DISCLAIMER: The video above was published on YouTube by Trace Study. The video is part of an amazing series of videos featuring prof. Kenji Komatsu from Toyama University teaching about the setting and maintenance of a Japanese plane (kanna). I do not own, nor I claim, any rights on these videos - all copyrights are owned by the respective owners.
The following is a non-literal translation that I have made, with my wife's help, to better understand prof. Komatsu's teachings. I am publishing this translation for the only purpose of sharing it with the woodworking community, hoping that this can be of help for other people as well. If you know of any ways to improve this translation, please feel free to contact me. Any contribution will be of great help to the whole community.
Sharpening of the kanna blade
Using abrasive powders
Sharpening with Silicon Carbide powder
Here is an alternative method without using diamond plates from the beginning. This is silicon carbide powder. We can sharpen with this powder on a flat steel plate. This one is finer.
If you need to flatten a lot, you use the coarser powder. If you need to flatten just a little, you can use the finer one.
We put a pinch of the powder here on this steel plate. We add a drop of water and we grind it using a genno. On this fine powder we add a bit more water and we sharpen.
We grind it because even the finer powder is still too rough, so we need to grind it into a finer powder.
Some people teach to use a coarse powder at the beginning, but if you do it, you will eventually get a wider pattern on the ura. To keep it like this we don’t want to use a powder that is too coarse. Because we need to sharpen as little as possible.
If you get it to be this shape, there is no way to undo it, it will always be like this.
This one here doesn’t touch, it's not flat. Sharpening on the finely ground powder a bit more. Now it almost touches. Here it doesn’t touch yet.
It’s the same result to use either this powder or the diamond plates. Just remember that when you find a place where it doesn’t touch, you need to hammer. In this case, here it doesn’t touch. So we need to hammer here on the back. Like this.
Don’t wash this powder away, just keep using it. This powder becomes finer as you work on it. You need to move like this, to avoid the step we saw before, which happens if you keep the blade in the same position relative to the edge of the steel plate.
With the ring and the little finger you almost raise the blade, shifting all the pressure on the tip of the blade.
Now it is almost flat.
Sharpen with white aluminum oxide powder
We wash away this powder, and we use white aluminum powder, which is much finer. We add a bit of water, and then we polish. When the powder turns dark grey, it means that it’s polishing.
When sharpening, the powder is pushed away, so move it back under the blade like this.
Maintenance of the metal plate
Here is a problem. Can you see that right in the middle this plate is not straight? This plate has become hollow in the middle with wear. When we use the powder, not only the blade is sharpened, but also the plate is worn away, so, once a while, we need flatten this plate.
This machine has blades that slowly flattens the plate by shaving a thin layer of metal away. This plate is perfectly flat.
We put the powder, add water and mix a bit.
Now it’s perfect. We could try to keep going like this until it becomes like a mirror, but even if we keep going like this it will not be perfect. We need to use something else.
Polishing to a mirror finish
Now you can see that it’s perfectly flat. Now it's time to finish the blade using a polishing stone.
First we need to make sure that the stone is flat, and then we start working on the stone.
Look… now it's beautifully flat.