convex edges

[Naf]

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!

[Ken S]

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

[Pietje]

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

[Erivan]

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

[cbwx34]

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....

[Ken S]

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

[tgbto]

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.

[cbwx34]

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.

[WimSpi]

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.

[tgbto]

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, which is a trove of useful information.

Cheers,

Nick.

[tgbto]

Damn me and my double clicking habits.

[WimSpi]

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