What is purpose of fullers on blades?

[thinreadline]

Originally Posted by Roland_M

Hi Fernando,

it's the same in Germany. We call it blood groove (Blut-Rinne). I think this term was invented by civilians or maybe authors of adventure books about war. My own father is one of the people which really believe, that the groove was made to led the blood away from body with more efficiency or similar stupid stuff.


Roland

This is a commonly held belief especially amongst those who have been in the armed forces. It is usually a fantasy promulgated by macho training sergeants to young recruits to scare or impress them ... even I was told this in the 1970s by several NCOs when I was in the Joint Services .

[Roland_M]

Originally Posted by thinreadline

This is a commonly held belief especially amongst those who have been in the armed forces. It is usually a fantasy promulgated by macho training sergeants to young recruits to scare or impress them ... even I was told this in the 1970s by several NCOs when I was in the Joint Services .

Thank you for your explanation. Maybe this is a kind of training sergeant running gag and they always have tears in their eyes from laughing, if the recruits believe that.

My father told this to me when I was six or so and I couldnt believe that. I have been thinking about this for a long time as a child, to find out whats wrong with this theory.

[thinreadline]

Originally Posted by Roland_M

Thank you for your explanation. Maybe this is a kind of training sergeant running gag and they always have tears in their eyes from laughing, if the recruits believe that.

My father told this to me when I was six or so and I couldnt believe that. I have been thinking about this for a long time as a child, to find out whats wrong with this theory.

Yes I think it is a common theme with training NCOs especially with young newbies

[Helleri]

I've heard it posited that a well shaped fuller or groove can increase shock absorption of a blade. On the surface of it, mechanically this does make sense. A semi circular dish is basically a vault. And vaults are good at redistributing pressure through deflection. The idea of this would be that when you hit something, that you are less likely to feel the shock of an impact through the handle as severely. However I've never seen any experiments that test this. And so I must treat this as speculation.

Blood letting and air pocketing have the same issue to my mind. They are simply theory craft that have not been tested to satisfaction that I have seen. Anything that can be submitted without evidence can be dismissed without evidence. In other words if you don't see proof you are seeing an opinion. That is not to mitigate the value of the informed or trusted opinion. As those can lead to tangential learning.

[motan]

Just few additional remarks:
I, and others, have mentioned strength/weight ratio as a factor. But this apparently very logical argument has a big hole in it. The main strength parameter to increase (per weight unit) is the lateral stiffness and this is NOT a good idea because it reduces lateral flexibility and increase the chances of bending and breaking. A lot has been done in sword forging to achieve the exact opposite-more flexibility, like in katanas (very functional swords indeed).
Air-pocketing and blood grooves do not make sense either if we consider physics and physiology. Suction (underpressure) does, in theory, because in battle situaton, a sword would have to be inserted and pulled out very quickly (much too fast for bleeding to develop). I beleive in empirical solutions, so this will remain a feeble theory untill experiments are conducted.
Why fullers then?

[David]

Originally Posted by motan

Just few additional remarks:
I, and others, have mentioned strength/weight ratio as a factor. But this apparently very logical argument has a big hole in it. The main strength parameter to increase (per weight unit) is the lateral stiffness and this is NOT a good idea because it reduces lateral flexibility and increase the chances of bending and breaking. A lot has been done in sword forging to achieve the exact opposite-more flexibility, like in katanas (very functional swords indeed).
Air-pocketing and blood grooves do not make sense either if we consider physics and physiology. Suction (underpressure) does, in theory, because in battle situaton, a sword would have to be inserted and pulled out very quickly (much too fast for bleeding to develop). I beleive in empirical solutions, so this will remain a feeble theory untill experiments are conducted.
Why fullers then?

Forgive me Motan, but it does seem to me that this question has indeed been adequately answered. The blood letting "answer" has been known to be a myth for many years know so i don't think any kind of experiments are necessary to prove this point.

[Timo Nieminen]

Originally Posted by motan

The main strength parameter to increase (per weight unit) is the lateral stiffness and this is NOT a good idea because it reduces lateral flexibility and increase the chances of bending and breaking. A lot has been done in sword forging to achieve the exact opposite-more flexibility, like in katanas (very functional swords indeed).

It's very easy to achieve more flexibility: just make the blade thinner. Thin enough, and the blade is very flexible and very difficult to break:
https://www.youtube.com/watch?v=eMAsCuDFSUI
(as long as the steel is resistant to metal fatigue). But too much flexibility reduces cutting and thrusting ability, since the blade will buckle instead of cutting/penetrating the target.

Stiffness is proportional to the thickness cubed, so the deflection of the blade by a given force is A/d^3, where d is the thickness, and A is some constant of proportionality. The strain (i.e., the proportional compression or elongation of the steel) is proportional to the deflection and the thickness, so deflection*B*d, where B is a constant of proportionality. The strain due to a given force is then A*B/d^2. If the strain exceeds the elastic limit, the blade will either break (at least crack, even if not completely break into two pieces) or take a set. Both things we wish to avoid. Note that a stiffer (i.e., thicker) blade reduces the strain - it's stronger, and less likely to break. It does not increase the chances of bending and breaking, but reduces them.

At this point, one should ask why, if this is the case, the thin very flexible blade doesn't break. The answer is that it's very difficult to apply a given force if the target gives with the force. Push on somebody hard - you can exert a lot of force. If they move away, you can't push that hard any more (without moving towards them). That's why the urumi survives. But if the blade wasn't able to get out of the way, you'd be able to press a small section of it into a tight kink without needing much force, and it would break or take a set. Since you probably don't have a urumi at hand, let alone one you're willing to destroy, you can try this with wire: compare the force you need to permanently bend thick wire vs thin wire (i.e., make the "blade" take a set). The thick wire needs more force; the thin wire needs to be bent further, but you can bend it further with less force.

The importance of stiffness for (a) supporting the blade against buckling while cutting, and (b) for strength explains why it common to see cutting-oriented swords with thin tips and thick bases. The last few inches near the tip can be about 1mm thick, while the base of the blade can be 10mm thick. A razor blade on a stiff 2.5 foot stick, rather than a 3 foot razor blade.

[kronckew]

the 'flexibility' and cutting ability of japanese blades is also a myth. if you are doing cutting with a katana, especially on stuff like bamboo, if you make a bad cut, you can bend the blade. even a master can have a bad day. with a soft spine and a hard edge, it is less likely to snap, which can get you killed in battle, a quick bend the other way can restore a bent blade, at least well enough to preserve your chances of living long enough to replace it.

japanese used layered and laminated steel to make the best of the best of the steels available to them, producing art accidentally in the process. modern mono-steels produce better swords for cutting competitions, less likely to bend or break and if made correctly, sharper - the mono steel allows a wider blade and thus a more acute edge angle.

they generally do not have bohi (fullers) to reduce cutting friction and move more weight balance out to the tip to improve the inertial forces. quick recovery to defend or re-attack is not a major requirement when cutting bamboo or pool noodles, or tatami mats.

fullers improve the strength to weight ratio, and are used for the same reason we use I-beams. a box beam (hollow square x-section) maximizes the lateral and vertical stiffness and strength to weight ratio, but it's be rather difficult to make a hollow square sword - 90 degree edges do not cut well (tho they may work for a stiletto or a pure thrusting blade/ small sword or cane sword).

fullers are used in sabres and swords because they work if done correctly, improving the balance and handling. bohi on samurai katana are also used because of the neat noise they add if you cut correctly (and don't bend your blade).