Tuesday, September 30, 2014

More thoughts on ura-dashi (tapping-out a Japanese plane blade)

A few points that I want to emphasize here. I'll step back .....

For this round of ura-dashi, these marks are all of it. What is that, like.... 20 strikes, maybe?

I am using a light hammer. My "anvil" is a lead filled can. I like that the lead is soft, because it cushions the blade. A chunk of hard metal works great, but feels..... Hard and slippery. The blade slides around more. A chunk of dense wood works well, too, but cushions a little bit TOO well. It will require more force from the hammer.

A turning point for me, was realizing that I WANT to make dents in the bevel of the blade. I want the sharp little edges of my hammer to dig in, and actually move the metal around. I am striking the blade pretty lightly, but because the hammer digs in, very little force is required.

I had read dozens of times about supporting the blade, using the face of the hammer obliquely, hit in the thicker 1/3 of the blade, don't hit the hard steel part, etc etc. My error was thinking that ura-dashi was a "bending" process. That you are hitting the end of the blade, to "bend" the edge down. You read of others having the same mis-perception, and wondering why they are having problems.

I forget where I first read about a "good" description of ura-dashi, but the essence of what you are doing is this.
  • You hit the soft iron
  • You create dents, that cause the surface to expand
  • The expansion causes the end/edge of the blade to deflect downward.
The anvil backing is nice to have, but you could actually do this on a flat surface (in theory, although I wouldn't want to). As soon as I stopped thinking in terms of "bending" and started thinking about dents and displacement, it all came together. After that, ura-dashi became fun and relatively simple.

I think that this line of thought needs to be emphasized more. 

It is difficult to see, but there is a little bump that protrudes to the left, and right under that, a dented area on the face of the bevel. The dented area looks like a smudge, or hazy spot (among the dust specs). Hitting, way down on the bevel, caused the metal to deform all the way at the top of the blade. That's pretty cool!

The amount of distortion is made evident while grinding.

Even though I was striking in the middle of the bevel face, the effect is amplified, and spreads clear to the edge. The picture above shows that even after a fair bit of grinding, the dented area remains. Small, well directed taps can give big results.

In this case, I am only focusing on that little bit, the zone in the bottom center. I eventually hope to get the entire edge to look like a flat line.

I am trying to sculpt a straight line, only shaping a minimum amount of metal. Something like this would be ideal....

Mandara-ya has the REAL info!

And again, wrought iron makes this a LOT easier! This blade.....

...needed a lot of work to hammer out the back hollow (urasuki). This blade is made with normal, soft iron, NOT wrought, and it was MUCH more difficult to move.

MANY little hammer strikes.

I am striking at the thicker portion of the blade, trying to affect a large area and volume. This is heavy shaping, and a skilled practitioner would use a large hammer.

The result was acceptable, not perfect, but...

The upper blade, not the lower.

Why bother?

Good blades have VERY hard steel. Now that we all have access to cheap diamond sharpening stones, the question of whether ura-dashi is worth the bother, seems moot. Not so, not so.

The blade is all important. Each blade is different, and the wooden dai is cut to fit the blade perfectly. If you change the shape of the blade too much, the fit will be ruined.

A different blade.

This is a blade that has probably never had ura-dashi performed on it, ever. The lighter colored areas represent steel that has been removed, effectively changing the shape of the blade, causing it to fit poorly. This is a blade that will not work as well as it could've.

What happened with this kanna, is that the blade essentially became thinner, and therefore fit loosely. To tighten things up, the user needed to add a thick paper shim to the blade bed. This works.

A better solution would be to preserve the original shape.

Worse than a sloppy fit, a bigger problem is that if you don't do ura-dashi, you might run out of steel.

At the outer corners of the blade, the steel is getting thinner. Some blades will show the steel nearly disappearing. You see this problem on chisels, too.

Another blade. This one was owned by someone who took care of his tools.

The urasuki is getting very small, but still maintains its proper shape.

When you look to the bevel, you can see that the line of hagane is even, and of full thickness.

The longer one might not last as long as the shorter of the two.

This kanna looks old, but has seen little use.

The beautiful blade was forged by a very talented blacksmith.

 Look at how thin the steel is! The owner of this tool needs to be VERY diligent in performing ura-dashi. This is a high end kanna. Very nice!

It's not that hard to do. A little hammer with sharp edges.....


  1. Hi Jason,

    I'm really enjoying your blog and this last post is most helpful! Thanks for sharing your work and explorations.


    1. Hi Matt!

      I know that I spend WAY too much time thinking about this stuff, realizing that I've got questions of my own, and then searching for answers from people who actually know what they are talking about. It's hard to find good, detailed information (beyond the basics, at least). I read everything that I can find, and I wish that I could find even more! If more people could pass on what they have learned, we wouldn't be in this situation of needing to reinvent the wheel. So to that end.....

      For better, or for worse, I am just writing out whatever I can think of. You never know what will be useful, but it is gratifying to me that you might find something of use. Please let me know if I can contribute anything in particular. I just love this stuff!



Like all of us, I am figuring this out as I go, so when you see something that is incorrect or flat out wrong (and you will!), let me know. This is a learning process. Real people and names, please. Constructive comments and questions are very welcome, but hate speak/politics are not! Life (get one!) is too short.

Thanks, Jason