What is purpose of fullers on blades?

thinreadline · 23rd June 2016, 03:18 PM

Quote:

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 · 23rd June 2016, 09:53 PM

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 · 23rd June 2016, 11:17 PM

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 · 24th June 2016, 12:48 AM

Quote:

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 · 24th June 2016, 12:07 PM

Quote:

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 · 24th June 2016, 01:26 PM

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).

motan · 28th June 2016, 10:43 PM

To Timo Nieminen and Kronkew. I also anderstand the basic physics. After all, this is no rocket science. But this does not explain everything. A cutting blade should have a balance between strength, flexibility and cutting power. The question is whether specific blades fit their specific purpose. Beams are made to absorb lateral forces, while cutting tools are generally made to absorb forces parallel to the sides of the blade (although bad hit can exert lateral force too). A blade that has thick and stiff spine will have less cutting power and less flexibility, while a thin blade will cut better, but might brake more easily. I personally think that ethnographic weapons take their shape mostly from tradition and esthetics and less so from function. Can you explain the fantastic shapes of oceanic clubs by their function? Can you explaing the totally different geometries of Ottoman kilij and Persian shamshir by different in their use? I can't.
Just as anecdote: when I was young, I worked in a banana plantation for sevral years. Instead of machete, we used Victorinox butcher knives of the largest size available. Why? Because their cutting power is far superior and you could do much more work in a day. However, they did brake occationally and were expensive to replace.

motan · 28th June 2016, 10:58 PM

Sorry David but you misunderstood me. Naturally, there is no reason to test the "blood grooves" theory in practice because it just doesn't make sense in the first place. I was talking about something else. I presonally don't think fullers reduce the resistance to stabbing. However, they can, at least in theory, reduce the resistance to pulling out the weapon AFTER a stab if it needs to be withdrawn quickly. Outer pressure should push the flesh onto the blade and the vacuum created by pulling may resist it. Fullers can relieve this vacuum by letting air and fluid IN. This is however all theory. I know these forces exist but I have no idea if they play a role in practice.

kronckew · 29th June 2016, 08:33 AM

the ultimate 'blood grooved' 'shiv' knife: take a piece of steel tubing of appropriate length, bed or chair leg, about a foot should do. cut the tube at an acute angle from one end but not too acute such that you have a sturdy tip & leave al least 5 inches for the grip. sharpen. wrap grip in friction tape. push forcibly into a target containing fluid. possibly an unwilling 'volunteer'. see what happens.

i seem to recall peasants in 'seven samurai' making spears from bamboo like that. ruined the bandits day.

p.s. - to summerize my thoughts, metal was expensive, smithing techniques learned on bronze and copper weapons that maintained stiffness while reducing the required amt. of material for a specific length weapon by adding grooves and fullers would as a side effect produce a artistic appearance that might be carried on even into stronger materials where they technically might not be needed, tho if you are making a few hundred thousand sabres for queen and country, the material savings in steel, weight carried by the trooper and horse, extra calories to carry heavy swords, and transport of spares, etc. could be worth a few shekels.

Jim McDougall · 29th June 2016, 06:02 PM

Quote:

Originally Posted by motan

Sorry David but you misunderstood me. Naturally, there is no reason to test the "blood grooves" theory in practice because it just doesn't make sense in the first place. I was talking about something else. I presonally don't think fullers reduce the resistance to stabbing. However, they can, at least in theory, reduce the resistance to pulling out the weapon AFTER a stab if it needs to be withdrawn quickly. Outer pressure should push the flesh onto the blade and the vacuum created by pulling may resist it. Fullers can relieve this vacuum by letting air and fluid IN. This is however all theory. I know these forces exist but I have no idea if they play a role in practice.

Outstanding riposte Motan!!!!
And very well explained......very much enjoying the discussion and elaboration on dynamics and physics here on this topic.

23rd June 2016, 11:17 PM	#3
motan Member Join Date: Apr 2016 Location: Jerusalem Posts: 274	Why fullers? 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?

28th June 2016, 10:58 PM	#8
motan Member Join Date: Apr 2016 Location: Jerusalem Posts: 274	To David Sorry David but you misunderstood me. Naturally, there is no reason to test the "blood grooves" theory in practice because it just doesn't make sense in the first place. I was talking about something else. I presonally don't think fullers reduce the resistance to stabbing. However, they can, at least in theory, reduce the resistance to pulling out the weapon AFTER a stab if it needs to be withdrawn quickly. Outer pressure should push the flesh onto the blade and the vacuum created by pulling may resist it. Fullers can relieve this vacuum by letting air and fluid IN. This is however all theory. I know these forces exist but I have no idea if they play a role in practice.

29th June 2016, 08:33 AM	#9
kronckew Member Join Date: Mar 2006 Location: Room 101, Glos. UK Posts: 4,216	the ultimate 'blood grooved' 'shiv' knife: take a piece of steel tubing of appropriate length, bed or chair leg, about a foot should do. cut the tube at an acute angle from one end but not too acute such that you have a sturdy tip & leave al least 5 inches for the grip. sharpen. wrap grip in friction tape. push forcibly into a target containing fluid. possibly an unwilling 'volunteer'. see what happens. i seem to recall peasants in 'seven samurai' making spears from bamboo like that. ruined the bandits day. p.s. - to summerize my thoughts, metal was expensive, smithing techniques learned on bronze and copper weapons that maintained stiffness while reducing the required amt. of material for a specific length weapon by adding grooves and fullers would as a side effect produce a artistic appearance that might be carried on even into stronger materials where they technically might not be needed, tho if you are making a few hundred thousand sabres for queen and country, the material savings in steel, weight carried by the trooper and horse, extra calories to carry heavy swords, and transport of spares, etc. could be worth a few shekels. Last edited by kronckew; 29th June 2016 at 08:44 AM.

23rd June 2016, 09:53 PM	#2
Helleri Member Join Date: Jan 2016 Location: Chino, CA. Posts: 219	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.

24th June 2016, 01:26 PM	#6
kronckew Member Join Date: Mar 2006 Location: Room 101, Glos. UK Posts: 4,216	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).

28th June 2016, 10:43 PM	#7
motan Member Join Date: Apr 2016 Location: Jerusalem Posts: 274	To Timo Nieminen and Kronkew. I also anderstand the basic physics. After all, this is no rocket science. But this does not explain everything. A cutting blade should have a balance between strength, flexibility and cutting power. The question is whether specific blades fit their specific purpose. Beams are made to absorb lateral forces, while cutting tools are generally made to absorb forces parallel to the sides of the blade (although bad hit can exert lateral force too). A blade that has thick and stiff spine will have less cutting power and less flexibility, while a thin blade will cut better, but might brake more easily. I personally think that ethnographic weapons take their shape mostly from tradition and esthetics and less so from function. Can you explain the fantastic shapes of oceanic clubs by their function? Can you explaing the totally different geometries of Ottoman kilij and Persian shamshir by different in their use? I can't. Just as anecdote: when I was young, I worked in a banana plantation for sevral years. Instead of machete, we used Victorinox butcher knives of the largest size available. Why? Because their cutting power is far superior and you could do much more work in a day. However, they did brake occationally and were expensive to replace.