Convexity, which side?

So recently I took my Zakuri 135mm funayuki to some stones to clean up the bevel and even up the edge a little, and quickly noticed a much greater degree of convexity close to the edge on the left side of the blade (with "Zakuri" stamp) than on the right.

This seemed counterintuitive to me. If I were doing a more convex edge, I'd have imagined the right side of the blade would get the greater convex so that it could push away thin slices and avoid sticking.

Knife cuts pretty brilliantly though. Super easy cutter, almost perfect food release. Might just be a coincidence.

Then started to flatten the bevel of my other Zakuri, a Tosagata bocho, and the same convexity appeared, though not as obvious. I haven't finished reworking that knife yet, but now I'm curious. Is a left-side convex the norm for thin knives that use convexity for food release? Or is it just an accident with the two Zakuri knives?

 

I'll go out on a long limb here and say that for double-bevel knives, asymmetry is really rare and usually unintentional. With the exception of a couple of Aritsugu A Types, I've never been able to detect it. I've never heard or seen a knifemaker talk about intentionally grinding blades asymmetrically. People put up choil shots that they claim show asymmetry, but (a) I often can't see it, and (b) choil shots show about .001% of how a blade is ground.

Okay, so I put it out there. Prove me wrong.

 

OK.
How about that

I enjoy thinning knives out, and so I thinned a few, and on every single one of them I try to convex the left side, or the side where the sliced object would be laying.

To me, this is because
(and this is just imho, i never read about it anywhere or cannot prove it with right by any objective proof apart from Shigefusas choilshots)
the way one is percepting easiness of cutting, has to do with how both sides of the blade are performing.
Lets say your right side is pushing the slices well and theres no sticktion, but the left is flat and it gets stuck to the produce each time, you are not as effective as you could be, right?

And that is why when I regrind a blade, I try to set a obtuse angle on the left side, for a right hand user. That way, imho, the left side will push itself away from the produce being cut, making no sticking there= if some of the slices from the other side get stuck, they are already separated from the bigger piece, right?

I know Dan was amused by that, when I laid my hands on his Zensho, but I think the knife works OK...

 

Well said, and I've seen enough of your work to respect your skills. Do you ever use a belt? If not, isn't it really hard to convex on just stones?

 

I've intentionally ground asymmetry into two blades before.

I made a honesuki once....that was severely asymmetrical to the right side.

I've also ground a gyuto with intentional asymmetry of about 70/30. I marked the edge with blue marking liquid and drew a center line closer to the one side. It turned out good me thinks.....customer liked it so that's my only barometer.

 

I didn't like some knives I tried that were ground with a strong convex on the left, I could feel the blade being pushed away from the food towards where I was slicing. it had a very weird feeling. Now a smaller convex on the left side near the edge, yeah, that works w/o moving the whole blade. I have a blade that is almost a flat grind on the left with a little convexing above the edge, convexed a bit more throughout on the right, cuts like a dream. I tried a single bevel gyuto once, 1 potato and the knife was on it's way back. LOL, just not a good thing! So I like a flatter grind on the left with some convexing above the edge, or a very gradual convex on the left, but not a strong convex on the left, pushes too much on the blade for my preferences.

 

Grab a straight edge and place it on each side of the knife to see which side has less surface contact.

 

Yup, straight edge will tell the story! Many times they will be slightly asymmetric (60/40), but the grind heights are very close to the same on both sides. Left will often be flatter ground than the right side with the convexing near the edge and the right will be ground a bit more with a little stronger convex to the whole side usually from what I have seen. Choil shots are hard to get a good one because if the choil/camera aren't exactly aligned, it can make it look different than it is, or if the blade isn't perpendicular.

 

I've used a credit card slid up and down the blade as a straightedge on every knife I've owned, and many I haven't. As I said, it's really rare that I can detect a difference between the two sides of a double-bevel knife.

Got any pics or youtube links?

 

Thanks Bill. I use the belt for that all the time. I did it on the stones before and it is possible, just takes 20x as much time. Plus you have to have many different stones, whereas Im trying to cut down on my setup. I went from 10 stones to 4, and then Im trying to use just two for regular sharpening.
Biggest problem with this kinda job is, you have to have some soft 1K-ish stone like King or something to remove scratches and preparation for final polishing, and this one I dont have. Recently I used one awesome 800 at Maxims, and this worked wonders too.
After belt Im removing scratches with sanding paper.



I think I wasn't clear enough as to what is being convexed. Im sorry.
So, on the left side, I would try to hit just a centimeter or one and a half from the edge. Deending on the blade height really.

 

Thanks for the responses. For what it's worth, I cannot easily see convexity when looking at the choil; it's a subtle thing that shows when grinding the bevel down on the stone. When laying the bevel flat on the stone, the right-side scratches go all the way down to the cutting edge bevel, while the left-side scratches stop 5-10mm from the edge, and I need to lift the knife a tiny bit to hit that part of the bevel.

Once the bevel's cleaned up, I switch to 12-micron polishing film with a layer or two of paper underneath to give it just a tiny bit of give, and that allows me to reintroduce a tiny bit of convexity to the bevel on both sides; something a little like bieniek's image, though very subtle. It also puts a slightly more even finish on the bevel than I get on a flat stone.

 

Here you go.

 

Cool. What kind of knife is that? And is that how it came from the maker? [I'm not denying that users can put asymmetrical bevels on knives when sharpening; just saying that knifemakers don't typically grind them asymmetric.]

 

It's a Hiromoto AS 150mm petty. This is how they arrive stock, no mods to the steel. That's how my AS 240 gyuto came too. Really, the only "J" knives I've had that were symmetrical were Shun and Miyabi.

 

I should have recognized the Hiromoto---I've had a couple of them. The cladding was somewhat asymmetrical on one, totally symmetric on the other, but that didn't really have anything to do with an asymmetric grind. The only really asymmetrical grinds I've seen were on Aritsugus, as mentioned.

Guess we'll have to agree to disagree.