Convex edge sharpening

[troutfisher13111]

What happens If I sharpen a convex edge like a regular edge, across a stone? I am just getting good at freehand sharpening and I don't want to take the time to learn something new. This has been keeping me away from knives with convex edges. But I really like the looks of the Knives of Alaska "alpha wolf". Thanks.

 

[cbwx34]

Convex edges can be maintained freehand, primarily with a leather strop, and if you need a bit more, you can lay a piece of sandpaper on top of the leather.

Check out these sites:
http://www.barkriverknives.com/convex.htm
http://home.nycap.rr.com/sosak/convex.htm

You can also sharpen the very edge with a regular stone, (essentially a microbevel) or if you follow the directions on the 2nd link, you can convex sharpen on a regular stone.

cbw

 

[sevenedges]

You can eventually change the convex edge over to an assemetrical edge by simply sharpening out the convex edge. Infact you will eventually have to if you only use a flat stone. otherwise you will have an edge with no relief and a very obtuse cutting edge. Or you can use a slack belt sander every once in a while after several sharpenings of just the very cutting edge, to maintain the convex edge. I personally like the assm. edge because I personally think it cuts better, even though you trade some durability of the cutting edge. however, due to the fact that i use my knives so much i keep a convex edge on almost all of my working knives. you won't really have to learn something new. just sharpen the same way you normally would although it will take some serious time to get the assm edge on the knife the first time. After the inital sharpening it will all be the same as you are used to. hope this makes a little sense. I would recommend using a belt sander with no slack using a platen (backing of the belt on the belt sander) the first time, if you are comfortable with that. or maybe find someone to do it for you. a knife shop. for example, then you will be in business don't let the edge stop you from buying a knife you really like. my 2 cent
Jeff

 

[gud4u]

Just because a knife comes with a 'special convex edge' doesn't mean you can't put whatever edge you wish on that blade.

The development of your hand-honing skills gives you complete freedom to put whatever edge you wish on that blade. That's why you're working on your hand-honing skills, isn't it?

Many knives delivered with 'special convex edges' no longer have those edges.

There is absolutely nothing magical or mystical or special about convex eedges.

 

[db]

It is pretty easy to resharpen a convex edge free hand with bench stones. It's my prefered method to sharpen them. I even have converted knives to convex grinds. Here is a link to how I do it. It works for both resharpening or converting.
http://www.bladeforums.com/forums/showpost.php?p=4001268&postcount=1

 

[Cliff Stamp]

What happens If I sharpen a convex edge like a regular edge, across a stone?

Assuming you sharpen at a similar angle to the existing apec edge angle, nothing. A convex bevel is simply a series of flat bevels and can usually be well approximated by even two flat bevels for the apex/shoulder. Lots of people with convex knives sharpen the edge with flat bevels, both production knives like Fallkniven and custom makers like Fowler. Don't look at the curvature, look at the cross section.

Infact you will eventually have to if you only use a flat stone. otherwise you will have an edge with no relief and a very obtuse cutting edge.

You could apply a flat bevel which is more acute than the existing convex bevel if so desired. You can also adjust the relief with a flat bevel if so desired to reduce that over the convex bevel. For example you could take a convex bevel which sweeps out from 15 degrees to 25 to two flat bevels of 10 and 15 which would be a much more acute edge.

-Cliff

 

[sevenedges]

Cliff is exactly right. There are indeed several options. So please don't let the convex edge discourage you. if you don't quite understand what any of us are saying please don't hesitate to ask.

 

[Cliff Stamp]

Mel Sorg was one of the first people I saw speak out against the idea that convex edges had to be sharpened on belt sanders. This was back on KnifeForums before there was a BladeForums and the perspective on convex edges was horrible with as noted in the above, lots of myths and exaggeration. One of the silliest examples of this was when Thompson argued that comparisons against his blades were unfair because the other blade had a cantled edge which while it did raise the performance significant was obviously too hard to use-sharpen and thus was an unfair comparison. A lot of makers still say things like this implying that convex bevels are very difficult to apply and thus you are getting a rare knife. Busse has shown how you can even machine convex bevels if you want.

Sorg noted how it could be done simply on flat benchstones by just rotating the knife. His goal wasn't to obtain a convex edge specifically, meaning he wanted a curvature, he simply wanted the edge as thin as possible and found that if the blade was full flat it would not be durable enough so he rolled it a little on the draw on the stone to increase the angle a little. You can get the same effect with v-grinds with micro-bevels. The main point is simply to not focus on the curvature as much as the thickness and the angle of the edge and of course the rest of the blade. There are lots of convex ground knives which don't cut well because they are simply too thick and made to be splitters rather than cutters.

-Cliff

 

[Ben Dover]

Just for a challenge, I tried the Bark River tutorial method and reprofiled a great big Cold Steel SRK knife to convex. It was quite a bit of work, but starting with 180 grit sandpaper, and a mousepad, and working up to 1500 grit, and then a leather strop loaded with CrO, I managed to get a push cutting edge on it.

I would not recommend this on a big knife, unless, like me, yoi're doing it as a personal challenge and learning experience.

A fairly rough edge is, IMHO, far more practical on a big survival knife. But I did learn how to reprofile an edge to convex, and I proved to myself that I could do it. And it makes my convex knives easier to sharpen now that I know how to do it.

BTW, I now use a piece of leather instead of a mousepad for a "backing" for the sandpaper.

 

[Dog of War]

IIRC, wasn't it Bill Moran himself who recommended sharpening convex edges on a regular stone, simply making every fourth or fifth sharpening stroke at a more acute angle to remove or round off the shoulder behind the edge?

 

[GibsonFan]

IIRC, wasn't it Bill Moran himself who recommended sharpening convex edges on a regular stone, simply making every fourth or fifth sharpening stroke at a more acute angle to remove or round off the shoulder behind the edge?

I think you're right, at least Moran is the first I knew of like that. Maybe it was Loveless? I'm not sure, honestly. I recall back in the 80's reading about "appleseed" grinds, but I don't recall who it was proposing it.

Troutfisher13111, I suggest buying a cheapo knife with a grind and bevel similar to what you use now. Practice your technique on that in case you're concerned about over-grinding your good blade. You'll find it's not nearly as difficult as some people make it sound; just be cautious and patient.

You may find you don't like the looks of the convex grind, in which case your good knife will still be "stock" and you'll have learned something with little expense. On the knives I've done this to, including a tomahawk, I find the convex edge somewhat better than the original flat bevel in both cutting performance and edge-retention, as well as being very easy to touch up. I currently do all my sharpening and profiling on flat, handheld stones/diamond plates. There's no reason at all you can't do it this way, especially since you've already learned to keep a nice even bevel by hand. Personally, I shy away from any sharpening technique that involves more tools or gadgets than I can fit in my pocket, or the pouch on a sheath.

 

[Cliff Stamp]

IIRC, wasn't it Bill Moran himself who recommended sharpening convex edges on a regular stone, simply making every fourth or fifth sharpening stroke at a more acute angle to remove or round off the shoulder behind the edge?

He actually had the edge named after him so he obviously popularized it for US knifemakers/users. It is a very old grind though, it is how parangs are sharpened for example. Traditionally using a somewhat more complicated geometry as there are two arcs, one which sweeps up to the spine and another which sweeps down the the edge and they are of different curvatures. The blades are usually full sharpened (spine to edge) on flat stones which also keeps the blade relatively free of rust.

Fikes demonstrates the method you describe in his knife testing video. I was showed basically the same thing by a malaysian knifemaker about a half a dozen years ago. The big problem I have is tending to increase the curvature too much at the very edge. This isn't noticed right away as the change is too small, <0.5 a degree. But after a year or so the edge can end up quite obtuse as all the bits add up.

I have settled on using a really soft waterstone to periodically recut the primary because it will naturally wear enough in honing to induce a 1-2 degree curvature and this is usually what I want from the main shoulder to apex grind. When I then move to finer stone I then focus on the apex itself and thus I end up with a geometry which is about 8 at the shoulder, 12 through the main bevel, 14 at the apex, and a tiny micro at 20.

This is for a large chopping blade, the process is similar for smaller blades but the angles are much reduced and the usually don't get the final really obtuse micro. I started adding the additional v-ground micro after reading Cook's book on axes and thinking about the force responce optomization. It tends to make a large difference on heavy impact knives.

-Cliff

 

[GibsonFan]

On the knives I've done this to, including a tomahawk...

I should point out that my knives/hawk are not a true convex, all the way to the spine. I "feathered" the cutting bevel back to give a partial convex profile up to about 1/2" up from the edge. The remainder of the blades have their original shape/finish. I feel this gives most if not all the performance improvements of a full convex grind with a lot less work. A happy medium, if you will.

 

[Cliff Stamp]

I should point out that my knives/hawk are not a true convex, all the way to the spine.

Most wood cutting axes are not aside from the ones which are designed only for really soft woods. Most have hollow primary grinds and only the edge is convex.

-Cliff

 

[thombrogan]

I am just getting good at freehand sharpening and I don't want to take the time to learn something new.

Just drink too much coffee and hand sharpen.

All of my freehand edges are convex because I'm jittery, have limbs which move in arcs and bodily tissues which compress, and I am uncoordinated. They're all sharp, though.

Convex, concave, flat, what matters is that the knife cuts what you want it to cut and how you want it to cut without driving you insane. Everything else is the subject of seven pages of flame wars for fun and extravagance.

 

[Dog of War]

Fikes demonstrates the method you describe in his knife testing video. I was showed basically the same thing by a malaysian knifemaker about a half a dozen years ago. The big problem I have is tending to increase the curvature too much at the very edge. This isn't noticed right away as the change is too small, <0.5 a degree. But after a year or so the edge can end up quite obtuse as all the bits add up.

I find this to be even more of a problem if you use abrasive paper with a soft backing like mousepad. Emery paper or cloth on a stiff rubber backing, or even on wood, works better for me plus you can sharpen edge leading which has much less tendency to form a burr.

I have settled on using a really soft waterstone to periodically recut the primary because it will naturally wear enough in honing to induce a 1-2 degree curvature and this is usually what I want from the main shoulder to apex grind. When I then move to finer stone I then focus on the apex itself and thus I end up with a geometry which is about 8 at the shoulder, 12 through the main bevel, 14 at the apex, and a tiny micro at 20.

Somewhat similar technique here, no waterstones, but been adding microbevels to all my blades convex included for a long time mostly because it's fast and effective plus I'm lazy. But even otherwise I don't think of convex edges as being all that different from flat beveled edges .... as you've pointed out a number of times cross section is what matters.

The "appleseed" term sometimes used for a Moran grind is a good image IMO especially when dealing with fairly acute edges, because the pointed part of an apple seed corresponding to the edge of a blade isn't a radical convex. When I create or restore a convex edge of maybe 12 degrees or less it's pretty much a flat bevel that's had the shoulder aggressively convexed.

Just drink too much coffee and hand sharpen.

All of my freehand edges are convex because I'm jittery, have limbs which move in arcs and bodily tissues which compress, and I am uncoordinated. They're all sharp, though.

My "unmedicated" freehand tends to do this, so I usually have a few beers before sharpening. Couple cups of coffee and my plain edge blades start becoming serrated.

 

[Cliff Stamp]

... what matters is that the knife cuts what you want it to cut and how you want it to cut

Indeed, the biggest problem is lack of details. Statements like "Convex edges cut well." are like "ATS-34 is a good steel." there is no actual information and it just leads to misinformation. In comparison to "The minimal profile I find productive on a whittling knife in 1095 at 66 HRC has a 8 degree shoulder and a 12 degree apex bevel." In general knife discussion has moved beyond knives cutting due to steel (for the most part) and it would be nice to see generalizations of curvature removed from such nonsensical descriptions as well. The properties of cutting ability, durability and such are critical to cross section which should be the subject of the discussions. Curvature does play a role but it is secondary and will be swamped out by even slight differences in cross section due to properties like stiffness being cubic in thickness.

-Cliff

 

[GibsonFan]

The "appleseed" term sometimes used for a Moran grind is a good image IMO especially when dealing with fairly acute edges, because the pointed part of an apple seed corresponding to the edge of a blade isn't a radical convex. When I create or restore a convex edge of maybe 12 degrees or less it's pretty much a flat bevel that's had the shoulder aggressively convexed.

Yeah, that's what I meant to say!

When I sharpen in this manner I try for a nice smooth contour. I think the trick is, to imagine the current edge as an absolute point (assuming it's reasonably sharp and straight). Then picture easing the existing grind on the flat of the blade, into that point.

Anyway, that's how I do it, and I find I can actually get a more acute angle right at the edge this way. My knives don't go dull any faster because of it, as far as I can tell.

I haven't tried a mousepad or other soft surface as a substrate for sharpening yet. I like a nice flat stone because it provides a constant reference point.

 

[Dog of War]

When I sharpen in this manner I try for a nice smooth contour. I think the trick is, to imagine the current edge as an absolute point (assuming it's reasonably sharp and straight). Then picture easing the existing grind on the flat of the blade, into that point.

I agree. You're not looking to convex the point, or edge apex, but rather the steel behind it. For me this is easier to do if I'm using a stone, or paper backed by a fairly solid surface - that "constant reference point" as you say.

Consistent with some of Cliff's comments, I do take a bit different approach with blades I want to run more obtuse, particularly something like a hatchet or ax that isn't very hard and wants to burr. These I like to sharpen edge leading using emery paper on a firm rubber sanding block .... working edge leading does help I believe in not letting the edge get too obtuse too quickly, simply because you have to work somewhat carefully otherwise you'll ruin/cut the paper. Of course after a while the edge will become more obtuse than I want, then it's back to the coarse stone to get the edge back down again.

 

[Cliff Stamp]

Of course after a while the edge will become more obtuse than I want, then it's back to the coarse stone to get the edge back down again.

I have found, much to my delight, that the slight rounding that using a 200 grit waterstone produces just by its wear it pretty much the exact amount of curvature I find optimal for edge bevels. This means all I have to do is basically hone them flat and pick up the 1-2 degrees for the apex bevel by default.

Using the waterstones makes it a lot easier because a harder oilstone would mean I would have to rock the blade on the bevel to get the different angles. I do that sometimes, but the waterstone does it naturally. You can even get stones with different wear rates to produce different levels of curvature.

I assume the really soft 80 grit ones might give a 2-3 degree increase. The really fine natural chinese I have does a bare minimum of < a degree. You can't really measure it, but you can see it under magnification if you watch the way the light reflects.

-Cliff