Convex Edge Bevels

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cliff355

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Apr 19, 2003
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Well, I recently returned from a trip to the woods where I did considerable chopping on frozen wood with my convex-ground khuks. It appears that I may have to alter the grind on a couple of them. They have all been convex ground with the convex profile running the entire width of the original edge bevel. It appears that this may result in too thin an edge on khuks with a wide bevel.

In particular, my 20" 2 lb. (Shanker) AK demonstrated a need for a thicker edge. This is a wide, thin blade and in spite of my grinding on it the blade did a good job. It didn't dent, chip or roll on wood frozen bone-hard at -5F. However, the sweet-spot cutting edge got a bit wavy and these waves were too severe to correct with the Chakmak.

Two passes on each side with a 400 grit belt at an increased bevel angle eliminated the waves, and I believe repeated sharpenings will result in an edge which is essentially bullet-proof. Shanker did an excellent job in hardening this tool, and it rings like a bayonet when tapped with a nail.

Anyway, on wider blades in the future, I will consider convexing less than the full width of the edge bevel and see how that goes.
 
Originally posted by cliff355
It appears that I may have to alter the grind on a couple of them. They have all been convex ground with the convex profile running the entire width of the original edge bevel. It appears that this may result in too thin an edge on khuks with a wide bevel.

Anyway, on wider blades in the future, I will consider convexing less than the full width of the edge bevel and see how that goes.
Click to expand...

That's the main idea in convexing the edges to begin with as the full bevel is too thin to start with and with the flat or semi-hollow ground edges it is necessary to convex the edges only leaveing all the thickness you can within reason.
We, those of us promoting the convex edge, should have made that more clear from the get go.:(
Even though a thin convex edge is stronger than any other, a chopper still needs ample material behind the base edge to keep from bending or waving.
We have talked about it a little as in having to remove material from the edge to get back to a thicker part of the edge, but unless it's really clarified I can see how it could be easily mis-construed.:(
 
I experienced the same problem with my Kumar WWII. I convexed it across the full width of the bevel and it chipped and waved within the first couple of chops on a piece of firewood. I raised the bevel then sent it out on a pass-around and after extensive use by five different people it came back with minimal damage. There were a few small waves and dents, but considering the volume of cutting it had done without sharpening that wasn't unexpected. I raised the angle just a little more the last time I sharpened it and have yet to retest. I'll add some observations to the review thread once I get the chance to do some more cutting.
 
I have a BAS that I convexed. The angle is steeper than the overall beveled area done by the Kami. Despite the steepness it will still shave thicker hair. I think that when cutting it loses it's razor sharpness quicker than one sharpened along the full bevel, but it seems to actually chop better the longer you use it and never rolls or waves like some of the thin edges.
 
I'm real impressed with all you guys out there putting on convex edges and giving them a true beating...it's stuff like this that helps "work the bugs out".

One thing to remember is that if you have an "X" backed A/O belt or a Blue Zirc belt (firm, not much flex) then you will need to push harder to get the right bevel angle.

I think that's one reason why some people have a real grievance with convex edges is that you can't "measure the final angle" like you can with a flat ground bevel. Not all convex edges are the same. In fact, you could say they're all different. The harder you push, the thicker your edge will be - seems counterintuitive, but that's how it is.

The trade off is that on a belt sander, the harder you push, the faster it removes material, and the deeper the scratches get. It's a give and take.

That said, I've had to convex khuks that were hollow ground and fullered so much that the belt only touched the spine and the top of the edge bevel...but I still wouldn't call the bevel "thin". I put a lot of pressure on it against the sander with a stiff belt, and it convexed up nicely.

Putting on convex edges is great because you have so many different options and it allows for individual creativity. Doesn't take much creativity to hold a piece of steel at 25 degrees...:rolleyes:
 
I have an old 1" X 42" belt sander that I quit using because the belt speed was too fast to controll. I'm thinking of changing the pulleys to slow it down, and using it without a platten behind the belt to convex edges. Any thoughts?
Also... How far back can you grind without fear of running out of hard steel and into the softer part?
Brian
 
I use a strop with a handle that you can screw in and out to adjust the leather tension and then cut a strip of the wet dry you guys turned me onto on top. If it is adjusted real loose the bevel is steeper. If tight the bevel is not so steep unless I press real hard. Works pretty well.
 
Pen, I have to say that it does take a bit of creativity to do a convex bevel with nothing but hand tools though. Not to mention a lot of elbow grease!
 
Originally posted by Roadrunner
it does take a bit of creativity to do a convex bevel with nothing but hand tools though. Not to mention a lot of elbow grease!
Click to expand...
Actually.......
Use a carbide scaper along the length of the edge &
it goes pretty fast with lots of control.
You can go pretty heavy removal with high pressure.
Lots of control with light pressure.
You do have to get the feel for not chattering the scraper--
depends on vertical & horizontal angles,
& firmness of pressure.
Scratches left behind are lighter than you might expect.
 
Thanks for the tip ddean. I seem to remember you posting that before, I haven't tried it yet though. Maybe on my new Kobra. :D
 
RR:

The amount of elbow grease necessary may depend on the thickness of the khuk blade and width of the edge bevel. Convexing an entire edge bevel takes effort, and some khuks may not benefit from this. If the resultant edge would be too thin, addressing the last 1/4" of bevel might give the best results and require less effort.

On the other hand, my 21" Gelbu Special is 1/2" thick at the spine with a fairly narrow blade, and convexing the whole edge bevel was a good move. Nothing can hurt that edge other than a rock.
 
Originally posted by cliff355
RR:

The amount of elbow grease necessary may depend on the thickness of the khuk blade and width of the edge bevel. Convexing an entire edge bevel takes effort, and some khuks may not benefit from this. If the resultant edge would be too thin, addressing the last 1/4" of bevel might give the best results and require less effort.

On the other hand, my 21" Gelbu Special is 1/2" thick at the spine with a fairly narrow blade, and convexing the whole edge bevel was a good move. Nothing can hurt that edge other than a rock.
Click to expand...

I think the curved part of the blade seems to work better if I just take a diamond steel rod and sharpen it. I am talking about the part you use to peel bark off. I have been trying to to the convex edge from after where the curve essentially ends to the tip.

I major bitch I have been finding is that on some of the new ones I have gotten there is almost NO edge on the last 2" or so before the tip and the curved part after the cho is dull as hell. Ironically one that I paid full price for was the dullest I have ever gotten. After about 10 days of working on it (by hand) in the evenings while watching the news it will shave now, but even if I counted the time I put in on it at $5 an hour I doubt I could get it back when I sold it.
I think that maybe dullness should be a reason to make them blems.
 
Chronic dullness has been a bitch of mine in recent times as well. The high quantities of elbow grease I mentioned were expended just getting the bevels to meet, not really changing the shape of the entire bevel. I had to resort to 60 grit sandpaper, which leaves huge scratches that take a lot of time to work out. On some of my working khuks I've just left them be. I polished the edges but left the 60 grit scratches farther up the secondary bevel. It looks nasty but cuts great, which is my main concern anyway. :cool:
 
"The high quantities of elbow grease I mentioned were expended just getting the bevels to meet,"


Exactly. I couldn't figure out how to say it but that was how dull mine was. the top of the edge where the bevels meet was flat almost.
 
Convex bevels on heavy choppers like that should have apex angles of about 15 degrees or so. This is the angle which the convex bevel forms in the last 1/8". Above this you can generally run them as high as you can go, it is very rare to see a ripple deeper than 1/8" unless the steel is very poor.

To determine the edge angle at the apex just use the same method for flat ground bevels, the difference will be minimal as the curvature is close to zero over that small a distance. If you want to be really specific you can map out the entire convex profile, for example see :

http://www.physics.mun.ca/~sstamp/knives/brush_blade_recurve_parrell.html

The final listed curvature is actually still too steep, Glen did a fine job on the hardening and it still won't ripple which means it could likely go down further. However the last profile was ground flat to the belt under light pressure so thinning it out further isn't practical unless I ran a stiffer belt. I may put hollows in the primary when I get a wet wheel grinder.

-Cliff
 
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