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 hira-kanna is used for final smoothing of timber. In advanced mode it is sharpened with slightly rounded corners to avoid any scratches left on the timber. Anyways this is a tool that is used to flatten surfaces.
A problem that many people have is that the corners of the blade seem to protrude more than the rest of the edge of the blade, and end up leaving scratches on the material being planes.
Why this can happen?
When we sharpen on a stone that is hollow in the middle the blade corners are slightly rounded. On the other hand, when we sharpen on a stone that is higher in the middle the opposite is achieved. The blade corners end up sticking out more than the rest of the blade edge (“U” shaped edge). This is one of the possible reasons.
Another reason is that when the blade is inserted in the dai, if the back of the blade touches the middle of the bed too much, it pushes the blade in the middle. In the long term this continuous pressure bends the blade, which then comes out of the mouth with the edge slightly bent with the corners protruding more than the rest of the blade. This is the second possible reason.
The last reason is that the dai bottom is curved as I am showing. I already explained how you use the dai-naoshi-kanna to remove some material in certain areas at the bottom of a dai. But, if you use it like this… the bottom becomes rounded like this. The sides end up being slightly lower so when the blade enters the corners come out first.
These are the three possible reasons for this issue. Therefore, you need to make sure that the sharpening stone is flat - this is the most important thing. Moreover, you need to check if the middle area of the bed is too high - if so, you need to remove a bit with a file, a float or a chisel.
To avoid each of these possible causes of this problem, we need to check:
If you make sure that these problems are in check, you will be able to achieve better performance with the hira-kanna and you will be able to obtain really flat surfaces.