Hello and welcome to my blog! The process of tapping out, known as ura-dashi, is unique to Japanese tools and most commonly used on plane irons. This post will focus on the efforts I’ve made to chase after an effect of careful ura-dashi maintenance known as Ito-ura or thread edge.
Japanese tools are made for the user to easily create a very sharp edge, their design and construction provide for that once they are properly set up. It’s the setup part, however, that presents some challenges and can be daunting at times. Once those steps are completed, the regular maintenance of a properly setup blade is very easy so long as it is performed as needed.
Japanese tools are constructed of hard steel (hagane) laminated to soft iron (jigane), the iron often being very old wrought. Initially one might perceive this interest in old wrought iron as being purely aesthetic, but with regular maintenance of many tools comes an appreciation of those plane irons which feature a nice old wrought for more than just their looks, but because old wrought iron is usually very malleable. Chisels on the other hand are not normally backed with very soft iron, instead the smith will work toward a balance between strength and ease of maintenance.
Japanese tools feature a hollow, known as urasuki, on their back or ura. The ura is maintained through a process known as ura-dashi. The goal of ura-dashi is to expand the jigane with hammer blows until the hagane deforms. This deformation is what ultimately allows for a thin landing at the cutting edge and also allows the landing to be maintained throughout the life of the tool.
The chisels shown here are a displaying the result of both tapping out and slowly changing the bevel angle (higher). The chisel on the right is further along in the process. These chisels are by Hidari Konobu, the head blacksmith Saitou-san prepares his tsuki-nomi in a way which makes them similar to plane blades in that they can be setup and maintained with ura-dashi. In other words, these are not normal chisels and I do not suggest using ura-dashi on regular chisels except in rare circumstances on wide chisels.
Initially upon tapping out, with success, the steel will begin to form a new landing. As the landing is blended into the remainder of the uraba it begins to become uniform. The left again contrasts the right in that the right side is further along.
The goal of ura-dashi is to form a landing in a way which can maintained with ease and one which presents the furthest tip of the cutting edge to the stone. If this is not done, or if the landing is made very large, the tip of the cutting edge will be recessed from the stone and the burr created from sharpening never quite removed. The result will be a semi-sharpness that will be unimpressive. One way to guarantee that the extreme edge is on the same plane as the rest of the landing is to create an effect called ‘ito-ura’ or thread edge. Ito-ura is a situation in which the landing is very thin and so contacts the sharpening stone without failure.
During regular use of a chisel or plane blade the blade wears on both the bevel side and the back side to form the rounded profile of a dull blade. On the backside of the plane iron this is known as a ‘wear bevel’. The wear bevel is just that, a bevel, and will not contact the stone when the back is placed flat on the stone. Instead of wearing away at the back to remove this, the bevel is reduced until the wear on both sides of the iron are removed. The flatness of the contact surface on the back then becomes critical in effectively removing the burr, any part of the back which is out of flat will cause the landing to be uneven and so part of the burr will be retained. One way to ensure that the backside is perfectly flat is to have a very small contact area which is maintained only by the finish stone, the finish stone being one which is kept absolutely flat.
This is very similar to maintenance of western blades by hollow grinding the cutting edge, the hollow allows a thin contact point at the front and back of the cutting edge. Those two contact points, being very small will wear quickly and evenly to form a thin landing that is even and flat.
Earlier this year I spoke with Jim Blauvelt, an experienced tea house carpenter and blacksmith, who recommended some changes in my ura-dashi routine. Jim suggested that I revisit using the corner of my anvil’s surface and he also suggested that I change the shape of my hammer’s face. The face I was using was a hard edged square face, and Jim suggested reshaping that in the shape of a cross peen hammer. The result would be that each hammer blow would be more effective and that it would create a rounded divot rather than a sharp divot.
Rather than trying to position my blade on a rounded corner of my anvil, I decided that it was time for a support that would be much more intuitive to use. My father offered up a hardy insert for the anvil. I polished it up and prepared it for use. The original surface hardness long gone and what remained is soft iron. I thought it might be hardened, as many of these tools are, but it dents very easily and so likely the hardening has been removed from misuse in it’s previous life in a high school metal shop.
I repaired the surface, not only for aesthetic reasons but because I do not want rust transferring from the tool to the back of the plane iron.
It was time to once again proceed with ura-dashi. The first patient, one that had presented some difficulties. The blade simply would not budge with the previous process, and so the improvements made allowed it to move predictably. Eventually I created a very thin landing.
A little more ura-dashi.
And the final result, ito-ura, or thread edge.
Now having realized ito-ura, I’ve been chasing that effect in my other plane blades. The resulting sharpness from ito-ura is markedly improved over the same edge and a wider landing. Many plane blades arrive from the maker with a land already too large to really be considered ito-ura. I’ve taken to carefully working the ura to ensure that the landing does not increase drastically which would put the goal of ito-ura even further away. The first step in doing this is similar to how I did the work previously. Using my granite checking plate to search for teeter-totter.
The low side then being shimmed up and raised by carefully working the center of the blade to deform the soft iron and reposition the steel so that it seats along the entire outside landing.
The ura is then marked, and the rubbed on the granite check plate to see where the landing contacts. Shown here the landing contacts very far behind the cutting edge. If one were to start flattening the sides would grow dramatically before the back would become flat making for difficult future maintenance of this blade. However, I will proceed with ura-dashi and work the blade until the cutting edge touches along its edge before using a sharpening stone.
Once the blade shows contract evenly along the edge, based on my sharpie marker test, the blade is then brought to the finest sharpening stone and the back worked lightly until an even finish results. The result is a thin edge around the landing, rather than a thicker edge that would result from working the the blade from a low grit stone on up.
The thicker edges, on the two blades shown on the right for example, are being slowly worked back until a thin thread edge remains.
Given enough regular use and sharpening the edge will begin to recede showing inconsistencies in the landing. As the edge disappears those spots will be tapped out and eventually the remaining landing will be very even and thin. This landing is thinner in the center, and so it will be worked with a bias toward the center until the landing is both thin and straight across. It will take many sharpening sessions but over the course of those sessions it will improve.
I hope that you have enjoyed reading this post and I look forward to your comments. My regular work will proceed very soon. I’ve been building pieces which I’ve covered in detail in the recent past and so I’ve decided not to write about them specifically. Once completed I will finish up my shoji shortly there-after and detail the steps taken in completing the shoji work.