When you are sharpening single bevel knives like Yanagiba, Deba, and Usuba, there are definite right ways and definite wrong ways of doing it. If you sharpen single bevel knives the wrong way, not only will the knife not get as sharp as it should, but you can also ruin the knife forever.
Single bevel knife sharpening is quite simple. All you need to do is to follow the bevel that is already there. Problems arise when it is sharpened like a regular Western-style knife. I’ll explain here why you do not want to do that.
As you can see, the knife is made with two different type of steel, soft and hard. Hard steel is is used for the edge while soft steel protects the hard steel from breaking. There is also a part called Urasuki, which is the concave part on the back and Uraoshi which are the tiny flat areas at the top and bottom edges of the knife.
When you sharpen single bevel knives, you want to sharpen the Kireha and Uraoshi parts flat on the stone, then on the last finishing stone, you put a Koba or micro bevel on the Kireha side. The Koba is like a hem on fabric, it makes the edge stronger. When you sharpen the Kireha part, you want to make sure the Shinogi remains nice and straight. The width of the Kireha stays the same as as you sharpen. What this means is that the Shinogi should move up the knife at the same rate that the edge steel comes off. See the diagram below.
The height of the Uraoshi parts should not exceed 2mm or so. If you sharpen the Uraoshi too much, you shorten the life of the knife. The concave area, called the Urasuki, has 2 functions. One is to create an air pocket so that food does not stick to the blade. The second is that the steel on the back of the knife is quite hard, and having a concave area makes it easier to sharpen because you don’t have to take as much steel off.
This section is to illustrate what is required to repair an improperly sharpened single bevel. As you will see, it is a very long and arduous process, one that I don’t recommend home sharpeners attempt.
A customer brought a single bevel knife into the shop for repair. Instead of following the Kireha or Uraoshi, it was sharpened like a normal Western-style double bevel knife from both sides. Repairing a knife damaged this way is quite challenging.
If a knife is sharpened like this too often, it can ruin the knife forever. As mentioned above, these knives are made with two different hardnesses of steel and the softer steel is not appropriate for the edge at all. If this knife is sharpened like this for too long, the softer steel will soon reach to the edge and ruin the whole knife.
First you need to get the Uraoshi back. To do this, place the knife flat on its back side and grind until the inside beveled part of the edge is gone. Because the back side is all hard steel, it is a long and difficult process.
Then, to make it easier to maintain in the future, I made a new Urasuki. I did this using unconventional tools, and it was very challenging. I do not recommend home sharpeners attempt this, this should be done by professional knife makers only.
The Kireha bevel was also sharpened at the wrong angle, so I removed a great deal of steel from the front side and made the bevel more acute. You can see in the above diagram how much steel needed to be removed to accomplish this.
While we learned a lot about how to repair single bevels, doing this repair, we would rather not see knives come in like this in the first place. What we do recommend for people interested in learning about sharpening single bevels is to join us for our Advanced Sharpening Class. If you’re still learning about sharpening, and need to learn conventional knife sharpening we also offer a standardSharpening Class where even true beginners can start learning the art of knife sharpening.
GlossaryHagane: harder, edge steel
Jigane: softer, protective steel
Kireha: the flat part from the Shinogi line to the edge
Koba: micro bevel. Usually hair thin.
Shinogi: the line where the bevel starts.
Uraoshi: the small flat parts on the back (<2mm)
Urasuki: the concave part on the back