traditional katana physics

[wnease]

I'm looking for first hand info on some of the handling characteristics of traditional katanas, specifically balance point, mass, and harmonics. I understand that over hundreds of years the priorities of sword users changed and so the characteristics of the sword changed. I'm primarily interested in cutting performance.

 

[Watchful]

I have a little bit of knowledge here, but that will be dwarfed by Bors here who knows a heckuva lot more than me.

 

[Bors]

I'm looking for first hand info on some of the handling characteristics of traditional katanas, specifically balance point, mass, and harmonics. I understand that over hundreds of years the priorities of sword users changed and so the characteristics of the sword changed. I'm primarily interested in cutting performance.

You could fill up volumes with each point in your question LOL.


Keeping in mind you’re a sword maker so as not to ramble on to much since your interested in mainly cutting performance I would start by researching a 17th century smith by the name of Nagasone Kotetsu. His swords seem to be held in high regard for cutting ability.
This is one of his swords. The Tameshigiri inscription says it cut through three bodies. Tameshigiri at that time was performed on live prisoners or criminals.

The handle and furniture on the swords were subject to change. A change in rank might require a different handle wrap as well as different fittings. The sword might have two handles one for EDC and one for court. The material used for the furniture varied greatly from horn to silver, gold and iron, in addition simple or complex designs also effected the weight of the furniture. The balance of the sword can be affected buy theses changes. Presumably an attempt would be made to match the handles so as not to effect the balance to greatly but then again maybe not.

Very complex swords...

 

[wnease]

neat, the fullers are very deep, any idea how thick swords of this time were? looking at the way the planes intersect they must not have distal taper....

 

[MValborg]

i was under the impression that the balance of a katana was centered towards the belly of the blade, but ive yet to wield a true traditional quality version, so i have to assume.

 

[FortyTwoBlades]

neat, the fullers are very deep, any idea how thick swords of this time were? looking at the way the planes intersect they must not have distal taper....

There usually is distal taper in Japanese pieces, but one thing to bear in mind when examining nihonto is that since the sharpening process removes an entire skin from the blade, those blades that have come down to us may be a lot lighter or thinner than they were when new. It always presents something of a puzzle.

 

[Bors]

neat, the fullers are very deep, any idea how thick swords of this time were? looking at the way the planes intersect they must not have distal taper....

I have not ran across any hard stats on the blade thicknesses. It is true that the polishing process does remove material however you must also take into account that polishing is not done frequently. These blades were taken exceptionally good care of during their working life.

This blade was designed for exceptional cutting and looks to be fairly long. The fullers would have been necessary to help reduce the overall weight. While not immediately apparent in the photo the blade probably has a distal taper. Mass distribution in the blade is controlled with fullers, distal tapering and tapering.

Harmonics is controlled buy the curvature in the blade and the differential tempering. The curvature in the blade will dampen the fundamental frequency. The higher low frequencies are dampened by the hard area of the edge and the high frequencies are dampened buy the softer spine.

Incorrect angle (edge axis off line) at impact, poor grip, poor follow through, moving target and sword design all contribute Harmonics.

These pictures illustrate harmonics during cutting. Some swords are very forgiving as this one is and some are not forgiving at all.

This is what a perfectly executed cut should look and sound like.



The ringing is something that Albion swords seem to do which is not detrimental to performance and is IMHO pleasing to the ears. Listen carefully to the solid impact the edge axis is spot on and all the momentum of the blade and the applied force is concentrated at the edge and not wasted. The blade remains quite while passing through the target.

 

[wnease]

Hmmn, thanks for the info. I personally have terrible form when cutting, but very high impact speed, so I end up overbuilding my blades to withstand the shock (it's taught me a lot about harmonics and handles lol). I primarily cut 2x4's and 3" and 4" used nylon fast rope and tow cable, with great success, and am trying to design to the softer media that katana would cut, though the price of tatami is prohibitive for experiment.

I'm glad you mentioned the harmonic damping of the soft back... I once vac molded 4 layers of carbon fiber to the sides of a banite blade and it did modify the harmonics significantly!

Curvatures also varied through the ages... do you suppose that was due to aesthetics or technical requirements?

The fullers on the kotetsu pieces are functional at reducing mass, unlike many of the fine grooves commonly seen.

 

[senoBDEC]

curvatures were primarily a product of
A) heat treat method (clay-coat on back)
B) method of carry (horseback, tucked inside obi, length, etc).

There is no doubt that, within those parameters, there were a fair amount of variation re: curvature probably due to aesthetics.

e.g. tanto and wakizashi (or shoto) tended to be not very curved, while daito (katana or tachi) tended to have curvature centered around either the middle of the blade (symmetrical crescent shape - tucked into obi, left side, edge up.) or nearer the handle (tachi - due to cavalry-saber style hanging from belt, edge down and drawn while on horseback - also tended to be longer and less wide).

There's quite a bit of good discussion of this on the Swordmaker's Forum on swordforum.com IIRC. Free to register .

 

[Shootist]

The sori or curvature aids in the physics of cutting. It promotes a natural slicing motion instead of a chopping one. It is correct that the sori for a tachi was more pronounced as they were mainly worn on horseback and they were also worn with the ha (cutting edge) facing downward.

 

[senoBDEC]

I thought that it was the two-handed grip and it use that tended to promote slicing motion... the rear hand tends to "pull" as the hand moves.

That said, I suppose the curved blade means the edge tends to intersect and be pulled through the target. How significant this is though, I'm not sure. After all, chopping and slicing are really more technique-based rather than related to the geometry. Curved edges can chop as easily as they can slice - and the "natural slicing motion" is more based on a slightly wider grip combined with the method of powering the strike and the forward (COG) balance on most katana.

 

[Bors]

I thought that it was the two-handed grip and it use that tended to promote slicing motion... the rear hand tends to "pull" as the hand moves.

That said, I suppose the curved blade means the edge tends to intersect and be pulled through the target. How significant this is though, I'm not sure. After all, chopping and slicing are really more technique-based rather than related to the geometry. Curved edges can chop as easily as they can slice - and the "natural slicing motion" is more based on a slightly wider grip combined with the method of powering the strike and the forward (COG) balance on most katana.

The two handed motion does not promote the slicing action. How the sword is moved at the target determines chop or cut. More of the body's energy can be directed into the blade with a two handed grip. The curved and straight blades can chop and slice. Arguably the curved blade slices better because of geometry.

I pulled this from "The Book of the Sword" by Sir Richard Burton which illustrates some of the geometry differences between the straight blade and the curved one.

The curvature of the blade was not solely the function of the quenching process. The quenching process would only curve the blade so much. If additional curvature was required this would have to be considered during the forging process. Court and yes fashion also dictated to some extent how a sword should look. If the sitting ruler favored strongly curved blades political etiquette required everyone in court to also favor strongly curved blades. This sort of thing was also seen in Europe as well as many other places.

 

[FortyTwoBlades]

There's enough to this discussion that whole books can (and have) been written on it, but one of the things demonstrated in the picture that Bors posted is a demonstration of effective sectional density--i.e. how much mass is behind a surface area. If you were to draw a vertical line through the blade at the point of impact you would find a larger mass behind the curved blade as opposed to a straight blade of equal width thanks to the curvature aiding in mass placement. More mass behind the surface area = greater energy of the blow going into the cut. However, it's all a trade off. A straighter blade gives greater thrusting potential (though you may still make circular thrusts with a curved blade). Likewise with two blades of equal mass, width, and thickness, a curved blade will have less reach than the straight blade. A longer reach can equal increased tip velocity, but that's a whole different discussion.

I'm sure I'm butchering the explanation massively, but I bet Bors can correct me on anything I'm messing up.

 

[MValborg]

i was sorta hoping he would fill me in on the balance thing

 

[FortyTwoBlades]

On cutting swords it's common to find the balance to be about 6" in front of the guard. This varies greatly, of course, but it's a good ballpark.

 

[MValborg]

whoa my knock off is balanced like that, score!

 

[the possum]

The few nihonto I've been able to handle were around 1/3" thick at the base. Search for Kasane- this is the term for thickness measurement. The Kashima sisters routinely list thickness and other useful dimensions of real antiques on their site; check their catalogue and gallery for lots of info: http://www.ksky.ne.jp./~sumie99/ I took measurements of my great uncle's swords that may give you an idea of the distal taper if you're interested. The older one had been polished extensively, so it got thinner towards the tip, and had its dynamic balance point right around the yokote. His newer (1939 IIRC) sword had less taper and the dynamic balance point was about a third of the way back from the tip. They don't generally exibit much harmonic stuff compared to European swords since they're rather thick and stiff.

I pulled this from "The Book of the Sword" by Sir Richard Burton which illustrates some of the geometry differences between the straight blade and the curved one.

Just note the diagram is greatly exaggerated. The spine does not get any thinner when draw-cutting, and at least on blades with only a modest amount of curvature (or straight) you won't be cutting at a 45 degree angle (if the blade meets the target relatively straight).

...one of the things demonstrated in the picture that Bors posted is a demonstration of effective sectional density--i.e. how much mass is behind a surface area. If you were to draw a vertical line through the blade at the point of impact you would find a larger mass behind the curved blade as opposed to a straight blade of equal width thanks to the curvature aiding in mass placement.

Not sure I'd worry about this in the grand scheme of things. If you really want more sectional density, just make the blade wider from the start, etc.
Certain mediums, such as skin, just react better to a slicing/drawing type cut by nature. I can take a sharp pocketknife and press it firmly against my skin, but I don't dare draw it across my skin under that same pressure!

On cutting swords it's common to find the balance to be about 6" in front of the guard.

Keep in mind how little the balance point alone can tell us about a sword's dynamic handling and impact properties. I can take a bar and attach two 1 pound weights at either end, and another bar with just a 2 pound weight in the center. Both will have the same length, mass, and balance point, but will handle totally differently. The moments of inertia are what determine circular motions/behavior.

 

[Bors]

Balance or placement of the COG is governed in general by usage and personal preference although other factors may influence it’s placement. Placing the COG closer to the point increases blade presence (the blade feels heavy), increases the energy imparted into the target and slows down the reactiveness of the blade. Moving the COG closer to the guard has the apposite effect that is the blade presence lightens the amount of energy decreases but the sword is more reactive or faster.

If you were inclined to be facing heavily armored opponents then it might be prudent to field a sword that moved a little slower but hit harder. If you were not inclined to be facing armored opponents then a faster sword might be on order. The COG is important to the handling of the sword while harmonic dampening is important to how the sword reacts during and after impact.

 

[the possum]

Placing the COG closer to the point increases blade presence (the blade feels heavy), increases the energy imparted into the target and slows down the reactiveness of the blade. Moving the COG closer to the guard has the apposite effect that is the blade presence lightens the amount of energy decreases but the sword is more reactive or faster.

I thought we were about on the same page the last time we had this discussion, but now I see we're still not quite there.

The COG is a result of the compromises in mass distribution to control the handling and impact qualities; not the other way around. If the COG was really the only thing controlling handling behavior, then both of the weighted bars in my example would handle the same. But in fact they do not, despite having the same COG, mass, etc. I can share other examples if this is not clear enough.

 

[FortyTwoBlades]

If you were inclined to be facing heavily armored opponents then it might be prudent to field a sword that moved a little slower but hit harder.

If you were planing on facing armored opponents I'd suggest something other than a sword. Axes, maces, and polearms are kind of the only route to take short of wrestling your adversary to the ground via half-swording and then stabbing them in the neck once they're on the ground.

Possum, I realize that there's more at work than just sectional density, but it does play a role. I know what you mean about chopping vs. slicing, as I often demonstrate this to others. The biggest advantage of the curved blade for draw cutting is that the blade follows more in the arc of the swing, creating a more exaggerated drag or pull. But even a sword that's straight as an arrow will, of course, be slicing slightly even if doing a direct chop just as knives that have been freehand sharpened will always be slightly convexed.

The real question is how much slicing ability is actually needed at the sacrifice of thrusting--this is obviously a matter of personal preference, but I think that it's important to try striking a balance to maximize the versatility of the blade. If you only cut halfway through someone they still die plenty fast.