A q. about convex grinds

[cotedupy]

This is some copy taken from a knifemaker regarding a 150mm double-bevel Deba, 6mm at the spine:

'The small but weighty blade features a thick spine and convex grind to give it the strength needed for tasks such as gutting and filleting fish, cutting through shellfish, jointing chickens and other butchery tasks. Despite their strong convex grinds X has ground these knives super thin behind the edge to make resharpening incredibly easy.'

I know that a convex ground blade can be very thin behind the edge, but on a reasonably chunky knife like this? And would it not somewhat negate the point of a knife suitable for butchery?

I know f all about the intricacies of grinding, so interested to hear what others thought...

 

[Matus]

Convex grind gives you the most robustness at the cutting edge - are you sure you are not mixing up convex and concave (hollow) grinds?

 

[KenHash]

Wondering if this blade has a convex grind and a beveled edge, not a convex edge.

 

[cotedupy]

Convex grind gives you the most robustness at the cutting edge - are you sure you are not mixing up convex and concave (hollow) grinds?

I was just kind've struggling to picture a strong convex grind on quite a thick small knife, that was also very thin right behind the edge. And then wondering if that would slightly obviate the point of having that convexity (?)

(As I say - I don't know much about this kind of thing!)

 

[RDalman]

I think what they're trying to describe is that the knives don’t have a chunky micro. What is thin in context gets a bit "how long is a string"?

 

[cotedupy]

I think what they're trying to describe is that the knives don’t have a chunky micro. What is thin in context gets a bit "how long is a string"?

Ta. I imagine you're right, and I was perhaps slightly over-thinking what they meant by it.

 

[bbrooks008]

Or they could just be making that part up (pretending that they aren't sacrificing thinness on a thick knife with a "strong convex grind") so that you will buy the knife

 

[Jason183]

I was just kind've struggling to picture a strong convex grind on quite a thick small knife, that was also very thin right behind the edge. And then wondering if that would slightly obviate the point of having that convexity (?)

(As I say - I don't know much about this kind of thing!)

IDK what that deba grind looks liked but What you described reminds me of Kato grind, strong convex but still super thin at the edge

Here’s an example of strong convexity with thick edge( Tojiro western Deba)

 

[cotedupy]

Or they could just be making that part up (pretending that they aren't sacrificing thinness on a thick knife with a "strong convex grind") so that you will buy the knife

Haha! Possibly. Though they are a very good maker, I have a few myself, so I'll give them the benefit of the doubt. I know the 'X' in question so I could've really asked him... if he ever answered texts or emails.

 

[cotedupy]

IDK what that deba grind looks liked but What you described reminds me of Kato grind, strong convex but still super thin at the edge View attachment 125904
Here’s an example of strong convexity with thick edge( Tojiro western Deba)
View attachment 125905

Ah thank you for the pics, they actually show why I was a little confused... as I would've thought if you're going to be using a double-bevel deba for going through fish and chicken bones you might not want it super thin at the edge (?). And instead more like your 2nd pic.

Though as RD said above it's probably all relative anyway, and I was thinking a bit too much about it.

 

[Jason183]

Ah thank you for the pics, they actually show why I was a little confused... as I would've thought if you're going to be using a double-bevel deba for going through fish and chicken bones you might not want it super thin at the edge (?). And instead more like your 2nd pic.

Though as RD said above it's probably all relative anyway, and I was thinking a bit too much about it.

Yes, the 2nd geometry will be more robust, but will not be easy to sharpen and thinning once it gets dull, I had Tojiro western Deba before, it’s really painful to sharpen and thinning after few used(VG10), steels also matters, if it’s AEB-L, it would of be much easier to sharpen.

 

[ian]

IDK what that deba grind looks liked but What you described reminds me of Kato grind, strong convex but still super thin at the edge View attachment 125904
Here’s an example of strong convexity with thick edge( Tojiro western Deba)
View attachment 125905

I don't know how one can call the first choil shot a "strong convex". The left side (in the picture) is basically flat, maybe with a little curvature. The right side has some convexity near the edge and maybe a little concavity further down. Idk. Maybe the Kato has a nice convex grind away from the heel, but I don't see it being very visible in the picture.

 

[Jason183]

I don't know how one can call the first choil shot a "strong convex". The left side (in the picture) is basically flat, maybe with a little curvature. The right side has some convexity near the edge and maybe a little concavity further down. Idk. Maybe the Kato has a nice convex grind away from the heel, but I don't see it being very visible in the picture.

View attachment 125932

It’s hard to see from the picture, since it’s only half of the geometry shown, from what I’ve remember, Kato features full convexity grind from spine to cutting edge, starts with robust spine and gets extremely thin behind the edge, Kato might be a bad example for this, since the distal taper changes along the spine, but what OP describes reminds me something similar like Kato grind would able to achieve this.

maby this is better example than Kato based on choil shot, Kaeru WH

 

[Carl Kotte]

I think what they're trying to describe is that the knives don’t have a chunky micro. What is thin in context gets a bit "how long is a string"?

17!

 

[ian]

It’s hard to see from the picture, since it’s only half of the geometry shown, from what I’ve remember, Kato features full convexity grind from spine to cutting edge, starts with robust spine and gets extremely thin behind the edge, Kato might be a bad example for this, since the distal taper changes along the way, but what OP describes reminds me something similar like Kato grind would able to achieve this.
View attachment 125943

Yea, most people seem to describe Kato as having a spine to edge convex but being thin bte, exactly like you say. I've never held one; was just saying that the choil shot doesn't illustrate that.

It's funny, we often think of strongly convex knives as fat workhorses. One advantage of a knife with a thin spine is that you can actually pack more convexity close to the edge than you can with a fat spined knife, without having it be too thick bte. I feel like you can see the convexity in this Ginga choil in the top third of the picture. Often most of the blade face is kinda flat, though. If you have a really thick knife and you want it to be convex ground and thin bte, it seems like you've got to make the grind reasonably flat near the edge, but then you can distribute the convexity more on the rest of the blade face. Maybe I'm misunderstanding something though.

 

[stringer]

I haven't held a Kato. But, in my opinion, the best performing grinds use asymmetrical grinds to achieve something that has a lot of convexity but is also really thin behind the edge. The thickness from the middle of the knife up to the spine doesn't really matter as much. That is why that knives can be very thick at the spine but still thin behind the edge and still convex. Here's a few choil shots I will try and show what I mean.

The knives I really like usually have geometry that looks like this

This is for right handed. If you are looking at the choil, gently accelerating convexity on the left side spine to edge. Super flat or slightly concave on the opposite side ground high up the knife. If my knives don't start with this geometry then they will eventually get it because this is how I sharpen them. I think that this gets you something that is thin behind the edge and also convexed some for strength and durability (especially when combined with a microbevel at the apex).

Watanabe is gentle convex on one side and gentle concave on the other to result in very thin behind the edge

Shi.han grind is similar. Gentle convex on one side, extremely flat on the other, not quite as thin behind the edge as watanabe. But give me a few more sharpening sessions and it will give it a run for its money.

My Tosa Nakiri takes a different approach. Kind of an inverted coke bottle shaped figure from the factory. Thick at the spine. Concave spine to middle where there's a slight thickening and then concave again from the middle down (slightly concave bevels). Extremely thin behind the edge. Too thin. This isn't stock, I have flattened and started to convex the bevels to strengthen them. But it does show how you can get some nice convexity out of a fairly thin knife.

And lastly. Here is a forgecraft 10" that I have put a lot of work into. This knife is less than 2mm thick at the spine. Still plenty of room for convexity on the food facing side.

 

[ian]

Yea, asymmetry for the win, for sure.

 

[McMan]

Here's a question...
Does anyone add convexity by thinning from the spine down to the mid-blade?
I've never tried it. But it seems like it could be a good idea in certain (extreme) cases.

 

[Boynutman]

@stringer That is a lot of background and informative examples! But it has me confused. I always figured that the carrot facing side (for me lh side as I am a righty) could be flat and the 'slice' facing side should be convexed/beveled/... to aid in food release.
Looking at your pics and reading your a righty, it looks like I have it mixed up?
Am I missing something here?

 

[stringer]

@stringer That is a lot of background and informative examples! But it has me confused. I always figured that the carrot facing side (for me lh side as I am a righty) could be flat and the 'slice' facing side should be convexed/beveled/... to aid in food release.
Looking at your pics and reading your a righty, it looks like I have it mixed up?
Am I missing something here?

It should be convex on the side where the slices are moving away from. Flat grind or slightly concave on the part closest to the hand that holds the food.

 

[stringer]

It should be convex on the side where the slices are moving away from. Flat grind or slightly concave on the part closest to the hand that holds the food.

Left and right is confusing because choil shots are upside down from how we usually think about it.

 

[captaincaed]

The knives I really like usually have geometry that looks like this

Kippington, I didn't recognize you from your new portrait.

Nice analysis. This was the HHT nakiri?

 

[stringer]

Kippington, I didn't recognize you from your new portrait.

Nice analysis. This was the HHT nakiri?

Yes

 

[M1k3]

@stringer That is a lot of background and informative examples! But it has me confused. I always figured that the carrot facing side (for me lh side as I am a righty) could be flat and the 'slice' facing side should be convexed/beveled/... to aid in food release.
Looking at your pics and reading your a righty, it looks like I have it mixed up?
Am I missing something here?

Just remember, flat surface + flat surface =

potato suctioned.

Flat surface + convex or concave surface =

 

[captaincaed]

When the left (whole food) side is too flat and sticks, I get a "danger" signal from the fingers on my non-knife hand. Breaks the rhythm, pops food up, gets my fingers in the way.

 

[M1k3]

When the left (whole food) side is too flat and sticks, I get a "danger" signal from the fingers on my non-knife hand. Breaks the rhythm, pops food up, gets my fingers in the way.

Just all around an annoying pain the ass!

 

[stringer]

When the left (whole food) side is too flat and sticks, I get a "danger" signal from the fingers on my non-knife hand. Breaks the rhythm, pops food up, gets my fingers in the way.

On the Watanabe you can see the shoulder he leaves on the slightly concave side. That's what keeps the knife from stictioning to the held food.

 

[captaincaed]

On the Watanabe you can see the shoulder he leaves on the slightly concave side. That's what keeps the knife from stictioning to the held food.

I wonder a bit about what explains effective concavity vs. unhelpful.

A PA with a slight left-side hollow gave me more of this sticking. It was intentionally added to adjust for a right-side hook, and added thickness. I think it was just behind the edge. Is your concavity higher up?

I also have a wide-bevel that is quite thin in all measurable ways, but was splitting potatoes. Turned out it was suction from small concavities in blade road. I think that's a different situation, but the left-side hollow's location and size seems important

 

[ian]

I like a little convexity on the whole food side too, just not as much as on the cut food side. Wat does do the slight concave thing well, though. It's weird, though, I kinda don't like how Wats cut anymore. I mean, they go through food really well, and release reasonably well, but it's like an uninteresting cut in some way. Boring! Probably it's just my mind wanting to think that Wats are boring, and therefore thinking I can tell a difference in how they cut.

 

[ian]

I wonder a bit about what explains effective concavity vs. unhelpful.

A PA with a slight left-side hollow gave me more of this sticking. It was intentionally added to adjust for a right-side hook, and added thickness. I think it was just behind the edge. Is your concavity higher up?

I also have a wide-bevel that is quite thin in all measurable ways, but was splitting potatoes. Turned out it was suction from small concavities in blade road. I think that's a different situation, but the left-side hollow's location and size seems important

The Wats I tried had a concavity that ran from about 3mm above the edge to 10mm above the edge, or something like that. Was pretty slight, and there was a slight shoulder above the concavity.