Hand Tools #6: Japanese Woodworking Tools - Part 4: Chisels
Posted by Tobias Lochner on
It's hot here in Swellendam, really hot...39º! My maple trees are happy and in full foliage, their exquisite colours defy description and in my limited spare time I am creating three grandfather clocks for clients.
This article will be the last part of my Japanese Woodworking tools blog series.
I hope and trust that the series has been informative and has helped to introduce you to the wonderful world of Japanese craftsmanship.
Japanese Woodworking Tools - Part 4 - Chisels
As I have said many times, 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 and easier the chisel will be to control.
Japanese chisels (Nomi) have an unfounded reputation of being suitable only for softwoods. Although traditional Japanese woodwork is generally done in softwoods, these wonderful chisels work in any woods, the same as their Western counterparts.
Japanese Chisels (Nomi) come in a variety of styles, usually based on their intended use. Most Japanese woodworking chisels 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 head 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), used for most workshop tasks, are available in the widest variety of widths and typically have blade lengths between 50mm and 75mm. 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, Atsunomi are used by carpenters and cabinetmakers to make large joints. Blades are usually about 12mm to 50mm in width, but may be as narrow as 3mm and as wide as 90mm.
MukomachiNomi (Mortise Chisels) are striking chisels intended to make small mortises or grooves. The narrow blade has a neck 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 are push chisels like paring chisels, but with an offset blade or "cranknecked 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 Western chisels.
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 are manufactured by Hitachi in Japan.
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 is somewhat like 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 in their manufacture. The repeated hammering in this process causes the carbides that are 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 an entire chisel were made of a very hard steel, it’s very likely that the chisel would snap under use.
This is most probably the main reason why Western chisels aren’t often hardened past a Rockwell hardness of 60-62, because O1 and A2 steels will also become brittle if they are treated to be extremely hard.
For you budding metallurgists out there in woodworking land, here is some more information on Japanese tool steels used in making chisels:
Shirogami White steel (White Paper Steel) is named for the wrapping paper used by its manufacturer! It is used to make tools that can be sharpened to an excellent edge with good quality natural stones or fired ceramic stones.
It is a carbon steel with only very small amounts of the impurities P (phosphorus) and S (sulfur). It has a very narrow range of temperatures for hardening and thus requires the blacksmith to be extremely skilled. There are three forms of White Paper Steel, each with a different carbon content:
#1 (1.2- 1.4% Carbon)
#2 (1.0-1.2% Carbon)
#3 (0.8-0.9% Carbon)
Aogami Blue steel (Blue Paper Steel) also contains very little Phosphorus and Sulfur, but W (tungsten) and Cr (Chromium) are added to make the hardening temperature less critical and to increase wear resistance for longer lasting sharpness.
There are two grades of Blue Paper Steel, each with a with different carbon content:
#1 (1.2-1.4% Carbon)
#2 (1.0-1.2% Carbon)
There is one more type called Super Blue Steel, with more W and Cr, plus Mo (molybdenum) for additional toughness and wear resistance. This also widens the hardening step; it can be cooled in oil instead of water.
In the power grinding steps of manufacture, the white and blue steels can be distinguished by their sparks. White steel produces many bright sparks, but blue steel produces fewer, smaller sparks.
Kigami Yellow steel, also known as “Yellow Paper Steel”, because it (you guessed it...) comes wrapped in Yellow Paper... contains fewer impurities than the JIS SK steel, and this steel is commonly used for saw blades and kitchen knives. JIS SK Steel JIS (Japanese Industrial Standard) SK steel contains a lot of P, S and other impurities, but is very easy to temper and is most often used for manufacturing cheaper knives and tools.
So much for learning about paper wrappings...
Back to our actual Japanese chisels. As I stated earlier, 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.
Here is where the Japanese blacksmiths really excel, they weld-forge the very hard blue or white steel onto 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 with excellent cutting characteristics.
In addition, sharpening the bevel of a laminated steel Japanese chisel is easier than a high quality Western Chisel, 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.
Another thing to 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. Simply prepare them correctly, sharpen them properly, keep them finely honed and they will give you many years of pleasure and service.
Should you have any questions or comment regarding Japanese chisels and achieving their proper initial setup and sharpening, please use the comments section below the blog article. Alternatively you can call Johan Pieterse at Toolcraft on 021-705-1247 and he will give you all the information you need.