Convex grind (sharpening)...

[Martyn]

as this drew some interest in the poll, I thought I would expand a little further with a pic....



I mentioned that I thought sharpening a true convex grind was tricky, perhaps the above pic will help explain why. In blade profile A, you can see the purple lines that represent how the convex grind should be sharpened to maintain the grind profile. You can see as the blade is progressively worn away, much of the surface of the blade needs to be worked on, in order to keep the original profile.

On profile B, you can see what I think is a natural tendancy to sharpen only the edge of the convex grind. The net result is a progressively more obtuse blade profile, which has been corrupted away from the original shape.

It's perfectly possible to produce a sharp edge working the knife as in example B, without even realising you're altering the profile (speaking from experience on this point). I think properly sharpening a convex grind is as much an art an making the knife. You cant get jigs for this, so it HAS to be done by hand. Even the hoodoo hone, (which is a good solution) with it's conforming shape, still leaves much for the user to get wrong. Even with a conforming edge, it's perfectly possible to overwork the edge and get that more obtuse profile.

Though I have to confess, I'm probably being a little puritanical about this. In reality, It'd probably take you thousands of sharpenings to change the profile to the extent shown in example B. So long as the knife is sharp after the fact, I dont suppose it matters much. You could always periodically take the whole blade to task and reprofile it, if this became an issue.

I'd love to hear your tips & techniques for getting a good edge on this blade design, as I do believe it's one of the best possible form-factors you can get. I'm tempted to buy into something with a full convex grind (or failing that, make one), but am still cautious about the sharpening difficulties. Opinions?

[TAHAWK]

The convex sharpening "pro" on the HI Forum at BladeForums, "Pendentive," uses abrasive paper on mouse pads or similar "giving" surface and a slack belt sander. He has quite a lengthy thread on the subject. I've seen a CD he made showing the results: he swings a big khukri at the edge of a sheet of paper and it just slices right through like there was nothing there. Phew!

[Hoodoo]

The convex edge is one of my all-time favorites. I discovered the incredible ability of this grind some years ago while comparing my Marbles Sport 99 to a bunch of other knives. I was totally amazed at the wood carving ability of that little knife, even ground with relatively thick blade stock. What I use is basically a mouse pad glued to a piece of wood and then sandpaper placed over that. If you make it out of cedar or similar light wood, you have something for the field that is very light weight. A handful of precut sandpaper pieces in different grits weighs nearly nothing. Add a piece of leather and you have a great strop. If you eliminate the mouse pad, you also have something you can sharpen flat or convex ground knives on as well. This is often all I take into the field anymore.

For the full convex grind, I sharpen the whole blade but not up to the spine. If you do that, you will cut into the spine and weaken the blade. I also like to convex the edges on flat ground blades. This can make a world of difference in cutting efficiency and all my whittling pocket knives have convexed edges.

Here are some variations on a theme:









This one is made out of aluminum:

[Ed]

I've got to make myself one of those..... so simple :super:

Ed

[TAHAWK]

Good stuff.

[coutel]

I am lucky that I have a slack belt that I can use to convex the edge....Run the belt so that it brushes the edge forwards (spine to edge).

Very easy to get a wire edge (usualy on a 400 grit) then just give it a final strop.

I maintain it on a ceramic rod or a stone......and sometimes strop it on a leather belt to polish it......

Other than that, the Hoodoo system looks excellant.

[boaty]

Hoodoo, are you using guitar plectrums for shims to keep your sandpaper in place? What a great idea!

For convexing the edges on my knives I just use my normal oil/water stones and just imagine that the knife is suspended from a piece of string so its passage over the stone describes an arc - harder to describe than to do.

This is particularly easy on my knives with a scandi-grind

[Hoodoo]

boaty, I use plectrums but I also use sharpened popsicle sticks. Whatever works, eh?