convex grind

gull wing

gull wing

Joined
May 4, 2002
Messages
2,536
How do you "convex" a flat ground blade? I read an article a while back about a person who had a flat ground blade that did not cut as well as he thought it should. He said after he "convexed" the blade it cut much better.
I can't find that article now so I can't ask. I could see maby, a second bevel? Has anyone done this?
 
I know that Buzzbait "convexed" his Military... You might want to check with him.
Cris
 
I think this will answer:

http://home.nycap.rr.com/sosak/convex.htm

Maybe this too:

http://outdoors.free.fr/s_article.php?id_article=66

Try it on something like an inexpensive Mora knife from Ericson and see how you like it. I use a couple of similar knives in the kitchen, and blending in the bevel like this made the knives perform much better for slicing thicker things and went a long way towards compensating for the fact that they are really too thick-bladed to perform as well as a purpose-made paring knife. But for carving (as opposed to chopping or splitting) wood, I prefer the zero-edge flat bevel.

Of course, this totally removes the "grind line" of the bevel, which many consider, when "crisp" and evenly executed to be a display of knifemaking skill and worthy of a premium price. But just because a feature is in fashion, or requires great skill to execute well, doesn't necessarily mean it will make the tool perform better for a given task.

....Now, where did I put that Nomex suit???
 
firkin

Thank you. I thought the same thing about how other people see the convex grind. I am finishing up my first knife and I made it a full convex grind. I look at it and say "Man whoever did that didn't know how to grind. There isn't a crisp line on the thing" Oh in this case it is true that I don't know how to grind well enough (I have been practicing flat and hollow grinding) to have perfect crisp lines but I am proud that I got everything even and the same thickness over the whole length of the blade that I made. There I said it. I wanted say it but thought that I was wrong about the whole thing...geee...I hope you did mean that. Got room in that suit for me?
 
I felt like you years ago Feth. Laurence convinced me that the convex grind has it's place. I use it on certain knives (and swords) that need a tough edge that still cuts well. The sword I just finished has a reflexed blade. The 1/3 of the blade forward of the reflex is a full convex and it tapers to a full flat grind.

In testing I found it worked just like I wanted it to. I used it on my Dandridge Dummy with PVC Bones and it cut through them and still shaved hair with both the flat and convex. I am convinced the convex cuts things like bones (PVC and natural) better because it doesn't stick.

It also gives you something to do with the slack belt grinder you wondered why you made!:D
 
Thank you Mr. Napp. That sounds like a great sword. You wouldn't happen to have any pictures would you?
 
Lots of room in the suit Feth, I'm not the only one in it.--you'll find that lot's of folks appreciate convex grinds on working knives, once they get their brains around sharpening them.

To me it's pretty simple: making a deep cut requires the blade to displace the material being cut. Like the hull of a (displacement) ship going through the water. Don't see many ships with hollows in the bow or distinct corners where the bow flares out to the sides. Any shapes like that are designed to convert the hull to a non-displacement hull which avoids going through the water and moves on top(which takes a lot of power but pays off in speed)--exactly what one doesn't want the knife to do when making a cut.

For some applications like planing, skinning or cutting thinner material, this doesn't apply. Method or ease of sharpening, or looks can then be more important. For display/collecting it distills down to individual and/or collective taste.

But somebody that can perfectly execute a "crisp", even, flat or hollow grind has to have a head start on executing an even convex grind it they want to. Starting with "squared-up" stock just about always makes getting even curvature easier, at least for anything I've done regardless of the material. (Not that I've made any blades, just worked on 'em.)
 
You wouldn't happen to have any pictures would you?

I haven't taken any pictures yet but will try tomorrow. I haven't been building much the last year so this is one of the few.
 
mr.Ed Fowler's famous cutting knives are convex grinds. you dont hear many say anything about a lack of crisp grind lines there? i have many convex grinds on my site. the woodsman bowie for one.i convex and full vee to the top flat grind. my designs are for work. i do admire many other styles but at the end of the day do they work? i belive it was Mr. fowler that describe the process by saying. look at a convex grind as a series of flat grinds. also the way i do convex grinds the final edge is on the very face of the blade. you can cut a full circle.
 
RHINOKNIVES

You know. It is kind of ironic. I in fact look at your site at one point and saw your woodsman bowie. I like how it looks myself and thought to myself that I hope my knife turned out as well. Mine is not a bowie shape but has that same kind of look". I will post a pic when finished. Thanks everyone.
 
Having a convex edge is a no-brainer, as far as I'm concerned. It's the toughest edge I know. It's been said that there are only 1 or 2 things a serrated knife can do that a well-sharpened plain-edged knife can't. I think the same applies to the convex edge. There are only a few applications that are better suited to other edges, while a convex edge tackles most all of what I do with my knives (exceptions being paring and filleting/skinning). It's better for woodcarving, box cutting, rope cutting, chopping, hard push-cuts, etc.

The difference is in the radius of your curve (think cross-section). If you start out with a thick piece of steel and push real hard while slack-belt grinding, you'll get a "fatter" edge (smaller radius) than if you flat grind first to a stubby edge and then slack belt the last 1/16" off.

I think some avoid the convex edge because of concerns about being able to properly sharpen and maintain it.

Sharpening and maintainence is terribly simple, just requires sandpaper and a soft-backed surface (a cushioned block, or even your hand). Restoring the finish just requires patience and the right types of sandpaper, and stropping compound.

I just finished making a CD with videos on how to sharpen this way. I took a 15" khukuri and blunted it by rubbing the edge on concrete and a metal rail. Even gave it a few dings. Within 20-30 minutes I had it back to shaving sharp using just sandpaper and a strop.

Here are a few samplers: (at low resolution - sorry)

Rubbing the knife on the concrete (did it a few more times off camera)

Scraping the rail

Proof of dullness test

Sharpening with a block 400 & 600 grit paper

Polishing by hand 1200 & 2000 grit paper, followed by a stropping by hand with green compound.

2x4 test Actually, the third time I filmed this. Had problems with the camera on the first 2 tries. You can see the paper from the first one on the chair (under the one I grabbed). No stropping or sharpening in between. ;)



Ok....I'm off my soapbox now. :D
 
You must log in or register to reply here.
Back
Top