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Gianluca Zanini




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PostPosted: Tue 31 Jul, 2007 12:28 pm    Post subject: CoP / PoP Calculation         Reply with quote

Hello,

does anybody knows how to calculate the "point of percussion"?
I have read some articles about it, and I was wondering if it can be calculated with a particular formula by using the measurement and weight, or is there a particular practical method which involve some particular striking/cut testing?
Thanks in advance for your help

Gianluca
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Richard C G





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PostPosted: Tue 31 Jul, 2007 12:38 pm    Post subject:         Reply with quote

My guess is no. You have to know the stiffness of the material, as well as the stiffness of the individual blade, so you have to know the profile in both dimensions. This is probably only one reason out of several. It is most likely easier to do it experimentally.

Any simple equation (as far as I know) to calculate such things would assume a rod.

Not too many swords are shaped like rods.

If anyone says yes, I will be interested in seeing the equations(s).
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Nathan Robinson
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PostPosted: Tue 31 Jul, 2007 12:41 pm    Post subject:         Reply with quote

For swords, the Center of Percussion can be determined easily.

Read this:


Understanding Blade Properties

An article by Patrick Kelly

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Gianluca Zanini




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PostPosted: Tue 31 Jul, 2007 1:13 pm    Post subject:         Reply with quote

Nathan Robinson wrote:
For swords, the Center of Percussion can be determined easily. Read this: http://www.myArmoury.com/feature_properties.html


thank you so much! I had just a superficial reading of the articles on your web site.
Yes it is quite easy to determinate. Sometime one imagine the problems greater than what it is! Worried

Gian
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Gianluca Zanini




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PostPosted: Tue 31 Jul, 2007 1:21 pm    Post subject:         Reply with quote

Richard C G wrote:
My guess is no. You have to know the stiffness of the material, as well as the stiffness of the individual blade, so you have to know the profile in both dimensions. This is probably only one reason out of several. It is most likely easier to do it experimentally.

Any simple equation (as far as I know) to calculate such things would assume a rod.

Not too many swords are shaped like rods.

If anyone says yes, I will be interested in seeing the equations(s).


I found this on http://en.wikipedia.org/wiki/Center_of_percussion and I silly thought it might be possible to use it for a sword, but after some trials I realize there were to many variables to determinate, such as the stiffness, mass inertia etc etc.

Thanks

Gian
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Richard C G





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PostPosted: Tue 31 Jul, 2007 1:23 pm    Post subject:         Reply with quote

The original post asked if it was possible to "calculate" not to "determine" the point of percussion.

I stand by my answer, although the poster seemed satisfied with finding out that it can be determined empirically.

If anyone has any magic equations, I would still like to see them.


Last edited by Richard C G on Tue 31 Jul, 2007 1:25 pm; edited 1 time in total
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Nathan Robinson
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PostPosted: Tue 31 Jul, 2007 1:24 pm    Post subject:         Reply with quote

Richard C G wrote:
The original post asked if it was possible to "calculate" not to "determine" the center of percussion.

I stand by my answer, although the poster seemed satisfied with finding out that it can be determined empirically.

If anyone has any magic equations, I would still like to see them.


he also said, "...or is there a particular practical method which involve some particular striking/cut testing?".

Hence my very pragmatic answer...

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Richard C G





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PostPosted: Tue 31 Jul, 2007 1:28 pm    Post subject:         Reply with quote

OOPS! So he did.

My response was intended to be definitive, not argumentative.
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Vincent Le Chevalier




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PostPosted: Tue 31 Jul, 2007 1:29 pm    Post subject:         Reply with quote

If what you're looking for is the "harmonic" Center of Percussion (the one linked to blade vibrations), Richard is right. It would indeed be a rather heavy calculation, needing a lot of data on the sword. It is however easy to measure.

In fact I'm not even aware of a way to compute how the harmonic CoP varies when you had or remove mass somewhere on the sword. For all I know the only way to get that is to try and see.

Note that there is another, less used definition of the CoP, based on inertia. I call it the dynamic CoP. It is not discussed in the featured article, but that's the one the wikipedia page speaks about (the harmonic CoP is a term rather confined to swords enthusiasts, it seems). It is equally easy to measure, it is in fact the pivot point associated to your hand (see http://www.albion-swords.com/swords/sword-terms.htm for the definition and the way to measure pivot points, two thirds down the page).

Sometimes they are the same, sometimes not. The dynamic CoP exists on anything from stick to swords to axes. The harmonic CoP exists only on flexible blades (for example it is not obvious on katanas as far as I know). Working out where the dynamic CoP moves when you add or remove mass is possible mathematically, though the resolution is a bit long to explain it can be done by hand with some simple geometry.

As I said in my recent "literature review" (http://www.myArmoury.com/talk/viewtopic.php?t=10758) the question of the relevance of both of these CoP for cutting is still open. I personally consider the dynamic CoP as more useful to figure out handling, alas it is almost never given in reviews...

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Nathan Robinson
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PostPosted: Tue 31 Jul, 2007 1:30 pm    Post subject:         Reply with quote

Hi Richard-

Sorry if I sounded defensive. I took no offense to your post and was happy that you shared with us all. Sometimes when I'm in "work mode" I get far too "matter of fact" and short and to the point in my posts.... I'm trying to take a moment to share some helpful stuff here but likely don't have the time to really do more than little snippets here and there. Worried

Good intentions and all that...

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Chad Arnow
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PostPosted: Tue 31 Jul, 2007 1:33 pm    Post subject:         Reply with quote

I'm not sure if anyone has bothered to come up with a formula for something with so many variables (length, width, amount and rate of profile taper, thickness, amount and rate of distal taper, cross-section, tightness of assembly, weight/shape of all the components, etc., etc.) when it's so much easier to whack the side of the pommel. Happy

Since it's a vibrational issue, the easiest way to find it will be to make the sword vibrate by whacking the pommel. Then you can see where the waves cancel out to no (or little) vibration.

Happy

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Chad Arnow
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PostPosted: Tue 31 Jul, 2007 1:52 pm    Post subject:         Reply with quote

Vincent,
I think it's easier to stick to terminology like "pivot point" rather than "dynamic CoP." Not every sword's pivot point is made to be struck with. In some cases the CoP and the blade pivot point coincide. Sometimes, they don't and shouldn't. CoP is a static vibration thing. Pivot points are dynamic rotational things. Terms like "pivot point" and "point of rotation" don't cloud the issue.

As for why pivot points (hilt and blade) are never given in reviews, it's a much newer concept than CoP or PoB. Not everyone knows about it or cares about it or realizes why it may be important. For our reviews, we have a set of standard stats we try to collect for everything. Pivot points aren't among them, as most people won't know how to make sense out of them. Also, our standard set of stats was compiled before people started really talking about rotational points. We're not going to be able to go back and get pivot points for all our previously published reviews. Happy As has been discussed, PoB is not the best stat, yet it's so common we always include it for A) consistency, and B) because people still want to see it.

The primer articles on swords linked above don't include pivot points simply because no one was really talking about them when the articles were written. Way back when, weight was the most important stat. For a while after that, PoB and CoP were the hot stats to know. Then, distal taper rates were big. Now, some people want pivot points or other things. In 2 years, there will be other measurements. Happy

For me, I have never made a purchase decision based on PoB or CoP or even pivot points. All of these measurements are a by-product of the design of a sword. They all work together to make a great sword, an okay sword, or a bad sword. To take one measurement or other out of context doesn't always help. What's more important to me is to be able to hold it myself or to be able to read the opinions of those I trust when they talk about their handling experiences. The handful of dry stats seen in reviews mean much less to me than the paragraphs relating how the sword performed in hand. Happy

When cutting, I will try to determine the CoP just so I can make sure I'm cutting with the part of the blade best suited to deal with the shock of impact. That's the only use I have for knowing the CoP these days. Happy

Happy

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Angus Trim




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PostPosted: Tue 31 Jul, 2007 2:07 pm    Post subject:         Reply with quote

Vincent Le Chevalier wrote:

In fact I'm not even aware of a way to compute how the harmonic CoP varies when you had or remove mass somewhere on the sword. For all I know the only way to get that is to try and see.


Regards


A lot of things can cause small, and not so small changes in the position of the "primary node"....

Is the blade double edged, or single edged?

If double edged, is it fullered? Where does the fuller "break out"? Interesting enough, I noted on a couple of swords once when I was experimenting, that double fullers seem to move the cop down towards the tip an inch.....

If single edged...is there a false edge? How long is the false edge? Distal taper if any?

Yes, distal taper, both single and double edged blades feel the effect of distal taper.....

Then....... believe it or not, a pommel's weight can effect things........

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Vincent Le Chevalier




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PostPosted: Tue 31 Jul, 2007 2:35 pm    Post subject:         Reply with quote

Chad, thanks for your answer... I hope my post did not come out has reproachful or anything, maybe I wrote too fast as well Happy

Chad Arnow wrote:
I think it's easier to stick to terminology like "pivot point" rather than "dynamic CoP." Not every sword's pivot point is made to be struck with. In some cases the CoP and the blade pivot point coincide. Sometimes, they don't and shouldn't. CoP is a static vibration thing. Pivot points are dynamic rotational things. Terms like "pivot point" and "point of rotation" don't cloud the issue.


Problem is, there is an infinite number of pivot points, as many as there are points on the sword where a force can be applied (from the user or the target). As a matter of fact, pivot points relative to the back and front of the hand on the hilt are different and sometimes quite far apart. So I came up with "dynamic CoP" when I needed to refer to the specific pivot point associated to the spot in your hand where you don't want to get a shock (for me it's between the index and middle finger, empirically), as a short-cut. I'm quite unsatisfied with the term as well...

Technically, looking at how other disciplines use the terms, I think it's the harmonic CoP that should change name and be called the sweet spot or something like that, but there is no chance to change that, ever Wink

Quote:
As for why pivot points (hilt and blade) are never given in reviews, it's a much newer concept than CoP or PoB. Not everyone knows about it or cares about it or realizes why it may be important. For our reviews, we have a set of standard stats we try to collect for everything. Pivot points aren't among them, as most people won't know how to make sense out of them. Also, our standard set of stats was compiled before people started really talking about rotational points. We're not going to be able to go back and get pivot points for all our previously published reviews. Happy As has been discussed, PoB is not the best stat, yet it's so common we always include it for A) consistency, and B) because people still want to see it.


Just to clarify, I was in no way criticizing past reviews or anything. I know it will stay the same for a long time... But if I want that to change, the only way for me is to talk about it, so that people realize the interest of other stats Happy

Quote:
The primer articles on swords linked above don't include pivot points simply because no one was really talking about them when the articles were written.


And I was not criticizing the articles either. In my opinion, what is found in this kind of articles should be the basis of what is generally understood inside the community. Pivot points clearly do not fit that yet... And their applications have not been discussed in sufficient depth.

Quote:
Way back when, weight was the most important stat. For a while after that, PoB and CoP were the hot stats to know. Then, distal taper rates were big. Now, some people want pivot points or other things. In 2 years, there will be other measurements. Happy


Quite possible, but I have faith in the advances of science Big Grin In fact what drives me in the subject is not the values down to the fifth decimal for any particular sword, but rather the better understanding of how all weapons work.

Quote:
For me, I have never made a purchase decision based on PoB or CoP or even pivot points. [...] The handful of dry stats seen in reviews mean much less to me than the paragraphs relating how the sword performed in hand. Happy


Well I'm not proposing to reduce the reviews to five numbers either Big Grin In fact, currently I act just as you do, I don't even read the stats but skip directly to the paragraphs below... The difference is that I don't have many occasions to handle the swords I fancy. If I had values for pivot points, I think I could build a semi-reasonable feel simulator that would give me an idea of how the sword behaves, which would be a step forward. Well that's an ongoing experiment of mine, I suppose I'll have more to say about that in the future...

Thanks for your work!

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Gianluca Zanini




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PostPosted: Tue 31 Jul, 2007 2:36 pm    Post subject:         Reply with quote

Dear Sirs,

I thank you very much for your precious information. My lack of technical knowledge of the matter is evident, beside my difficulties to approach the topic with my english.
The empiric method is enough for me and I was not going to unveil any magic formula.
My "argumentative" reply was not meant to contradict any of your competent contributes neither.

kind regards

Gianluca Zanini
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Vincent Le Chevalier




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PostPosted: Tue 31 Jul, 2007 2:45 pm    Post subject:         Reply with quote

Angus Trim wrote:
A lot of things can cause small, and not so small changes in the position of the "primary node"....


Well I see how all the things you listed can impact the node's location, but my specific point was that there are no way to theoretically figure that out with any amount of precision... For example, why a given change has a small impact, and another a big impact... I don't know, a way to figure out how the blade should be changed if the pommel is not the same? Getting the same effect with different modifications? Things like that...

Regards,

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Jean Thibodeau




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PostPosted: Tue 31 Jul, 2007 2:59 pm    Post subject:         Reply with quote

Chad Arnow wrote:
When cutting, I will try to determine the CoP just so I can make sure I'm cutting with the part of the blade best suited to deal with the shock of impact. That's the only use I have for knowing the CoP these days. Happy


( Edited: Note I was writing this before I read Vincent's posts as we were probably composing our posts at the same time ).

CoP, pivot point I'm not sure which I'm maybe testing for but here is a description of what I have noticed that should be related to test cutting without actually cutting that is purely tactile and best done holding something like a block of wood in one hand as a striking surface. ( I've done this with the back of a single edged sword directly on my lightly padded arm: With anything sharp go very very light or heavily tape the edges).

Also one can test the technique with a waster to just get what it feels like: Take the sword ( Or waster )

1) Hit lightly with the tip of the sword: At the tip the blow feels weak and mushy and one is normally well forward of the best part of the blade to use in a cut. The blow seems like it as little follow through and sort of bouncy.

2) Hit lightly with the forte of the blade just in front of the guard: At this point of the blade the blow feel very heavy and the blade want to strongly rotate with the blow but also feel like a dead weight. Very much doesn't bounce.

3) Working one's way from the tip down to the hilt and back up there is a sweet spot that feels strong and penetrating.
And I think this corresponds to the CoP or pivot point but I'm not sure what I'm measuring but I can sure feel it when I'm at or close to this sweet spot.

With something extremely weighted at one end like a mace this sweet spot is at the head or just below it.

Actually if one uses a mace to test to understand what this feels like in an exaggerated way: hold the mace at the end of the handle and " gently " tap your arm with the handle about 6" to 8" above the hand and what I'm describing in (2) will be very obviously felt.

So this best place on a blade to cut can be found directly by how it feels as an impact and may be what this topic is talking about or some point related to pivot points, CoP and CoB as a combination of all of these. I guess some people will find this by just doing some real cutting tests ! But if just picking up a sword where and when cutting wouldn't be practical this can give the same information I think.

This is just something I noticed decades ago just playing with stuff but never really thought about until now to put into words i.e. a sensory observation never translated to a verbal/intellectual way of thinking about it.

Hope this make sense or cause some interesting discussion to happen and maybe a different way to find the best part of a sword to cut with.

You can easily give up your freedom. You have to fight hard to get it back!


Last edited by Jean Thibodeau on Tue 31 Jul, 2007 3:12 pm; edited 6 times in total
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Chad Arnow
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PostPosted: Tue 31 Jul, 2007 3:00 pm    Post subject:         Reply with quote

Vincent Le Chevalier wrote:
Chad, thanks for your answer... I hope my post did not come out has reproachful or anything, maybe I wrote too fast as well Happy


Not at all; I'm not upset about anything, just explaining where we are and what my own personal feelings are. Happy

Wat fascinates me is how much the understanding of sword properties has changed in the last 10 years as well as how far we still have to go.

Happy

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PostPosted: Tue 31 Jul, 2007 4:35 pm    Post subject:         Reply with quote

Vincent Le Chevalier wrote:
Angus Trim wrote:
A lot of things can cause small, and not so small changes in the position of the "primary node"....


Well I see how all the things you listed can impact the node's location, but my specific point was that there are no way to theoretically figure that out with any amount of precision... For example, why a given change has a small impact, and another a big impact... I don't know, a way to figure out how the blade should be changed if the pommel is not the same? Getting the same effect with different modifications? Things like that...

Regards,


Hi Vincent

I was just more or less supporting what you said...... I wouldn't try to come up with a way to determine the node with a math formula........

But, most cut and thrust, or cutting double edged swords will have the primary node at 2/3 the distance from the cross to the tip. Backsword blades without any fullering or false edge can be as far as 3/4 the length...... type XV's are typically "inside" 2/3 the distance....

All assuming decent harmonics.........

And the "impacts" can go from there........

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PostPosted: Tue 31 Jul, 2007 4:38 pm    Post subject:         Reply with quote

Chad Arnow wrote:


Wat fascinates me is how much the understanding of sword properties has changed in the last 10 years as well as how far we still have to go.


And that's only what's already been written....... there's a lot more to share, just going to have to get the right conversation going.

swords are fun
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