Friday, September 19, 2014

How (not) to ChuMansu

Thanks to Jason from Mypeculiarnature for the link to the secret technique of  長勝

(ChuMasaru acording to google translate.)

The idea is to give a positive rake angle to the cross cut tooth, quite similar to the western saw. 

It seems that in general (factory made) japanese saws come with a negative rake, wie so:

Positive and negative are defined (in my world at least) inverting the saw and measuring the angle from the vertical and the cut is from right to left. Here a real japanese drawing:

See the original here

So, A has a negative rake, while C has a positive rake. 

After Jason's bet, I tried to file C-style teeth in my saw.  It was late, so I just did it fast and changed the angle of the upper part of the tooth. Bad idea, it looked like an ugly "S"

This morning I leveled the teeth before filling again the positive angle rake. It was not enough. And I think I have tendinitis on the left hand for holding the saw... maybe it's time to make a vise. 

Anyway, I went running today, and after 5 minutes: Cabum! It came to my mind, this is the way to do it:

Start with your original angle. File down half of the teeth's height. THEN, file the new angle. Believe me, starting directly from A to C is simply stupid.  The figures are at different height to show how much you need to remove from your saw to get the new geometry. Note that from A to B the gullets do not change, they only go down when you start filling the new angles to get C.

I tried to photograph two pieces, one cut with a disposable blade of negative rake, and one with my implementation of positive rake, but the pictures look very similar. However, the feeling of both cuts is different. Cutting with ChuMansu leave a much smoother surface, this is very noticeable in soft-woods; there is less tear out on the end grain, if that makes any sense. Or think that you put a small (edit: skew?) low angle block plane on every tooth, this is kind of what you get. 

Tomorrow I will follow my own advice and will file down the teeth to 1/2 of their size, and will try again. That is it for today.  

1 comment:

  1. Haha! I love it! Great work Sebastian!

    I REALLY need to get going, to try to catch up with you in terms of your saw filing progress. This is exactly the inspiration that I need.