bowed edge repair - help!

[Choppin]

Hi all -

I recently received this nakiri from Japan and when I took it to the stones I noticed the edge is slightly bowed. When sharpening, only the blue areas (heel and tip) touch the stone, but not the red part. I lowered the angle to polish the bevels and noticed something similar - the red (middle) area was less abraded than the blue areas. Holding the knife with the right side facing up, the edge is U shaped (an exaggeration of course).

The spine looks straight to me (hard to be sure due to the distal taper, but see the pic below). I suspect the issue is not a bow but an overgrind on the right side, that goes from the edge up to the shinogi. The KU is worn down at the shinogi and I can feel the shoulder there is much less pronounced than on the tip and heel areas.

I paid like $80 so no worries if it's a loss but otherwise it's a great knife and I'd like to try fixing if possible. I have no idea how to proceed though, never done this kind of repair.

Here are the alternatives I have in mind:

- Bend the knife at the spine, trying to revert the bow. But if the spine is straight, I might end up with a straight edge and bent spine? Would that be even effective if the spine isn’t the issue?

- Hammer the blade right in the middle of the shinogi on the left side, trying to straighten everything from the shinogi to the edge. Like Carter shows here:

- Fix it on the stones, regrinding and reprofiling the whole knife (might not be worth the effort / time?)

I’d really appreciate any thoughts and ideas. Thanks.

 

[KimBronnum]

It might not be repairable. If you grind it “straight” you may “run out of” core steel, if you know what I mean? It ought to be returned, but I see Your point reg. Price. I don’t see any other Way than grinding it flat on a stone. But I don’t know if it is a good idea Let US know what you do.

 

[tostadas]

It looks pretty minor. I'd just try to bend it back gently. Lay the front part of the blade flat on a sturdy surface and put a little pressure near the bend.

Have you checked sharpening on the other side? Another possibility is that maybe you just have a section thats ground slightly more than the others. And a full sharpening will take care of that if this is the case.

 

[Choppin]

duplicated post

 

[Choppin]

Have you checked sharpening on the other side? Another possibility is that maybe you just have a section thats ground slightly more than the others. And a full sharpening will take care of that if this is the case.

the other side sharpens well, it sits a bit convex on the stone so all of the edge is touched by the stone. the issue is deburring, because the opposite side is not fully touched by the stone (it's concave)...

I've done a full sharpening and it didn't solve it. The overgrind goes up to the shinogi line at least, it's not just on the edge. I'll try bending the knife!

 

[ethompson]

Are you checking the straightness from the spine or the edge? The edge is what matters. Tricky to develop a good eye for it though.

 

[Choppin]

Are you checking the straightness from the spine or the edge? The edge is what matters. Tricky to develop a good eye for it though.

seems really tricky, I'm just beginning training my eye for this. it reminds me of my first woodworking lessons (my other expensive hobby lol), learning to check for straightness in wood planks and also the difference between 1.0mm and 0.5mm thickness... it seems madness at first but then it starts sinking in... "a-ha!, that's what the instructor means..."

I'm checking both - spine looks straight but edge is definitely bent... I can see a slightly bent and the fact that the middle part of the edge on the right side doesn't touch the stone when sharpening is the confirmation for me

 

[HumbleHomeCook]

I'd lay that right side flat on your cutting board (handle hanging off). Get down to eye level with edge and slowly raise the spine until the edge is making contact with the board. See if you can detect a bend.

 

[TB_London]

Some tappy taps with a hammer on the convex side will straighten it up, being laminated makes it much more straightforward than dealing with mono. If you find it hard to judge the bend by eye use a straight edge like a metal rule

 

[Choppin]

Some tappy taps with a hammer on the convex side will straighten it up, being laminated makes it much more straightforward than dealing with mono. If you find it hard to judge the bend by eye use a straight edge like a metal rule

thanks, I’ll try that! hammering makes me less nervous than bending the blade lol

@HumbleHomeCook I tried this but on a glass plate and the bend is visible.

 

[tostadas]

thanks, I’ll try that! hammering makes me less nervous than bending the blade lol

@HumbleHomeCook I tried this but on a glass plate and the bend is visible.

I think bending is lower risk than hammering. I've cracked a blade before by tapping it once.

 

[refcast]

When you say sharpening, do you mean wide bevel sharpening? The edge profile doesn't look like it has a big recurve, so it should contact with normal double bevel sharpening, which is done at a higher angle than wide bevel stuff. If you're wondering why that red part doesn't contact while wide bevel sharpening . . . Either the blade is bent, or it was overground there, or the sharpener ground the blade while it was bent, and then straightened it after (which isn't good practice but it happens). But the edge spine doesn't look very bent. Maybe the tiniest bend but pretty much straight

I've gotten double bevel blades like that a couple of times from knife japan @ethompson got a yanagiba like that.

To "correct" that, I'd have to keep on sharpening the blue parts, but it's not necessarily "wrong" or unusable as it is. Definitely not perfect or ideal though

 

[Choppin]

When you say sharpening, do you mean wide bevel sharpening? The edge profile doesn't look like it has a big recurve, so it should contact with normal double bevel sharpening, which is done at a higher angle than wide bevel stuff.

I should have specified that - when sharpening at a 13-15 degrees angle (normal double bevel sharpening) the very edge only touches the stone on the blue areas. The edge on the red area is left untouched, even after several passes on a SG500, applying pressure there. No burr, no visible bevel. This is what concerns me the most because it makes sharpening a pain (and it should be pretty straightforward with a carbon nakiri).

When laying the bevels flat on the stone (wide bevel polishing/sharpening) the red area gets abraded much less than the blue areas… in my photo the original grind marks are more evident in the red area.

Either the blade is bent, or it was overground there, or the sharpener ground the blade while it was bent, and then straightened it after (which isn't good practice but it happens). But the edge spine doesn't look very bent. Maybe the tiniest bend but pretty much straight

mmm good points. There is for sure an overgrind there. The worn-off KU and eased shoulders are clear evidence IMO.

 

[M1k3]

Kojibo time. Sounds like you got a warp at the edge.

 

[Choppin]

a few tappy taps and bendy bends later…

hard to tell if the edge is 100% straight, but upon visual inspection I can’t see the bow anymore. I applied Sharpie to the edge and gave it 2-3 passes on a SG2000 at 13-15 degree angle and got full edge contact! uhul!

I still have to deal with the overgrind if I want a decent polish but at least I can sharpen it now.

 

[TB_London]

Success

 

[Delat]

@Skylar303