Puukos

I remember one puukko that had lost over two thirds of blades width. It took 15 years daily use (by one that likes her puukko sharp) to wear it down. A friend used emery (I'm not sure of this word but it's one povergrinder type) to sharpen it. He spoiled the heat treat and that blade started to wear faster(it was repeatedly sharpened with that emery after that). It would probably have lasted easily 20+ years without mistreat.

 
Thanks for the info Tommi. About the fastest wear I have ever heard of was when Mel Sorg told me about his fathers blades. He used to use convex no edge bevel ground blades. As his work involved a lot of precise paper / carbdoard cutting, his blades were sharpened very often. Mel commented that he wore the blades out every 2-3 years.

-Cliff
 
She cut mostly vegetables (carrots, potatoes..). They are not hard for edge IMO so the dulling came form edge 'hitting' the wooden cutting board.
 
When you sharpen a puukko it doesn't get narrower any faster than sharpening a knife with a secondary bevel. The difference is the edge geometry stays constant so you can keep using a puukko without any loss of performance (or ease of sharpening) until it's worn down to a sliver. You can do that with other grinds too but only if you regrind the primary bevel periodically.



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-Cougar Allen :{)
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This post is not merely the author's opinions; it is the trrrrrruth. This post is intended to cause dissension and unrest and upset people, and ultimately drive them mad. Please do not misinterpret my intentions in posting this.
 
Assuming you grind along the same angle as the makers primary grind. The blade will be reduced in width at a rate inversely proportional to the angle of the grind times the rate at which it is reduced in thickness. That it to say it will get narrower much faster than it will get thinner. This translates to secondary beveled blades as well. You obviously don't lose thickness of blade stock here though, just width (with the same relation).

-Cliff
 
You're going to keep sharpening at that edge until the little nicks are gone and it's sharp again, and then you're going to stop. Regardless of what kind of grind that knife has, the width of the blade is going to be reduced to the depth of the nicks.

If it's a puukko grind and you keep the original angle, the cutting geometry stays exactly the same; the only change is the flat part of the blade gets narrowed. Eventually you'll find there's no more flat part left, but that'll take a long time and a lot of use as long as you don't get carried away and keep sharpening after it's already sharp.

If it's a conventional grind, the edge gets thicker every time you sharpen it. That happens faster with a saber grind than a full flat grind or a hollow grind, but it happens to 'em all. After a while you find that knife doesn't cut like it used to, not when you're cutting to any depth in a resistant material, anyway.

Then you have several options -- you can just put up with it, you can regrind the primary bevel and return the knife to like-new performance but with the flat part a little narrower (or the spine a little thinner, if it's a full flat grind and you maintain that geometry -- I don't unless I want the blade thinner and I'm not worried about strength), or you can add a third bevel between the primary and secondary bevels. That last is the easiest way short-term; all you have to do is sharpen at a very acute angle and stop before you get to the edge and then sharpen at your normal angle. Then you'll probably be able to sharpen it once or twice in the normal way, just the edge bevel, before the edge gets too thick again. You can get good performance that way; it's just that sharpening takes a little longer....

I like to keep a knfe very sharp at all times and yet I don't always want to undertake a major sharpening job when it starts getting a tiny bit duller than it could be. Sharpening a puukko like Tommi's grandfather did it works well for me -- I can give it a tiny secondary bevel in a matter of seconds and hardly notice any difference in performance, do that a number of times, and then when I feel like sharpening, when I have time and I'm in the mood, restore it to a full puukko grind with no secondary bevel -- even then I give it some convexity toward the edge when I strop to finish; that makes the edge significantly stronger without hurting performance to speak of.

I do something similar with saber ground blades -- I keep a secondary bevel on them at all times, but I grind at the primary bevel occasionally so the edge never gets too thick -- again, I can do it at my convenience; it's not the kind of thing you have to do right now; it won't make much difference whether I do it this time or next time I sharpen that knife or the time after, just so I get around to it eventually.

I believe the ideal geometry is a perfect parabola. It's inconvenient to regrind and repolish the entire blade all the way back to the spine every time you touch up the edge a little, but you can get a much closer approximation to a parabola than most knives without a lot of trouble. I believe the shape is more critical the closer you get to the edge -- even if all you do is strop after you've ground a flat edge bevel, that makes it significantly convex where it counts the most, right at the very edge -- especially if you use tripoli on your strop (that's a relatively coarse compound to use on a strop).

A lot of people seem to have the idea that convex edges are always blunt ... well, hardware store axes are blunt, edged weapons designed for use against armor were blunt.... When you do some experimenting and try to find how acute you can make an edge for whatever particular purposes you use that knife for, you'll find you can make a parabolic edge more acute without it nicking or rolling on you than any other shape -- and the closer you come to a parabola the more acute you can make it.

It seems to be easiest to grind an angle first and then round it to approximate a parabola. When you grind a puukko on a flat stone you can see the edge of the bevel and it's easy to get it right. Then strop on backed leather with tripoli, bearing down more toward the edge, and you get a subtle hyperbola -- you could round off the angle away from the edge, too, to make it closer to a parabola rather than a hyperbola, but it doesn't seem to matter much; the hyperbolic edge performs very well.



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-Cougar Allen :{)
--------------------------------------
This post is not merely the author's opinions; it is the trrrrrruth. This post is intended to cause dissension and unrest and upset people, and ultimately drive them mad. Please do not misinterpret my intentions in posting this.
 
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