"Concave" edge grinding

W

Wiredude

Well-Known Member
So as I often do, I get lots and lots of ideas all the time, and usually they don't make it a whole lot further than maybe a scribble on a sheet of paper, but anyhow...
I'm tossing around the idea of making a trainer karambit, just because I can, and it seems like it could be a fun toy to boot.
But that got me to thinking. I'm guessing grinding a (hooked/concave/whatever-you-wanna-call-it) edge presents at least somewhat of a challenge.
I would think a karambit might be a somewhat extreeme example, but it could also apply to a kukri, or a swedge/sharpened clip on a clip point blade.
Would anyone care to jump in and comment/share experiences on doing this? Just thinking, and really, way ahead of myself, but still...
 
Hi,

Grinding a concave edge is challenging. I don't believe it is necessary though. It would be easier to change the basic geometry of the knife. The reason I say this is; a concave edge by itself is a weak configuration. The apex itself is what does the cutting, the area just above the apex is where resistance occurs. You would think with a concave edge grind the resistance would be lessened, but that is not so unless the material being cut is very thin. If a blade with a concave edge is used to cut leather, for instance, the shoulders at the top of the concave radius would drag and lesson the effectiveness of the cutting action.
I like the way you are approaching this. Every knife should be designed from the edge backwards. Ask yourself @ the onset of building a blade; "how do you want the edge to cut" "what do you want the edge to cut" "how strong does the edge need to be" "what shape should the blade be in order to perform the appointed task and what choice of steel is best.

This approach to knife design results in a blade that cuts well, is strong relative to the task and has an edge that is easy to sharpen, relative to the work it performs.

I grind mostly "flat" edges because with the products I make and use, they are easily produced and maintained. I put convex edges on competition knives and camp knives because both of these knives require an edge that will not "wedge" itself into the material being "chopped" Both camp knives, machetes and competition knives support this type of edge.

All other blades I grind a "flat" edge on the knife. Kitchen knives and hunters, fillet knives and whittlers all get flat edges.

I have found that convex edges cannot be reproduced or maintained at the same geometry with any certainty. Flat edges on the other hand can be reproduced and maintained with certainty by any one with basic sharpening skills.

Please don't let my comments restrain you from building a knife with a concave edge. I have done so myself, its good experience and only adds to your skills as a knife maker.

Regards, Fred Rowe
 
Are you talking about a recurve or an arc like a karambit? Most grind on the edge of the flat platen or the contact wheel . You can also grind it perpendicular on a contact wheel. I do this for some karambits. It gets easier the more you do it but it can be a pain when first trying it.
 
Thank you for the replies, both of you.
@Fred Rowe, not entirely sure from your response, but it sounds to me like you're refering to the hollow grind vs. flat grind vs. convex grind debate. That wasn't actually what I was refering to, though I appreciate the thoughts regardless, more food for thought is a good thing. I was more refering to a edge that is "concave" when the knife is viewed in profile. Specifically more extreeme examples as in many Karambit designs.

@ J. Rosa - Yes, exactly as in a karambit. I was kind of thinking it was also called a recurve edge, but wasn't totally sure. I'm guessing though, that just how difficult the bevel is to grind increases as the recurve arc gets tighter.

Thanks again
Steve
 
Wiredude said:
Thank you for the replies, both of you.
@Fred Rowe, not entirely sure from your response, but it sounds to me like you're refering to the hollow grind vs. flat grind vs. convex grind debate. That wasn't actually what I was refering to, though I appreciate the thoughts regardless, more food for thought is a good thing. I was more refering to a edge that is "concave" when the knife is viewed in profile. Specifically more extreeme examples as in many Karambit designs.

@ J. Rosa - Yes, exactly as in a karambit. I was kind of thinking it was also called a recurve edge, but wasn't totally sure. I'm guessing though, that just how difficult the bevel is to grind increases as the recurve arc gets tighter.

Thanks again
Steve
Click to expand...

If we are talking about an actual cutting edge being ground on a wheel giving it a concave shape, then that is what my comments were referring to.

It is a weak design for an edge, that can be improved upon by changing the geometry of the blade overall.

Fred
 
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