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Quantifying the Properties of Your Sword |
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If you want an objective evaluation of your sword you need to quantify its properties
by numbers. As it turns out, you will need quite a few numbers. Some describe simple
properties like the mass or length, some describe properties not so well known (like the moment of inertia). Some numbers
are easy to get by simple measurements, others can't be measured directly and must be found by calculations or in some indirect
experimental way. Worse, some desirable numbers may not be available at all, for example because you would need to take
your sword apart in order to get then.
Here I will cover all properties of interest and describe the ways to obtain
the respective numbers.
Let's start with the easy to get properties / numbers: |
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1 |
The mass of your sword and its parts.
If you have access to a halfway decent balance, you can weigh your sword. Always possible; can't be easier.
It might
be more difficult or even impossible, however, to obtain the mass of parts since not
all swords can be as easily disassembled as for example a katana. The mass of the pommel is of particular interest when
it comes to fine-tuning the balance of a sword but that number might not be easy to obtain. If you cannot take off the pommel
or other parts, I can't help you either. There is just no easy way to do that. Your best bet would be to calculate the mass
from the volume and the density of the material used. |
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Everything relating to the geometry
that can be measured by a ruler.
You can easily measure the length of the blade, its width and thickness, and so on.
However, if the blade is tapered in any way, contains fullers, etc., just a few numbers will not describe the geometry in
sufficient detail. The same goes for curved blades.
What does "sufficient detail" mean? That you could (in
principle) compute the relevant properties as discussed here . That
means that you can supply all data needed by the "Weapon
Dynamics Computer" provided by Vincent Le Chevalier
from his homepage. |
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3 |
Finding the center of mass (COM) comes
next. I gave you two modules on that already - one in the backbone and
one in the science section - so I won't say much here. The procedure
can be a bit tricky for curved swords, in particular when the center of mass is actually outside
the blade. Resting the back of the sword on your finger then will do the trick, but that only shows the "projection"
of the true COM (in or below you finger) onto the sword.
Be careful! It is a bit tricky to balance the sword, measure
some distance on it, and write it down without dropping the sword. Could be dangerous! Do employ help here. |
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Now we come to your swords moment of inertia,
a key parameter. Neither easy to grasp nor easy to measure. Once more, I have dedicated already two modules to this topic,
one in the backbone, one in the science
section. Here I only need to stress that the moment of inertia of your sword is a number that only
makes sense if given together with a pivot point or more precisely, the rotation
axis running through the pivot point. |
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How do me measure the moment of inertia with respect to some rotation axis? Well,
by making the sword rotate around this axis. That is most easily done with a katana, just use a sturdy nail in the wall
instead of the mekugi, the pin that holds the hilt to the blade as shown, or by "wiring" your sword somehow to
a pivot point. The position of the pivot point doesn't matter (within reason), but you must know the precise distance p
between the pivot point chosen and the center of mass. |
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| Swing your sword! |
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Now give your sword a push and let it swing back and forth like a pendulum. Measure
the time T it takes for one full swing. Do it by measuring the time for 10 cycles or more and divide the time
by the number of cycles - it's more precise. From that you get the frequency n = 1/T
or the cycle frequency w = 2pn.
The moment of inertia IA for the axis chosen, and the moment
of inertia ICOM for the center of mass then come out as: |
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| IA |
= |
T2 · m · g · p
4p2 |
| | = |
m · g · p
w2
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| ICOM |
= | I
A – m · p2 |
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How you get that equation is shown in the science
module. If you know the mass mand the distance p between the center of mass and the pivot
point, you can now calculate the moment of inertia for any axis parallel to the one
used, including the axis through the center of mass as shown. Just replace the distance p in the lower equation
by the distance pivot point - new axis. |
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Let's move on to the percussion point.
Once more, two modules to this topic exist, one in the backbone, one
in the science section. We need to know here that there isn't "a"
percussion point but only a pivot point - percussion point couple. If we make the standard
assumption that the pivot point is at the hilt close to the cross guard, the position of "the" percussion point
then is somewhere down on the blade. It is this position we are after. |
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The easiest way to obtain the percussion point is to calculate it. All you need
to do is to find the moment of inertia IPP for the pivot point close to the cross guard, which
you do by using the relations given above. Then you determine the distance pPC between the COM
and the pivot point. The distance s C between your cross guard pivot point and the percussion point
is then given the following equation (derived in the science module). |
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There are also two direct ways to find the percussions point of your sword: Hit
something with it or make it waggle. The idea is to hit a piece of wood with the impact point moving up / down on the blade.
You then will just feel when you reach the percussion point because then you feel "nothing"
in your hand. Matt Easton of Schola Gladiatoria demonstrates that very nicely in his Youtube
video mentioned elsewhere. Here is a screen shot: |
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 | | Matt Easton finds the percussion point |
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What you get is up to your sensitivity but if you feel it (or rather not feel
it), you have the percussion point or at least that point that feels most comfortable for hitting something.
Otherwise
go for the waggle test. Hold your sword loosely around the pivot point and move it quickly back and forth. The sword then
moves as shown below. If you look at the pictures here , it is obvious
why. It takes a bit of practicing and the precise location is not all that easy to see. You do get a clear impression about
the approximate position, however. |
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 | | The "waggle" test for finding the percussion point |
Source: From Vincent Le Chevalier's homepage 1)
Reproduced with gracious permission of the author |
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It remains to find the position of the vibration nodes.
However, I have nothing to add to what I already presented in the main
part. |
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© H. Föll (Iron, Steel and Swords script)
With frame
True Name