Question about the definition of a flat spot

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RamenMonster69
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Question about the definition of a flat spot

Post by RamenMonster69 » Fri Jul 13, 2018 11:57 am

Hello, question I'd be curious to get community input on. It may be dumb, but I'm wondering exactly how literal is the definition of the flat spot. I enjoy using the mid section of the knife for push cutting. For many knives I have such as the Kono HD2 270 and TF Nashiji 240, I find that they are often described as having a nice flat spot, but its ever so slightly curved, i.e. there's not 6 inches that is 100% flat level. The knives definitely have less of a flat spot than many mass produced knives like the equivalent length Shuns, Kasumis, or Globals. Is this deliberate?

I tend to notice this more as I use the knife. My suspicion is that its mainly me getting to know the intricacies of the specific blade profile mores and being slightly OCD about it. However I also I mainly do sharpening tune ups with shapton 4k-16k stones as a substitute for honing. I tend to follow a diagonal pullling across the entire length of the knife for these tune ups for the knife rather than up and down per section thats the width of the stone, similar to how one would strop on a leather. I want to make sure that these slight variations aren't reducing the flat spot by just enough to make it more noticeable. Also hopefully since this is a definitional question I put it in the right general discussion area rather than a secondary area such as sharpening. Thanks.

jmcnelly85
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Re: Question about the definition of a flat spot

Post by jmcnelly85 » Fri Jul 13, 2018 12:56 pm

What’s flat to one person might not be flat to another; however, some knives are 100% dead flat in some places.

Any form of sharpening can have an affect on profile, a common one to see is tall heels, continuous curve throughout, and wonky tip-belly transitions among others, pay particular attention to even attention to all parts of the blade and even amounts of pressure as you go. Fixing and tuning profiles isn’t all that difficult if something does happen.

RamenMonster69
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Re: Question about the definition of a flat spot

Post by RamenMonster69 » Fri Jul 13, 2018 1:04 pm

Right so when someone says a flat spot they aren't talking always about if you put the knife flat on the board an entire area being exactly in contact with the board? That's more my question? Just that its quite close? I can see how that's not always a bad thing with the natural movement of the arm, more trying to understand.

Lepus
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Re: Question about the definition of a flat spot

Post by Lepus » Fri Jul 13, 2018 1:24 pm

You’re right to consider putting this in sharpening, but also right to put it here. Sharpening is only one part of this equation.

I think you are correct to be concerned that your technique is rounding off your flat spots, but that is something you can fix with a quick trip to a coarse stone. The sort of technique you describe brings to mind how I was taught to sharpen in culinary school. I am sure some people get consistent results with it, but I can’t see how it would make keeping a flat spot flat easy.

I would not expect any Teruyasu Fujiwara to conform to any precise standard. They are notoriously inconsistent. Disregarding that, though, I have seen many knives that are very flat, but seldom any knives that are ruler flat. I just checked my Watanabe, Goko Western, Konosuke Fujiyama, and Kanehiro 240mm gyutos and not a single one is actually dead flat at the heel. They are all flat enough to chop monstrously; when they hit a board and a fraction of a millimeter of the blade sinks in they have at least 7cm of cutting edge in the board. But placed gently on top of a flat surface and in front of some sort of illumination they all let light peak through, usually with a little back belly.

jmcnelly85
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Re: Question about the definition of a flat spot

Post by jmcnelly85 » Fri Jul 13, 2018 1:30 pm

I’m sure some people refer to 100% flat, but in a lot of cases it can mean relatively flat. For instance, I love a good flat spot, but in use I’d rather have one that’s really close to flat for good portion without being 100% flat. The heel can be dead flat but after a bit a little bit of something can be a bit more forgiving and versatile if technique isn’t spot on. A dead flat knife with good technique can be very fun and effective, though.

RamenMonster69
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Re: Question about the definition of a flat spot

Post by RamenMonster69 » Fri Jul 13, 2018 2:05 pm

Yes I would characterize both knives as having about 2-3 inches of dead flat followed by near flat. Does that tend to be the norm? I mainly notice it if say I'm finishing off a shallot after a grid cut, and if I'm cutting with the middle section of the blade, there may be a slight section that doesn't go quite go through at one end of it if I'm not conscious of it if that makes sense. I know there are weird issues with TFs sometimes, I would say with mine, I felt it was very flat compared to what I had before, but that its changed over time. I wonder if this is just me adapting to how I feel about that. I know that's the case with sharpness as I've gotten more into the higher end Japanese knives.

Lepus to what you're saying, is when I feel it needs a full progression sharpening? I start off with a 500 grit shapton glass typically, is that coarse enough? I try to follow the JKI videos for it. I have an atoma 400 and a Shapton pro 120, I rather avoid using them. I do always finish a knife at each stone progression with the stropping movement, but I doubt a few strokes would throw off it that much at the end.

Sounds like its a slight mix of terminology, deliberate design, and sharpening technique, so keep an eye out during full progression sharpening but don't sweat too much?

Lepus
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Re: Question about the definition of a flat spot

Post by Lepus » Fri Jul 13, 2018 2:28 pm

If you have 70mm+ of a dead flat area that's already more than I think most knives have. All the knives I mentioned before are pretty flat for around that length, but again, not dead flat. There are quite a few gyutos that have absolutely no true flat spot, including many Kurosakis. The Yahiko Nashiji I tried had an insanely long flat section and I couldn't use it all. Someone could use it to julienne both halves of at once as a party trick, but just taking the time to line all that up would take as much time as julienning both halves separately.

If you really want the middle section of the blade to be that flat and it fits your technique, the Yahiko would be a good investment. I would say most cooks using that section of the blade are trying to do something more precise that is only possible with the help of some grind taper toward the tip, so they tend to push or thrust cut, which is a bit looser and doesn't depend on the profile being quite as flat.

I think the best area for a secondary flat spot on a 240mm+ knife is actually near the tip. The Gihei blue #2 240mm, and possibly other profiles I have not used, has a wonderful true flat spot right at the tip that was hell on wheels for fine dicing shallots. It was like having a santoku built in to a larger knife. I sold the knife but would like one in a more exotic steel eventually.
RamenMonster69 wrote:
Fri Jul 13, 2018 2:05 pm
Sounds like its a slight mix of terminology, deliberate design, and sharpening technique, so keep an eye out during full progression sharpening but don't sweat too much?
I'd say that's a healthy attitude. If you do want a flatter spot, you can sharpen it in, though I may not suggest it unless you're comfortable shifting the entire blade road up. The more I think about what you've written the more I think you are acclimating as you suspect.

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