Sharpening Part 10 – The Ura 浦

Related image
Geographic Ura
Chisel Ura

If a craftsman wants to do good work, he must first sharpen his tools.

Confucius, The Analects

We talked about the Ura previously in post No. 9. It is a defining detail in most Japanese woodworking blades, and one we must understand if we are to efficiently sharpen them. In his post we will look into this important feature in more detail.

What is the Ura?

Japanese plane and chisel blades have a unique and intelligent design feature at what is called the “flat” on Western plane and chisel blades, called the “Ura” (pronounced oo-rah).

Ura translates into the English language as “bay,” as in a protected area where the sea meets the shore. At the center of the ura is a hollow-ground, depressed area in the hard steel hagane layer that serves two purposes. 

One purpose of this detail is to make it easier to keep the blade’s “flat” (the shiny areas surrounding the depression) planar (in the same plane).

If you pay attention when sharpening your wide Western chisel and plane blades lacking the Ura detail you will notice that, after many sharpening sessions, the blade’s flat, which was once planar, becomes convex with a high point at the flat’s center making it difficult to keep the extreme cutting edge, especially the corners of the blade, in close contact with the sharpening stone. Yikes!

This doesn’t occur because you don’t know how to sharpen your blades, or due to pernicious pranks by pesky Pixies, but simply because your sharpening stones/platens/paper tend to abrade the blade’s perimeter more aggressively than the center. The resulting curvature makes it more difficult to polish the flat’s extreme cutting edge. Major buzzkill.

Because of the Ura, Japanese woodworking blades are quickly fettled initially and tend to stay planar without a second thought for many years of hard use, an important benefit if you count your time worth anything.

Another purpose of the Ura is to reduce the square inches or square millimeters of hard steel you must polish during each sharpening session. As you can see from the photo above, the shiny perimeter land is all that touches the sharpening stone. Compare this with the black area which doesn’t touch the stone. That’s a lot of hard steel you don’t have to deal with. Besides making the job easier, it also saves a lot of time when sharpening and helps one’s expensive sharpening stones last longer. Time is money and stones ain’t cheap, as my old foreman used to say.

Even if you don’t use your tools to make a living, at least remember that time spent sharpening is time stolen from the pleasure of making wooden objects.

The Downside Of the Ura

The Ura detail is not all blue bunnies and fairy farts, however, because it does have one unavoidable downside: Over many sharpening sessions the Ura unavoidably becomes gradually shallower, and the lands surrounding the Ura on four sides become correspondingly wider. It is not uncommon to see old chisels and plane blades with the depressed area of the Ura almost disapeared. You can postpone this day by sharpening the Ura wisely. However, in the worst case where the Ura disappears entirely, you will still be left with an entirely usable Western-style flat, so not all is lost.

In the case of plane blades, unless the plane’s ura is subjected to a brutal sharpening regime, the land that forms the cutting edge (called the “Ito ura” meaning “strand” as in a flat area on a riverside, in Japanese) tends to gradually become narrower, and even disappear entirely after numerous sharpenings. Of course, when this happens, the blade loses its cutting edge, and the land must be restored by “uradashi” aka “tapping out” or bending the cutting edge towards the ura side, and then grinding it flat to form a new ito-ura land. Tapping out a blade requires some caution, but is not difficult. I will not deal with this aspect of blade maintenance in this post.

In the case of chisels, which have smaller and shallower ura compared to wider plane blades, the land at the cutting edge does not typically require tapping out, although it’s certainly possible to tap out wider chisel blades. Narrow chisel blades, on the other hand, are difficult to tap out without damaging them due to the rigidity produced by the hard steel layer (detailed in the previous post in this series) wrapped up the blade’s sides.

Mitsuura Chisels

Ichimatsu Nomi Ura (by Kiyotada). After many years of hard use, the multiple ura (aka “mitsuura”) on this oft-sharpened chisel used to pare precision joints has become shallower and the planar lands have become wider. Still entirely useful, it now takes more effort to sharpen than when new.
Spearpoint Mitsuura chisels made by Sukemaru using EDM technology. Sadly, Mr. Usui no longer produces them.

Some chisels are made with multiple ura, typically called “mitsuura” meaning “triple ura.” Mitsuura chisels are more difficult to sharpen because the area of hardened steel that must be polished is larger. The Ura of mitsuura chisels also tend to wear-out quicker than single-ura chisels because each individual ura is shallower in depth than standard Ura. I am not a fan of multiple ura except in a few specific applications.

In the next stage of our journey into the mysteries of sharpening, we will wander through the metaphysical realms of the “Fae.” Be sure to have a brass bench dog in your pocket when we leave the well-lighted pathways.

YMHOS

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Links to Other Posts in the “Sharpening” Series

One thought on “Sharpening Part 10 – The Ura 浦

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