Modifying my khukri's balance for improved performance

[the possum]

This discussion leads me to become interested in a comparison of older blades compared to modern reproductions. Have we actually seen a loss in functionality even with all the modern materials and methods of manufacture because makers have become removed from the blade and are copying designs without a knowledge of the reasoning and thus changing "trivial" features which may not actually be so inconsequential.
-Cliff

I hesitate in even commenting on this, because I don't want this thread to turn into some kind of flame war. But I guess in the end it matters to performance, so here's a couple thoughts.

I have seen many times in the tactical blade community, comments praising knives that I think would have lousy dynamic balance. Often phrases like "all business" and "no frills" are used to describe a knife that is just a slab of steel with some angular grinds. I recall at least once someone even putting down blades with tapers because of this mentality. I'm not gonna go find the quote 'cause I don't want to start a ruckus, so if anyone wants to dismiss this as a "strawman" argument, go right ahead.

But when I see comments like that, I have to wonder if we really know more about dynamic blade use than the folks who depended on them as weapons in the days before guns. I guess they think the ability to swing the knife quicker is just an "extra frill". Likewise, they must think the ability to get even more chopping power without having to lug around extra weight on the belt (and weight is often cited as a top priority for soldiers) is just an extra frill. Why do they think these features were used on serious fighting blades for centuries?

Maybe this belongs in whine & cheese. I dunno. Hopefully it will get enough exposure here that at least some of the above mentioned straw men will start thinking about it critically.

 

[Cliff Stamp]

Often phrases like "all business" and "no frills" are used to describe a knife that is just a slab of steel with some angular grinds.

As with anything there is a lot of hype used to sell knives which is based on ease of manufacture. The critical problem is the reluctance to question makers and *demand* proof of claims of superiority. Where is the materials data? Where are the reference blades/steels to benchmark the "impressive" performance? Makers coming into the dicussion then have to either call the hype or go along with it. What is needed is many more makers like Cashen writing such articles as the following which directly contradicts claims made by makers praised for knowledge of such topics :

http://www.cashenblades.com/hype.html

Plus guys like Roman Landes willing to publically critize the heavy carbide steels strongly promoted by Crucible and many makers. Landes notes they are directly inferior to typcially labed "lower end" steels because steels like S60V are prone to carbide tearout at low angles. Thus you are paying more and getting less. Guys like Landes and Cashen actually have the facts to support their arguements in *published* articles and books. The hype against it still goes on even with no contradictory data being cited simply because of the willingness to accept maker claims on faith and ignore the blindingly obvious contradictions.

Why do they think these features were used on serious fighting blades for centuries?

In general there is little such research and considerations. Consider for example the massive amount of myths about sharpening in spite of the wealth of work done by Swaim and Talmadge which directly refutes it. If people are ignorant of work done just 10 years ago you really can't expect generations of past knowledge to be appreciated. Right now there are simply far too many makers saying balance is all about center of mass. This gives the arguement such a high inertia that it is going to take a long time to stop its snowballing unless a number of big name makers take public and active stands against it, calling other makers on ignorance of dynamic issues just like Cashen did with forging and Landes did with carbide tear out.

-Cliff

 

[the possum]

Right now there are simply far too many makers saying balance is all about center of mass. This gives the arguement such a high inertia that it is going to take a long time to stop its snowballing unless a number of big name makers take public and active stands against it, calling other makers on ignorance of dynamic issues just like Cashen did with forging and Landes did with carbide tear out.

I only make a couple knives a year at most, so even if I did try doing something like that, I'd never be taken seriously. Obviously, since I'm not cranking out a huge number of blades and don't have my name in print, I wouldn't know anything.

And even if I did, I have absolutely no desire start turmoil over the issue. I just can't stand being disliked that much.

 

[Cliff Stamp]

I just can't stand being disliked that much.

Popularity is the bane is objective thought.

Hype only lasts so long though and once a crack is made there is no repairing it and eventually the walls will come tumbling down. Once this starts people radically shift viewpoints because you don't want to be the one under the rubble when the majority viewpoint has shifted. This is especially true when the arguement has been made public in a large venue and there are *no* opposing viewpoints put forth :

http://forums.swordforum.com/showthread.php?s=&threadid=68731

Where is the contradiction from the legions of makers refuting this? There are surely enough of them over the years who have argued for hammer forging being directly superior and massively so as in multiple to one performance. If your opinion is based on facts and objective reality then why not go there, present the data and/or show Cashen where his arguement is flawed. There is no cost of travel, you are not flying across country and it takes no more time/effort to post there than anywhere else.

Similar with the above commentary on balance, you can actually measure all aspects of balance quantitatively as noted in the above article. It isn't a subjective matter any more than the mass or the stock thickness. Now there is always user preference, what you want and what you are capable of, and this is a different issue. I would not want a blade for cutting thick wood balanced the same as one for cutting brush. Not speaking again of mass or center of mass, but mass distrubution and noting specifically of tip control, blade speed and ideal impact points.

-Cliff

 

[the possum]

Getting back to the topic, the other day I was wracking my brain trying to understand something in that article (Sword Impacts and Motions by Mr. Turner) that upon re-reading didn't seem right. I think I may have it now, but figure it would be good to bounce ideas of someone else.

At the top of section 2, he draws comparisons between sword impacts and levers, using this illustration:

He then goes on to provide a formula for finding the "inertia" of the blade at the tip, or any other place you care to calculate, based on the distance between the balance point and rotational center, total distance between the rotational center and its corresponding impact node (length of the pendulum), and total weight. I ran the calculations and found that after I modified my khukri, its inertia at the new "sweet spot" is .589. (I don't even know the proper description here- if this should be expressed as pounds or foot-pounds, as I converted my measurements to feet and pounds.) Before, its inertia at this same point was .779. I ran the numbers for other areas including the tip and belly, and likewise found that the inertia had decreased overall. The new tip inertia is .335 pounds(?). By coincidence, the tip inertia of my bowie (which has a longer blade) is around .316 pounds. Mr. Turner postulates that "If forced to guess at a typical example of tip inertia, I'd go with something close to the mass of a baseball, or 5.12 ounces," which is .32 pounds.

After further studying the illustration of the lever above, an even simpler idea occurred to me. If we're just trying to find the inertia at the "effort", then why not just position a scale there? So I stepped out to our seed weighing scale, and rested the tip of the blade on the scale, and supported/balanced the rear of the knife on my finger at the rotational center. (i.e., I rested the crossguard of my bowie on my finger, and the bolster of my khuk.) The scale read just over 5 ounces. Hmmmm...

Now, since I've decreased the inertia of my khuk all along the blade, it would seem at first glance that chopping power should go down. But I think there are several additional factors to explain why this is not the case, as observed.

The "inertia" described above does not account for blade speed when swung. I have decreased the blade's inertia, so I can swing it faster, thus actually increasing power.

The blade is moving faster further out towards the tip. Even if the inertia were higher before my modifications, it does no good if it numbs my hand to try using it that way. Now that I've changed the rotational centers, I can make use of the faster tip.

According to the above, any time you decrease the distance between the balance point and rotational center, and increase the distance between the rotational center and its corresponding impact node, inertia will go down. However, it still depends on how you accomplish this. It will have very different results if you do this with a pommel, or just with tapers and blade mass. Think again about my scale example above- any time you add weight to the butt of the knife while the guard is resting across your finger, the scale will say the tip gets lighter. So yeah, it makes the blade seem lighter to swing. But, because of the rotational principles, on impact the weight in the pommel will act the same as though it were out on the blade instead of behind the grip. You can (and usually do naturally) make your swing and the impact occur in different centers. During a cut that is totally wrist rotation, the knife would be spinning around the center of your grip. That 5 ounce pommel is less than 2" away from the rotational center then, so it takes very little to get it up to speed. On a snap cut, the rotational center may actually be at the pommel, so then it doesn't hinder you at all. But when the belly/tip/impact node strikes something, the knife will want to rotate around the guard then. Now, the pommel is over 4 1/2" from the rotational center, (with my khukri; it would be more like 6" on my bowie) so it resists the torque and adds to the blow. On impact, it would be the same as though that 5 ounces is 4 1/2" out on the blade. A tip impact would make it seem further out on the blade. (since I didn't get that impact node quite all the way to the tip)

Lots here to mull over. Not sure I've got it entirely, but that's what I'm thinkin' for now.

 

[Cliff Stamp]

He then goes on to provide a formula for finding the "inertia" of the blade at the tip, or any other place you care to calculate, based on the distance between the balance point and rotational center, total distance between the rotational center and its corresponding impact node (length of the pendulum), and total weight.

What he is calculating is the force needed to counteract the torque due to the center of mass. The torque from the force at the tip (force times distance to point of rotation) must exactly equal the torque from the center of mass (weight of sword times center of mass to point of rotation). Divide this equality by the distance from the point of rotation to the impact point and you have this "effective mass". Yes you can just weigh it directly, this is in fact a standard first year physics lab, unfortunately done with meter sticks and not swords.

Now, since I've decreased the inertia of my khuk all along the blade, it would seem at first glance that chopping power should go down.

Yes this is one of the places where people start to wander on misconceptions. At some point the mass and inertia of the blade will have increased to such an extent that you can't cut very well with it. Cutting ability is dependent on impact energy (mass times speed time speed) and further directly influenced by speed because the edge has to be moving far faster than the object will during impact or else it just pushed the object rather than cuts it. Thus for optimal cutting power you want the maximum inertial which you can move at very high speeds. You won't get very good cutting power with a fillet blade or a small splitting axe because one is far too light and the other far too heavy and thus they suffer from the two general drawbacks in extremes. I have actually done test cuts with both and you can note failures of two different types. This then immediately points out that user physical ability is going to be critical in determing the ideal weight, but balance is more general assuming that the same impact points are required.

But when the belly/tip/impact node strikes something, the knife will want to rotate around the guard then. Now, the pommel is over 4 1/2" from the rotational center, (with my khukri; it would be more like 6" on my bowie) so it resists the torque and adds to the blow.

There is quite a lot going on and note how he specifically says

"The pommel on a straight cutting blade makes the sword hit harder, and by a very significant amount, while only making a small degradation in the sword’s maneuverability."

When you start a cut you are rotating the knife and your force is very close to the pommel and it doesn't take much to move it either by rotation or just in a linear direction. However when the tip of a sword takes an impact what has to stop the pommel is the internal force of the sword (which will generate an equal and opposite counter force on the object). Now the pommel doesn't know the sword has hit anything and it keeps moving forward and thus ends up moving ahead, this then snaps back as the steel is elastic which drives the sword deeper in to the cut. This assumes that the impact point isn't one which induces rotation around the pommel in which case it does nothing aside from slightly raising the overall mass of the sword.

-Cliff

 

[the possum]

What he is calculating is the force needed to counteract the torque due to the center of mass. The torque from the force at the tip (force times distance to point of rotation) must exactly equal the torque from the center of mass (weight of sword times center of mass to point of rotation). Divide this equality by the distance from the point of rotation to the impact point and you have this "effective mass". Yes you can just weigh it directly, this is in fact a standard first year physics lab, unfortunately done with meter sticks and not swords.

I think I've got it now. But just to be sure, by decreasing this "effective mass" out on the blade, this is what actually makes the blade feel lighter and quicker to swing/rotate, correct? This is what determines the moment of inertia?

There is quite a lot going on...

When you start a cut you are rotating the knife and your force is very close to the pommel and it doesn't take much to move it either by rotation or just in a linear direction.

I guess I was getting at another point that I hadn't considered much before. The pommel adds power to a tip cut, but it does also reduce that "effective mass" out on the blade. (assuming I'm understanding the above correctly) Lots of people notice how the pommel makes the blade feel quicker, and leave it at that. But, let's look at this from another angle. If we didn't add the pommel, the blade will still feel slower. If we wanted the same quickness in the blade, then we'd have to remove mass from the blade to get there.

So the difference made by the pommel in the end is actually larger than it appears at first glance. We don't just start out with a baseline power level and get more by sticking on a pommel- without it, we'd have to actively decrease the power to get the same handling qualities.

 

[Cliff Stamp]

But just to be sure, by decreasing this "effective mass" out on the blade, this is what actually makes the blade feel lighter and quicker to swing/rotate, correct?

To a point yes, the blade does take more force to rotate if the effective mass is higher, however the parallel isn't exact between the two.

This is what determines the moment of inertia?

The effective mass is used to discuss impacts on the blade from the point of view of it moving as a whole in a stright line, the moment of inertia however controls the reaction to rotation from a force. They are dependent on the same variables but in slightly different ways. When you ground material off the blade then both the effective mass at the tip and moment of inertia of the blade decreased. However when you added the pommel then the effective tip mass decreased but the moment of inertia actually increased, though not by a lot as noted below. When you switched the mass from far out on the blade to on back of the pommel then a number of major things happened :

1) the center of mass moved closer to the handle

This means that statically the blade feels lighter as less force is required to move it in a straight line.

2) the mass you moved is much closer to the point of rotation

This means the blade is much easier to rotate because the moment of inertia is much smaller. The change moveing weight from far out to the pommel can be massive because moment of inertia of a piece of mass depends on the square of the length from the mass to the point of rotation. So for example if you were to take a gram from ten inches infront of your index finger and move it to four inches behind it then the moment of inertia of that bit is reduced to 15% of its origional amount. If you go out to 15 inches on the blade and transfer a gram there to a pommel four inches back from your index finger then that bit has its moment of inertia reduced down to 7%.

3) the mass you moved is much further from the point of impact

This is the really cool part. An impact on the blade easily stops the mass of the blade next to the point of the impact, but the pommel is quite far away and it is moving forward too and the tip of the blade can't suddenly stop and the pommel move unhindered so the pommel gets yanked back by the sword and it doesn't appreciate that and pulls back which drives the sword hard into the target. Since the pommel is really far away from the point of impact it takes a lot of force to move it and thus it can make a large influence on the cutting power of tip cuts. Yet as you noted it is quite close to the hand so you can move it without much effort and with much less effort than you could with it out on the blade. Essentially you have moved around the mass so it is easier for you to move but harder for the objects you hit to move.

[pommel]

If we wanted the same quickness in the blade, then we'd have to remove mass from the blade to get there.

Yes, and can you even get the same ideal impact point then? You get more power, ease of movement and better tip precision and vibration optomization. I think you may also be able to feel and react in hand to the movement of the pommel and thus use it to power into cuts after the point of contact, based on work I have done lately, not with pommels, just extrapolating off of some related concepts.

-Cliff

 

[Ebbtide]

Thank you Possum for pointing me to this thread and the link as well.
The illustrations and explanation on the link are a huge help to this visually oriented guy.

Alot to digest, but I'm on it.