilated into the Cult of Convex.

(just applied a minty-fresh, sharp & shiny convex to a new Buck folder today, BTW...)


David

 

[Twindog]

What you guys are saying is that every so-called flat edge is really a convex edge.

And convex edges are superior.

Therefore, all of our edges are superior because flat edges and convex edges are the same. Enjoy your Kool-Aid.

 

[banksy]

We labor so hard, building elaborate jigs to sharpen straight bevels on knife blades, and in the end, a slightly convex hand sharpened blade is just as sharp (sharper maybe) and stronger than the flat bevel. In the final analysis , what we have is a James Steward made for TV movie, Fools Parade.

I am not surprised to find out I have come full circle. Seems all of life is abundant with ending up where one starts.

I think jigs have been developed to help people who are unable to sharpen freehand, and whether they have been unable to produce a sharp V edge, or a sharp convex edge is irrelevant.
Clearly, many people can't produce either in the time scale they have allowed themselves. How many times have you heard, 'I struggled all my life trying to freehand sharpen, then I discovered the (name of jig here) and produced a hair poppin, tree toppin, mirror polished razor edge first time' ?
I don't think they are fools.

 

[Obsessed with Edges]

What you guys are saying is that every so-called flat edge is really a convex edge.

And convex edges are superior.

Therefore, all of our edges are superior because flat edges and convex edges are the same. Enjoy your Kool-Aid.

Speaking only for myself, all I'm saying is it's not a big deal either way. I simply like convex, mainly just because it's very easy to create and maintain it (I do both in a 'stropping' fashion, using sandpaper instead of stones, which suits my own abilities best). I haven't made any assumptions as to which is superior in performance or durability, because I also simply believe that sharp is sharp. I've had equally sharp edges either way, with no significant difference in durability; that just comes down to thickness of steel behind the edge, and shape doesn't impact durability significantly anyway; the edge angle, steel type and heat-treat does most of that. And it doesn't really matter what the bevels look like, leading up to the edge; the apex still has to be crisp. At the closest magnification, if we were looking at only the last 5-10 (or 10-20, 20-30?) microns of bevel behind the apex, we likely wouldn't see any difference in the shape of the edge itself anyway, because all sharp edges still eventually have to apex in a very crisp 'V'.

The only point I was trying to make, is that it's very, very easy (and likely) to introduce at least a tiny bit of convex into every freehand edge. Beyond that, there's really no reason to infer anything either superior or inferior about edge quality, in doing that. It simply happens, and it's not a big deal. I'd bet this is what Murray Carter was getting at in the first place. It's a fallacy to assume that any human has the absolute & precise control to produce a geometrically & perfectly 'flat' bevel on a blade in the course of grinding a new edge on it by hand. It certainly can look flat enough to our uncalibrated eyes, but too many folks seem to assume it's perfectly flat when it really can't be, as produced by the human hand alone.


David

 

[HeavyHanded]

Yeah, but I guess what I was getting at with my long-winded post is that there's a differnece between convex that's just rounded by happen-stance, and one that deliberately rounded to form a specific shape.

Is a good response, I'm not sure there's a lot of disagreement here to any real degree. I tend to think of intentionally convexed edges as being deliberate only as far as the shape of the back bevel goes (and one of the reasons I stopped doing them on a conformable surface), so I can control the shape of that series of overlapping radii. At the cutting edge itself I don't believe I can see any difference from a convex to a V bevel under magnification - the last couple of millimeters are going to be as flat as I can make them in both cases.

I also like to attack all my freehand edges with the intention of making them more acute than how I expect them to finish. If I want to end with 28 inclusive, I shoot for 26. Much like the mechanical technique I've come to adopt, its all about reducing tolerances while dealing with the limitations of the equipment (me).

 

[HeavyHanded]

Thought I'd add a little of my own theory to the discussion. I've long felt that since tactile and audible feedback represent the only useful sensations to work with when freehanding to precision, the limitations should be fairly easy to identify. I do not believe one can readily discern between angle variations that fall within the margin of error defined by the abrasive. By this I mean for example, say a 40u abrasive is making 5u troughs, the minimum possible amount of variation is going to be just under 5u. At higher angles (where you are crossing the theoretical plane of that bevel side) you will feel the edge catching 100% on the abrasive. At lower angles you will either feel it catch on the shoulder, or there will be a complete absence of catch along the edge. Keep in mind, you can feel the difference whether moving into the abrasive or moving away from it, though there is a bit more feedback when edge leading.

With a convex you can still feel the variation, so the lack of a shoulder to frame the level of feedback is nice, but hardly essential.

In my diagram, #1 shows the underlying undisturbed steel that falls 100% within the grind troughs. The zone to the outside of this is the region where the troughs are, so only a percentage of steel remains. This also illustrates why, when at the grinding phase, burr removal is limited to material that falls outside this region - any unsupported steel (bur) that is within this region will have to be removed with a loose abrasive or one on a conformable surface, as it will be bordered by shielding projections.

#2 shows how the abrasives can tear troughs out of the steel within the margin of error based on abrasive size. For the most part, any passes that fall within this amount of deviation will go largely undetected to the senses.

#3 is a funhouse blow up of the possible grind path deviations, and

#4 funhouse blow-up shows where the highest percentage of steel will remain after the grinding process and how that convexity is already built into the scratch pattern.

The size of the abrasive is going to regulate this to some extent - virtually every time you freehand to a finer abrasive, you will initially uncover more convexity than you thought was there from a visual examination of the coarser bevel. It doesn't matter what the absolute abrasive size is. In the diagram #2, that bevel to the naked eye is going to look flat even though from a percentage of material removal, its already convexed but "hidden" within the grind troughs.

This is why I always remind myself and recommend to newbies, always work from the shoulder out. This is the most reliable way to maintain the original edge geometry as you refine the scratch pattern. If there's no shoulder (full convex) work the back bevel for awhile before advancing on the apex. Its possible to "ride" the convex portion and grind it down to meet the target geometry, but now you're working from the area with the least possible tactile feedback - keeping in mind that with every jump to a finer abrasive there will be less feedback anyway.




Hope this makes sense...

 

[willis]

I have a couple of books Ed Fowler wrote and he says by laying the blade a little flatter on about every third stroke is how he hones his knives, which come convex ground.

 

[Twindog]

I have a couple of books Ed Fowler wrote and he says by laying the blade a little flatter on about every third stroke is how he hones his knives, which come convex ground.

If Ed's honing strokes were otherwise at a consistent angle, then with the approach that you're describing, he would just be rounding off the shoulder on a flat edge, rather than creating a convex edge.

The problem with convex edges is that you never really know what kind of edge you're dealing with. I've yet to see on this forum anyone precisely define their convex edge well enough for someone else to recreate that same edge. But I see people quite often clearly defining a V edge that anyone would reproduce on their own knife.

 

[Obsessed with Edges]

I think 'convex edge' is sort of a misnomer anyway. To me, it's misleading, in the sense that deliberately taking much, if any, rounding all the way out to the edge (apex) is likely to result in an edge angle that's excessively wide, if not simply rounded off at the apex. I'd never deliberately try to take it that far. HH's description of starting the grinding further back on the shoulders of the bevels, and carefully & gradually working toward the edge, is more in-line with what I do (I've referred to it as 'sneaking up' on the apex, without significantly altering it's crisp shape). I've also made an effort to do all of the finishing strokes on the firmest backing possible, and edge-trailing, to minimize any chance of blunting or rounding the apex itself.


David

 

[HeavyHanded]

If Ed's honing strokes were otherwise at a consistent angle, then with the approach that you're describing, he would just be rounding off the shoulder on a flat edge, rather than creating a convex edge.

The problem with convex edges is that you never really know what kind of edge you're dealing with. I've yet to see on this forum anyone precisely define their convex edge well enough for someone else to recreate that same edge. But I see people quite often clearly defining a V edge that anyone would reproduce on their own knife.

Its really not that difficult, at least not if you're working off a hard surface. You start with knowing to a reasonable degree of accuracy, what is the inclusive apex angle. Then figure how far back the convex goes, and factor in how thick the spine is or how thick the stock is where the convex starts. Bottom line if you know the inclusive, the rest is relatively easy to estimate.

Its no different with a V edge, you can describe it all you want, but whether it finishes off a back bevel with a hollow grind, sabre grind, full flat grind, Scandi, and the relationship between how thick the spine is and how broad the side of the blade is on any particular tool will have a profound effect on how it cuts. Knowing the inclusive apex angle is just a starting point, same as with a convex.

Grinding off the shoulder is about 85% of what goes into a convex edge. The other 15% is grinding off each transition till its cosmetically smoothed out - no Kool Aid required.

 

[KennyB]

Thought I'd add a little of my own theory to the discussion. I've long felt that since tactile and audible feedback represent the only useful sensations to work with when freehanding to precision, the limitations should be fairly easy to identify. I do not believe one can readily discern between angle variations that fall within the margin of error defined by the abrasive. By this I mean for example, say a 40u abrasive is making 5u troughs, the minimum possible amount of variation is going to be just under 5u. At higher angles (where you are crossing the theoretical plane of that bevel side) you will feel the edge catching 100% on the abrasive. At lower angles you will either feel it catch on the shoulder, or there will be a complete absence of catch along the edge. Keep in mind, you can feel the difference whether moving into the abrasive or moving away from it, though there is a bit more feedback when edge leading.

With a convex you can still feel the variation, so the lack of a shoulder to frame the level of feedback is nice, but hardly essential.

In my diagram, #1 shows the underlying undisturbed steel that falls 100% within the grind troughs. The zone to the outside of this is the region where the troughs are, so only a percentage of steel remains. This also illustrates why, when at the grinding phase, burr removal is limited to material that falls outside this region - any unsupported steel (bur) that is within this region will have to be removed with a loose abrasive or one on a conformable surface, as it will be bordered by shielding projections.

#2 shows how the abrasives can tear troughs out of the steel within the margin of error based on abrasive size. For the most part, any passes that fall within this amount of deviation will go largely undetected to the senses.

#3 is a funhouse blow up of the possible grind path deviations, and

#4 funhouse blow-up shows where the highest percentage of steel will remain after the grinding process and how that convexity is already built into the scratch pattern.

The size of the abrasive is going to regulate this to some extent - virtually every time you freehand to a finer abrasive, you will initially uncover more convexity than you thought was there from a visual examination of the coarser bevel. It doesn't matter what the absolute abrasive size is. In the diagram #2, that bevel to the naked eye is going to look flat even though from a percentage of material removal, its already convexed but "hidden" within the grind troughs.

This is why I always remind myself and recommend to newbies, always work from the shoulder out. This is the most reliable way to maintain the original edge geometry as you refine the scratch pattern. If there's no shoulder (full convex) work the back bevel for awhile before advancing on the apex. Its possible to "ride" the convex portion and grind it down to meet the target geometry, but now you're working from the area with the least possible tactile feedback - keeping in mind that with every jump to a finer abrasive there will be less feedback anyway.




Hope this makes sense...

Heh, I notice this all the time. I try to practically "finish" an edge on a coarse hone when restablishing a bevel and any time I think I have a V edge that's "really" flat, taking it to the finger abrasive reveals the true nature of it.

I also don't think it's too hard to describe a convex edge in such a way that another forum member could reproduce it.

Here, I will describe how I do it and I will wager someone can easily reproduce it...

1. Start with a V bevel ground to 20-30 inclusive with a bevel width of ( just for example ) .100"
2. Grind in a new more acute angle about 5-10 degrees lower than the initial.
3. Knock off the shoulders of the old bevel, to form a new bevel above the old. Establish the new bevel so that it travels 25-50% down the original. (So with our example, .025" to .050" wide bevel)
4. Now increase from the original V bevel angle by 5-10 degrees and grind in another new bevel. This one however should be much smaller, essentially as close to .020" as you can get.
5. Blend the bevel shoulders together by beginning at the edge angle used in step 4, and lowering the spine through the stroke.

None of these numbers are exact but easy to replicate and the results while not exactly the same by person-to-person, will be very similar. This is roughly the same process I use for my kitchen knives, though of course the dimensions are different. But I don't think it's a very complicated procedure

 

[Twindog]

I also don't think it's too hard to describe a convex edge in such a way that another forum member could reproduce it.

Here, I will describe how I do it and I will wager someone can easily reproduce it...

1. Start with a V bevel ground to 20-30 inclusive with a bevel width of ( just for example ) .100"
2. Grind in a new more acute angle about 5-10 degrees lower than the initial.
3. Knock off the shoulders of the old bevel, to form a new bevel above the old. Establish the new bevel so that it travels 25-50% down the original. (So with our example, .025" to .050" wide bevel)
4. Now increase from the original V bevel angle by 5-10 degrees and grind in another new bevel. This one however should be much smaller, essentially as close to .020" as you can get.
5. Blend the bevel shoulders together by beginning at the edge angle used in step 4, and lowering the spine through the stroke.

None of these numbers are exact but easy to replicate and the results while not exactly the same by person-to-person, will be very similar. This is roughly the same process I use for my kitchen knives, though of course the dimensions are different. But I don't think it's a very complicated procedure

Your process of producing a convex edge is extremely complicated, and even with all those instructions, there is not enough information to mathematically model the final edge.

Your example starts with an insanely thick edge -- producing an edge bevel more than a quarter of an inch tall for the 20-degree angle. Your next step makes the edge even taller, but you don't give enough information to know how much taller. Even my ZT 0560CBCF, reground to a 30-degree edge, is just 0.1 inches tall.

But here's the bottom line: You can make whatever convex edge you want, and I can quickly and easily convert it to a V edge that will have a more acute edge angle and cut better.

Convex and V edges come in an infinite variety of edge profiles, including edge profiles so similar that you'd need an electron microscope to detect a difference in profiles.

What I object to is the common notion expressed in this thread that convex edges are somehow better, sharper or more robust than V edges. No such general statement can be made. You can only compare specific profiles -- or hybrid profiles -- of each type.

Edges are bounded by three points: the two edge shoulders and the apex. When convex edges are compared to V edges bounded by those same three points, the V edge will be more acute and the convex edge will be more robust. But those differences can be reduced to almost zero, depending on the arcs that further define the convex edge.

As a general matter, neither convex edges nor V edges are superior to the other.

 

[HeavyHanded]

But here's the bottom line: You can make whatever convex edge you want, and I can quickly and easily convert it to a V edge that will have a more acute edge angle and cut better.


Edges are bounded by three points: the two edge shoulders and the apex. When convex edges are compared to V edges bounded by those same three points, the V edge will be more acute and the convex edge will be more robust. But those differences can be reduced to almost zero, depending on the arcs that further define the convex edge.

As a general matter, neither convex edges nor V edges are superior to the other.

I am not sure where these assumptions come from. A convex is nothing more than a V bevel with shoulders removed and blended into the back bevel. Starting at the same inclusive angle, it is not possible to make a convex into a more acute V bevel. You can grind away and make it the same, but not more acute, or your grind path will extend far up the face of the back bevel and you will end up with a much thinner full flat grind or possibly a very aggressive Scandi. The whole point of the convex is to make a more acute angle with less friction going into a cut, not necessarily a tougher one unless its convexed right to the apex - something that is liable to happen with convex or V bevel if stropped aggressively. In any event its visible and detectable, and capable of being remedied in the exact same manner.

I am no big booster of one over the other in general, but for heavy chopping and carving, a convex edge is thought of as superior by so many that one needs extraordinary examples to "disprove" it IMHO. My own experience backs this up 100%.

 

[David Martin]

HH, Looking at your diagrams #3 & 4 this depiction and it's subsequent description, perhaps inadvertently express why one should use the least number of stones possible to obtain a sharp edge. Because the conclusion is 'the more strokes & stones one uses = more chances for the edge to become more convexed. One could easily gather in this discussion to stay as much away from stropping as possible. Utilizing the stone to remove the burr. DM