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.
Maintenance of the dai (kanna body)
This is the bottom of the dai (in Japanese this is called shita-ba). To flatten the shita-ba, we use this tool, a special type of kanna called the dai-naoshi-kanna, together with this straight edge.
Dai-naoshi-kanna can have various types of profiles. The reason why this one has steps all along the body is to be able to flatten along the mouth of the blade and avoid cutting in front of the mouth.
Depending on the size of the kanna you want to flatten, different sizes of dai-naoshi-kanna are used.
Some dai-naoshi-kanna have contact points only on the front and the rear and are hollow in the middle. Others touch in three points. I prefer to use a flat bottom. Even here it’s perfectly flat.
The blade is inserted at 90 degrees, so when we want to extract the blade we need to hammer on top of the rear of the dai-naoshi-kanna
Sharpening of the blade of the dai-naoshi-kanna is not the same angle as a regular kanna. A regular kanna is sharpened using a 30-32 degrees angle. This one is sharpened at more than 40 degrees.
The blade needs to protrude very little from the body.
If you plan to use a kanna on large flat pieces of timber, two contact points are fine. If you plan to use it on thinner pieces of timber, three contact points are needed, for better stability.
Where it touches is here. In this area, between contact points, you need to remove a little of material. Here, behind the mouth, also you need to remove a little all the way to the end.
I have a vice, so I clamp the dai in the vice.
If you see any scratches in the bottom of the dai, it means that the blade of the dai-naoshi-kanna is not completely straight, so you need to adjust it.
Here, close to the mouth, we need to go perfectly straight, and we remove all behind the mouth.
When we use the kanna, the timber we work on always goes through the middle part of the plane, so it changes the shape of the bottom of the dai. Therefore we need to adjust quite often the bottom of the dai.
This is how you use the dai-naoshi-kanna.
Check with the straight edge
To check the bottom of the dai, we use this special straight edge (in Japanese this is called the shita-ba-jougi). You place it like this, against a light, to see the light that passes through.
To check for twisted body we also need to check along the diagonals.
The most important thing to check is that in front of the mouth the contact point is perfectly flat
here. This is how we see if it’s correct.
It is difficult to remove this area along the mouth, so I prefer to use the blade like this as a scraper.
Rounding both sides of the blade
In my case, I rounded the corners of the blade. I do this to avoid the scratches that the corner could leave on the bottom of the dai.