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. 
Sharpening of the kanna blade
Uradashi: Setting and maintenance of the kanna blade
Initial check of the blade for a new kanna
This is a new plane I just bought. Let me take out the ura. We need to make sure this is perfectly flat.
Question: When we buy a new kanna is it not already flat?
Often it is not perfectly flat. The best way to find out is to use a sharpening stone to check where the blade touches the stone and where it doesn’t touch. There are a few ways to do this. We can use a flat metal plate (called kanna ban). We add some white alluminum powder.
Since this is just a test, just a little powder is enough. We want to see and understand where the blade touches the metal plate, and where it doesn't. If you can’t see well, you can use a black marker.
When doing this test, it’s important to just work on the edge of the blade. We can see that, in this case, the blade doesn’t touch in the middle.

Sharpening with diamond plate
There are different possible scratch patterns. The scratch pattern on this particular blade is similar to the first one of these - it doesn’t touch in the middle. The second one here doesn’t touch in the right corner. The third one here doesn’t touch on both corners. Now that we know where the blade is high and where it is low. The next step is to use a fine diamond plate.
One way would be to just use the diamond plate until everything is flat and touches perfectly. You don’t need to move too slow and carefully at this stage. But even if we went ahead like this, it would be very difficult to reach all the high areas. In this case the best way is to hammer the other side of the blade.

Flattening the blade by hammering with the genno
Here in the middle this blade doesn’t touch, so we need to hammer behind here. Let’s mark where we need to hammer. We need to hammer along this line. And a little more here. Whenever you hammer, you do it on a larger area than where you think you need it.
We are not trying to bend the blade by hammering. We hammer the soft metal and this forces the harder steel to be pushed out. If you hammer too much, let me show you what can happen. Here, on this blade, you can see a small crack. If you hammer too close to the hard steel of the edge of the blade, you can break it. Like this.
The metal on the back is soft and it stretches, pushing out the tip of the blade. We are not trying to fix the issue all in a single hammering session. We hammer a bit and test again.
Now we can see that the blade touches here, but still it doesn’t touch here. Where it’s shiny, it touches well. So we need to hammer a bit more here.
Removing the blade from my hand, you can see how the finger moves left to right, supporting the blade. The hammer (genno) always hits in the same place, and I move the blade left and right.
Let’s test again. Now it touches even more, we just need to do a bit more.

Hammering with a “takka”
You don't have to do this by hand. There is a tool that you can use to do this, called a takka. It holds the blade in place here, and this is the movement.
Here there is a small area that is slightly higher, and the hammer hits right here gently. This machine, though, hits always along the same line so it’s easier to break the blade. Hitting with a random pattern is much better to avoid breaking the blade. If you hit all along the same line it’s possible to break the blade. You can use this machine, but it’s better to do it by hand.
Let’s test again. Here is another way to hold the blade. This way allows to press stronger.
Now it touches all along the edge perfectly.
This diamond plate is fine, but still rough for the next steps, so next we need to use finer grits.

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