Sen and Sen-Dai for Hand Scraping Saws



This is going to be a post with a lot of pictures, hopefully it won’t take forever to load on your web browser. To start, a method for measuring saw plate thickness, basically a red-neck deep throat dial micrometer using a drill press. By setting the zero on the dial to the thinnest parts of this saw I could write the deviation in thousandths directly on the plate, a grid of numbers. Supposedly this kind of saw plate variation of thickness for western panel saws is unusual, but it offers me a good opportunity to discuss the Japanese tools used to hand thickness nokogiri.


With the grid of the saw mapped I built up a topographical map to better visually represent the variation in thickness. After all, if you’re going to scrape a saw evenly you need to know where to take off metal. This saw already went through quite a bit of hammer straightening, but the problems with the uneven thickness of the plate are making it difficult to get that last 10% of straight. The normal fashion for gauging proper thickness on Japanese saws is to bend the saw and observe the curve, and I can attest that this is, with appropriate experience a la Yataiki, an extrememly accurate way to thickness saws to within a thousandth of an inch, based on some of his saws that I have looked at with the dial micrometer.


The tools used to scrape the saw plates are sen. You’ll find that these were once very common tools to all of the tool making blacksmith trades, including katana and kanna. Some of the sen pictured here are specialized for saw making, like the one at the very bottom of the frame, but there are others that could be used for hand scraping the ura (hollow) on the backs of Japanese chisels and kanna, or the flute on the sides of a katana.


This particular set uses laminate construction with Swedish steel.


And the final stage of scraping involves a lot of work hand burnishing the surface with a lot of pressure and elbow grease.

What kind of Swedish steel I would very much like to know, seeing as saying Swedish steel is about as useful as saying they are made from high carbon steel, there’s a lot of different kinds out there these days.


Lets start with a frame of reference for what the hell I’m talking about. This is a photo of Yataiki thicknessing a saw at the sen-dai. Beautiful metal shavings, no? The sen-dai comprises both the board the saw is resting on and the staple vise used to hold the saw flat. The large staple goes over the sen board and is mounted into a foundation block of poured concrete in the ground, a large block about two feet wide by four feet long, very stable.


For holding the saw down flat to be scraped on the sen-dai there are lots of little spring clamps and wedges. These are all used under the staple of the sen dai. The ball bearing is for rolling vigerously in hand to prevent blisters, the loop of steel is a way of binding the handles of a pair of tongs when forging. The little rectangular wedges hold the sen board against the wedge beneath it that gives it the proper downward angle for work.


For holding the opposite side of the saw nearest where you would be seated are more spring clamps, elegant little pieces of spring steel that slip over the edge of the sen board.


Saw makers don’t make one saw at a time, its more of a production affair. Here is a good stock of rough forged blanks, ready for rough grinding after the tangs are forge welded on.


An elegant spring clamp in use.


And under the staple the various wedges. The spring wedge holding the tang down has a curl at the other end used as a snell, for tapping the wedge loose.


This is just a mock up of the sen-dai. There is one size of board for larger saws.


And a smaller, thinner board for dozuki. Both of these are made from Kashi, Japanese white oak, the same wood used in plane dai.


The staple spring clamp for dozuki have a variation with a little stop cut at the end that the blade butts up against.


That allows for working right up to the end of the saw plate.


In the past all of the thicknessing would have been done by hand. More modern methods involve a rough surface grinding to remove most of the excess material. Here is a ryoba saw, rough ground and tempered. Beautiful colour.


Apparently dozuki are differentially tempered, softer along the tooth edge.


What you’re seeing here is a dozuki blade that’s been hammer straightened after tempering with two different kinds of hammers. Fascinating surface!


Note: This post has been edited to correct an earlier mistake, referring to Japanese Oak as Keyaki (a type of Japanese Elm) instead of Kashi.

8 thoughts on “Sen and Sen-Dai for Hand Scraping Saws”

  1. I think that you just advanced our understanding here by a factor of 10! I have seen so many pictures of the sen/dai in use, but you just completed the picture. Thank you! Again!

    The temper pics are excellent, very illustrative. Very cool to see how the dozuki maintains full temper along the back and body. Stiff and resilient, but softer at the tooth, to keep the teeth from snapping off.

    Mahalo for sharing!

  2. Wonderful knowledge! Definitely linking this to /r/blacksmithing, they love tempering pics and such over there. Perhaps /r/woodworking too.

    Looks to be the same color my steel is. I love all this knowledge, but it’s a little like looking into a void, with how much a setup needs! Also reminding me I shouldn’t take saw-making too seriously, if I can’t tell the colors- then again I’ve heard there are tempering crayons available. Just today, I found out our living room is colored gold…I thought it was tan?

    Will you be adding the back to the dozuki? I’d love to see all the pics for that process, I want to try making a copper-backed dozuki one day.

  3. How interesting! I find it amazing that hand saws were so completely “hand made” even into modern times in Japan, where it seems that western saws have been a factory affair much longer…I wander if the earlier western saws were ever made by a smith and scraped in a similar fashion to the Japanese saws? Perhaps the older style frame saws in style from the Roman era onwards were made that way?

    1. I doubt it… romans knew files back then but I haven’t seen any sen-like tool in europe. So I vote for draw filed surfaces, if they somehow finished the saws. I don’t think a knowledge like this could be “lost” in the western world, they just never learnt it. Hell, we are still trying to explain people why japanese saws are better and they still don’t listen 😛

  4. Just found your blog again, Gabe. Love it! Thank you. Thought I’d point out that you refer to the support boards as being made of keyaki and then clarify that they are Japanese white oak. I looked at the pictures, and indeed, they appear to be white oak. I’m sure you know keyaki is Elm (Zelkova Serrata) and oak is (Akashi). K and K — an easy mistake to make on the go. And I knew what you meant, but thought you’d wish to know of this contradiction in case an edit could help others who don’t.

    1. You’re right, I totally confused the two. Good catch! Akashi, thought that was funny, sounds like “O Kashi”, as if you were saying venerable kashi, very appropriate.

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