convex edges

[Ken S]

Hi, Perra.

Very nice knives. I looked on the link you provided to the fallkniven website. The one knife was selling for 3750 SEK, or $362 US Dollars. I am interested in this quality of knife; however, for me, the interest is only academic. My most recent knife purchases have been Victorinox kitchen knives or Buck pocket knives, in the $20 to $60 US price range.
These knives are generally thinner. I don't know if convexing them would be practical. I also don't know anyone who would have fallkniven quality knives.

Admittedly, I am not a knife enthusiast. My favorite film camera was a Leica M3, so I can understand how some people prize fine knives.


i hope to learn more about convexing. It only seemed like a possibility with the introduction of the new .kJ jigs.

Ken

[Perra]

Hi Ken
Glad you liked them. These are fine quality knives that are also manufactured for the Swedish army, air force and Swedish special forces. They are usually thicker than, for example, fine kitchen knives and therefore convex edges work very well. The same width as the ones we call forest knives.
And it is also true that convex knives have a larger tip angle than, for example, kitchen knives. It is part of the function. I never grind convex edges on thin knives but see an advantage in having it on thicker blades.

As you can see in my previous pictures I have built my own grinding jig where I can grind convex edges with controlled angles and radius.

Should be interesting to test kj-45 but I see a problem that I think can be difficult to solve and that is to copy or follow an existing convex edge. This is usually what I do when I sharpen convex knives. But we'll see if it works.

I also take photos myself but with a Nikon. Not as nice as with a Leica

[GKC]

As Ken posits, the value proposition (time/steel used vs. utility of knife) of a convex grind depends on the intended use. The fellow whose YouTube video I linked earlier (Virtuovice) has a number of quite carefully documented tests of different convex and scandi grinds specifically for bushcraft uses such as featherstick-making, splitting kindling ("batonning") and cleaning game.

For example, in making a featherstick to start a campfire, he demonstrates how the concave edge is superior (in his view) given the rotation of the wrist as it moves down the stick. He does time trials and examines the quality of the result. (He is somewhat obsessive about it, but he seems to know that.) Also, he field-butchers the deer he hunts (in Japan) and is quite particular about the specific convex grinds on his various hunting knives, which he finds better because they don't nick the bones as much (he is a surgeon in his day job, so I accept his views on this).

One of the features of a convex grind that is said to be helpful for splitting and carving tasks is that the thickness behind the apex pushes the material away which (a) aids in the task at hand, which involves physically separating the material and (b) keeps the apex away from the material for the completion of the cut, which stops the apex from dulling unnecessarily. Obviously, this is not what you want for many blade uses, but it is said to be an advantage for those particular uses.

DLT Trading (Bark River Knives) is one of the champions of the convex grind. Owner Mike Stewart is pretty adamant about the advantages of the convex grind. If anyone wants to hear his case, here is a link to one of his videos. Knife makers typically use a slack belt to make a convex edge, but in this video Mike Stewart notes that it was originally done on large grinding wheels by rocking the blade back and forth:

https://www.youtube.com/watch?v=lkV32RmQOXA&ab_channel=DLTTrading%2CLLC.

Gord

[3D Anvil]

Another big proponent of convex grinds is Rockstead -- the Japanese maker of absolutely stunning, and stunningly expensive fixed blade and folding knives.  Many of their knives feature a zero-edge, full convex grinds.  They mostly do mirror polished ZDP-189 hardened to 67 hrc.  I've never handled one, but owners say that the knives come extremely sharp, and they hold their edges like nothing else.  Which is a good thing, because you pretty much have to ship them back to Japan to get them resharpened properly.

https://www.rockstead.jp/

[Naf]

On a practical basis, how many knives would really benefit from having a convexed edge compared with the shallow concave standard edge of a Tormek wheel?

Ken

Since this entire topic really resonated with me, I gonna come at this one from a different angle than many of you, no pun intended. My answer to KenS original question is: Many knives would benefit from that, but not all of them.  (Clearly, caveats apply.)

I not able debate the semantics to the degree some of you are able to yet, in this particular arena, but I've never much feared any arena enough (obviously by now), and over the years I've held my own in many.  I agree with quite a number of things said by many of you (dont recall seeing one I disagreed with, acutally) in this topic, yet I compelled to add the following. (Many people read here to learn and won't post; thank you to all who eventually find the courage to post.)

Having been handed an axe and file in front of pile of saw-cut wood before I was 10, I learned what I didn't fully understnad (or certainly couldn't yet verbalize at least) for maybe twenty years.  Convex edges are much better for splitting than chopping.  Don't trust me... if you haven't yet, try it; just be careful with the convex axe / chopping part especially if you coming down on it and rember keep the head out of way. (As far as slicing, I haven't tested that on BESS tester yet, but hope to get to that someday.)  I realize KenS questions was about knives but the same principle applies. It sure seems to me it actually is really that simple. I mean, I really learn lots from all you just reading here (you welcome for all times I decide stay stilent), but I've also learned lots from taking things out with me and using (not that you havn't) "other than the manufacture specify" over the years.  So for me, personally, the answer above seems obvious;  however, the less obvious answer is the next question:  On a practical basis, how many PEOPLE would really benefit from having a convexed edge KNIFE WITH THEM compared with the shallow concave standard edge FROM a Tormek wheel? 

(Clearly myriad possible scenarios exist and we not debate them all; this is of course a Tormek forum about sharpening thus all other answers to this topic already I NOT attempting compete with.) Roughly thirty years ago, I preferred putting convex edges on any thicker knives (which I guess would call 3/16" and up, these days) due to edge durability, but I had much more strength then. Fast forward, now I prefer to have a concave edge on larger knives than I used to. (In most of my own outdoor usage scenarios, I find that I chop, or cut cross-grain, much more often than split, or cut with grain; I know I'm only talking wood here, but if you want cut your potatoes and onions with a 5/16" convex edge knife, it's your knife, your food, your fingers, so I really not care what you want do at end of day. Matter of fact, I have a footlong+ one I might lend you if I can watch. Ping me.)

If I gonna have at least a hand axe with me, it probably weighs more, at least 25d minimum (for flat ones, others 30/35d depending), and highly likely convex than it's counterpart knife that might be nearly its equal, but might also be concave (or V of course) even at 20d; I can swing it easier/faster than axe and it can cut much more deeply with same energy and much less fatigue even with less weight since concave, but still got axe for bigger jobs. (But I'm an outdoor knife/axe enthusiast who can easilt fix the tools I can manage damage.  If you can't fix as easily, or don't want to as often, you might really prefer convex edges.)  If I taking just a knife, then it probably 25d, probably 1/4" thick, or more, and definitely doesn't have a concave edge; it either V or more likely convex, depending (there KenS word he was expecting) on the knife length/weight, since its increased speed and own weight sure seem contribute to its own damage as well as to what it hits. But I routinely grab something different from the last time out, aside from my favorite bag/belt/boot knives which are much more acute angles... and I'm very lucky to have the choices.  But there it is... I have repeatedly proven to myself over many hears I much prefer always having the choice of using a convex edge, or a V, or concave edge. I guess I think of it like a sledgehammer... when you need one and don't have one, well good luck. The larger you go the more possibilities become practical, including convex edges, especially at 25d or more, and it certainlyu more than worth it, IMHO. KenS, I skip the part about plans to transform sledgehammers with my old SG wheels in future. Wrapping up...

Specifically regarding doing convex edges with Tormek, that I'll leave to you and I comeback this in future; sticking for now with my other tried and true sharpening tools... not saying the capability isn't there... in case you can't tell from my other posts... so far I've been quite impressed with most of my Tormek gear... and, for some sharpening, so far, I'll avoid it.  As I said when I started  posting, I'm experienced with sharpening, but completely new to Tormek, as of roughly several months ago, so I'm still learning a LOT.  Thank you all for helping me with that!  Since you're discussing convex knives, I hope I raised more questions than answers, so you don't simply dismiss it (to encourage your further investigation of its usefulness).

[Naf]

Dammit. Anybody know why some my posts end up double-spaced with line breaks between paragraphs removed?  Sorry!

There, I fix it. I would have never guessed that what the backspace key for.  Thank you. Clearly I was more interested in why it continues to happen to me here (only ever in this forum[size=78%]) than how fix each message after the fact.  Seems to only happen when I type a response within Notepad, then copy/paste the plain text into this forum's message box, attempting to avoid other oddities that occur when simply entering text there.  I'm sure no one else ever notice anything odd there. [/size]

[Ken S]

Naf,
You can go back to your reply; click on "modify"; and put the cursor before the line you want to bring up. Press your backspace key. One line at a time, this should clean up your post.

My thickest knife is my Henckels eight inch chef's knife. It's hardly thick, compared with outdoor knives. Therefore, I have no knives which require convexing; I probably won't have actual experience, only an enthusiastic academic interest. Therefore, I will remain on the sidelines as an observer in future conversations about convexing.

Ken

[GKC]

Ken, if your query is about when it might be sensible to convert (regrind) a flat or hollow ground knife to convex, I can't think of it ever being worth it for me. There are excellent suppliers of convex ground knives (e.g. Bark River) and few of us are going to get close to the near-flawless convex grinds that they put on their knives at the factory. If we do, it will be on a slack belt, not a wheel. (Convex-ground knives do tend to be on the expensive end of the scale, and someone on a budget might convert a hefty-but-economical Mora (scandi grind) to convex to avoid the expense of a pricey Bark River or Fallkniven. I would just keep the scandi grind, but if I wanted an inexpensive convex, that is what I would do, and I would do it on a belt.)

With that said, some knife people like a convex micro bevel on whatever grind they have, and the KJ-45 might be able to do that without too much work, I am keeping an open mind until I get the jig. I expect to use both the KJs and SVMs unless I find one to be superior in all applications, which would surprise me.

Gord

[Ken S]

Good post, Gord.

I agree with you. I suspect that after the initial curiosity, most knives we sharpen convex probably started out that way, at least for the kitchen knife and pocket knife crowd like me. This topic has opened my mind to new possibilities for convexing. There is always something to learn.

Regardless of how much convexing we do, the automatic self centering will prove useful. Like you, I think I will probably use whichever jig seems most useful for the task at hand.

Ken

[3D Anvil]

I think the best way to get a convex edge is to freehand sharpen on stones.  Unless you use some kind of angle guide, you will naturally get a convex edge with freehand sharpening even if you aren't trying to ... because no one can hold a perfect angle.  You can be intentionally even looser if you want a deeper convex. 

For an even more convex edge, people use sandpaper on a soft substrate like foam or leather.  For example: https://www.youtube.com/watch?v=NYu8wpV1JM8

[tgbto]

I think a slack belt is a best way as it is its intended purpose, as opposed to the byproduct of the intrinsic limits of the technique used.

[Ken S]

Tgb,

I agree that using a slack belt seems a very logical way to do convex grinding. I happen to have two belt grinders, a Viel and a Kalamazoo, both equipped with Penn State Industries variable speed motors. They are very useful complements to the Tormek. Following Steve Bottorff's advice, I would reach for one of these first for things like convexing an ax, trimming a bolster, or straightening out an indented curve. However, I think the KJ-45 deserves a faur chance before we decide that it won't work for convexing. (If I did not happen to have a belt grinder, the KJ-45 would definitely be my choice!)

Ken

[3D Anvil]

The problem with convexing a knife--as opposed to an axe--on a belt grinder, is heat.  You are laying a lot of metal on that belt, so there's a pretty high risk of de-tempering the edge.  It can be done, but you have to be very careful.  Good idea to dunk the blade in cold water between passes.

[Ken S]

I agree. That is why I use the variable speed motors. Also, I switched to the smaller Viel drive pulley. The smaller circumference drive pulley cuts the speed by 1/3 in addition to the speed reduction with the variable speed motor.
(The Kalamazoo comes with the smaller drive pulley.)

Ken

[tgbto]

However, I think the KJ-45 deserves a faur chance before we decide that it won't work for convexing.

I was referring to the previous post stating that the best way to convex was freehand on a benchstone. I did not say the KJ-45 could not convex. Will it be "the best way to convex" though?

In any case, I think that someone willing to convex a not-too-thick knife the KJ-45 way could do it very well by sharpening with the older jig : doing a few passes with reduced projection distances by turning the handle by, say, half a turn each time.