Sharpening Part 9 – Hard Steel & Soft Iron 鍛接

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 as part of the “forge-welding” process. An acidic flux powder has been placed in-between and on the metals in preparation for laminating them together into a single blade.

Men are like steel. When they lose their temper, they lose their worth.

Chuck Norris

While Beloved Customers are of course familiar with the features of the high-quality woodworking blades we purvey, some Gentle Readers may have little knowledge of the important details of Japanese woodworking tools. So in this article 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.

Your humble servant believes an understanding of these basic facts will aid Beloved Customer’s sharpening efforts, or will at least tickle Gentle Reader’s 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 1850’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 production costs by minimizing the amount of precious steel used to make axe, scythe, plane and chisel etc. blades. This was achieved 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 because of some navel-gazing infatuation with the archaic, but because it has serious advantages.

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/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.

Here is the key point to understand: When a blade made from a lamination of high-carbon and low/carbon steel is quenched, the sudden temperature change causes the high-carbon steel layer to become hard, even brittle, while the softer low/no carbon layer is unaffected and remains soft.

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

Why go to so much trouble? One advantage of this construction is that it allows the cutting edge to be made much harder than is possible in the case of an non-laminated blade therefore staying sharper longer in use than softer blade. But why does lamination make this possible? Consider the absolute fact that a chisel blade made of uniform material heat treated to a uniform hardness of, say, HRC65 might cut very well, and stay sharp a long time, it will always break in use. Not just chip, but actually break in half. The softer low/no carbon jigane layer supports and protects the hard high-carbon layer preventing it from rupturing. Such durability is one hell of an advantage.

Another benefit of laminated construction is ease of sharpening. Remember, the harder a piece of steel is, and the larger the its area, the more work it takes to abrade it. But in the case of a laminated blade, the amount of hard-steel exposed at the bevel the user must abrade is just the relatively thin strip of shiny metal seen in the chisel photos above and below. Please also recall that the darker low/no carbon layer jigane is dead soft and melts away on the sharpening stones without much effort.

So the laminated construction of hard hagane to soft jigane produces a blade that is tough but at the same time hard, one that will become very sharp and stay sharp a relatively long time thereby improving work quality and productivity while at the same time reducing the time spent sharpening.

BTW, this is not a technique that was invented in Japan, it’s just the Japanese blacksmiths that continue to employ this ancient and clearly superior technique, at least, that is, for a little while longer. A word to the wise.

A 42mm Hidarino Ichihiro Oiirenomi

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 in the bowels of 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 iron ore in a crucible and pouring it into molds “casting” a strip, bar, or ingot of high-carbon steel which is then forged to make the blade, hence the name.

This became possible only when the technology required to reliably and fully melt steel to a liquid state on an industrial scale was developed. Such steel was also called “Crucible Steel” after the crucible container used to melt iron ore.

This technology was widely used in the United States and Europe through the 1870’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 reused 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.

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 of chisels was historically a simple flat plate in Western blades. This is still 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 them well.

If Beloved Customer will carefully consider the blades pictured in the four photographs above, you will notice the lighter-colored hard steel lamination wrapped up the chisel’s sides 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, Japanese carving chisels are not typically made this way, and are consequently structurally weaker.

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 your humble servant’s knowledge. We will look at this design detail more in the next article in this series.

The Point

What does any of this have to do with sharpening? Allow me to focus what we have discussed previously.

These design details cleverly turn potential disadvantages 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 65~66 HRc in hardness. Western blades are made of a single piece of steel heat-treated to approximately 50~55 HRc to make the tool softer/tougher thereby limiting breakage. The extra hardness of the Japanese blade helps it 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. Oh my, that would be bad.

The softer low-carbon/no-carbon steel or iron jigane 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 (or “flat” as it is called in Western chisels) is all one-piece of hard steel. Without the hollow-ground ura depression, you would need to abrade all that hard steel at one time to initially flatten and regularly sharpen the blade, a necessity I guarantee would ruin your mellow mood even if you consumed 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) surrounding the ura. This is exceedingly good.

The ura depression makes it easier and quicker to not only sharpen the blade, but also 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, including the final article in this series.

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

It is not my intention to be fulsome, but I confess that I covet your skull.”
Sir Arthur Conan Doyle, The Hound of the Baskervilles

If you have questions or would like to learn more about our tools, please click the “Pricelist” link here or at the top of the page and use the “Contact Us” form located immediately below.

Please share your insights and comments with everyone in the form located further below labeled “Leave a Reply.” We aren’t evil Google, fascist facebook, or thuggish Twitter and so won’t sell, share, or profitably “misplace” your information. If I lie may I cough up a hairball during every meal.

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

24mm Mentori Usunomi by Sukezane (Face View)

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.

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)
24mm Mentori Usunomi by Sukezane (Ura View)

Description

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

The mentori usunomi has a streamlined cross-section similar to 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 using hours on end.

In comparison, lacking the steel crown and mushroomed handle, the usunomi is more comfortable to use. More importantly, the blades and handles of these chisels are longer and lighter in weight providing superior angular control for precision paring operations.

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. But Japanese paring chisels like the usunomi have a few potential advantages worth considering.

The most significant advantage is that the steel cutting edges of Japanese paring chisels are much harder. The paring chisels our blacksmiths forge are around 65~66 HRc in hardness, whereas Western paring chisels are usually around 55 HRc. A Western style paring chisel with its thin blade of uniform steel hardened to 65 HRc would easily snap in half in practical use.

This extra-hard lamination is hand-forged by our blacksmith from Hitachi Metal’s Yasugi Shirogami No.1 steel (aka “White Label Steel”), an exceptionally pure high-carbon steel that 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, especially important in the case of the hard steel used in our chisels. 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 un-reinforced 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 nature of the wood you need to pare and the type of paring you intend. For instance, paring end the grain of maple may require a steeper angle than when paring the long grain of pine.

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 after 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 of Mitsuura
30mm Unsunomi by Nagamitsu – View of Face
30mm Unsunomi by Nagamitsu – Closeup 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 used with 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 peregrine to pull a plow, neither oiirenomi nor atsunomi are as effective as the usunomi for paring and cleaning joints.

A Shire Horse and His Little Friend. Stout, heavy and strong is good for some jobs, but…
Slim, light, fast and sharp is better for others.

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

If you have questions or would like to learn more about our tools, please click the see the “Pricelist” link here or at the top of the page and use the “Contact Us” form located immediately below.

Please share your insights and comments with everyone by using the form located further below labeled “Leave a Reply.” We aren’t evil Google, fascist facebook, or thuggish Twitter and so won’t sell, share, or profitably “misplace” your information. If I lie may a shire horse polish his hooves on my back.