Sharpening Part 9 – Hard Steel & Soft Iron 鍛接

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A piece of hot high-carbon steel, which will become the cutting edge, has been placed on the orange-hot low-carbon steel body of a knife. An acidic flux powder has been placed in-between and on the metals in preparation for forge laminating them together into a single blade.

“It is our choices, Harry, that show what we truly are, far more than our abilities.” 

J.K. Rowling, Harry Potter and the Chamber of Secrets

If you are reading this, it’s safe to assume you are interested in sharpening woodworking blades. You may have little experience with Japanese tools, and even then you may not be aware of some of their important details. In this post we will try to remedy that by examining some simple historical points common to woodworking blades around the world, as well as some details that make Japanese blades unique.

I believe an understanding of these basic facts will you aid your sharpening efforts, or will at least tickle your interest in Japanese blades. Please comment and let me know your thoughts.

Laminated Bi-Metal Construction

As discussed in previous posts in this series, before technological advances in the 1800’s steel was difficult to make and expensive. Consequently, it was standard practice not only in Japan, but everywhere including Europe and the United States, to reduce costs by minimizing the amount of precious steel used to make axe, scythe, plane and chisel etc. blades by laminating smallish pieces of high-carbon steel to softer and much cheaper wrought-iron bodies through a process called “forge welding.”

Most chisel and plane blade blacksmiths in Japan continue to employ this lamination technique even today, not out of some navel-gazing preference for the archaic, but because it has serious advantages.

A 30mm Hidarino Ichihiro Atsunomi, approximately 12″ OAL.

The best Japanese plane and chisel blades are generally comprised of a layer of very hard high-carbon steel called “hagane” (鋼) in Japanese, forge-welded to a softer low-carbon (ideally no-carbon) iron body called “jigane” (地金). We discussed both of these metals in the previous two posts in the series. Here and here.

Why go to so much trouble? One advantage of this construction is that it allows the cutting edge to be made much harder, and therefore cut effectively longer than a blade of uniform hardness. For instance, a blade made entirely of steel hardened to HRC65 might cut very well, but it would break or shatter in use. And even if it did not break, it would be time consuming and irritating to sharpen such a wide expanse of hard steel. Remember, the harder a piece of steel is the more work it takes to abrade it.

A 42mm Hidarino Ichihiro Oiirenomi

By combining a thin layer of this very hard steel with a thicker layer of soft low-carbon steel or wrought iron the blade can be made thick, rigid, resistant to breaking, and will hold a sharp edge relatively longer while still being easy to sharpen. This once-common ancient structure is clearly superior to all other structural systems for planes and chisels at least.

Laminated Blades in the West

If you have examined antique plane blades with wooden bodies you may have noticed many have blades stamped ” Warranted Cast Steel”

Despite being designated “cast steel” in England and America in past centuries, unlike Conan’s Daddy’s sword, or the orc blades made by in Isengard, plane, chisel and saw blades with this mark were not “cast” by pouring molten metal into a mold to form a blade. Rather the process to make the steel involved melting steel in a crucible and pouring it into molds “casting” a piece of high-carbon steel which is then forged to make the blade, hence the name.

This technology was widely used in the United States and Europe through the 1860’s. In fact, one steel mill is said to have been producing crucible steel until the 1960’s. Toolmanblog has an interesting summary on cast steel.

With few exceptions, these plane blades have a thin piece of high-carbon steel forge-welded to a soft wrought iron body, very similar to Japanese plane blades. I have used a couple of these antique blades to make Krenovian planes and testify of their excellent cutting ability.

Chisels were also once made in Europe using this same lamination technique, although fewer examples remain extant.

Axes, hatchets, and many farming implements were also mass-produced up until the 1920’s in the US using a variation of this same technique with a “bit” of steel forming the cutting edge laminated to or sandwiched inside a body of low-carbon steel or wrought iron. Axes are still made this way in Japan. It’s a proven technique with a lot of advantages, but it does require a skilled blacksmith to pull off successfully.

The point I am trying to make is that blades made using forge-welded laminated technology were the very best available in Europe and the United States for many centuries. It is sad that this superior technology has been discarded and forgotten except in Japan, but wars and economics change everything while people remain the same.

Here is a link to a blog post by Paul Sellers where he praises the old chisels and laments the new.

U-Channel Construction

A closeup of the 42mm Hidarino Ichihiro Oiirenomi showing the lamination line between the steel cutting layer and low-carbon steel body of the blade
The same 42mm Hidarino Ichihiro Oiirenomi. Notice the hard-steel lamination wrapped up the blade’s sides to add rigidity.
A 30mm Hidarino Ichihiro Atsunomi, approximately 12″ OAL. Notice the hard steel lamination forming the cutting edge at the bevel. This is a beautiful lamination.
A beautiful hand-filed shoulder detail typical of Yamazaki-san’s work

The shape of the hard steel cutting layer laminated to the softer low-carbon steel (or wrought iron) body was historically a simple flat plate in Western blades. This is also the case for Japanese plane blades, axes, and farming implements. But if you imagine Japanese blacksmiths would be satisfied with such a simple design for all applications, you don’t know the Japanese mind well.

Notice the lighter-colored hard steel lamination wrapped up the chisel’s sides in the four photographs above forming a “U channel” of hardened steel adding necessary rigidity and strength. This is a critical detail for Japanese chisels intended to be struck with a hammer. Interestingly, carving chisels are not typically made this way.

Plane blades are not subjected to the high loads chisels experience and so would not benefit from this structural detail.

The Ura

Japanese chisel and plane blades, among others, typically have a hollow-ground depression called the “Ura” (pronounced “ooh-rah”) which translates to “ocean” or “bay,” located at what is called the “flat” on Western blades. Notice the polished hard steel lamination extending from the cutting edge to several millimeters up the neck. The black area surrounded by the shiny lands is the same hard metal, but has been ground to form a hollow called the “ura.”

This clever and effective design detail is unique to Japanese tools to the best of my knowledge. We will look at this design detail more in the next post in this series.

The Point

What does any of this have to do with sharpening? Glad you asked. This design has some potential disadvantages that have been cleverly turned into distinct advantages you need to understand when sharpening Japanese woodworking blades.

For instance, the layer of high-carbon steel laminated into our chisels and planes is usually 64~65 HRc in hardness. The typical Western blade is made much softer at 50~55 HRc to avoid breakage. This extra hardness makes the blade stay sharper longer, an important benefit if your time is worth anything. This is good.

But if the entire blade were made of a solid piece of this extra-hard steel, it would a royal pain in the tukus to sharpen, I guarantee you. It would also break. That would be bad.

The softer low-carbon/no-carbon steel or iron body, however, is much softer and easily abraded making it possible to keep the hard steel layer thin, and therefore easily abraded, while protecting it from breaking. This is good.

Unlike the blade’s bevel, however, the ura is all one-piece of hard steel. Without the ura depression, you would need to abrade all that hard steel to initially flatten and regularly sharpen the blade, a necessity I guarantee would ruin your mellow mood without massive quantities of controlled substances. But with the addition of the ura detail, we only need to abrade the perimeter planar lands (the shiny areas in the photos above) around the ura. This is exceedingly good.

The ura depression makes it easier and quicker to not only sharpen the blade, but also to to keep the “flat” planar (in a single plane). Without the ura, such a hard blade would be difficult to maintain planar and frustrating to sharpen. With the addition of the ura, the blade is genius.

An important skill to learn when sharpening Japanese blades is how to maintain the lamination and ura effectively. We will discuss this important subject more in future posts.

Conclusion

If you didn’t learn at least three new things from this post then you are either very smart or weren’t paying attention. ¯\_(ツ)_/¯

In the next installment in this bodice-ripping tale of romance and derring-do we will examine the hollow-ground “Ura” in more detail. It’s important enough to deserve a special post.

YMHOS

Links to Other Posts in the “Sharpening” Series

Sharpening Japanese Woodworking Tools Part 1

Sharpening Part 2 – The Journey

Sharpening Part 3 – Philosophy

Sharpening Part 4 – ‘Nando and the Sword Sharpener

Sharpening Part 5 – The Sharp Edge

Sharpening Part 6 – The Mystery of Steel

Sharpening Part 7 – The Alchemy of Hard Steel 鋼

Sharpening Part 8 – Soft Iron 地金

Sharpening Part 9 – Hard Steel & Soft Iron 鍛接

Sharpening Part 10 – The Ura 浦

Sharpening Part 11 – Supernatural Bevel Angles

Sharpening Part 12 – Skewampus Blades, Curved Cutting Edges, and Monkeyshines

Sharpening Part 13 – Nitty Gritty

Sharpening Part 14 – Natural Sharpening Stones

Sharpening Part 15 – The Most Important Stone

Sharpening Part 16 – Pixie Dust

Sharpening Part 17 – Gear

Sharpening Part 18 – The Nagura Stone

Sharpening Part 19 – Maintaining Sharpening Stones

Sharpening Part 20 – Flattening and Polishing the Ura

Sharpening Part 21 – The Bulging Bevel

Sharpening Part 22 – The Double-bevel Blues

Sharpening Part 23 – Stance & Grip

Sharpening Part 24 – Sharpening Direction

Sharpening Part 25 – Short Strokes

Sharpening Part 27 – The Entire Face

Sharpening Part 28 – The Minuscule Burr

Sharpening Part 29 – An Example

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The Varieties of Japanese Chisels Part 12 – The Usunomi Paring Chisel (薄鑿)

Our thoughts flow to our hands; our tools become as part of our bodies, the blade of our bodies.

Tsunekazu Nishioka, Temple Carpenter, Horyuji Temple Restoration, Nara Japan.
Tsunekazu Nishioka

In the first post in this series, we examined the two main categories of Japanese chisels: the tatakinomi designed to be struck with hammer, and the tsukinomi used to pare wood without using a hammer. Beginning with this post we will shift our focus to several varieties of tsukinomi.

If you need to cut precise joints in wood, then you need both striking and paring chisels.

The most popular variety of tsukinomi is the mentori usunomi (面取り薄鑿)which translates to “beveled thin chisel.” The name is appropriate as the blade is long and thin and the neck gently tapered.

42mm Mentori Usunomi by Sukezane (Side View)
42mm Mentori Usunomi by Sukezane (Face View)
42mm Mentori Usunomi by Sukezane (Ura View)
42mm Mentori Usunomi by Sukezane (Face View)
24mm Mentori Usunomi by Sukezane (Face View)
24mm Mentori Usunomi by Sukezane (Face Closeup)
24mm Mentori Usunomi by Sukezane (Ura View)
24mm Mentori Usunomi by Sukezane (Ura Closeup)

Description

Just as with oiirenomi, the blades of tsukinomi can be made with different profiles, such as the rectangular cross-section of the kakuuchi, or the more triangular cross-section of the shinogi.

The usunomi has the more streamlined cross-section of the mentori oiirenomi with two bevels ground into the right and left sides of the blade’s face, flowing over the shoulders and feathering into the neck.

An atsunomi or oiirenomi can pare joints, of course, but the steel crown and mushroomed wood fibers on the handle’s end make them uncomfortable for such jobs. More importantly, the blades and handles of these chisels are often too short to provide adequate angular control. In short, the usunomi is more comfortable to use, and pares wood more powerfully and more precisely.

Western paring chisels by comparison are even thinner and have longer blades than Japanese paring chisels. There can be no denying they do a fine job. Japanese paring chisels like the usunomi have a few potential advantages worth considering, however.

The most significant advantage is that the steel cutting edges of Japanese paring chisels are much harder. The paring chisels my blacksmiths forge are around 65~66 Rc , whereas Western paring chisels are usually around 55 Rc. A Western style paring chisel with its thin blade of uniform steel hardened to 65 Rc would easily snap in half if stressed. This extra-hard steel makes possible an edge that stays sharper longer, with the result that, given the same number of sharpening opportunities and time in a given workday, a professional-grade usunomi will help you do more hours of high-quality work than a softer blade. For craftsmen that use their tools to feed their families this higher-level of performance is not something to be sniffed at.

The second advantage of the Japanese paring chisel is their hollow-ground ura which makes it easier to maintain a flat bearing surface. If you haven’t used Japanese chisels, this claim may sound unlikely. But please recall that there are narrow lands surrounding the ura, all in the same plane, that create a flat bearing surface to guide the chisel.

Usage

This tool is well-suited to reaching into narrow mortises and other wood joints to clean and pare surfaces roughed out by axe, adze, saw and tatakinomi to precise tolerances.

It excels at trimming mortise side walls and end walls. And shaving tenon cheeks and shoulders to precise dimensions without causing spelching or cutting too deeply as shoulder planes are wont to do is a piece of cake.

In addition, the longer blade and flat face of the usunomi make it ideal for paring angles, such a 45° mitres, in combination with wooden guide blocks or jigs.

The usunomi may be struck with the heel of the hand, but never with a hammer or mallet. The slender neck, thin blade, and unreinforced handle will simply not accept such abuse gracefully.

Chisels intended to be struck with a hammer typically perform best with a cutting edge bevel of 27~30°. Any shallower and the hard steel at the cutting edge may chip instantly dulling the tool. However, the cutting edges of usunomi along with other tsukinomi are not normally subjected to the high stresses chisels motivated with hammers must endure, so the cutting efficiency can be increased by lowering the angle to 24° or so without creating problems, depending of course, on the wood you need to pare and the type of paring you intend. For instance, paring end grain may require a steeper angle than long grain.

If you have used long-bladed Western chisels hard for a few years, you will have no doubt experienced your chisel’s flat becoming somewhat rounded over many sharpenings. This occurs because, for various reasons, the center portion of the blade’s flat is abraded at a slower rate when being sharpened than the blade’s perimeter, resulting in distortion regardless of whether you keep your stones perfectly flat or not.

Obviously, a chisel with a flat that is banana-shaped lengthwise and crosswise is not ideal for paring flat surfaces, but there is a bigger problem. Namely, it is simply more difficult and time-consuming  to create a sharp edge on a blade with a curved flat than one with a true flat. A flat like this begs for amateurish tricks using rulers, etc.. of the sort professionals would be embarrassed to use. A friend once scathingly described these techniques as “training wheels.” Oh my.

The ura on the Japanese chisel is specifically designed to deal with this shortcoming, and it does a great job of it.

30mm Unsunomi by Nagamitsu – View ofFace
30mm Unsunomi by Nagamitsu – View of Mitsuura

The 30mm usunomi in the photo above has an ura with three hollow-ground areas instead of one. This detail is called a ” mitsuura” ミツ浦 meaning ”triple ura.” It has the advantage of providing a larger bearing surface than the standard ura does, one that is helpful when using wooden jigs for paring to precise angles, for instance. It also helps the ura index better when paring large surfaces, especially with chisel blades wider than 24mm.

Some people prefer chisels with the mitsuura detail for their appearance. I admit mitsuura look sexy, but I am not a fan of using this detail unless it is truly necessary because of the downsides I will not deal with in this already overlong post.

If I can liken the atsunomi to a shire horse, then the usunomi is a falcon. Both are beautiful powerful animals, but just as one wouldn’t use a draught horse to chase down a rabbit, or a peregrin to pull a plow, neither oiirenomi nor atsunomi are as effective as the usunomi for paring and cleaning joints.

The usunomi is one of those tools that is a pleasure to use.

Among woodworking tools, the usunomi is special: as it becomes part of your hand, you will discover that neither the blade nor your hand but your mind is shaping the wood.

YMHOS

Links to Other Posts in this Series

The Varieties of Japanese Chisels Part 1 – The Main Categories

The Varieties of Japanese Chisels Part 2 – The Mentori Oiirenomi (面取追入鑿)

The Varieties of Japanese Chisels Part 3 – The Shinogi Oiirenomi (鎬追入鑿)

The Varieties of Japanese Chisels Part 4 – Kakuuchi Oiirenomi (角打追入鑿)

The Varieties of Japanese Chisels Part 5 – High-Speed Steel Oiirenomi (HSS 追入鑿)

The Varieties of Japanese Chisels Part 6 – The Mortise Chisel (Mukomachi Nomi 向待鑿)

The Varieties of Japanese Chisels Part 7 – The Nihon Mukomachi Nomi (二本向待鑿)

The Varieties of Japanese Chisels Part 8 – The Atsunomi (厚鑿)

The Varieties of Japanese Chisels Part 9 – The Uchimaru Nomi Gouge (内丸鑿)

The Varieties of Japanese Chisels Part 10 – The Sotomaru Nomi Incannel Gouge (外丸鑿)

The Varieties of Japanese Chisels Part 11 – The Tsuba Nomi Guard Chisel (鍔鑿)

The Varieties of Japanese Chisels Part 13 – The Shinogi Usunomi 鎬薄鑿 Paring Chisel

The Varieties of Japanese Chisels Part 14 – Kote Nomi (鏝鑿Trowel Chisel)

The Varieties of Japanese Chisels Part 15 – Ootsuki Nomi 大突き鑿

If you have questions or would like to learn more about our tools, please use the questions form located immediately below. Please share your insights and comments with everyone in the form located further below labeled “Leave a Reply.” Your information will remain confidential (we’re not evil Google or incompetent facebook).