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Jared Smith




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PostPosted: Wed 07 Jun, 2006 6:46 pm    Post subject: Use of the term CoP and controversy         Reply with quote

In hope of starting a discussion of broad interest I decided to see if anyone wished to explore and attempt to contribute to the Center of Percussion (CoP) topic/ definition in more depth.

At least one issue I am wondering about now is how we determine our CoP values in the myArmoury reviews, and how the manufacturers are doing it.


I will save you from a lengthy read and point out going into this that the definition of CoP given in Patrick Kelly’s “Understanding Blade Properties” article… http://www.myArmoury.com/feature_properties.html closely matches what sporting goods companies refer to the “sweet spot” of a bat or racket. It is actually a complex interaction of harmonics, mass distribution, and “effective pivot point” based on how an average athlete really pivots a club like implement using their wrists and arms when they swing it.


One ARMA article proposes a “waggle test” predicated on a “pivot point”, which I am not sure is realistic in terms of handling. http://www.thearma.org/spotlight/GTA/motions_and_impacts.htm

Baseball/ softball bat true hitting distance performance are based on a “sweet spot” which is something distinctly different from the CoP determined by mass distribution relative to the center of the grip area. This discrepancy results from the fact that in a realistic swing, the athlete pivots a club like object around a location that is actually behind the center of his grip (typically at their wrists or even farther up the arm.) I speculate that there could be some analogy between this “sweet spot” and the best point for a sword blade to strike a target intended for cutting. http://www.kettering.edu/~drussell/bats-new/cop.html
http://www.kettering.edu/~drussell/bats-new/sweetspot.html
Actually finding the true “sweet spot”, and rating a piece of sporting equipment (with legal acceptance from a governing tournament sanctioning organization), is a fairly difficult thing. Relatively scientific tests that simulate true usage pivot locations are typically employed (and required by law) in addition to validation by handling performance trials using experienced athletes. The overall certification process usually takes over half a year in the case of a “tournament certified” baseball or softball bat which may be produced annually in quantities of around 1000 bats priced at roughly $300 to $500 each. I know since my spouse worked at one such company and had to handle customer relations and technical explanations for one year when recall issues were imposed by the governing organization (governing organization rule changes imposed several million dollars worth of accumulated expense due to months of technical testing, alteration, and recertification efforts of about 100,000 bats previously considered legal for the affected company.)


An engineer (regarded locally as a genius) I greatly respect balanced my first premium sword vertically with pommel resting in his palm. He then “wobbled” it in the cutting plane and pronounced an “effective CoP” to be at an appropriate location (about 8” from the tip of a roughly 36” long blade.) He also struck the blade with his hand and a stick along various locations (trying not to contact it other than where struck while supported at pommel) and orientations pronouncing that vibration was pretty well dampened near the “desired cutting location for striking blows” based on visual judgment of the blade vibrations. His whole examination took about 10 seconds. Although this individual does not collect swords, within my regional engineering area he is recognized as a genius. At the time, his analysis did not seem quite right to me in terms of physics and grip location. What he stated as the important “effective CoP” actually was an entirely different location than this premium manufacturer’s stated CoP for the sword. As I have thought it over many times since, it seems increasingly amazing to me how he quickly and decisively he chose this method of assessment.

Some here may strongly disagree. However, I offer some controversial stuff to discuss.

Best impact performance as proven in sporting goods applications is often not the mathematical CoP based on an assumed pivot point at the center of grip combined with mass distribution calculations made based on that grip’s center.

Vibration and “perceived hand shock / reaction forces” within bats and rackets typically are minimized if impact occurs at the true “sweet spot” that delivers maximum impact energy to a ball as wielded by someone proficient with that type of tool. This is difficult to define in terms of CoP unless there is a convincing assertion of where the true pivot point is when a competent athlete swings the bat or racket.

Absence of evidence is not necessarily evidence of absence!
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Angus Trim




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PostPosted: Wed 07 Jun, 2006 8:20 pm    Post subject:         Reply with quote

Hi Jared

I've seen a lot of discussion over the years over where the best place to strike a target with a sword is...and it depends. Depends on the sword and on the target........

Take for instance, the typical AT cutting sword. The tip is generally shaped for cutting, the edge geometry generally a good one for cutting.

If the target is something like a milk jug filled with water, the best part of the blade to use for cutting is the tip. a 2L pop bottle filled with water, just behind the tip. Cutting Mugen Dachi tatami mats, just behind the tip. Cutting 1/4 inch plywood though, and targets that offer that much or more resistance, then the best place to cut with on the blade, is the node.

This isn't done with any math, or any real armchair calculation. This is figured the old fashioned experimental way.... cutting with said sword against several different targets, and recording the results.........

If function is the main criteria, its been my experience that most quality modern swords intended to cut with react similarly......

swords are fun
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Craig Johnson
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PostPosted: Wed 07 Jun, 2006 9:36 pm    Post subject: CoP and what I have learned trying to figure it all out         Reply with quote

Jared Smith wrote:
... as wielded by someone proficient with that type of tool. ....


Hi Jared

The topic is a very interesting and complex issue. As you well describe, the quantitative elements that can be measured to describe whats happening in the swing of an object by the user are many and interrelated. The largest variable in the whole process is defined in the quote above. The design parameters of any given sword are set once it is finished in the makers shop, barring after market alteration or tuning. The variable that will affect the efficiency of the cut from one time to the next or from one cutter to the next is the hand on the grip or as I like to call it the ground grip interface.

Any given sword will have an ability to achieve a specific maximized individual cut each time it is swung, if it does not achieve this optimum on each swing, the variable is the swingee. Each specific sword will have a different most efficient cut. The really accomplished swordsman is able to recognize what this maybe by the heft and "feel" of the piece in their hand. Any two given accomplished swordsman will have am individualized ideal of what they want in a sword. This is the great conundrum of swords. The best or least well made sword is only as good as the hand on the grip. This is why your engineer quickly felt he knew what it was in the sword he liked.

Now we give ourselves a term like CoP that has a couple of lose definitions and is used freely in discussions where one sometimes does not really know what the point of description actually initial and it can get confusing and less than helpful. If we are going to call it the point where the most energy is transferred from edge to target on a given length of blade where the edge contacts at a right angle to the surface of the target under a given load of force with known vectors of travel across the plain of the edges and any vectors along the edge itself due to the arcing nature of a cut with a given rotation point that is a function of the sword size we can get a comparison from one blade to another, kind of Confused

But I think the comparative nature of the discussion is less helpful to the user than the understanding of how the sword works well can be to anyone who practices the use of the sword and can use the information to analysis how they use a sword and what traits they are most comfortable with when they have several swords to choice from. This I think is the real value in this type of discussion. As Gus pointed out the intended target or range of targets will adjust the point on any given blade where you will get the best cut. Looking at what you want from a sword and choosing a piece to fit these needs is probably the way a swordman of old would have looked at the problem and one I think we do not do as much as we should today.

Best and worst part of the whole deal is every now and then you pick up a sword and you know that it is just right for you and often you can not explain why Happy

Good topic

Best
Craig
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Marton Pap




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PostPosted: Thu 08 Jun, 2006 12:51 pm    Post subject:         Reply with quote

Hi!
As I see, the main difference in determining the location of CoP is where you want the pivot point to be, namely the cross guard (ARMA), the center of the grip (baseball bats), the pommel (the engineer) or even further back. As the ARMA article points out, if we put the CoP to the tip to cause the sword pivot around the cross, the use of a pommel makes sense, and increases cutting power.
Regards
Marton
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Jared Smith




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PostPosted: Thu 08 Jun, 2006 4:13 pm    Post subject:         Reply with quote

Thanks for the interesting inputs.

I had overlooked how different types of targets (thick wood, thin plastic jugs filled with water, etc.) might signficantly alter the percieved "sweet spot" of a sword. I suppose this has to be true since resistance and hand shock resulting from a target the sword does not pass through (maybe a tree or thick piece of opponents armour) is going to become a bigger factor!

Another key aspect of this topic is "Does everyone publish their CoP numbers based on the same method?" Specifically, are well all basing this on the guard as the pivot point, or are the manufacturer's doing their best to assess all of these difficult interactions (vibrational nodes, effective pivot points, qualitative test/assesment, etc.) and basically estimating the "sweet spot" of the sword?

Some might say there is negligible difference between the sweet spot and the physics CoP based on a non-debatable pivot point (pommel center, guard, etc.) I have a suspicion that discrepancies could become more significant as grip length increases (hand and a half...true two handed grip.) Those who hit well with a bat should contemplate where they place their hands when pitching for themselves (tossing a ball up one handed and swinging the bat one handed.) One handed grip location is instinctively adjusted several inches differently than where hands are placed when hitting two handed.

Absence of evidence is not necessarily evidence of absence!
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Chad Arnow
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PostPosted: Thu 08 Jun, 2006 4:31 pm    Post subject:         Reply with quote

When we publish reviews here, we use that term as defined in Patrick's piece as the primary node: the place on the blade that will transmit the least vibration to the hand(s) during a strike. It's determined by holding the grip lightly and whacking the pommel. The place on the blade where the undulations seem non-existent becomes the COP (that's perhaps an overly simplified answer). It's a static measurement based entirely on vibrational properties of a non-moving sword. As far as I know, everyone calculates it basically the same way. It's not based on any pivot points; it's simply a sword-at-rest static statistic.

Like any measurement, it's only part of the story. What is at least as important as COP and POB is a sword's pivot point and its overall mass distribution. Location of the pivot point will vary from sword to sword and won't always line up with the COP. This thread has great info on pivot points and dynamic properties: http://www.myArmoury.com/talk/viewtopic.php?t=3154

Happy

ChadA

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Steve Grisetti




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PostPosted: Thu 08 Jun, 2006 5:09 pm    Post subject:         Reply with quote

Chad Arnow wrote:
...we use that term as defined in Patrick's piece as the primary node: the place on the blade that will transmit the least vibration to the hand(s) during a strike. It's determined by holding the grip lightly and whacking the pommel. The place on the blade where the undulations seem non-existent becomes the COP....

I tried this technique with my Arms & Armor German Bastard Sword, and measured approximately 22-1/2 inches from the guard - the same measurement that Bill Grandy recorded in his GBS review. Cool

But, what if you have a short sword, such as a Roman gladius. I tried this technique with my Albion (1st Gen) Mainz. In that case, depending on how I whack the pommel, I might feel a little bit of blade vibration, but I sure can't detect the node. I get a similar non-result with a 21-inch bladed hanger that I have. If you have a short, stiff sword, how relevant is the Center of Percussion? (I note that Patrick Kelly did not record COP data for his Triumvirate of Gladii review.)

"...dismount thy tuck, be yare in thy preparation, for thy assailant is quick, skilful, and deadly."
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Vincent Le Chevalier




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PostPosted: Thu 08 Jun, 2006 6:28 pm    Post subject:         Reply with quote

Hi all !

In fact, I've been thinking about this issue since the publication of the cited ARMA article, so it's about time I share my thoughts... I promised a while ago to write an article regrouping what I've been able to establish, but decided to delay that until I have a coherent whole and not just bit and pieces.

I've understood, skimming through posts here and on SwordForum, that this was a controversial matter that has led to heated arguments in the past, so I shall try to remain as fair and neutral as possible. OK, it's an important subject, but not one worth fighting about Wink

I will not develop any computations or mathematical theories here, but I pray readers to believe that I've done computations wherever they were required... I'm finishing engineering studies, and it's been great fun to apply what I learned at school to swords Happy

Right, enough of an introduction, let's get into it.

My belief is that we are confusing two things in this. When speaking about COP, COB, pivot points, blade harmonics, etc. people ordinarily think of two aspects : behaviour during impact or cutting, and handling properties. I think that trying to apply the exact same theories and criterias to those problems is not a good idea.

True, there are common features. When analyzing the physical behaviour of a solid like a sword, it seems obvious to me (mostly coming from my engineering background) that one has to consider at least three caracteristics of the object : its mass, its center of gravity and its moment of inertia. The center of gravity tells you where the mass is on average, the moment of inertia tells you how far it is spread around this average position, that is, how difficult it is to make the object spin around the center of gravity. Indeed, it's a proven fact of mechanics that as long as you consider the objects do not bend or deform, those three quantities are enough to resolve completely the behaviour. So my idea would be to leave the blade harmonics (which result from its deformation) for the last step, and start by looking at those three caracteristics.

The main problem, you see, is that they are not all that easy to measure, especially the moment of inertia. The mass is easy, you just need scales... The center of gravity is not that hard either, it's just a matter of finding a static equilibrium, we've all done that. The moment of inertia is impossible to determine by a static measure, you have to put the weapon in motion to measure it. And even then, it's not easy to measure, you have to deduce it from indirect measures of length.

The easiest way to measure the moment of inertia (that I know of), is the waggle test described in the ARMA article. It allows you to determine a pair of pivot points, and from these, to deduce the moment of inertia. It's a reasonably accurate way of determining this, as far as I've been able to try. I like best doing that with the tip pointing to the ground, since I feel it's more safe and stable that way, but the idea is the same.

There are, however, two problems with that moment of inertia. First, it's relative to the center of gravity, and thus does not reflect the ease of rotation around any other point. And even then, it's not an intuitive thing at all. It is expressed in units to which we are not really used (hey, who has an idea of what a kg.m² feels like...). Thus, the definition of other, derived caracteristics, such as the COP or variants of it.

As far as impacts are concerned, I think the interesting quantity, which we all try to maximize, is the ratio between the energy of the weapon before the impact, and the energy that disappears in the target during the impact. That's where the troubles start. This is something that depends not only on the weapon, but on the target and on the motion of the weapon as well. I think it's reasonable in most cases to consider that the speed of the target is way inferior to that of the weapon. So we are left with : center of gravity, moment of inertia, mass of the sword, its motion (roughly, around which point in space it's rotating, and at what speed), and the mass of the target. After a painful enough computation, the result exhibits a number of interesting properties, that are fairly representative of what Angus Trim and your engineer friend said.

When the target is an infinite mass (that is, something that will not move upon impact no matter what, such as the plywood boards Angus refers to, I think) the optimal point impact wise is the pivot point associated to the center of rotation of the sword just before the impact. In my personal experience (but I'm no expert swordman) it's difficult to make the weapon rotate about a point closer to the blade than the pommel, without loosing much energy before the impact... So your friend was just determining the pivot point relative to the pommel, and it's truly the most efficient point if you manage to get your weapon to rotate about the pommel. Now, the second thing is that when the target is not of infinite mass, this "perfect" point moves toward the end of the blade. This is I think coherent with what Gus said about dependency on the target...

But there is another thing with impacts, it's the effect you feel on your hand. It's not something I'm quite sure of, since all the calculations I've made rely on models for the hand that are, I believe, much too simplistic. Nevertheless, there is a sure thing, when you hit sufficiently close to the pivot point associated to your hand, you feel no shock at all. From my experience and computations, it's not so much a sweet spot but a sweet zone, and so is the area of "perfect" impact described earlier, thus there is always a significant overlap, of which a skilled swordman will naturally take advantage...

Blade harmonics are mostly significant when the impact happens sideways on the blade, which is not really expected when you are cutting, except if you missed your cut and end up with a bad edge alignment. In this matter, I mostly share the point of view of the author of the cited ARMA article. I believe that the effect is second order with respect to the properties mentioned before... What confuses the matter is that the node of vibration can be quite close to the interesting pivot points, specially on cutting swords, so that people might well be looking at the right point but with the wrong explanation.

Now, on to the handling properties.

Interestingly enough, the "dry" feel of the sword seems to be widely believed as related only to the position of the center of gravity. As far as I understood the various reviews I read, the center of percussion and/or blade harmonics are given mostly to evaluate the behaviour during impact, and not during handling and motion. True, the static feel of a weapon (what you feel when holding it motionless) is related only to the center of gravity. But, well, most of the time your weapon is moving, so my guess would be that we miss a significant part here... It's the section I'm still not completely satisfied with in my study, but I do have some interesting results. Specifically, I found that the perceived blade mass of weapons, when you consider only actions relative to one point (your hand), is just the ratio of the distance between your hand and the center of gravity, and the distance between your hand and its associated pivot point. It seems to work best, maybe for psychological reasons, when you choose the index as a reference. It does not tell the whole story however, since it does not take into account the fact the hand is not a point... Still working on that, but I would appreciate if the collectors here could try this with their swords, and tell me if they feel it's significant. I think it is, I've chosen iaitos and bokens already with that, based on what I know I like as a blade mass. I've used it to compare all the weapons I've come accross, and I'm already quite happy with the result...

I believe as well, but that would need some neurological evidence, that the tip control you have is directly linked to the position of your hand's pivot point relative to the tip. I've seen evidence of this comparing sabers, cutting swords, thrusting swords, even foils and bokens... I like to compute a "tip behaviour" caracteristic, as the ratio of the distance from COG to the pivot point of the hand, and the distance from COG to tip. I used that as well for iaitos and bokens. Seems to work decently, try it yourself and please tell me what you think...

Now, I'm writing about that, because I think that when it comes to measurements, we tend to focus far too much on impact properties. I mean, a sword will do significant damage when it hits with a good edge alignment anyway, the dependency upon the point of impact is only a slight variation... On the other hand, handling caracteristics do vary, and affect what moves you can do with the weapon. In short, a good sword and a good axe both cut efficiently, but you should not handle an axe like a sword...

Sorry for the extra-long post (partially off-topic, maybe), but I feel that subject is well worth a lengthy explanation, and I've been working a lot on it so it's hard to stop Happy I'd appreciate any feedback and questions, of course, and I shall still try my best at writing an article on that, but I really couldn't resist posting a reply Wink

Cheers

--
Vincent
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Angus Trim




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PostPosted: Thu 08 Jun, 2006 8:20 pm    Post subject:         Reply with quote

Hi Vincent

I hope you don't mind a bit of a disagreement with your analysis.......I agree it looks good if what we're talking about is stiff swords like katanas, or something like crowbars.... But most western swords aren't as still as katanas.........

There are both videos and stills, of antiques { a saber cut, from horseback, on a melon, in Australia} and modern swords that show both the edge alignment of the strike, and the blade action during and after the cut...... Quite often what you see, is a blade whipping both during the cut and afterwards {even with a strike with the edge alignment being spot on}. Quite often, the cutter is actually unaware of this happening. If the nodes are in the kind of alignment that a few of us call "harmonically balanced", then in a good cut its likely the cutter will be unaware of the blade action.

Years ago, before I began swordmaking, I was a sword reviewer. At one time, I had 7 type XIIs from 3 different makers, all within 2 inches of blade length, and 3oz in weight. But cutting in 1/4 inch plywood, one was outstanding, with consistent cuts in the 8.5 to 9 inch range, and one was poor in comparison, averaging 5 inch depth of cut. Most of the other swords were grouped around 7 inches, but it varied. I had newly met Tinker at that time, and mentioned my findings to him. At that time, he explained "harmonic balance" to me, and suggested I look over things again. I did that, and funny thing was that the "harmonically balanced" sword cut deepest, the one that stung my hand was worst, and where the hilt node was, also pretty much explained the rest of them too.........

It gets better........

Having experimented for years on this, a few years ago, I discovered that one needed to have the distal taper work with the wave action of the blade. If the character of the blade was just a bit off, and the sword cut something with enough resistance, it could very easily take a set in this one spot. The first time I saw this, I didn't believe it.... I made a model of sword that tested out pretty country fair. But the first three that were sold all came back bent, all in exactly the same spot. The heat treat was spot on......... After blade redesign, the model became quite successful.

After that experience, I tested new models for it, and managed to catch several before releasing them. Having made over 250 models {many of them one offs}, I've done a lot of experimenting and cutting.........The distal taper and the wave action of the blade have to compliment one another for best results........

Its my opinion that skipping harmonic balance in the discussion of the physical properties of the western sword is the biggest mistake some people make. You don't need to worry about it with kats, they're too stiff. But then again, even baseball bats have "sweet spots" where there is little to no vibration on impact, and these are the best spots to strike the baseball.......

The reason I mentioned target choice earlier, is if the target doesn't really give the blade any resistance, the blade can be thru the target before there is any sideways action to the blade. Once you get that resistance, then harmonics impact cutting performance, and durability..........

The reason I mention the blade node as the cop, is that I used to cut 1/4 inch plywood to measure some of this. Cutting on that node always produced the best results. Cut outside the node, cut inside the node, the cuts would measure less, and you'd get more blade action {depending on rigidity of course}........

The node and lethality of a sword cut? Not so much...... In unarmored combat, the tip is/ would be every bit as lethal as the cop. Particularly with a tip geometry for cutting....... The tip wouldn't do as well cutting in plywood, but would be more than good enough to take a throat out, remove a wrist, etc..........

swords are fun
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Jared Smith




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PostPosted: Thu 08 Jun, 2006 10:28 pm    Post subject:         Reply with quote

I suspect the statistic that is so often published as “CoP” in sword reviews is the correct and relevant dimension. It is probably extremely important that lessons from people like Angus should not be discarded.

I am not sure “CoP” is the right term for it. (Sort of advocating something like “sweet spot” here which is what other industries have adopted in recognition of the difference.) I admit that at some points we have to accept compromises (recent sword "point" versus "tip" terminology topic comes to mind.) Then again, not everything needs to fall apart technically, and has not in sporting goods industries that face a very similar issue as what I hope some might address here. If no one really cares for physics and precisely defined terms, the issue may go the same way as the automotive term for “radiator” (a device which in fact rejects something like 10% of it heat by radiation, mostly by convection… should have been called a “convector” or simply “heat exchanger.”)

A physics book will state nothing about vibrations resulting from striking something at the mathematical CoP. In fact, weird demonstration devices could be constructed for which CoP is “close to” the worst possible place for impact in terms of vibration nodes. If rotational form is absolutely perfect, the vibrations theoretically do not have to occur. Such an object would probably be worthless as a sword, but is possible.

One of my first regular paying jobs was at a golf course (almost two decades ago.) I noticed an old gentlemen (estimate octogenarian) who occasionally visited the driving range and consistently hit balls further than anyone else with deceptively gentle effort. One night I attempted to emulate him with no success. A couple of evenings later he returned with a “teaching tool” intended specifically for me. This tool was a wooden handle, with a rubber strap the length of a shaft appropriate for my size, and a club like metal head at the end of the strap. His teaching tool would reward only perfect form with good hits. It is not a perfect analogy, but an example of how CoP does not have to correspond with a variety of other complex and desirable aspects (stiffness for thrusting, vibration, forgiveness of alignment, etc.) of a club-sword like object’s physical characteristics.

Absence of evidence is not necessarily evidence of absence!
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Vincent Le Chevalier




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PostPosted: Thu 08 Jun, 2006 10:44 pm    Post subject:         Reply with quote

Angus Trim wrote:

I hope you don't mind a bit of a disagreement with your analysis.......


I surely don't Happy I posted mainly to have feedback from knowledgeable and experienced people...

Angus Trim wrote:

I agree it looks good if what we're talking about is stiff swords like katanas, or something like crowbars.... But most western swords aren't as still as katanas.........
[...]
Quite often what you see, is a blade whipping both during the cut and afterwards {even with a strike with the edge alignment being spot on}. Quite often, the cutter is actually unaware of this happening. If the nodes are in the kind of alignment that a few of us call "harmonically balanced", then in a good cut its likely the cutter will be unaware of the blade action.

Years ago, before I began swordmaking, I was a sword reviewer. At one time, I had 7 type XIIs from 3 different makers, all within 2 inches of blade length, and 3oz in weight. But cutting in 1/4 inch plywood, one was outstanding, with consistent cuts in the 8.5 to 9 inch range, and one was poor in comparison, averaging 5 inch depth of cut. Most of the other swords were grouped around 7 inches, but it varied. I had newly met Tinker at that time, and mentioned my findings to him. At that time, he explained "harmonic balance" to me, and suggested I look over things again. I did that, and funny thing was that the "harmonically balanced" sword cut deepest, the one that stung my hand was worst, and where the hilt node was, also pretty much explained the rest of them too.........


Yes, I'm not saying that the harmonic balance is not important when cutting. I believe that a good blade should be harmonically balanced indeed, in the sense that there should be a node at the hand location. I don't believe, however, that the location of the other node is significant. Partly, because it is not really adjustable as the pivot points are (not totally sure of that, but have you ever been able to move the harmonic COP very far away from about two third of the blade ?). Partly, because there is a bunch of weapons (wooden weapons, axes, katanas, gladii, and so on) that do not exhibit this behaviour, and that can still be good or poor cutters/impacters...

It can be noted as well that there could be coincidences in the behaviour. I mean, a sword poorly balanced from the point of view of inertia can be poorly balanced harmonically. Conversely, when the maker designs a sword to be perfectly balanced harmonically, he may be adjusting inconsciously the balance in terms of pivot points as well...

I own a type XI from you, in fact, and I have been experimenting with it. As far as I could see, the harmonic node and the pivot point relative to the handle are very close. I think that's not a random thing, and I'd like to make such measures on other cutting swords as well. The only other "real" european sword I own is the Milanese rapier from Arms & Armor, and it's not the same thing at all.

Angus Trim wrote:

Having experimented for years on this, a few years ago, I discovered that one needed to have the distal taper work with the wave action of the blade. If the character of the blade was just a bit off, and the sword cut something with enough resistance, it could very easily take a set in this one spot. The first time I saw this, I didn't believe it.... I made a model of sword that tested out pretty country fair. But the first three that were sold all came back bent, all in exactly the same spot. The heat treat was spot on......... After blade redesign, the model became quite successful.


I'm not really surprised by that. Vibrations are commonly accepted as being a major cause of stress and fatigue failures. Indeed, the blade geometry has to take that into account (I'm not saying it's easy though Wink ).

Angus Trim wrote:

Its my opinion that skipping harmonic balance in the discussion of the physical properties of the western sword is the biggest mistake some people make. You don't need to worry about it with kats, they're too stiff. But then again, even baseball bats have "sweet spots" where there is little to no vibration on impact, and these are the best spots to strike the baseball.......


Once again, I don't mean to discard or reject any discussion on harmonic balance. Obviously it's something significant when your weapon is flexible enough, when a hard impact is made. But basing all the analysis of impacts upon this I think is a mistake, because it's not something that can be applied to all impact weapons.

And moreover, it says not much about handling properties. Unless you have a foil or a super-flexible jian for exemple, you're probably not going to make the blade vibrate much through dry handling. Not enough to affect your moves anyway. So we need a way to compare handling as well, and that way I believe should be based on pivot points.

Angus Trim wrote:

The node and lethality of a sword cut? Not so much...... In unarmored combat, the tip is/ would be every bit as lethal as the cop. Particularly with a tip geometry for cutting....... The tip wouldn't do as well cutting in plywood, but would be more than good enough to take a throat out, remove a wrist, etc..........


On this I fully agree. Unfortunately, we will never really know if warriors in the past were paying much attention to that node... But I'm quite sure tip cuts were used a lot. Especially when youre target is trying to avoid your blow Wink

Thanks for the input !

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PostPosted: Thu 08 Jun, 2006 11:42 pm    Post subject:         Reply with quote

Jared Smith wrote:
I suspect the statistic that is so often published as “CoP” in sword reviews is the correct and relevant dimension. It is probably extremely important that lessons from people like Angus should not be discarded.


I have not payed a great deal of attention to the COP statistics in reviews since a few years now... Not that I don't value the harmonic properties, but my feel is that there are other more important points. Plus, as you stated in your original post, the definition is never really clear, and seems to vary according to individuals. That, and it does not apply to all the weapons I'm interested in. I'm more looking at what reviewers actually say about handling, even though this is not a numerical, precise information.

I want to state clearly once again that I'm not meaning to "discard" the opinion of anyone. I just believe in the virtues of discussion Big Grin

Jared Smith wrote:
One of my first regular paying jobs was at a golf course (almost two decades ago.) I noticed an old gentlemen (estimate octogenarian) who occasionally visited the driving range and consistently hit balls further than anyone else with deceptively gentle effort. One night I attempted to emulate him with no success. A couple of evenings later he returned with a “teaching tool” intended specifically for me. This tool was a wooden handle, with a rubber strap the length of a shaft appropriate for my size, and a club like metal head at the end of the strap. His teaching tool would reward only perfect form with good hits. It is not a perfect analogy, but an example of how CoP does not have to correspond with a variety of other complex and desirable aspects (stiffness for thrusting, vibration, forgiveness of alignment, etc.) of a club-sword like object’s physical characteristics.


Note that in the case of this practice tool, the COP has probably not much to do with vibration nodes either... I suspect the rubber is going to disturb completely the pattern.
If I may dare an explanation at the value of this training tool... I think it's intended to enforce the use of pendulum-like movements. Indeed, you will not be able to move the head much by applying torque on the handle, since if I understood correctly your description, the rubber will just bend. Whereas if you handle the thing like a pendulum, you will be able to make it move as if it were a solid piece. Something akin to what I've learned practicing japanese swordmanship : you achieve a good cut by letting the mass of the blade do the hard work, just letting the weapon rotate in your hands, rather than forcefully applying torque on the handle. But then I know next to nothing about golf, so maybe I'm completely wrong on that Wink

Note also, that the pendulum properties of an object are completely summed-up by pivot points, and have generaly nothing to do with harmonics... And your old gentleman seemed to value these greatly for training.

Thanks a lot for starting a thread about all this...

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Marton Pap




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PostPosted: Fri 09 Jun, 2006 4:31 am    Post subject:         Reply with quote

Really interesting!
This practicing tool reminds me what the Döbringer manuscripts says:

Know also that a good fencer should
before all things know his sword and be able
to grip it well with both hands, between the
cross guard and the pommel since you will
then be safer than if you did grip it with
one hand on the pommel. And you will also
strike harder and truer, with the pommel
swinging itself and turning in the strike you
will strike harder than if you were holding
the pommel. When you pull the pommel
in the strike you will not come as perfect or
as strongly.

This and the practice tool together underlines the importance of the pivot point related to the crossguard. As I see now it should be adjusted close to the tip with the mass of the pommel, and the blade should be light enough and the vibration node carefully adjusted far from the cross, so the pommelweight doesn't pull it back much, thus the two points (pivot and vibration) can stay close together and close to the tip.
The other thing I have been thinking about recently is the taper and the node, like Angus mentioned. If the node is mainly influenced by the taper and less by the shape [hexagonal, diamond etc.]of the cross section, than it would be extremely interesting to carefully measure the change of the cross section size [square millimeter] along the blade of some original swords. Does anyone know about such research? I think laserscanning is precise enough but I think it is quite expensive.

I really look forward to read your opinions!

(It would be exciting if the swords have the same overall taper, and even more if they don't have the same but could be categorized into two or three families.)
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PostPosted: Fri 09 Jun, 2006 7:44 am    Post subject: Web of complexity         Reply with quote

Excellent discussion guys

Quote:
Jared Smith wrote:
I am not sure ?CoP? is the right term for it. (Sort of advocating something like ?sweet spot? here which is what other industries have adopted in recognition of the difference.)


I agree Jared. I have long disliked the term COP as it really is not a defined term in the context of physics and the literal meaning center of percussion would be the center point of any contact regardless of the characteristics we are trying to evaluate. "Sweet Spot" does give a better indication of what the total package we are trying to cover maybe. I have used the term optimal striking point and most people get that, even none sword types which is something that might be good, as we do not want to get to far down a path of terminology the rest of the world can not relate to.

Vincents comments are very good and I have looked at many of these same issues in depth as well. A few years ago Gus, Tinker, Peter, Jason, and myself spent an evening discussing the varying virtues of these elements and the possibilities of what could or could not be codified and measured and end up being meaningful. The help of a theoretical physicist was not only enlightening but downright scary when the math started to be perceived Happy The one thing that really struck me from that discussion was how a tiny change in a sword was detected in the hand but if we had been tryting to codify it it would hae been difficult to see in numbers. The change was noticeable in the moving hand.

From that time on I have had several discussions with these guys on an individual basis and the interesting thing is we all still find elements we had not though of as yet and find examples that challenge our perceived understanding of the whole what makes a good sword question.

When we look at the pivot point discussions I agree that they are strong factors in the equation but I am often dissatisfied with the moving stick model or even the simple sword arc model. It seems to me that the sword, as any tool used for striking, adjusts its function when the hand is applied to the tool and it is not just the hand. When one starts to model a sword in motion is quickly becomes apparent that the style of grip, wrist/elbow/shoulder/waist action all impart factors that are not insignificant to the swords action and resulting numbers as far as what really happens in a sword cut in air or against a target.

As I thought more about this I started to feel that there is another dimension to this all. It is what I call the rotational point of the sword. This is the point on the blade that the sword wants to move around as you manipulate the sword in action. This is what I guess you might call the "active center of gravity" for want of a better term. (if anyone has one please suggest it) This will change per user and as different parts of the swing are emphasized or concentrated on i.e. grip, wrist, elbow, shoulder, waist. Now, to my mind, this has more to do with how the sword works in combat than some of the other factors, as the relationship of two three dimensional objects moving around each other in space is what combat comes down to and the function of a sword is to work in that environment with a certain degree of success. The result of how that sword interacts with the other sword/target is what it was designed for and very difficult to apply measurable numbers to. ( let me know what you think of the above as it is more an amorphous idea in the subconscious than a fully blown active theory)

Vincents comment about the ease of a good sword cut is very telling in this as when most practitioners get a really good strike against a target it will usually feel more effortless than if they are really trying to force a monster strike or cut. In a lot of personal testing I have done it seems to me the relaxed muscle motion and controlled strength of the cutter is what is the key factor to a swords efficiency.

Well enough for now if I keep going I will get off on a tangent and never get back

Big Grin
Best
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Joel Whitmore




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PostPosted: Fri 09 Jun, 2006 7:59 am    Post subject:         Reply with quote

[quote="Vincent Le Chevalier"]
Angus Trim wrote:


Unless you have a foil or a super-flexible jian for exemple, you're probably not going to make the blade vibrate much through dry handling. Not enough to affect your moves anyway. So we need a way to compare handling as well, and that way I believe should be based on pivot points.



Interesting discussion here and I ran across this statement from Vincent (who obviously has a keen scientific mind). When you move into two-handed swords the blade vibration does affect handling. Not so much the blade vibrting as when struck, but when just cutting, the sheer length of some of these blades affects your swing. One has to be very very precise with movements due to the blade flexing during the swing. Many times I have taken my A&A English Two _hander to cut and dropping the tip on the downward swing just an inch or so causes the blade the flex and thus a misaligned cut.

Also, I think there is a danger here of "over quantifying" this matter. From an engineering standpoint the action of a sword blade (especially while moving or cutting) is a hugely complicated matter. Presonaly I don't think one would get any more performance enhancement out of a sword but running sims on a supercomputer than an experienced smith like Gus testing and refining his product. As far as I know ( and that is limited) no one has done serious scientific or engineering studies on the dynamics of a sword blade. I think it would be an interesting engineering exercise, but I can't see anyone spending that much time and money for an object that is basically meaningless in today's society. For me, it just gives me more and more appreciation to the medieval smiths who made swords hundreds of years ago.

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PostPosted: Fri 09 Jun, 2006 3:38 pm    Post subject:         Reply with quote

I honestly just happened to run across this relevant article.
http://en.wikipedia.org/wiki/Center_of_Percussion

I personally consider vibrations relevant to handling. I will admit that for typical longswords, vibration is primarily in a plane perpendicular to the cutting arc, and probably amounts to less than 5% of the total performance. That is not insignificant when one considers how many times a sword could have been swung (historical accounts of durations of battles and sword edges hacked up like saw blades after battles tell us a little...) A 2- 5% difference may be a life and death issue over the course of a few hundred swings with unimaginable fatigue progressing throughout such a day of battle.

I also admit that a "sweet spot" on athletic equipment is not the same for everyone. Those labels on bats or discolorations on racket strings (tyically 3 to 5" long) are generally large enough (based on testing) to encompass a large range of athletes' styles who have shown any decent performance capability with the equipment though. There is a statistical exact spot for the label as well. That "statistical center" of the sweet spot is actually pretty darn close to the perfect sweet spot for something like 75% of bat users according to the company my spouse worked at. When tasked to "reverse engineer" what the effective pivot point of a bat must be based on the empiraclly proven "sweet spot", it is generally much closer to the pommel like end of the bat, than it is to the grip area center. This tends to be an amazingly consistent truth of a wide range of club like objects (rackets, bats, golf clubs, etc.)

Absence of evidence is not necessarily evidence of absence!
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PostPosted: Fri 09 Jun, 2006 4:28 pm    Post subject:         Reply with quote

A couple of more thoughts and speculation here. Let me preface it by saying I consider the Peter Johnson's and Angus Trim's to be the masters. My opinion does not count for 1% in my own estimation of what their opinions count for.

(Another over-simplification.) Take a piece of standard round pipe of very uniform dimensions. Hold it with your hand exactly in the center. Twirl it like a batton. In this instance there are two "CoP"s located at each tip of the balanced spinning piece of pipe/ batton. Hold it any where else, there is only one CoP. In fact, there are infinate CoPs for anything depending on how you grip it and what the real "effective center of rotation" actually is. CoP is entirely dependent on an assumption. There are an infinate number of CoPs for anything, including swords. They are greatly different in the case of "murder strokes" than they are for typical strikes. "CoP" without a qualification of what it is in relation too is meaningless.

Let's just say some correct expert has determined this perfect spot on a sword blade to hit some type of specific target with. I fancy myself better (admit I offer no proof) than a mental retard with regard to logistics. What will be of interest to me is how far that point is from the guard of the sword. That represents to me how far I can be between my last line of defense and my opponent while I do him optimal harm. Not all sword like objects have guards. But for those that do, what interests me is how far that "sweet spot" is out in front of that guard. When I read reviews, I often don/'t know exactly what the "CoP" or what ever you want to call it is being dimensioned from. I would like to see some type of "important blade point / sweet spot" defined with respect to sword guards. Right now, I am not certain what various manufacturer's are giving this dimension in relationship too.

Lastly, "CoP" does little credit to all that craftsmen who have spent years of making, testing, and refining have actually invested into these works of art. I do not doubt that they ascertain in mere seconds exactly what is important at some specific point on the blade of a sword.

Absence of evidence is not necessarily evidence of absence!
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PostPosted: Fri 09 Jun, 2006 4:39 pm    Post subject:         Reply with quote

As far as I have seen, any maker who talks about the CoP is referring to the blade node where vibrations are best dispersed. As far as I've seen, everyone measures it as a distance from the guard. I've never seen any confusion over this.

This stat does have some relevance, hence its inclusion in so many places. I check the CoP before cutting with a new sword, because my experience (whatever it may be) has shown that more powerful blows generally come from the CoP. More comfortable blows certainly do. Knowing where it is certainly helps me. Happy I've used swords where off-CoP cuts vibrate badly, usually when I've struck behind (guard-ward, not tip-ward, of) the CoP. Certainly some swords and cutting media respond very well to slashes by the tip and other outer-part-of-the-blade cuts as well.

It's just one stat of many. How it mates up with pivot points and how a sword's overall mass is distributed is much more important. While it is relevant, I wouldn't try to make too much out of it.

It's a static measurement that shows a region on the blade where many cuts will be effective. How well the sword moves is determined by balance, weight, and mass distribution more than CoP.

But that's just my two cents. Happy

Happy

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PostPosted: Fri 09 Jun, 2006 5:24 pm    Post subject:         Reply with quote

Chad Arnow wrote:
As far as I have seen, any maker who talks about the CoP is referring to the blade node where vibrations are best dispersed. As far as I've seen, everyone measures it as a distance from the guard. I've never seen any confusion over this.)


Can someone provide links on this site or the prominent manufacuterrs's web sites where this fact is clearly stated? I don't challenge it, I just think it not particularly clear where it is measured from at the momet.

Absence of evidence is not necessarily evidence of absence!
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PostPosted: Fri 09 Jun, 2006 9:17 pm    Post subject:         Reply with quote

Jared Smith wrote:
Can someone provide links on this site or the prominent manufacuterrs's web sites where this fact is clearly stated? I don't challenge it, I just think it not particularly clear where it is measured from at the momet.


Albion's stats don't indicate where the measurements are taken from. Gus, A&A, Del Tin, and MRL don't seem to mention that stat at all or with any consistency. Etc., etc. So Albion looks like they might be the only "prominent manufacturer" that regularly lists it (or one of the few if I missed a couple), and they don't clearly define where that measurement is taken from (that I saw).

As for this site, if you check the figures in our reviews and collection galleries, we state "from guard" for our CoP and PoB measurements. If you cross-check our measurements with Albion's for reviews of their products, within a reasonable degree of variance, they line up. So we can deduce that Albion also measures from the guard. Happy

I think measuring it from any other location than the guard would make it more confusing and even less helpful. Grip and pommel lengths will vary widely, so knowing the distance from one of those won't help. Knowing the distance from the tip won't help unless you know the length of the blade anyway. I seem to remember at some point that folks used to believe that the CoP should lie in a certain range on many blades, perhaps 2/3 of the way between guard and tip or something. But my memory is fuzzy and people don't typically talk in terms like that any more, probably because CoP is not the most telling stat by itself and because it varies by sword type and purpose.

Measuring from the guard seem to make the most sense especially since we're effectively talking about how far away from your forward hand (just behind the guard) the target needs to be to be struck with the spot where the blade's vibrations are best dealt with.

Happy

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