Recently, a customer/friend asked me a question about how single bevel knives are properly sharpened and if the concavity from the large wheels the knives are made on has any functional purpose. Here was my answer:
The concave part of the blade road is a function of how the knives are made. Often, they go from a wheel to a buffer and are finished with minimal stone contact, so the low spot in the center of the blade road remains. As a rule of thumb (though not always 100% accurate), the more expensive knives have less of this and to less of a degree than less expensive knives. Knives that have had honbadzuke performed (by either the knifemaker or retailer) should not have this, though sharpeners in Japan are less uptight about this than most customers in the west are. Honbadzuke and regular sharpening are designed to remove these low spots (and high spots for that matter) over time.
In terms of correct single bevel sharpening, there are really two ways people go about things. The first is generally regarded as the most technically correct and best for performance, while the second is easier to do, and still correct. The first type of sharpening is hamaguri sharpening. In doing this, the blade road is sharpened in 2 parts and then blended together. The first sharpening is from the shinogi line down about 1/2 way or 2/3 the way down the blade road. In this first sharpening, the shinogi line should be moved up the same amount of height you intend to remove from the edge of the knife. The second sharpening is of the edge and the area just behind the edge. During this sharpening, you remove metal from the edge and form a burr. The two angles of these first and second sharpening are almost the same, so the difference comes mainly from finger placement and pressure, rather than lifting up the edge. These two bevels are then blended together to create a hamaguri edge. The curvature should be very subtle.
The second kind of sharpening is often referred to as beta togi. This is the kind of sharpening where the blade road is entirely flat, and, when sharpened, is laid flat on the stone. Compared to hamaguri, this kind of sharpening is less time consuming and easier to do. It yields a thinner edge, with slightly better cutting performance, but lacks in edge retention, toughness, and food release when compared to hamaguri edges. Even in this kind of sharpening, the high and low spots are removed over time, which indicates proper sharpening.
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