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    Username Post: Single Bevel Knives and Geometry        (Topic#844351)
    C9
    Master Member KnifeNut!
    *
    02-10-09 06:05.15 - Post#1761096    



    I notice that geometry plays a large role in double bevel knives, but how does it play a role in single bevel knives?

    We have 5 parts of the geometry

    Thickness/Thinness
    Front Face (above the shinogi)
    Shinogi
    Bevel & Niku
    Urasuki

    As for the urasuki, and to a large extent the front face and thickness, we neither have effective means to modify them to our advantage, nor do we generally modify them intentionally.

    The parts we can modify are the shinogi, which the primary aim is simply to keep it sharp, just like the edge, and the bevel. However, the most significant part of bevel geometry is often removed in favor of a flat bevel, the niku.

    So how important is geometry to single bevel knives? Beyond making a quality knife, with a well ground urasuki, at our desired thickness, what else is there for the maker to do in the design of single bevel geometry? We often change the bevel to suit out needs, but rarely the overall geometry.
     


    DwarvenChef
    Master Member KnifeNut!
    *
    02-10-09 07:21.35 - Post#1761164    


        In response to C9

    Ok 10 mins after I wake up..... this post hurts the head... I'm going back to bed....

    Hiromoto AS Addict

    "Thats not a stain you fool, it's Patina



     
    olpappy
    Master Member KnifeNut!
    *
    02-10-09 07:28.47 - Post#1761170    


        In response to C9

    I would think all the things that go into "a well made knife" as you mentioned: A well ground urasaki, carefully shaped shinogi, bevel and edge geometry. The care and skill that goes into grinding those lines are what makes the difference between a higher quality, more expensive knife and a lesser one.

    The maker determines the actual angle of the edge bevel, as well as the thickness of the blade.

    You're right that once those grind lines are set, there's very little the end user does to change them, in fact most of the time the goal should be to preserve those original lines, shinogi and urasaki. Although the end use has to sharpen the blade road, there are basically two options, either do it completely flat, or put a hamaguri clamshell (convex) edge on it. The former will be thinner, perhaps sharper, but less durable; the latter will be more durable and probably very close in sharpness.
     
    Loup Garou
    Master Member KnifeNut!
    *
    02-10-09 17:03.27 - Post#1761536    


        In response to C9

    The curvature of the urasuki, as in the cross section between it and the front, is supposed to act as a shock absorber. If you grind it too flat, it actually weakens the edge.

    Niku is separate from thickness, in that a blade can be thin but have a bulging side surface which increases strength. I read recently that thin edges (5-10 degrees) can actually last longer and be more durable due to less force required in cutting – however, they are far less durable for lateral loading, as in you can’t twist the knife. But this is with reference to a double bevelled geometry. A single bevel has an extremely broad bevel where a real narrow angle <5 degrees might weaken geometry. A curvature seems to be the geometry that people who make the knives, either the makers or manufacturers, furnish on their own knives. It looks elegant and can enable a deba to cut up chickens, but for a dedicated knife like a yanagi isn’t necessary.

    The shinogi can be altered for a deba which has a tall profile. But for a usuba or yanagi you can only alter it so far before it interferes with the spine of the knife. The practice of not grinding or incorrectly grinding knives would only make sense if it was done to keep the price down, which it sometimes isn’t. Also, the idea that the best knives are not sharpened so that the user can fashion an edge to his own liking isn’t strictly true, because you can’t alter the bevel line.

    If you genuinely wish to fashion your own edge from a factory ground knife, I’ve heard you can request that the manufacturer positions the shinogi closer to the edge, and then you hand sharpen it back manually and get a clean line. I haven’t requested this, but I believe it is acceptable ettiquette and a done thing. If you get someone to translate, you should be able to request a more obtuse grind (= lower position of shinogi) from most companies.

    The front face doesn’t really have any value on the knife, except for creating a transition at the bevel to allow food to fall away. A full height grind increases sharpness (or decreases the edge angle) but reduces strength. The bevelled design is to increase strength… because a full height grind on a single bevel would be pretty thin. Even single bevel gyutos like the Momijis have a bevel, and these aren’t thick knives.
     
    kcma
    Master Member KnifeNut!
    *
    02-10-09 18:13.37 - Post#1761571    


        In response to Loup Garou

    the thinness of these single bevel knives are so underrated. the angle doesn't look that small. but ppl fail to realize that you have to subtract the angle from the concave. yes there's a little flatness, but it's small, and the edge acts like it's not there. and that's also the reason you don't sharpen the back of the knife.

    and yes, don't twist and turn these knives
     
    DocNightfall
    Master Member KnifeNut!
    *
    02-10-09 20:57.00 - Post#1761600    


        In response to kcma

    People often neglect the importance of the bevel not being flat.

    When you make a straight slice, you leave a flat surface on the food. If the bevel is also flat, you end up with two flat surfaces pressed against each other. This creates a lot of stiction and drag.

    A little bit of niku or convexity, evenly distributed over the entire bevel, goes a long way towards reducing that stickiness. It seems counter-intuitive that a slight increase in bevel angle at the very edge would result in so much greater cutting efficiency-- but in the case of single bevel knives, because the bevel is so wide, what goes on behind the edge is far more important than the final angle.

    With a good niku, the cut food slowly lets go as you slice and falls away completely at the shinogi.

    I seem to recall Korin's video describing how to create a good niku by raising the angle a bit from flat and sharpening at the edge, then LOWERING the angle a bit from flat and sharpening at the line of the shinogi, effectively giving a three-step bevel. Then the angle transitions are blurred to give a smooth convex. The video gave a lot of warnings about the technique of sharpening at the shinogi, saying that it was not for beginners. You have to be able to grind at the portion of the bevel just next to the shinogi line without rounding off the line itself.

    -----
    David


     
    BertMor
    Master Member KnifeNut!
    *
    02-11-09 00:48.17 - Post#1761663    


        In response to DocNightfall

    I just don't understand how this is accomplished. The blade road is flat ( well not really its is a tad concave because of the round grinding wheel). If you were to raise the bed near the shinogi, won't you just round off the shinogi?

    I've got the Korin video, and I am still completely confused about how to sharpen a single bevel properly to maximize its performance.

    I tried using my yanagi to 'butterfly' a small side of salmon, and the drag was pretty high. I felt that using a suji would have been a better tool for the task, but I wanted to see what a yanagi could do.

    Any advice?
    Bert C=:-)

    Why? Because footballs don't have wheels!


     
    C9
    Master Member KnifeNut!
    *
    02-11-09 01:15.03 - Post#1761671    


        In response to BertMor

    I think the idea is to have a flat area that meets the blade face to create a shinogi, so that instead of rounding the shongi like an edge, you're sharpening a bevel, similar to a bevel on the edge.

    Also, if you take a look at the way they use the stone wheels, they have horizontal wheels, and often times the knife will be perpendicular to the position it would be in to create a concave grind.
     
    DocNightfall
    Master Member KnifeNut!
    *
    02-11-09 01:15.39 - Post#1761672    


        In response to BertMor

    If your blade road was previously flattened (which it shouldn't be), you lower the angle just a tiny tiny bit so you actually grind on the shinogi. But you have to keep the angle very consistent as you sharpen here so that the shinogi transition line remains a distinct angle and doesn't get rounded. As mentioned above, you create a three-step bevel and smooth out the transitions. That's a good way to reconstruct the hamaguriba if it has been obliterated for any reason.

    Lower end knives with minimal hand-finishing tend to leave the factory with a hollow ground primary bevel and a secondary to create a working edge. The owner is expected to do the hand finishing themselves. You flatten out the hollow grind, then grind another back bevel near the shinogi at a very slightly lower angle and you end up with the three step bevel. Smooth out the transitions from back bevel to primary to secondary, and you end up with a single hamaguriba bevel.

    If you already have a well-formed hamaguriba edge, all you do is shift the finger pressure closer to the edge to sharpen that portion, and shift the finger pressure closer to the shinogi to sharpen there as well. The bevel will roll on its convexity and the area right underneath the finger pressure will be in contact with the stone.

    -----
    David


     
    DocNightfall
    Master Member KnifeNut!
    *
    02-11-09 01:27.17 - Post#1761681    


        In response to DocNightfall

    What I've described above is what I recall of the Korin video's sharpening method. It's kinda foggy to me and I may have dreamed up some parts.

    What I personally use is the method shown in the Suisin videos. Link below,

    http://www.suisin.co.jp/Japanese/to...

    -----
    David


     


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