What is thin behind the edge, anyway? and Kujira

[GorillaGrunt]

I think I remember seeing a thread on this before, but I’m wondering about it again. How do you all decide whether to call a knife thin behind the edge or not? Some are obviously so, like the lasers e.g. Shibata, and some are obviously not, like the German knives e.g. Wusthof. I’d say Mazaki are, and the Sukenari I’ve used are not. I judge based on how it feels and sounds going through onions, carrots, and sweet potatoes or yuca root if I have the latter two, and the height of the edge bevel (standardized 15* angle if I’m comparing or thinning).

But then there’s some ringers: the edge bevels on my Ikeda honyaki and Martell look like they’re not super thin behind the edge, but sure cut like they are; my Yukimitsu, no matter how much I thin it so far, looks like it barely has a microbevel but acts like it’s much thicker. Other features of the grind/blade geometry as a whole?

Finally, I got a Masakage Kujira from the recent batch. Thought at first it was like a good Takeda, turned out to be more like a bad Takeda. I always forget to take before shots, but the choil view looked like one I’ve seen of a Hiromune Takaba — good for a sword but not what I need in a kitchen. I decided I’d keep it as an expensive project knife and eventually learn how to refinish it too. Here are side and choil views of where I’m at. It’s quiet through carrots, doesn’t resist in Brussels sprouts and I’m happy with it, which means it probably needs one or two more sessions: what does the crowd think?

 

[dgib7994]

That certainly looks better than the choil shots I saw of that knife on Knifewear.

IMO the best way to test for edge thinness is to roll it over your thumbnail. You will see the steel deflect in accordance w/ the radius of your thumbnail and return to true. Since "Stiffness" (how hard it is to bend the steel) is dependent on cross-sectional geometry, only knives that are very thin at the edge will pass this test.

Other features of a blade's geometry can influence wedging and performance even with a thin edge, most notably the shoulders of a wide bevel or a thick spine.

My $0.02

 

[dgib7994]

For your reference, here are 3 knives of mine that I consider thin and all pass the edge deflection test.

(in order: 1. Yoshimitsu Fugen white #1 tall nakiri, 2. Kagekiyo White #2 K-tip gyuto, 3. Gesshin Ginga white #2 gyuto)

 

[GorillaGrunt]

Is that Yoshimitsu S ground or forged geometry S, or just looks that way in the light? Are the other knives in that line like that too?

Thanks for the responses, please keep em coming

 

[Corradobrit1]

Thin behind the edge

 

[Kippington]

IMO the best way to test for edge thinness is to roll it over your thumbnail. You will see the steel deflect in accordance w/ the radius of your thumbnail and return to true. Since "Stiffness" (how hard it is to bend the steel) is dependent on cross-sectional geometry, only knives that are very thin at the edge will pass this test.

This is spot on. It's known as the "brass rod test" on non-kitchen (read: thicker) knives, but on truly thin knives you can do it with a fingernail quite easily.

You don't really want this on high hardness/low toughness steels though, as it starts to push the practical edge holding ability of the knife.

 

[Carl Kotte]

That’s thinitude!

 

[RDalman]

"You don't really want this on high hardness/low toughness steels though, as it starts to push the practical edge holding ability of theknife."

This depends on the heat treatment and user, but for me I disagree on that. And thickening up a edge is much much easier/quicker than thinning one.

 

[Kippington]

I mean, someone basicly asked me for a low-tempered laser. 65 HRC @ 150 degrees C, high carbon steel.
I dunno about you Robin, but I won't do it. Same goes for uber thin edges on 66-67 HRC ZDP-189. It's asking for trouble, IMO.

I'm happy to do it at 64 HRC and under though.

 

[RDalman]

I mean, someone basicly asked me for a low-tempered laser. 65 HRC @ 150 degrees C, high carbon steel.
I dunno about you Robin, but I won't do it. Same goes for uber thin edges on 66-67 HRC ZDP-189. It's asking for trouble, IMO.

I'm happy to do it at 64 HRC and under though.

Depends alot on austemp control and tempering also I think. Aged aeb-l at 65 is not bad in this regard. 1.2562 is brittle, but I have done it hard and superthin and those guys have not had big chipping troubles.

 

[Benuser]

A rule of thumb for a decent cutter, no laser: 0.2mm above the bevel, 0.5mm at 5mm from there, 1mm at 1cm.
Very fast checking by setting a micrometer at 1 or 0.5mm and see where the blade glides through it.

 

[Dan P.]

When thinness goes wrong:
https://www.instagram.com/p/BpFRx9xBUVa/?igshid=mwbm6zhmckey

 

[Nemo]

When thinness goes wrong:

Now, that's what I call a foil edge!