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Knife Design

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 "The glare of the morning sun
Deep snow on the peak
The sky brightens
Rivers of mist around Fuji defy description"

A swords design depends on its intended function, the cultural context from which the weapon arises and the tools, materials, working methods and proclivities of its maker. Considering all swords are required to perform the same basic functions, cut, thrust, parry and block, it seems surprising at first that thay should be so varied in design, both throughout time and across cultures.

Most peoples exhibit quite a variety of form in blade design. In the Philipines for example the variety of blade shapes is mind numbing. The Japanese on the other hand explored one form only and relentlessly persued perfection through a thousand permutations.

The progenitor of the modern katana, the tachi, was designed as a saber to be used by mounted archers who fought in the model of the Mongol. The tachi was used as a back up weapon after the archer's arrows were discharged.

As with most cavelry sabers the tachi has destinct curvature, generous length and is suspended edge down. It was also lighter in crossection than the modern katana.

Changes in battle tactics around 1400AD brought about a revolution in design that eventually led to a sword called an uchigatana. Due to changes in armour the blade became marginally heavier. Fighting on foot became more prevalent so the sword lost a little of its curvature and was shortened somewhat. Rapid deployment of the weapon became a priority so it was worn edge up in order that the draw and cut could be made in a single fluid movement.

The end product of all this was what we now call the katana, which has remained fundamenally unchanged since. This "shinto" or new sword retains the characteristics of the "koto" or old sword to the extent that to the casual observer, the two forms are not readily distinguishable.

The design of the blade is deceptivly simple. Single edged, moderately curved with a ridge line either side. Around these parameters of course we find a host of variations.

Fig 1.

In order to help describe these variations Japanese terms are used as well as English which alone is not sufficient to describe the Japanese sword.

The point or kissaki is the most important part of any sword, even those not designed for thrusting. Invariably when swords are of equal length, most contact was with the final inch or so of blade. Swords were generally wielded at arms length so cutting and trenchancy at the point was essential.

Kissaki are of very sturdy design and come in many types based on shape and size. Longer points penetrate soft materials efficiently, but are suseptible to damage against harder materials such as the lamellar plate armour of feudal Japan. Shorter points by contrast fare better but trade off on penetration. Point length also effects cutting ability. The most common varient, medium or "chu- kissaki", probably reflects most peoples recognition of these trade offs.

Fig 2

Edge curvature of the kissaki also effects cutting and penetration. A straight edge favours thrusting while the curved edge favours cutting. Note "fukura - kareru" and "fukura - tsuku" in Fig 2 Another varient "ikari - kissaki" offered greatly improved cutting to the detriment of thrusting and must have been a nightmare to resheath. So I thrust I make my point,(--grin--). Yeah I know it's a bit dry.!

Another feature of the kissaki is the temperline at this part of the blade. Called the boshi, it is classified separately from the hamon proper and its descriptions are based on shape and size.

The "yakote" is the frequently indistinct line which separates the kissaki from the rest of the blade. This line is established by carefull polishing and its presence is often more cosmetic than real. Nevertheless it is an important feature of the kissaki as it helps to define it and provides a, "virtual" or actual, second point.

The sword's cutting edge and it's support structure the spine are not simply a means of extending it's point. If such were the case a 3 inch blade on the end of a stick would suffice. The guy with the longest stick wins, right!. While as mentioned earlier, most cuts were made with the final few inches of blade, the extension of the cutting edge from point to grip is what separates the sword from its cousin the spear. Against a spear, once inside the arc described by the spear head one is relatively safe. But the edge of a sword blade is a different proposition.

The cutting edge or "ha" is the most visually arresting feature of the Japanese sword. Formed of almost full hard steel, clearly contrasting with the softer steel of the spine and separated from same by the "yakiba", the differentially hardened edge is one hallmark of a sword's quality.

Fig 3.

Steel becomes brittle on hardening but will not hold an edge when annealed. The "ha" and its deliniating "yakiba" are not simple cosmetic in nature but part of the Japanese solution to that eternal trade off between hardness and toughness. Yakiba, despite being more or less identical in their functional characteristics were produced in a wide variety of form mostly but not entirely related to the aesthetic whim of the smith. This is reflected in the use of the term "hamon" literally "edge name", the shape of the yakiba being one of the ways in which the smith was identified. An example of this is the "sanbonsugi" form which can be attributed to Kanemoto, or a student of his, with a fair degree of reliability. Over 200 different variations on yakiba are known to exist.

Features of the "yakiba" such as "ashi", ie. "lightening strikes" do however affect the functional characteristics of the blade and should be noted as they are an indication of quality. These are visible on the surface as thin points of martensite extending toward the edge. They have a tendancy to reduce the development of longtitudinal fractures.

Smiths developed such a high degree of control over the heatreating process that complex hamon could be produced at will. While some forms were restrained and formal, others were free and wild.

"Utsuri", bright cloudy areas of cementite, on the upper ji, are also a note worthy indication of complex hardening proceedures. Few modern smiths can produce such effects and even those who can, find it a haphazard and elusive phenomonon.

No effort was spared in the persuit of the ideal sword. Lamination was employed to increase shock resistance. These laminations are visible on close examination of the "ji". Fig 4.


Most are manifest as subtle water marks on the surface of a well polished blade. However, not all Japanese swords were laminated, especially factory made blades of the "Showa period" Handmade homogenous blades were also made during this period and before, especially when expedience was paramount. Such blades were quite servicable.

One smith made a point of the fact that his swords were made from imported Wootz from India. Thus they were therefore, not mechanical damascus but the true "Damascus" of legend. Such blades were not laminated but were forged from high/low carbon rounds cast in enclosed clay crucibles. These rounds looked rather like a bran muffin.

Not content with the foregoing most Smiths adopted the practice of forging sandwiches with varying carbon content in their constituent parts. In its simplest form this might take the appearance of a blade sized bimetal strip comprising of high carbon and low carbon steel. This process is still used in the manufacture of Japanese kitchen knives today. Another form and perhaps the most common involved wrapping a piece of high carbon steel around the low carbon, rather like wrapping a slice of bread around a piece of cheese and about the same size and shape as it happens. This "sandwich" was then pounded into the shape of a blade, "makuri-gitae", Fig 5

Fig 5.

Carried to it's ultimate conclusion as many as five different grades of steel might be used in a single blade. Each of these grades were folded independantly to some thousands of layers then all five were combined in a single block from which the blade was forged.

One aspect of blade design often overlooked is the effect that stress risers can have on performance.
Rapid changes in crossection lead to a buildup of stress at these transition points which if sufficient can fracture or break the blade. Such changes in crossection should be minimised and gradual.

Fig 6.

Hold a pencil in your fist as you might a knife. Now pull back on the point with your other hand until the pencil breaks. Most of the time the pencil will break where it leaves your fist. This is a graphic reminder that the area of transition, around the guard is the most critical part of any blade. A note worthy feature of the Japanese sword is that the notches as well as being minimised are situated about an inch forward of this area and so are less stressed than they might otherwise be. The Japanese sword is the only form to address this problem in such a novel manner. It is also the heaviest part of the blade. Contrast this with the Philipino kris. In fact the weakest part of a Japanese blade is the area around the peghole, and that is inside and somewhat buttressed by the grip. I must say here that I've always felt that the peg hole would serve better if situated closer to the butt. But typically it is situated on the second, or sometimes the third diamond behind the guard. This may have something to do with keeping the peg in the center of rotation as the sword pivots in the hand. Of course it also ensures that the peg is prevented from falling out.

While we are on the tang its worth noting the length, curvature, and taper of same.

Fig 7.

As can be seen in Fig 7 top row, nakago or tang come in quite a variety of shapes and sizes, but generally speaking all can be considered a derivative of the ubu form being for the most part cut down versions of same. The lower row shows tang end shapes. Tang length is between a 1/4 and a 1/3 of blade length for a katana length sword.

Fig 8.

Blade width affects acuity. A 1/4" thick blade only 3/8" wide is not very sharp. The same thickness but 2" wide could be quite sharp.

The angle at which a blade cuts, affects the way it cuts. The wedge angle at B, fig 8, is less than at A. The angle at B is smaller because the width of the blade is effectively greater, by virtue of the direction the blade is moving. Therefore curving a sword is one way to get a sharper edge without any extra weight penalty. It also yields more edge per blade extension.

To facilitate withdrawal, ideally the curvature of a sword should be based on the curve described by the withdrawing hand. This is the circumferance of a circle the radius of which is the length of the arm from the hand to the spine. At least this would seem to be the logical choice for a purely cutting sword. In practice however the radius is closer to the height of the weilder. This is especially so of shinto period swords. Many swords will be found to have a dual curvature

The length of the sword is directly related to the height of its wielder. It must be short enough to withdraw comfortably and it must not touch when swung at the ground. Tachi can afford to be longer since the weilder is mounted and therefore farther from the ground.

Grooves in the shinogi ji area of a blade lighten it and improve flexibility. Because of the I beam crossection, the strength/weight ratio is considerably improved. Generally the grooves were forged rather than carved into the blade. Forged grooves are superior to carved as the forging aligns the structure of the steel like the layers of a onion, the fibers following the direction of forging. Over 50% of grooved Koto period swords were carved while the figure is closer to 80% for shinto period swords. Working practices are obscured by somewhat frequent, after the fact, alteration for repair, lightening, or at the request of their current owner. Carved grooves can be identified by examining the grain on the inside of the groove. If the layers intersect the inside of the groove it is carved, if the layers follow the curve of the grove it is forged.

Fullered blades irrespective of length have better strength/weight ratios and flexibility is also improved. As long as they are properly constructed and designed handling and performance are enhanced. Even with the effect of leverage removed, there is still a decided advantage to fullering small blades.

Cultures from Turkey to the Philipinesand Europe to Africa made blades with single and double ribbed crossections. The Chora points to the efficacy of this design and is in effect, half an I beam. It is also light and sturdy and a joy to handle. An Arab bayonet I have is both flexibly and strong and its also gratifyingly light to the poor sod who has to drag it all over creation.

The Marples Ideal huntimg knife is not simply a throwback to the bayonet. Its strength handling and dependability made it very popular.

The Boye Basic would be too heavy and unweildy without its I beam construction and here the effect of leverage is eliminated as the weight reduction is inside the grip.

Not for the faint of heart, this blade making!. Breakage was a problem, why else go to such lengths? The solutions offered here are not unique to Japan. Other cultures have used some or all of these methods to varying degrees from time to time. The Vikings and early Germans made multiple alloy composites from prefolded billets which were later forged into blades. In the Philipines exquisite laminations were effected by the inclusion of layers of nickle. Philipine blades frequently show composite construction methods also.

Swedish and Finish cutlers regularly sandwich high carbon steel between two layers of low. Check out the Finish pukko next time the opportunity arises. Stainless steel is also being tri-laminated by the Sandanavian cutlers in more recent times.

The contribution that American cutlers have made to the "rediscovery" of lost processes cannot be over looked. In particular the rediscovery of Wootz. Oh joy!. The use of "Zone tempering" is also quite prevalent in the US among custom makers. And now that some cutlers are using power hammers and hydraulic presses, great pressures are attainable, so that laminations of stainless for mechanical damascus and composite are feasible. These are heady times!


Knife Design
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