Popular Mechanics (South Africa)
WOODWORKING: JAPANESE CHISELS
Part four, the last in our series on Japanese woodworking tools, cuts into the subject of hand chisels.
I’VE SAID IT BEFORE. The real working part of any chisel or blade is the meeting of two facets of a piece of steel. The keener that this meeting is, the sharper the instrument, and the easier the chisel will be to control.
Japanese chisels, or nomi, have an unfounded reputation for only being suitable for softwoods. Although traditional Japanese woodwork is generally done using softwoods, these wonderful chisels work in any woods, the same as their Western counterparts.
Japanese chisels come in a variety of styles, usually based on their intended use. Most are tanged and have a tapered ferrule where the tang enters the handle. If a chisel has a hoop, ring or ferrule (sagariwa) on top of the handle, it can be struck with a steel-headed Japanese hammer. If no top hoop is present, the chisel is intended to be pushed by hand (not struck) and may have a longer handle to facilitate this hand use.
Oire nomi (butt chisels), which are used for most workshop tasks, are available in the widest variety of widths and typically have blade lengths between 50 mm and 75 mm. Generally, they have thin blades and come with a top hoop so they can be struck.
Atsu nomi resemble butt chisels, but are thicker and stronger. They are used by carpenters and cabinetmakers to make large joints. Blades are usually about 12 mm to 50 mm in width, but may be as narrow as 3 mm or as wide as 90 mm.
Mukomachi nomi (mortise chisels) are striking chisels intended for making small mortises or grooves. The narrow blade has a neck that’s the same thickness as the cutting edge, which gives the blade the extra strength necessary to cut deeply. The blade’s cross section is rectangular in shape with a hollow back and slightly concave top and edges. This concavity reduces friction when withdrawing chips in a narrow mortise.
Tsuki nomi (paring chisels) are push chisels, used with two hands for cleaning mortises and smoothing joints. The blades are sharpened at a low angle for easier paring.
Kote nomi, like paring chisels, are push chisels but with an offset blade or ‘crank-necked’ blade. They are used for cleaning out long joints, such as housing joints or sliding dovetail joints. The offset blade makes it easier to clean the joint without interference from either the handle or the user’s hand.
Knowing and properly understanding what underlies the construction of a Japanese chisel will empower you to use these chisels for any woodworking task. The reality is that Japanese chisels can take an extremely sharp edge that lasts a long time, due mainly to the treatment and type of steel used for the cutting edge.
Japanese tool steel typically has a higher carbon content and relatively few alloying elements compared to Western tool steels. That leads to a higher carbide content in Japanese chisels when compared to their Western counterparts.
Today, the most common steels used in Japanese woodworking chisels are ‘white steel’ (shirogami) and ‘blue steel’ (aogami). Interestingly, the two names come not from any particular characteristic of the two steels, but rather from the colour of the paper used to package these steels, which is manufactured by Hitachi in Japan. (Editor’s note: Zoolander wasn’t involved in the naming, either.)
The main difference between the two is that blue steel (aogami) has specific alloying agents added that give it increased abrasion resistance and a more durable edge. White steel (shirogami), on the other hand, is easier to sharpen, and can also achieve a substantially durable edge. This can be likened to the difference between Western O1 and A2 steels.
Now we get the important part. The steel in Japanese chisels is hardened to a higher degree than most Western chisels. Japanese chisels with a Rockwell hardness of 64 or higher on the ‘C’ scale are not uncommon, whereas Western chisels typically have a Rockwell hardness of 60–62. The added hardness results in the edge being less likely to deform under impact, such as when chopping hard with the chisel.
Secondly, Japanese chisels also undergo a forge-welding process while they’re being manufactured. The repeated hammering in this process causes the carbides present in the steel to become very small and evenly distributed, which results in an extremely sharp and long-lasting edge. There is, however, a downside to this extremely hard steel that can take an incredible edge – it comes at the cost of being brittle. If a chisel were made of a very hard steel, it’s likely that it would snap under use.
This is most probably the main reason why Western chisels aren’t often hardened past a Rockwell grade of 60–62, because O1 and A2 steels will also become brittle if they are treated to be extremely hard.
As mentioned, if one was to make an entire chisel out of blue or white paper steel, it
would most likely snap during use due to the steel being brittle. This is where Japanese blacksmiths really excel – they weld-forge the very hard blue or white steel on to a softer iron backing layer.
After shaping, hardening and tempering, the high carbon steel becomes the cutting edge and the softer iron forms the structure for the handle and shank, and the all-important support for the cutting edge. Effectively, the lamination of the two steels offers the perfect marriage of high impact capability and excellent cutting characteristics.
In addition, sharpening the bevel of a laminated steel Japanese chisel is easier than a high-quality Western chisel. This is because only a small part of the bevel is made up of the blue or white paper (very hard) steel, while the balance of the bevel surface is softer steel.
During the forge-welding process of laminating the two steels together, a hollowing occurs on the back of the chisel blade, apparently due to differentiation in the cooling contraction rates of the steels. This is not a bad thing at all for us as users of these tools, as the chisel is then slightly hollow-ground in the central rear area during finishing.
It is, in fact, an advantage, as it is much easier to achieve a flat surface on the rear of the blade, simply because we are not having to grind away the entire surface to achieve flatness (as we have to do on our Western chisels).
As a closing thought, bear in mind that a reasonably good starter set of three Japanese oire nomi chisels will not break the bank, and in my opinion offers excellent value for money. Prepare them correctly, sharpen them properly, keep them finely honed, and they will give you many years of service, productivity and woodworking pleasure.