V - Sharpening Convex Edges

[The_KaramVit]

Has anyone tried sharpening convex edges the way we sharpen v edges? How has your experience been?

 

[Gaston444]

I had to do that to a Cold Steel San Mai III trailmaster, and it did improve chopping performance without causing any chipping. But the convex on that was so fat the bevel looked tall and ugly before the angle became closed enough to actually be useful...

I hate convexed edges, and have never seen one that worked for me, especially on big chopping knives... Fallkniven "Northern light" edges came close to being thin enough and so flattened into a V easier and better. Still did not like it, but the result looked and performed OK.

I did them freehand. I don't even know how convex edges are supposed to be sharpened, as they make no sense to me any way I look at it...

Gaston

 

[rifleman3030]

That will put a secondary bevel on the knife which kinda defeats the purpose of a convex in my opinion. I generally use sand paper on a soft giving surface and use trailing strokes that is an easy way to sharpen them, another way is when sharpening on a stone using rocking motions in a scrubbing stroke.

 

[bucketstove]

Has anyone tried sharpening convex edges the way we sharpen v edges? How has your experience been?

I don't understand the question

I sharpen using "oil" stones (alumox, diamond) ... I do three step, cut off weak metal, raise burr, cut it off (double angle or 40 degree ), then "microbevel" (15dps-20dps)

all my edges have convexity to them ... my 10dps edges are more like 10-12 dps
they start out more like 10-16dps and get closer to 10dps as I keep sharpening them

yes, I use a scrubbing stroke, go fast, and most often have stone in hand ... which adds a lot of rock'n'roll ... and I like it

 

[The_KaramVit]

I don't understand the question

I sharpen using "oil" stones (alumox, diamond) ... I do three step, cut off weak metal, raise burr, cut it off (double angle or 40 degree ), then "microbevel" (15dps-20dps)

all my edges have convexity to them ... my 10dps edges are more like 10-12 dps
they start out more like 10-16dps and get closer to 10dps as I keep sharpening them

yes, I use a scrubbing stroke, go fast, and most often have stone in hand ... which adds a lot of rock'n'roll ... and I like it

Gaston, Rifleman, thanks for your input. I've read up on many people using the sandpaper on a softer surface beneath and pushing down into it to follow the curvature of the convex geometry. I'm still trying to figure out if that's necessary for me or if i should reprofile to a V-edge for convenience (i already have flat stones that i am good with).

Bucketstove, i'm referring to knives with a convex primary grind profile and the sharpening of such knives. Some come with a zero convex grind, and some perhaps with a very very small V microbevel. Some examples of this type are many of the Bark River knives that are known for their convex primary bevels. This is of course in contrast to our more common primary flat or hollow (concave) ground edges. I hope this helps to clarify my question.

If i'm not mistaken, this "convexity" that you refer to that all your edges have would relate to your secondary bevels, assuming your knives do not feature zero-grinds. I agree completely with your statement if that is the case. I also sharpen my knives free hand and notice i cannot always get the angle perfectly straight, and therefore produce a somewhat convex secondary bevel.

 

[Father & Son Traditional Sharpening]

I traded Rob_Mob here at Blade Forums a Busse Basic 8 Limited Edition, which upon receipt he put a 19 degree flat bevel on the edge. It came with a slightly convex full height grind with a heavy convex edge that was at least 25-30 DPS. I had thinned the shoulders significantly while I had it and left a very narrow hint of the original apex bevel, so when he got it was virtually spot on for putting a shallower v-grind. This is what I would recommend doing, as the problem with putting a v-grind edge on a convex primary is the DPS increases as you approach the edge…. therefore you have to do quite a bit of thinning to keep a thin/acute v-grind edge.

 

[HeavyHanded]

I do all mine on a stone or on wet/dry over a very hard surface and shape the convex mechanically rather than by calibrating pressure on a conformable backing. I also avoid making V bevels on them and generally just shape and maintain with honing compound applied to paper, or lapping film over a hard surface. I run the convex out to the edge in a very shallow arc with the final edge being flat for the last mm or two. This is also how I shape my hatchets and larger choppers.

 

[Tim the Wizard]

They get sharper

Over time, a compound double bevel will come to resemble a convex edge, if the edge is maintained through stropping.

I have no issue with thinning a convex factory edge into a compound double bevel if the original edge is too obtuse. I have not noticed any real-world issues with this approach. After all, only the apex of the edge is making the initial cut.

I acknowledge you mentioned V-edge in the original post. What I am referring to is more of a double V.

 

[VermontEdge]

I actually tend to do the opposite: regardless of how a knife comes, I touch it up on my strop which, eventually, will start to convex them all. I find stropping an incredibly quick, easy and forgiving method of maintaining a crazy sharp edge.

 

[D.B Cooper]

I touch up my convex knives on my Sharpmaker like everything else. Then I strop between sharpenings. Works for me.

 

[bucketstove]

Bucketstove, i'm referring to knives with a convex primary grind profile and the sharpening of such knives. Some come with a zero convex grind, and some perhaps with a very very small V microbevel. Some examples of this type are many of the Bark River knives that are known for their convex primary bevels. This is of course in contrast to our more common primary flat or hollow (concave) ground edges. I hope this helps to clarify my question.

If i'm not mistaken, this "convexity" that you refer to that all your edges have would relate to your secondary bevels, assuming your knives do not feature zero-grinds. I agree completely with your statement if that is the case. I also sharpen my knives free hand and notice i cannot always get the angle perfectly straight, and therefore produce a somewhat convex secondary bevel.

I see ; no you aren't mistaken;

I also had a hollow grind folding knife ,
it goes from 3/32" thick ( 2.381mm ) to 1/32" thick ( 1.588mm ) over a height of 16/32" ( 12.7mm )
I've been flat grinding it by hand, the hollow is completely gone, but its not really flat but its close,
it is now convex
the stone I was using to do this dished ... so that helped form the convex

if you search for sharpening convex on bench stones, you can read about guys dishing stones intentionally

I imagine a steeper convex , more like a multi bevel, should be just as easy on a bench stone, just sharpen like you would a multi bevel, go fast , and it will be convexed enough

 

[HeavyHanded]

As a last thought on this, I have only applied a V bevel to an otherwise convex edge once and removed a bunch of steel to change it back. To get the cutting bevel back to the same angle I started at, I would have needed to remove even more flattening and thinning the primary. One of the reasons I stay away from conformable backings for convex edge work is so I can spend extra attention to the region just behind the edge. Once this starts to thicken, the cutting edge gets dragged along with it.

 

[Gaston444]

That will put a secondary bevel on the knife which kinda defeats the purpose of a convex in my opinion. I generally use sand paper on a soft giving surface and use trailing strokes that is an easy way to sharpen them, another way is when sharpening on a stone using rocking motions in a scrubbing stroke.

What exactly IS the purpose of a convex edge I wonder? Using a rocking motion while sharpening is exactly what I try to avoid at all cost, as it results in dull edges...: This is precisely why I sharpen parallel to the edge, to limit rocking motion as much as possible, finishing up in a diagonal motion to "blurr out" the early too deep fully parallel striations which can weaken the edge... The only point I see in a convex is a stronger edge, and you can do that better with a more open flat sided V micro bevel into the bigger V bevel...

I just don't see the point at all, except as an excuse for sloppy sharpening...

Gaston

 

[The_KaramVit]

What exactly IS the purpose of a convex edge I wonder? Using a rocking motion while sharpening is exactly what I try to avoid at all cost, as it results in dull edges...: This is precisely why I sharpen parallel to the edge, to limit rocking motion as much as possible, finishing up in a diagonal motion to "blurr out" the early too deep fully parallel striations which can weaken the edge... The only point I see in a convex is a stronger edge, and you can do that better with a more open flat sided V micro bevel into the bigger V bevel...

I just don't see the point at all, except as an excuse for sloppy sharpening...

Gaston

Edge strength and keeping more meat close to the edge could be one advantage of a convex grind (i speculate). As Triple T mentioned, adding microbevels could make the edge resemble a convex edge, and i think a convex edge can be thought of as having infinitely many microbevels.

VermontEdge mentioned another method popular to maintaining convex edges, only stropping. I think doing this effectively requires larger micron strop compounds such as the black and white colors. Unfortunately, i only have the green, which is far too fine. I haven't tried this out yet. This inquiry is really about seeing the limits of what i can get away with without buying any extra sharpening accessories other than the flat stones i already have.

 

[The_KaramVit]

As a last thought on this, I have only applied a V bevel to an otherwise convex edge once and removed a bunch of steel to change it back. To get the cutting bevel back to the same angle I started at, I would have needed to remove even more flattening and thinning the primary. One of the reasons I stay away from conformable backings for convex edge work is so I can spend extra attention to the region just behind the edge. Once this starts to thicken, the cutting edge gets dragged along with it.

Thanks for sharing, HeavyHanded. What is the reason you reverted back to the convex after converting to the V edge? Did you not get a good result from the V bevel?

 

[HeavyHanded]

Edge strength and keeping more meat close to the edge could be one advantage of a convex grind (i speculate). As Triple T mentioned, adding microbevels could make the edge resemble a convex edge, and i think a convex edge can be thought of as having infinitely many microbevels.

VermontEdge mentioned another method popular to maintaining convex edges, only stropping. I think doing this effectively requires larger micron strop compounds such as the black and white colors. Unfortunately, i only have the green, which is far too fine. I haven't tried this out yet. This inquiry is really about seeing the limits of what i can get away with without buying any extra sharpening accessories other than the flat stones i already have.

Thanks for sharing, HeavyHanded. What is the reason you reverted back to the convex after converting to the V edge? Did you not get a good result from the V bevel?

It didn't cut as well.

Applying a V bevel to a convex results in a more broad cutting edge, unless you grind off a ton of steel from the primary. Convex edges will have less steel behind the edge than a V bevel as long as the two edges terminate in the same degree value.

The V bevel on the convex creates more wedging than a V bevel with flat primary grind sides. A full convex will have less wedging than the other two, and considerably less than a hollow grind (as long as the terminal angle is the same). In general, felling axes are convex (better penetration), splitting mauls are flat sided or hollow ground (greater wedging force).

By grinding the edge on a hard flat surface you can shape the curve of the convex with relative precision, I make mine with a shallow arc that stays nice and thin behind the edge, this way the convex doesn't get broad behind the edge. You can use the Sharpie trick and make the lines radial - from spine to edge all up the length of the blade - this will show exactly where you're grinding. Cosmetically it is more challenging to craft the edge this way, but they tend to turn out better for cutting.

 

[chiral.grolim]

What exactly IS the purpose of a convex edge I wonder? ...The only point I see in a convex is a stronger edge, and you can do that better with a more open flat sided V micro bevel into the bigger V bevel...

The point of a convex edge is to maintain a thicker (stronger) apex region on a thinner (better penetrating) blade. Using a comparable flat microbevel to back-bevel produces an edge "shoulder" transition that results in a less smooth cutting action. If you cut down this shoulder with a flat transition-bevel, you end up with shoulders on either side of it, and if you cut those away, well then you are effectively creating a convex bevel behind your apex, so why not cut to the chase and just cut those bevels all at once? The result is a strong apex that thins out gradually as it goes back. Even Gillette has begun doing this in their razor blades: http://www.google.com/patents/WO2013010049A1?cl=en

 

[Fred.Rowe]

The point of a convex edge is to maintain a thicker (stronger) apex region on a thinner (better penetrating) blade. Using a comparable flat microbevel to back-bevel produces an edge "shoulder" transition that results in a less smooth cutting action. If you cut down this shoulder with a flat transition-bevel, you end up with shoulders on either side of it, and if you cut those away, well then you are effectively creating a convex bevel behind your apex, so why not cut to the chase and just cut those bevels all at once? The result is a strong apex that thins out gradually as it goes back. Even Gillette has begun doing this in their razor blades: http://www.google.com/patents/WO2013010049A1?cl=en

I did see Gillette was doing this with great success, but then again, razor blades are made by machines and are not ground free hand. This is where the rub comes into play. People like Martin and others on this forum who have mastered the skill of grinding convex cutting edges without the use of any guide system are the exception. They have spent years developing the skill that it takes to produce these free hand edges. I don't believe its something that the "average" sharpener has in them, its a developed skill that takes auditory feedback as well as the feedback that comes from the edge riding along the surface of the stone. The new person starting out trying to freehand an edge is lucky to contact the area at the apex a small percentage of the time. The rest of the time is spent searching for that sweet spot and in so doing, dulling and miss aligning the edge to the point it needs to be reground. I get many of these in my shop.
When its stated that a flat edge and a convex edge within the same geometry, the convex edge will be sharper, is not relating this comment to scientific fact. The "V" edge will be sharper, its in the science.
I'm with Gaston on this one; I much prefer a slight change in the angle of the "V" edge to compensate than to spend the time convexing an edge.

Unless you are someone who has mastered a freehand convexing technique, the average person is much better off using a set or controlled system and putting a "V" cutting edge on the blade.
The other problem I have with these edges that are made freehand, they are very hard to reproduce with any skill by the average person.

In a thread a while back I hesitated to state that "accomplished" freehand sharpening is truly an "ART" for the average person cannot do it well.

Of course this comes from a man that makes his living from the sale of controlled angle grinding and controlled angle sharpening. I can teach anyone both of these techniques to the point they would be considered "skilled" in a matter of an hour and they can repeat this skill, [not an art] over and over at their pleasure.

I applaud all of you that have mastered the skill of freehand sharpening, when you can teach me, in an hour, how to do it with skill, I'll change to your technique.

Regards, Fred

 

[HeavyHanded]

I did see Gillette was doing this with great success, but then again, razor blades are made by machines and are not ground free hand. This is where the rub comes into play. People like Martin and others on this forum who have mastered the skill of grinding convex cutting edges without the use of any guide system are the exception. They have spent years developing the skill that it takes to produce these free hand edges. I don't believe its something that the "average" sharpener has in them, its a developed skill that takes auditory feedback as well as the feedback that comes from the edge riding along the surface of the stone. The new person starting out trying to freehand an edge is lucky to contact the area at the apex a small percentage of the time. The rest of the time is spent searching for that sweet spot and in so doing, dulling and miss aligning the edge to the point it needs to be reground. I get many of these in my shop.
When its stated that a flat edge and a convex edge within the same geometry, the convex edge will be sharper, is not relating this comment to scientific fact. The "V" edge will be sharper, its in the science.
I'm with Gaston on this one; I much prefer a slight change in the angle of the "V" edge to compensate than to spend the time convexing an edge.

Unless you are someone who has mastered a freehand convexing technique, the average person is much better off using a set or controlled system and putting a "V" cutting edge on the blade.
The other problem I have with these edges that are made freehand, they are very hard to reproduce with any skill by the average person.

In a thread a while back I hesitated to state that "accomplished" freehand sharpening is truly an "ART" for the average person cannot do it well.

Of course this comes from a man that makes his living from the sale of controlled angle grinding and controlled angle sharpening. I can teach anyone both of these techniques to the point they would be considered "skilled" in a matter of an hour and they can repeat this skill, [not an art] over and over at their pleasure.

I applaud all of you that have mastered the skill of freehand sharpening, when you can teach me, in an hour, how to do it with skill, I'll change to your technique.

Regards, Fred

Fred, if we had an hour I'm sure we could get you well underway!

In all reality, it isn't so difficult applying a convex freehand as long as cosmetics aren't important. To make it blend nice takes a little more practice. At its most simple, just color the entire cutting bevel, shoulder, and primary going back toward the spine - a number of radial lines work fine as well as a bit of crosshatching if so inclined. Grind off everything but leave some color on the original cutting bevel. The cutting angle will be "discovered" when you grind off the last bit of Sharpie along the edge to cut a new bevel, this could even be verified with an angle wedge based on tactile feedback/observation of the ground edge. How aggressive or thinned out behind you want to get is a matter of tool use/preference. In practice this is easier than making a precise V bevel, especially if the primary needs thinning as well as the cutting bevel. All the grinding of the region behind the final approach needn't be terribly precise...unless cosmetics are very important. Blending of the scratch facets can be done just as with any other polishing of a curved surface, just lay off the final edge and approach.

References to this are very common in old-timey axe bit sharpening instructions, and the bit profile guides are still convex. The real trick is to keep it thinned out over time, much like a Scandi or woodworking chisels and irons - it can take a lot of steel removal to correct an overly broad convex just as an overly broadened chisel grind.

 

[Fred.Rowe]

Fred, if we had an hour I'm sure we could get you well underway!

In all reality, it isn't so difficult applying a convex freehand as long as cosmetics aren't important. To make it blend nice takes a little more practice. At its most simple, just color the entire cutting bevel, shoulder, and primary going back toward the spine - a number of radial lines work fine as well as a bit of crosshatching if so inclined. Grind off everything but leave some color on the original cutting bevel. The cutting angle will be "discovered" when you grind off the last bit of Sharpie along the edge to cut a new bevel, this could even be verified with an angle wedge based on tactile feedback/observation of the ground edge. How aggressive or thinned out behind you want to get is a matter of tool use/preference. In practice this is easier than making a precise V bevel, especially if the primary needs thinning as well as the cutting bevel. All the grinding of the region behind the final approach needn't be terribly precise...unless cosmetics are very important. Blending of the scratch facets can be done just as with any other polishing of a curved surface, just lay off the final edge and approach.

References to this are very common in old-timey axe bit sharpening instructions, and the bit profile guides are still convex. The real trick is to keep it thinned out over time, much like a Scandi or woodworking chisels and irons - it can take a lot of steel removal to correct an overly broad convex just as an overly broadened chisel grind.

This is how a brain surgeon explains how simple it is to separate conjoined twins at the top of the head. You don't give your self enough due credit. When you go about teaching someone to do this with skill you are imparting many many years of acquired skill, the lucky recipient will absorb a small piece of that skill. When I teach someone how to put a "flat V" edge on their knife using a set of degree wedges and an ERU, what I'm sharing is a very precise mechanical skill and they will absorb just about all the information the very first time out. I'm so confident of their success that I can ship all the skill they need inside a small flat rate box.

I would love to spend the afternoon with you exchanging ideas and techniques, it would be time well spent.

Best Regards, Fred