Random Thought Thread

True. However, in my model, my CG is forward of the specified CG. This is because my model has a density of .283 lb per cubic inch for steel. However, if I were to substitute brass (.307) for some of that steel in the handle, I could get it back where it "belongs".

If I were to make this design as an integral in one piece and make the entire thing solid steel it would be poorly balanced.

If I were to substitute some brass for some of that steel in the handle, it would actually be worse.
 
SharpBits said:
To be fair to his design, you have to remember that it was handed over to a government who went on to have the lowest bidders produce it.
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Quite a few government contract knives were perfectly good...and remain good even today.

The "truism" doesn't always hold water. It just gets repeated without necessarily having a basis in reality.
 
Nathan the Machinist said:
True. However, in my model, my CG is forward of the specified CG. This is because my model has a density of .283 lb per cubic inch for steel. However, if I were to substitute brass (.307) for some of that steel in the handle, I could get it back where it "belongs".

If I were to make this design as an integral in one piece and make the entire thing solid steel it would be poorly balanced.

If I were to substitute some brass for some of that steel in the handle, it would actually be worse.
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I'm really enjoying this FS design discussion much more than the usual boogers & beer random thoughts. Please share more, especially details on the modified FS design you're working on if you don't mind.
 
surfkiev said:
I'm really enjoying this FS design discussion much more than the usual boogers & beer random thoughts. Please share more, especially details on the modified FS design you're working on if you don't mind.
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If you do the math on the cross sections in the design print you will find that it starts out at around 16° per side, then transitions to 15 in the middle, and then returns back to 16 at the tip. I don't know if this was intentional or not. Parts of the print are under constrained. But it is a good design practice that is seldom seen today. Obtuse at the tip and the base, and narrower along the middle of the blade. This is the best way to distribute the material along the blade, from a functional and structural point of view.
 
If you were to look at the material used in the blade of nany historical dagger or sword, if that material were round, it would be a tapered cone.

However, in the shape of the blade, they tend to be closer to a uniform width with little taper and then it tapers rapidly at the very tip. When seen in plan view. This means the person forging the blade would have drawn the material out wider along the middle of the blade to achieve that shape with that material distribution. Functionally and structurally this is optimal and it is the natural consequence of the way they forged it and, they might not even be aware that they were twisting the primaries in a shallow Helix and then twisting it back to achieve that shape. This maximizes the strength to weight ratio of the blade, reinforces the tip, and makes the flex of the blade more uniform along its length rather than parabolic which would concentrate stress at the base of the blade.
 
Nathan the Machinist Nathan the Machinist maybe I missed it in your prior posts, but where should the optimal point of balance be in a dagger with the general dimensions of the FS knife? I think you mentioned somewhere that there should be weight in the tip and in the pommel, but does that mean equal weight at each end? If so, why is that more advantageous than putting the balance point near, or slightly forward of, the guard?
 
oldmanwilly said:
Nathan the Machinist Nathan the Machinist maybe I missed it in your prior posts, but where should the optimal point of balance be in a dagger with the general dimensions of the FS knife? I think you mentioned somewhere that there should be weight in the tip and in the pommel, but does that mean equal weight at each end? If so, why is that more advantageous than putting the balance point near, or slightly forward of, the guard?
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Historical weapons tended to be surprisingly light. That's one of the first things you'll notice when comparing a historical piece to a modern reproduction. Some of this may have been about avoiding wasting very expensive and hard to come by steel, but functionally it was better for your weapon to be as weightless as possible. That doesn't mean feel as weightless as possible, which is achieved with a piece with a low moment of inertia, but to actually be physically lighter.


Something like a utility knife such as the field knives, you want the knife to balance on the first finger. There is an argument to be made for having a balance between the first and second finger. This allows you to loosen your grip and reposition it without it falling out of your hand, among other things. It's less tiresome to use.

Swords are always blade heavy. This is not just the consequence of the length of the blade because that can be offset with the pommel, it is by design. When repositioning your blade to strike a Target, in order to keep the point on target while coming in at a different angle, you want the center of gravity in front of your hand to facilitate that rotation.* Real sword fights were very fast and over very quickly.

A knife like a fighter or a dagger, you frequently want to see that center of gravity near the ricasso guard transition. Farther forward for larger pieces because of the way they would be used, but never in the grip.

The difference between a knife and a sword is the kinetic aspect of a sword that does not exist in a knife. A dagger is somewhere in between.


*Edit: when used as a thrusting weapon, which is the most effective use of a sword.
 
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Knives with low moment of inertia will feel light and lively in your hand. This is a pretty cool thing to experience and it is very impressive when well executed. But it doesn't necessarily pay a dividend in real use. Frequently the opposite is true
 
Nathan the Machinist said:
Historical weapons tended to be surprisingly light. That's one of the first things you'll notice when comparing a historical piece to a modern reproduction. Some of this may have been about avoiding wasting very expensive and hard to come by steel, but functionally it was better for your weapon to be as weightless as possible. That doesn't mean feel as weightless as possible, which is achieved with a piece with a low moment of inertia, but to actually be physically lighter.


Something like a utility knife such as the field knives, you want the knife to balance on the first finger. There is an argument to be made for having a balance between the first and second finger. This allows you to loosen your grip and reposition it without it falling out of your hand, among other things. It's less tiresome to use.

Swords are always blade heavy. This is not just the consequence of the length of the blade because that can be offset with the pommel, it is by design. When repositioning your blade to strike a Target, in order to keep the point on target while coming in at a different angle, you want the center of gravity in front of your hand to facilitate that rotation. Real sword fights were very fast and over very quickly.

A knife like a fighter or a dagger, you frequently want to see that center of gravity near the ricasso guard transition. Farther forward for larger pieces because of the way they would be used, but never in the grip.

The difference between a knife and a sword is the kinetic aspect of a sword that does not exist in a knife. A dagger is somewhere in between.
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Thank you for clarifying that for me. I have a follow up question for you - what exactly does this mean?

Nathan the Machinist said:
The difference between a knife and a sword is the kinetic aspect of a sword that does not exist in a knife. A dagger is somewhere in between.
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Whereas the primary use of a dagger is penetration, and the primary use of a knife is cutting and/or chopping, wouldn't the use of a dagger be less kinetic? As in, a dagger will theoretically see its highest and best use being pushed into material then pulled back out while a knife will be moved around in all sorts of directions during normal use. Does "kinetic" refer to movement in general or does it have a more specific meaning in this context?
 
oldmanwilly said:
Thank you for clarifying that for me. I have a follow up question for you - what exactly does this mean?



Whereas the primary use of a dagger is penetration, and the primary use of a knife is cutting and/or chopping, wouldn't the use of a dagger be less kinetic? As in, a dagger will theoretically see its highest and best use being pushed into material then pulled back out while a knife will be moved around in all sorts of directions during normal use. Does "kinetic" refer to movement in general or does it have a more specific meaning in this context?
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Yes it is entirely true that a chopper has a large kinetic component. It's also the exception to the rule about weight because, on a chopper, heavier is often better. There are people in bladesports experimenting with tungsten steel because it weighs more than conventional steel which can produce a heavier blade within the blade sports geometry constraints.

However, most knives that are used as simple cutting tools, the lighter the better and a balance on the first finger is frequently preferred. Bob Loveless mastered this. If you're ever at blade show, there's a fellow named Jonathan near the front door that has a collection of Loveless knives that you can pick up and see what I'm talking about. They're so light and well balanced, it's lovely.

On a kitchen knife used in a pinch grip, that might put the center of gravity out in the blade between the index finger and thumb.

Weapons, however, are a different animal.

Swords have both the kinetic aspect of cutting from impact, and also rapid repositioning, and also the ability to block or shield without deflecting (unwanted rotation, putting your tip where you don't want it)

A dagger needs to be able to stab in a straight line without being deflected, which is why weight on the ends is helpful. Also, some daggers such as a Main Gauche need to resist deflection from the opponents sword and they will frequently have a sizable pommel to reduce unwanted rotation.

The functional needs of a modern tactical dagger have not evolved to be very different from a historical Main Gauche. While you're not going to parry someone's rapier with it, the basic needs of a dagger haven't changed much.
 
Nathan the Machinist said:
Something like a utility knife such as the field knives, you want the knife to balance on the first finger. There is an argument to be made for having a balance between the first and second finger. This allows you to loosen your grip and reposition it without it falling out of your hand, among other things. It's less tiresome to us
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Perhaps that's why Fairbairn designed it with a heavy handle? "The hilt should fit easily in your hand, and the blade should not be so heavy that it tends to drag the hilt from your fingers in a loose grip."
 
Nathan the Machinist said:
Yes it is entirely true that a chopper has a large kinetic component. It's also the exception to the rule about weight because, on a chopper, heavier is often better. There are people in bladesports experimenting with tungsten steel because it weighs more than conventional steel which can produce a heavier blade within the blade sports geometry constraints.

However, most knives that are used as simple cutting tools, the lighter the better and a balance on the first finger is frequently preferred. Bob Loveless mastered this. If you're ever at blade show, there's a fellow named Jonathan near the front door that has a collection of Loveless knives that you can pick up and see what I'm talking about. They're so light and well balanced, it's lovely.

On a kitchen knife used in a pinch grip, that might put the center of gravity out in the blade between the index finger and thumb.

Weapons, however, are a different animal.

Swords have both the kinetic aspect of cutting from impact, and also rapid repositioning, and also the ability to block or shield without deflecting (unwanted rotation, putting your tip where you don't want it)

A dagger needs to be able to stab in a straight line without being deflected, which is why weight on the ends is helpful. Also, some daggers such as a Main Gauche need to resist deflection from the opponents sword and they will frequently have a sizable pommel to reduce unwanted rotation.

The functional needs of a modern tactical dagger have not evolved to be very different from a historical Main Gauche. While you're not going to parry someone's rapier with it, the basic needs of a dagger haven't changed much.
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So weight on both ends help with inertia?

Or is that relevant

(I have no idea what I'm really talking about)
 
yoko said:
So weight on both ends help with inertia?

Or is that relevant

(I have no idea what I'm really talking about)
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Yes.

Think of taking a 12” stick and attaching two small weights to the stick. The stick is the same. The weights are the same. So the total weight remains the same.

What you can change, is the weight distribution. If you put the two weights next to each other, around where your index finger is, holding the stick like a knife with a 5” handle and 7” blade, it will ‘feel’ different than placing one weight on either end.

Weight towards the ends increases stability. Weight towards the middle will feel ‘livelier’, as less weight towards the end = less rotational inertia, so you can move the tip more easily.

On the one hand, it feels livelier when you do ‘knife ballet’ like that recent video someone posted of the old geezer in his kitchen.

OTOH, it also means that the tip is more easily deflected.

There are varying degrees, to the balance and weight distribution. Some of it may come down to preferences, but there is a physics and practicality component to it, which is what Nate is trying to explain.
 
SharpBits said:
Perhaps that's why Fairbairn designed it with a heavy handle? "The hilt should fit easily in your hand, and the blade should not be so heavy that it tends to drag the hilt from your fingers in a loose grip."
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Yes, I'm sure that's where his head was, however the length of the moment arm that much of that mass is acting upon is close to zero. A weighted pommel would be more effective and there is already a threaded nut on the end which could have been adapted into this. Although you would need a more robust tang than the 1/8" rod. I would argue it needs that, regardless.
 
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