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.
I have just bought this kanna, and I have done nothing to it, yet. Now we need to perform uradashi and sharpening. First let's look at the role of uragane. The blade scrapes the material like this. This is a model of the blade and the uragane. The shaving enters like this.
When the grain is straight and we move the kanna along the grain, there is no problem. But when we go against the grain the uragane is very useful. This other model is an enlargement of the very tip of the blade and the uragane.
When we go against the grain this part of the uragane bends the shaving away. Without uragane the main blade would dig into the reverse grain and cause tear out. It doesn’t shave the wood, it digs into it, the shaving becomes thicker and thicker, until it reaches the limit and it breaks, causing tear out.
The uragane prevents digging into the grain by bending the shaving forward.
Look, this is sakame bore (tear out). I pushed the blade forward too much to show you this.
To avoid we need to add uragane. This is where the surface was roughened by the tear out. After a few passed the tear out is gone. These holes in the shaving are where there was still some tear out. In Japanese this is called sakame bore.
After removing the tear out, at the end, to finish, we retract the blade a bit more, and then we move the uragane forward a bit. This way we have eliminated the tear out completely.This is the function of uragane.
When finishing the kanna-mi (blade) is retracted and the uragane is moved forward, closer to the tip of the blade.
When doing rough planing, the second bevel of the uragane is thicker and the uragane is retracted more. When finishing, the second bevel on the uragane is smaller and the uragane is moved forward, very close to the tip of the blade.