Case Study: Making a Hon-Yaki Nata

Nata (屶, directly translated “mountain sword”, or 鉈) come in various sizes and shapes, but the type most familiar in the west does the duty of a light brush hatchet or heavy camp knife. Common characteristics include thick spines and heavy blades, often with single beveled edges similar to Japanese wood chisels. This type work well for medium duty camp tasks, carving hatchet work, roughing and shaping, green wood work, and bamboo splitting. Similar to boat builder’s or timber framer’s slicks, they can make controlled straight slices due to their mass and chisel-like bevel. Another common variation has double sided bevels, cord wrapped integral handles, and curved or hooked blades for working in the rice fields.

This article will not aim to document the standard methods of nata handle construction, but will cover some of the considerations for undertaking a nihonto-fusion style method of mounting. Nor will this be a complete tutorial format, but rather an overview of some of the important points for the successful engineering of a knife in this manner.

Construction
Blade Geometry
Yaki-Ire
Handle Mounting
Final Assembly

Construction

Most nata are permanently mounted to a hardwood handle because it is faster and easier for production. However, this piece has been assembled in the takedown style using elements of nihonto handle engineering and features swordsmith style hon-yaki edge hardening rather than a thin steel edge laminated to an iron body.

Island Blacksmith: Charcoal forged nihonto fusion knives made from reclaimed and natural materials using traditional techniques

Blade Geometry

This style of nata has a straight edge and squared tip with a thick spine and a single beveled (kiriha-zukuri) edge similar to that of a Japanese wood chisel. The raw material in this case was exactly half of a reclaimed lawn mower blade, the tip next to the center hole and the worn-down former cutting edge area becoming the tang. When using lawn mower blades it is best to look for older ones as some of the new ones are made from air-hardening steel rather than simple carbon steel (this one was stamped “made in USA”).

Forged from half of an old lawn mower blade, nothing was wasted as the full volume of steel was reshaped into the nata blade.

When forging, the blade must be upset quite a bit from edge to spine to thicken the spine of the nata to about 1/4″ and reduce the depth of the blade to just under 2″. The volume of steel in this case produced a blade around 7″ long. The ura (back side of the blade, in this case towards a right-handed user’s left side) has a slightly (~1mm) concave shape forged into it for the purpose of flat sharpening, and the omote (front side, to the right of the user) bevel was forged in roughly and filed clean before hardening.

A slight concave is forged into the ura side for better performance and sharpening.

The edge is polished and honed. Beveled like a wood chisel, it has a very keen edge but retains plenty of mass behind it for heavier tasks.

The tang tapers slightly but not as much as a tanto might and has a rectangular rather than triangular cross section. Standard style nata must have shorter tangs due to the method of slitting the handle right through, but in this case the tang will be hidden so longer is preferable, this one is at 3.25″ and still leaves plenty of room for the lanyard eye. There should be a shoulder at top and bottom so the ferrule can sit flush against the blade.

The tang should taper and there should be two well-defined shoulders for the ferrule to rest firmly against.

Yaki-Ire

There are two approaches to the edge hardening, the standard and recommended one being to apply the clay mask an even distance from the edge on both sides, around 0.25″ or so (depth results will vary depending on the steel used). The goal is to form the hamon about halfway up the bevel on the omote side and the same distance on the ura, in a straight line (suguha). This should provide plenty of strength and support behind the hardened edge and make sharpening easier than with a fully through-hardened blade. If this approach is used, the blade can be sharpened on the ura or the omote.

In this case the clay was placed about 1/8″ from the edge on the omote and 0.5″ on the ura. This method creates a thinner hardened area on the bevel side but extends it farther up the flat side to prolong the sharpening life of the edge. Uneven clay application is a difficult technique as the uneven hamon pressure will bend the blade to one side (the omote), therefore the best approach if attempting this technique is to pre-bend the blade in the opposite direction so that it will be straight when hardened. If this approach is used, the blade is mostly sharpened on the omote and only slightly touched up on the ura.

Unlike most traditional tools, this blade is constructed and hardened in hon-yaki style, a difficult technique usually reserved for making swords. The clay mixture forms a heat mask that will insulate and toughen the body of the blade while the edge cools rapidly and becomes very hard.

In this case, the ura has a wider hamon area than the omote, but this will cause the blade to warp and should be compensated for in advance.

Handle Mounting

A forged steel ferrule is absolutely necessary for this type of knife due to the strain placed on the handle. The ferrule should be at least 1/16″ thick and preferably 3/32″. It should fit snugly against the top and bottom of the tang, sitting tightly in the notches and taper slightly larger as it goes back (the tang will be tapering slightly smaller as it goes back). The profile can be round or slightly oval.

The ferrule should rest squarely against the shoulders and very snugly on the tang. It must be quite thick for both strength and to allow for enough wood next to the tang in the finished handle.

The ferrule tapers slightly outwards as the tang tapers slightly inwards. This allows for a snug fit of both parts to each other and to the wood.

A tough hardwood block can be split and carved out to match the tang exactly and then glued back together before shaping the outside. The shoulder for the ferrule is carved first and then the handle shaped from it. The wood must fit exactly inside the ferrule as this intersection carries most of the force from the blade to the handle. When designing the handle, consider that the tang needs to have sufficient wood to the top and bottom to withstand the stress of chopping. In this case the handle is just over 5″ long. After shaping, the Sapele wood will be sanded smooth and coated with multiple thin wiped-on layers of natural urushi lacquer.

Refining the split and carved out Sapele wood handle blank. The fit must be perfect when finished.

Because of the rectangular cross section and flat sides of the tang, all of the carving can be done on one side. The benefit of this method is that the stress will not be on the joint, but fully contained in one of the halves of the wood block.

When shaping the handle, begin with the ferrule and always maintain a reasonable amount of wood above and below the tang. Frequent checking of alignment is important.

Final Assembly

The mekugi hole through the handle should taper slightly and the back of the tang hole should align perfectly with it when fully seated. The peg should be carved from strong bamboo and fit as exactly as possible to the tapered hole. The outside of the bamboo is the strongest part and should face towards the back of the handle. When looking at the end grain it is the area with the most holes or “dots” on it. When tapering the mekugi the outside edge of the bamboo should not be removed at all so that the strong side of a finished mekugi has lines running from top to bottom.

Note that the tang hole is slightly larger than the hole in the handle (this is also the small side of the mekugi-ana). This is to ensure that the peg tightens on the wood rather than jamming in the tang. The back of the tang hole, however must align exactly with the peg when fully seated so that the blade cannot move.

Note that the wood becomes paper thin as the tang widens and the ferrule narrows. Some of this can be removed if it becomes too thin to stay in place.

There is slightly more material on the edge side of the tang so the wood extends further up under the ferrule.

A natural waterstone polished hamon can be seen clearly at a certain angle of light. Note that the nata blade retains its full thickness all the way down to the start of the bevel.

Island Blacksmith: Charcoal forged nihonto fusion knives made from reclaimed and natural materials using traditional techniques

See the full specs of this nata here: Available Work: Hon-Yaki Nata