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
I'd say it has to be a thick knife if the convexity is to be noticeable.
By noticeable, I mean "having an effect on the cutting action". A x° apex on a convex edge will tend to have less metal "behind the edge" than the equivalent x° apex on a concave edge. The latter could hinder cutting deeply through dense material. So I'd say everything between a hunting/bushcraft knife and an axe.
I'm not sure why the "meat knife" they refer to in the video about the new knife jigs would be enhanced by the convex edge...
"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"
Ken, as you doubtless know, there are big "convex vs concave" (and "flat vs everything") debates on the knife forums. I have a convex bushcraft knife that I feel does all of its bushcraft tasks (including staying sharp) better for me than my scandi bushcraft knives (which I also love). But I would agree with tgbto that the advantages of convex edges--if the ones suggested in the debates are real--are likely to be most pronounced on thick blades, such as bushcraft knives and cleavers.
Though one of the other threads on the new KJ-45 knife jig queried whether one could get a smooth convex bevel on a Tormek, I think it should be quite doable, especially if one begins with a sufficiently coarse wheel. I note that the Japanese surgeon with the YouTube channel "virtuovice", who is one of the big champions of convex bevels (on hunting and bushcraft knives), does it by hand on waterstones. It is painful to watch how long it takes him, but he is a master at it. Here is a link to one of his videos, in which he propounds a theory in support of the superior toughness of a convex edge.
https://www.youtube.com/watch?v=XA5AM2Lb0iY&ab_channel=virtuovice
Gord
Good post, Gord.
Flat grinding was never an issue on the forum until diamond wheels. More recently, convex grinding has become an option with the new KJ knife jigs. The more I learn, the more questions pop up. Now I need to learn more about convexing bushcraft knives.................That's exciting; game on!
Ken
I think it will be difficult to get a nice convex edge with the new jig. It feels a bit "swaying" when you switch between the positions and difficult to move the knife in two directions at the same time and it becomes an, unwanted stop with the edge against the grinding wheel, which I find difficult to handle. As you can see on pic1-2, there is about 1.4 degrees (Slightly convex) difference between the two positions. It can work on a small edge, but on forest knives similar to F1 (https://fallkniven.se/en/home/) a medium convex or high convex is needed.
I think it is easier with a jig as in picture 31-33 which is more accurate for convex knives and where you have greater control at all angles. I will probably not grind convex with kj-45. It will be interesting to hear what you others think when you get the opportunity to test kj-45.
Correction:
cbwx34 helped me to understand one thing that i mist. Thanks cbwx34
I made a mistake when I wrote that it is 1,4 degrees convex.
I did not accounting for the change in the actual angle on the curved surface , just the angle change of the jig. There is probably neared 7 degrees in actual change. Sorry
Quote from: GKC on April 18, 2022, 03:36:52 AM
Here is a link to one of his videos, in which he propounds a theory in support of the superior toughness of a convex edge.
https://www.youtube.com/watch?v=XA5AM2Lb0iY&ab_channel=virtuovice
Gord
Well, that's an .... interesting theory. Mixing aerodynamics and structural physics in an innovative way, at least. Do you guys know of any research that would support the underlying assumption that for an equal apex angle, the convex edge is stronger (in terms of edge retention, I guess ?) than the straight edge ?
To be clear, and whatever the answer to the previous question, I find this theory highly suspicious. The fact that vegetable pressure crushing the bevel is the failure mode of the edge is IMHO cuckoo bananas. Unless we're talking hollow knives, but I don't think I have any...
This article does a good job explaining what I think the Japanese gentlemen in the video was talking about:
http://site.baryonyxknife.com/blog/2014/08/11/why-convex-edges-are-awesome-its-not-why-you-think/
The bottom line, according to the article, is that the advantage a convex edge of equivalent acuteness has is that it produces less drag when cutting, and thus requires less cutting effort. Makes sense to me, and it confirms some of my own observations.
Namely, I've noticed that knives I've free-hand sharpened (which have convex edges) sometimes seem to slice paper more cleanly than slab-sided edges from fixed-angle sharpening systems, even when the fixed-angle edge scores 20-30 points lower on a BESS tester. When I say more cleanly, I guess what I mean is that it's a quieter cut. I think that's because the convex edge doesn't have the sharp shoulder of the flat edge.
Ken,
Some chip carvers and axe users prefer this as they say it works better..
Rich
I totally understand what you say about the edge producing less drag and requiring less effort during the cut. That's not quite being "stronger" though, and the bevel is thinner...
As the article referenced by 3D Anvil says :
Quotelittle comparative testing has been done between two otherwise identical knives of equal edge angle with "V" and convex edges to provide substantive evidence for any sort of improvement in edge retention. If there is one it's simply that for equal amounts of force applied to each blade the thinner convex one will be able to cut deeper, and thus cut more material for equal energy expenditure, but the actual sharpness of the edge (or the thickness of the apex on the terminal "edge" bevel) will be affected about the same since it has the same edge angle and degree of wear resistance.
My experience is that there is a big difference between having a convex knife from the start compared to converting a straight knife to a convex one. A convex edge is stronger and more material on than on a straight edge as picture A shows. Picture B shows what happens when you sharpen a straight knife to convex. If you turn a convex edge into a straight edge, you also lose material just like turning a straight edge into a convex edge. It depends on what you start with.
On a convex original knife, the radius of the convex sphere is often smaller than if you turn a straight edge into a convex one because you cannot add material. The radius is drawn green in the image. The only way to reduce the radius is to move the tip so that the blade becomes less high.
Hi Perra,
It seems to me you're making the confusion that is referred to in 3DAnvil's posted link. And often made to support the fact that a convex edge is stronger. Both knives in your picture don't have the same apex angle. So yes, the knife with the higher angle will be "tougher" in that hitting it against a solid piece of wood will see it failing later.
... BUT :
- The point here is to compare two knives with the same apex angle, one being later convexed by thinning its shoulders (using the technique demonstrated with the KJ-45 or whichever other one)
- As a side note, Wootz's studies have also shown that apex angles higher than the optimal angle (ie that at which the edge stops failing from being too brittle/weak) actually result in lower edge retention.
Cheers,
Nick.
Hello,
I fully agree with tgbto:
Quote...compare two knives with the same apex angle...
I remember a similar discussion - What happens when sharpening a convex blade on a Tormek (if the angle is followed exactly) is shown here: https://forum.tormek.com/index.php?topic=4507
I recommend to draw the theory in CAD, then it is possible to simulate exactly what happens, and how the grinding method / grinding angle affects the result.
I'm not saying that there is no difference between convex and hollow grinds, but you need to compare comparables (same apex angle) and look at the variations that exist between these grinds. If you draw it, it doesn't look that bad, personally I would focus on the apex angle and blade thickness parameters. It doesn't make much sense to me to do a convex grind on thin blades or full flat / high flat blades, but let each person grind to their own...
jvh
Perra,
I understand and agree with your logic as you state here:
"On a convex original knife, the radius of the convex sphere is often smaller than if you turn a straight edge into a convex one because you cannot add material. The radius is drawn green in the image. The only way to reduce the radius is to move the tip so that the blade becomes less high."
My problem with it is where does one find convexed (or unsharpened) knives from the factory?
The practical question for me is, Do the practical benefits of convexing outweigh the extra steel needed to be ground off of a knife to convert a concave grind to a convex grind? The extra first sharpening would both slightly shorten the useful life of the knife and require more time. I suspect the answer to this question would start with, "It depends".
"Depends" means considerations like the intended use, future sharpening conditions, and the steel of the knife.
I can see possible benefits from convexing some knives.
Ken
Hi Ken
Here can you find convex knifes
https://fallkniven.se/en/home/
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
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
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
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/
Quote from: Ken S on April 10, 2022, 10:19:47 PM
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).
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]
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
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
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
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
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.
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
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.
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
Quote from: Ken S on May 19, 2022, 10:24:22 PM
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.
And for the sake of clarity, I agree good convex could probably be achieved by right person with either jig however they can do it best. However, I cannot. No way. That is why I must, at this time, stick with avoiding convex attempts on Tormek; rather, with something like TSProf K03. It not a Tormek bash AT ALL! It MY limitations!
Naf,
You are wise to concentrate on honing your present skills. I would offer the same advice to most of us, myself included. Learning to use the KJ automatically self centering feature will help all of us with very little learning frustration.
It will augment our SVM skills. Leave convexing to another day.
Ken
Hi Ken!
With the KJ-jigs it is possible to test it on knives.
My Zwilling Cleaver works very well with vegetables after putting on a 12° convex edge.
My Tojiro Santoku didn´t benefit of the convex. I had to grind the standart edge for 12°.
Pietje
Quote from: Ken S on April 10, 2022, 10:19:47 PM
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
This may sound as a question from a Newbie, and, well, let it be.
I have years of experience with German or French blades, but Orient is new to me.
I recently bought some Takamura Hamono VG-10 knives, pretty thin blades, which are sold as convex grind.
I use a T8, with Japanese stone, when required.
Will it make any difference, for the user, when I have to re-sharpen them ?
Thanks in advance.
EriVan
Quote from: Erivan on September 27, 2022, 04:19:50 PM
This may sound as a question from a Newbie, and, well, let it be.
I have years of experience with German or French blades, but Orient is new to me.
I recently bought some Takamura Hamono VG-10 knives, pretty thin blades, which are sold as convex grind.
I use a T8, with Japanese stone, when required.
Will it make any difference, for the user, when I have to re-sharpen them ?
Thanks in advance.
EriVan
Not sure if this will help... it's the only video I can think of for sharpening Japanese knives...
https://www.youtube.com/watch?v=dhpajIaBW0c
... this would make a great topic for a Tormek "Advanced Sharpening" video.... ;)
Good comments, CB! Sebastien and Wolfgang have announced their intention to do a second, more advanced online class with the new KJ-45 knife jig. Single bevel knives and convex edges might combine well with the new knife jig to fill class time.
Thanks for posting the link to the KG you tube. I have studied many of Wootz' (Vadim of Knife Grinders) you tubes. I must have glossed over or missed this one, probably because I don't own any Japanese knives. Wootz' use of the SJ wheel to polish just the very edge of the back is very clever. It reminds me of the David Charlesworth Ruler Trick to polish just the edge of plane backs. I believe Wootz' idea would also work well with plane blade backs.
There is always more to learn.
Ken
I don't think what is seen in the video is the kind of japanese knife @Erivan was referring to, as they were described as "pretty thin blades".
If we're talking gyutos/sujis and the like, the blade is so thin that convexity does not matter at all, those have edges that usually are but a fraction of a millimeter high. So the Tormek is a fine tool to sharpen those.
The convexing on japanese blades often happens because the sharpening process (or last part thereof) is done on a sharpening stone with a movement that naturally induces edge convexity. <edit> They are also sometimes ground on belt grinders which naturally convexes edges, but they can also use rotating stones for a perfectly flat initial grind </edit>
What is shown in the video is a different matter entirely, as Wootz ground a concave (granted, still slightly) edge on a *very thick* blade. The result is sharp BUT :
- It is done at a higher angle than the one intended for the blade OR it requires prior thinning of the blade along the entire edge (think 8" *wide* chisel... quite lengthy). And I don't wanna think what such thinning would do in the tip area, which is initially ground at a higher angle than the rest of the blade, with a very subtle blending into the main part of the blade.
- It leaves a terrible look on the blade, the newly ground area scarring the lower part of the edge.
- I would strongly advise against sharpening the backside on the Tormek, as it is designed to be sharpened on a flat benchstone with the back of the knife flat against the stone, and both spine and edge being sharpened at the same time. Sharpening only along the edge on the backside will mess up the geometry of the blade, most importantly on the tip. Even on a fine-grained stone, as the backside of the tip will be unsupported during sharpening.
To me this is a big "NO WAY". Traditional japanese knives (deba, yanagiba, usuba) must be sharpened on a stone, either by the owner who's willing to invest time in developing the technique and feel for this, or by a professional sharpener.
Quote from: tgbto on September 28, 2022, 09:08:58 AM
I don't think what is seen in the video is the kind of japanese knife @Erivan was referring to, as they were described as "pretty thin blades".
If we're talking gyutos/sujis and the like, the blade is so thin that convexity does not matter at all, those have edges that usually are but a fraction of a millimeter high. So the Tormek is a fine tool to sharpen those.
The convexing on japanese blades often happens because the sharpening process (or last part thereof) is done on a sharpening stone with a movement that naturally induces edge convexity. <edit> They are also sometimes ground on belt grinders which naturally convexes edges, but they can also use rotating stones for a perfectly flat initial grind </edit>
What is shown in the video is a different matter entirely, as Wootz ground a concave (granted, still slightly) edge on a *very thick* blade. The result is sharp BUT :
- It is done at a higher angle than the one intended for the blade OR it requires prior thinning of the blade along the entire edge (think 8" *wide* chisel... quite lengthy). And I don't wanna think what such thinning would do in the tip area, which is initially ground at a higher angle than the rest of the blade, with a very subtle blending into the main part of the blade.
- It leaves a terrible look on the blade, the newly ground area scarring the lower part of the edge.
- I would strongly advise against sharpening the backside on the Tormek, as it is designed to be sharpened on a flat benchstone with the back of the knife flat against the stone, and both spine and edge being sharpened at the same time. Sharpening only along the edge on the backside will mess up the geometry of the blade, most importantly on the tip. Even on a fine-grained stone, as the backside of the tip will be unsupported during sharpening.
To me this is a big "NO WAY". Traditional japanese knives (deba, yanagiba, usuba) must be sharpened on a stone, either by the owner who's willing to invest time in developing the technique and feel for this, or by a professional sharpener.
Good post... thanks for breaking it down and writing about the differences.
Quote from: GKC on April 18, 2022, 03:36:52 AM
Here is a link to one of his videos, in which he propounds a theory in support of the superior toughness of a convex edge.
https://www.youtube.com/watch?v=XA5AM2Lb0iY&ab_channel=virtuovice
When I see the decomposition of forces at a convex ground blade, it reminds me of masonry pointed arches of churches.
In itself, there is some truth in that, and certainly an interesting thought. But whether that is substantially noticeable in practice, I wonder. To my senses, it seems to me that a convex ground blade is more likely to "slip" over the material being cut.
But I say that as a woodworker, where you can't use convex-sharpened chisels very well.
I do say that I have no experience with this type of knife.
Quote from: WimSpi on September 28, 2022, 08:33:14 PM
When I see the decomposition of forces at a convex ground blade, it reminds me of masonry pointed arches of churches.
Well, as mentioned higher up in this post that would certainly be a worthy analogy IF the blades were hollow AND the failure mode of the edge was some kind of a structural collapse. As demonstrated on numerous occasions, the edge dulls either because of abrasion or because it gets bent on one side, which is not really what happens to gothic cathedrals. Wootz demonstrated that the bending/crumbling of the edge was at the first order caused by too acute an apex angle. The cathedrals architects used the pointed arch shape to be able to counter the tendency of round arches to fall on worshippers, so the more acute the sturdier...
As for slipping on the material, I'm not sure that's really a risk given that cooking knives are seldom used to carve thin slices out of a wood-hard material. Though it should be noted that the instrinsic geometry of traditional (slightly convex) japanese blades will tend to drive the knife into the material being cut (because of the huge edge assymetry, and convexity on the off-side helps). This is used in reverse with usubas when performing katsuramuki, where you peel a very long, very thin strip of daikon radish. I think concave grinding of such wide blades may have an impact on the behavior of the blade, and not a desirable one at that.
I would personally be happy to see a comparison of edge retention between knives with concave/convex edges with the same apex angle.
For those of you with an interest in SEM, a member of this forum mentioned the Science of Sharp blog1 (https://scienceofsharp.com/home/), which is a trove of useful information.
Cheers,
Nick.
Damn me and my double clicking habits.
Thank you for this instructive response Nick!.
My comparison also fails, because a knife is not hollow, as you explained very clearly.
I am also going to study your link
Wim