Modifying my khukri's balance for improved performance

[the possum]

When I got my HI khuk several months ago, I headed to the woods and tried it out on some fallen trees along with my big bowie. The khuk is an HI “dui chirra” model- 21” long and 32oz.

The knife felt rather heavy and slow compared to my bowie of course, so I had to chop at a slower pace to keep the fatigue down. This wouldn’t bother me so much if I got better chopping power in trade, but I didn’t. When using both blades at their “sweet spot”, they cut wood about equally, or perhaps the bowie was slightly better. The problem was, the khuk’s “sweet spot” (or optimal striking point) was only 10 7/8 inches ahead of the bolster, which is actually worse than it sounds since the sweet spot was so narrow. If I tried chopping even ¾” to either side of it, power went way down and shock went way up. If I tried cutting anywhere near the tip at all, it transmitted a huge amount of shock to my hand, turning my pinky finger numb and tingling within just a couple swings. This made it difficult to chop wood that was actually laying on the ground, (since the tip would strike the dirt before the belly made contact with wood) and I was more concerned about using it on coons, since I’m forced to use the tip area in that situation. I found that against the critters, its performance was noticeably less than my bowie for that reason. It was just not balanced for this type of work.

So I finally set about to correct what I perceived as a problem. I know that not everyone wants to use their blades the way I do, and that’s fine. This was about taking a tool that disappointed me from a performance standpoint, and changing it enough so I’d actually carry and use it. I also wanted to use it as an experiment to see if I really understood the concepts of “dynamic balance” well enough to actually put them in practice on a blade type I was not familiar with. My goal was to change the moment of inertial and rotational centers in order to move the “sweet spot” further out and make it easier to swing, without reducing chopping power.

The two fullers on the blade were very shallow; I’ve called them “cosmetic” elsewhere, since they don’t really reduce weight. So I clamped the knife to my workbench and started removing metal with a small angle grinder to deepen the fullers. (I’m not gonna post a close up shot of the blade, ‘cause I botched the grinds in a couple places. Stupid angle grinder…. It should still perform as a working tool though.) I kept checking my progress with a pendulum to see how much I was moving the optimal striking point, and going by feel. After deepening the fullers, I weighed the knife again and was quite surprised at how much weight I had actually removed from the end of the blade. Nearly 7 ounces total, for a finished weight of 25 oz. I was surprised that once this was done, I had only moved the balance point 1 inch closer to the handle, and the optimal striking point only slightly further out.

If I had just left it like this, I have a good feeling it’s handling characteristics or “dynamic balance” would be more like the antique it was based upon. According to this thread I found in the Archives, Berkley’s original (which the diu chirra was based on) only weighs 24oz, and is described as very lively in the hand. The fullers on the cast model sent to the Kamis ended up being thicker than the original.
It really swings much easier for me now, and even my wife (who is not into knives) couldn’t believe the difference. Two different things were at work here. By reducing weight from the tip, I’ve decreased the moment of inertia, and made the center of balance and optimal striking point further apart from each other. The fact that they are further apart may be more significant than where they ultimately lie. (still thinking about that one) The other thing is that I’ve simply removed a lot of weight out there, and now it’s just more compatible with my personal strength level. Someone else with a very strong wrist may have decided to quit grinding sooner.

 

[the possum]

It’s still not as fast as my bowie, though, and I haven’t really changed the optimal striking point much to improve tip cuts like I was wanting. In fact, I’ve reduced overall chopping power because I’m still forced to strike with the same spot on the edge, with less mass. So how do I get more chopping power at the tip? By adding a pommel. Just like with the weight I removed from the blade, it’s possible to go overboard and put too much weight in the pommel. Hence, “balance”. To determine how much the pommel should weigh, I set up the pendulum again, and started duct taping bullets from my reloading bench on the butt to see how much further out I could move the sweet spot. By adding several ounces of lead at the butt, the pendulum dropped several inches closer to the tip. I realized then that if I wanted to move the distal rotational center all the way to the tip, it would require too much weight at the pommel. So I basically just went by feel and added the maximum pommel weight I was comfortable with, and let the chips fall where they may. It’s worth noting that I tried this before I reground the blade, and even with 6 or 8 ounces of lead on the butt, the optimal striking point barely moved. There was just too much weight in the blade to compensate for, and really there still is now. By deepening the fullers, I’ve reduced the moment of inertia enough that now a much lighter pommel moves the sweet spot several inches. I added up the weight of bullets and figured how big a piece of metal I’d need, and started shaping a piece of scrap iron.

This is where my project got a bit more involved than I’d hoped it would. This khukri does not have a through tang, and I’m certain if I had just glued/screwed that heavy of a pommel on, it would have split the wood from the shock. The pommel is not just sitting there to bring the balance point closer to the hand; it takes a very active part in the chop by resisting the counter torque of the impact. So I removed the handle and soldered on a threaded bolt long enough to protrude out the back of the modified handle. I split the bolt and bent it to a Y shape so it has over an inch of contact with the sides of the tang.

Before gluing everything together permanently, I did some test whacks on scrap wood to make sure the joint would hold. Hopefully this will prove strong enough to prevent breakage, but time will tell.

I should mention that I’ve seen several folks recommend adding a pin through the handle of khuks made in this fashion to keep the blade secure. If they are all made like mine was, you’re wasting your time. The tang extends almost all the way to the back of the grip, and it’s held on with some kind of gray epoxy substance (JB Weld?). It’s a bit longer than the tang on my original from Atlanta Cutlery too, and that one had the longest tang out of over a dozen in the box at their showroom.

I used a hatchet to split the handle wood, and further used the poll to smash it. Even with the wood splintered it would not let go of the tang. With lots more effort I was finally able to remove most of it, but it did not pop off from the epoxy- the wood broke leaving a coating of fibers still embedded on the tang and pieces in the bolster. I started using a hammer to break off the epoxy, but it refused to just chip off. So I tried burning it off with a propane torch. I burned the wood fibers out but it still wouldn’t let go. I had to use a hammer and chisel to get enough out of the bolster to remove it, and then finally had to grind off the excess with a dremel. If a hatchet, hammer, chisel, grinder, and a friggin’ blowtorch have a hard time getting the handle off, I don’t think you have to worry about it flying off on its own.

After adding the 5 ounce pommel, I did another pendulum check to measure the sweet spot, and it moved out 2 ¼ inches, so now it’s in a much better position right on the curve of the tip for me. Also, besides just that raw number, some wood chopping proved that the sweet spot was much larger and more versatile now. I don’t notice the least vibration if I chop over 1” in front of the rotational center or about 5” behind it.

You’ll notice that I haven’t really discussed the center of mass (aka, balance point, center of gravity, point of balance, etc.) yet, even though lots of folks assume its location is what largely determines how the blade will handle. This is not entirely true. The way the blade feels when just holding it out in your hand, and the way it reacts during quick swinging and direction changes can be totally different. The “dynamic balance” and “static balance” are two separate ideas, and you can change them independently of each other. The balance point will end up wherever it needs to be as a side result of my real goals. FWIW, the balance point started out 6 inches ahead of the bolster, and then moved to 5” after I reground the blade. After adding the pommel in its finished form, it was only 3.5” in front of the bolster. So as you can see, counter to the conventional wisdom, I was able to gain chopping power at the tip even though the CoM got closer to the handle.

So here’s what my “new” khukri looks like.

When it’s all said and done, I’m not really sure we can call it a true khukri anymore, but I’m really not too concerned about that. I made the knife faster to swing and maneuver, less fatiguing to use, increased it chopping versatility, reduced hand shock, and made it more powerful for the kind of chopping I want to do. (i.e., chop closer to the tip.) Before I had 32 ounces, and had to strike about 10-11 inches in front of the bolster, and now I have 30 ounces, striking 13-14 inches out. At the very least, I didn’t give up any overall chopping power, as evidenced by the scrap lumber I used to gauge my progress.

Hope this generates some discussion.

 

[sodak]

Wow! You did a nice job, looks very finished. I'm anxious to see what your long term level of happiness will be. There's a lot more to this than meets they eye, isn't there?

 

[Thomas Linton]

Appearance-wise, I like the present look better than the original.

Did you etch the blade to determine where the hardened portion of the edge is located?

 

[the possum]

Nope, haven't etched it yet. Have some ferric chloride sitting in the garage though.

I understand what you're getting at here, and I realize that by chopping closer to the tip, I may no longer be in the hardest zone of the edge. I checked the blade with a file, and though it's definitely harder on the belly, the tip and rear portion of the edge are still at least as hard as the fodder knives/machetes I grew up with. So I'm willing to accept a little more sharpening in exchange for the increased versatility.

 

[Mark Nelson]

I like what you did with the khurk possum. Great job on the handle. I have considered doing something like that to a couple that I have but have not done so yet. If you ever get a chance pick up a Chitlangi. Out of all the khurks that I have or that have gone through my hands I think that it is my favorite style. May be more to you liking in how it handles and cuts. I have two and they both out cut many much heavier khurks and handle like a dream.

 

[Cliff Stamp]

Some of the questions asked in the following we have discussed before, I just think it would be worthwhile to have them noted in public. 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.

... move the “sweet spot” further out and make it easier to swing, without reducing chopping power.

And broaden the impact point comfort zone? What is the width of the relative sweet spot with the modified khukuri?

Nearly 7 ounces total, for a finished weight of 25 oz. I was surprised that once this was done, I had only moved the balance point 1 inch closer to the handle, and the optimal striking point only slightly further out.

...

In fact, I’ve reduced overall chopping power because I’m still forced to strike with the same spot on the edge, with less mass.

Can you offer a rough commentary on relative chopping power at this point?

Someone else with a very strong wrist may have decided to quit grinding sooner.

This is one of the foundation myths that really should have long since been destroyed. Arguing that balance can be achieved independent of a user is like saying the perfect shoe size is the same for everyone. Now that seems obviously absurd, but so is arguing the weight and distribution of a knife without input from the users physical abilities and desires. What about if you are exceptionally strong compared to someone who is much weaker but can move much faster. Surely the right knife "balance" for them is going to be very different.

So how do I get more chopping power at the tip? By adding a pommel.

I have seen at times very decorative ends on large brush knives. These were wood, but still given the size you would expect a lot of weight. I wonder if these animal heads and such are rooted in performance as much as tradition / religion. What happens then when we decide we don't want that "decoration" and just chop it off.

Just like with the weight I removed from the blade, it’s possible to go overboard and put too much weight in the pommel. Hence, “balance”.

You might want to note what happens if you move the impact point too far. What happens for example if you move it infront of the tip?

It’s worth noting that I tried this before I reground the blade, and even with 6 or 8 ounces of lead on the butt, the optimal striking point barely moved. There was just too much weight in the blade to compensate for, and really there still is now. By deepening the fullers, I’ve reduced the moment of inertia enough that now a much lighter pommel moves the sweet spot several inches.

This is worth quoting again, and reinforces the perspective of a "lighter" blade at a similar mass just from having a much different distribution rather than raw amount of material.

The “dynamic balance” and “static balance” are two separate ideas, and you can change them independently of each other. The balance point will end up wherever it needs to be as a side result of my real goals.

As a side note, there are some blades where the static balance is critical and the dynamic balance is largely irrelevant, most utility knives for example. Essentially any knife not used in a dynamic matter, which should be kind of obvious, but isn't really because this often isn't a point of contention at all.

FWIW, the balance point started out 6 inches ahead of the bolster, and then moved to 5” after I reground the blade. After adding the pommel in its finished form, it was only 3.5” in front of the bolster. So as you can see, counter to the conventional wisdom, I was able to gain chopping power at the tip even though the CoM got closer to the handle.

I made the knife faster to swing and maneuver, less fatiguing to use, increased it chopping versatility, reduced hand shock, and made it more powerful for the kind of chopping I want to do. (i.e., chop closer to the tip.) Before I had 32 ounces, and had to strike about 10-11 inches in front of the bolster, and now I have 30 ounces, striking 13-14 inches out.

I would be interested to see the effect on springy woods which demand a faster blade. Did you notice a difference in accuracy and tip precision?

For those interested, this is pretty much mandatory reading :

http://www.thearma.org/spotlight/GTA/motions_and_impacts.htm

-Cliff

 

[James Green Dragon]

This is an awesome thread IMO :thumbup:

I would like to see more of this with chopping comparissions. As well as related work with straighter blade types - something like a smatchet maybe. The modern day gladius...

Perhaps Mr. Possum can more fully explain the 'pendulum test' too - a picture is worth a thousand words - and right now I'm having trouble visualizing this part - I'd like to be able to experiment with the concept myself

...Okay, I know they explain it in the article but I'm still having a problem visualizing how to do it.

 

[the possum]

And broaden the impact point comfort zone? What is the width of the relative sweet spot with the modified khukuri?

As noted above, "Also, besides just that raw number, some wood chopping proved that the sweet spot was much larger and more versatile now. I don’t notice the least vibration if I chop over 1” in front of the rotational center or about 5” behind it." Before I modified things, "If I tried chopping even ¾” to either side of it, power went way down and shock went way up. If I tried cutting anywhere near the tip at all, it transmitted a huge amount of shock to my hand, turning my pinky finger numb and tingling within just a couple swings. " I may want to amend my comment though now that I've used it a little bit more. (I posted all this basically just after completing it.) If I go beyond the sweet spot towards the very tip, I still get hand shock, (as expected, since I wasn't able to move the distal rotational node all the way out) but it's not nearly as bad as before.

Can you offer a rough commentary on relative chopping power at this point?

I had a scrap length of lumber that I chopped a bit before starting, did a few more chops after the grinding, and a few more after finishing. This was my guage board; I did lots of other chopping as the work progressed. The cuts I made after the grinding were about the same or maybe a little less than before I started. This seems reasonable since I didn't really mess with the moment of inertia right at the tip much, and didn't move the distal rotational node. But the cuts after it was completed are indeed deeper on the whole. I only made 4, and each of them got deeper as I guess I warmed up. The shallowest notch was about the same depth as average for the first cuts. The deepest notch was more than twice as deep as the deepest notch from before I started. I'll see if I can get a picture in the next day or two.

I have seen at times very decorative ends on large brush knives. These were wood, but still given the size you would expect a lot of weight. I wonder if these animal heads and such are rooted in performance as much as tradition / religion

.

I have begun to wonder the same thing. Also look at middle eastern designs.

You might want to note what happens if you move the impact point too far. What happens for example if you move it infront of the tip?

Then it's impossible to strike anywhere with the edge without producing some amount of hand shock, and cutting power goes down, since you've actually made the tip's inertia a negative value. (At least I think that's why. Still pondering on that part.) This is what's wrong with my Windlass sword. Can you believe that the tip of a sharpened sword will literally bounce off a raccoon? You've also made it harder to induce rotations on the blade even though it feels lighter statically.

I would be interested to see the effect on springy woods which demand a faster blade. Did you notice a difference in accuracy and tip precision?

Haven't gotten to really use it enough yet, but I'll try to note it when I do. I really don't think it will make much difference for clearing brush, because then I have enough time to make a good cut, and my hand/eye coordination ain't exactly stellar to begin with. But, I think it will have a massive difference when I'm goin' after a coon, and he suddenly decides to change directions after I've already started the swing.

I would like to see more of this with chopping comparissions. As well as related work with straighter blade types - something like a smatchet maybe. The modern day gladius...

In the coming months, a friend has asked me to help him build a short double edged sword- much like a gladius. I plan to do lots of checking and experimenting with it as we go, and especially what actually happens when we add and fine tune the pommel.

Perhaps Mr. Possum can more fully explain the 'pendulum test' too - a picture is worth a thousand words - and right now I'm having trouble visualizing this part - I'd like to be able to experiment with the concept myself

I'll try to get back to you tomorrow- I have a couple pictures and even some video clips of it swinging. (anybody know of a good free site for hosting vid clips?) There are other ways to find the same info roughly, though. One is the "waggle test" as described in that article, and the other is just whacking on a piece of wood to find the "sweet spot". (though the chopping works best on blades desinged with the rotational node about a third back from the tip or less- otherwise the sweet spot gets broad and indistinct.) All three tests should return roughly the same result, but the pendulum test is more accurate if you want good numbers to start doing calculations. I came up with some numbers on the blade's inertia this way.

 

[James Green Dragon]

Cool :thumbup:

Try youtube.com or putfile.com...

 

[Kohai999]

I too have noticed a very big difference between the handling characteristics of my "antique" khukuris, and the HI ones, which I still love.

Cliff and Possum, do either of you know why the cho is so much higher on the blades of the current HI khukuris? I seems to me that having that 1"=/- difference of sharpened area truly impacts handling characteristics.

Best Regards,

STeven Garsson

 

[Cliff Stamp]

... why the cho is so much higher on the blades of the current HI khukuris?

Depends on who you ask. There was a huge blow up about this awhile ago where HI's khukuris were severely critized by a competitor and this was one of the focus points. The viewpoint noted by the kamis when asked was one of an extended choil for grip safety, that is arguable though.

[handle weights]

I really don't think it will make much difference for clearing brush, because then I have enough time to make a good cut, and my hand/eye coordination ain't exactly stellar to begin with.

Dong some work recently and trying to pay attention to method and not just doing everything instinctively, I can feel a large difference made by snap cutting and thus how a blade responds to rotation makes a big difference. Plus you really want to work with the tip for such work for maximum speed but if the ideal impact point is next to the choil this is problematic.

In contrast to the traditional blades, most machetes are just flat stock with slab handles simply as they are the cheapest way to manufacture them and you can't really expect fullers or a taper and a weighted pommel for $5. However it still would be interesting to compare the two versions. I would hope in general this discussion prompts moving balance beyond center of mass and hopefully we will see notes of the sweet spot location and versatility as well as dynamic/static issues and tip precision.

Concerning tip precision, I have noted in the past that some knives are very difficult to stab with accurately. I looked at it from the viewpoint of just blade length because it magnifies the tip deviation for a given angle. But considering the above it would have been interesting to note the other aspects of balance as well and see how they correlate. Can you see a difference on smaller knives as well or are we just talking about bowies and small swords.


-Cliff

 

[the possum]

James-

The article mentions letting a sword swing by its cross guard, but since the khukri doesn't have one, I bent a piece of aluminum wire and taped it to the handle. This way it has something to rock on.

Then you can just set it on something so it can swing, and loop the pendulum on the wire. Set them both swinging together, and adjust the string length until they stay in time. If the blade oscillates quicker, the string is too long. If the lead weight rushes ahead of the blade, the string is too short.

I think I got the video clip to upload. It's about 11 seconds long and 2.7 Mb, so if you're on dialup, if you can see the first few seconds, you don't need to download the whole thing. Swinging pendulum clip.

STeven-
For what I was trying to do here, I would rather have the shorter Cho with the edge getting closer to the handle, though really it's not that big a deal either way. This would just remove a bit more weight with the grinds, and that weight isn't really doing anything to help me where it's at. The rotational center and its impact node cannot exert force on each other. And this area should roughly be the rotational center for a tip cut.
Now, an arguement could be made for having a bit of mass in the guard if it's impact center isn't all the way out at the tip, since then it will be behind the rotational node with a tip impact, but this is still way less efficient than putting that mass in the pommel, where it will have more leverage at impact, and also will only take the same or less force to get rotating with the grip.

Dong some work recently and trying to pay attention to method and not just doing everything instinctively, I can feel a large difference made by snap cutting and thus how a blade responds to rotation makes a big difference.

Well, it would seem to me that when snapping the blade, it will be moving faster at impact than a straight chop with no blade rotation. With the straight chop, the blade is only moving as fast as your arm, but if you snap it around, you have the linear speed of your arm, plus the rotational speed of the blade snapping. Is that correct? At least it would explain the actual experience you noted. Additionally, by letting the blade cock back over (or even behind) your grip at the beginning of the swing, the mass is in better position to get moving. So blade rotation is beneficial both to get the blade moving, and with added force at impact. So it would be better to have a knife designed to accommodate that.

Concerning tip precision, I have noted in the past that some knives are very difficult to stab with accurately. I looked at it from the viewpoint of just blade length because it magnifies the tip deviation for a given angle. But considering the above it would have been interesting to note the other aspects of balance as well and see how they correlate. Can you see a difference on smaller knives as well or are we just talking about bowies and small swords.

I don't use "normal" sized knives much these days at all, so I cannot really say. Just about everything I do now that would involve thrusting is done with two feet of steel, or on rare occasions pocket folders- and even then I do far more cutting than thrusting. I'd think the principles would be the same, but more noticable on the longer blades. I do know James has mentioned having better point control and such with neutral balance in reverse grip, but I don't have a whole lot of experience in that field either. The sport fencing I did in college is 100% thrusting, but since I never used anything but practice foils, I have no real basis for comparison- yet I'm still pretty dang sure the ideas apply.

 

[James Green Dragon]

The video came out fine - thanks! :thumbup:

So, if I understand right, the weighted end of the pendulum represents the "sweet spot" of the blade (?) - is that right?

Thanks again amigo!

 

[Bors]

Looks like you did a nice job on it :thumbup:
However, the personalities of the two will always be somewhat different

 

[the possum]

So, if I understand right, the weighted end of the pendulum represents the "sweet spot" of the blade (?) - is that right?

Well, in a roundabout way- The pendulum represents the impact center for wherever you attach the end of the string. It seems for most chopping purposes, this will coincide with the "sweet spot" if the string is attached at the guard or slightly behind at the first finger.

If you attach the string elsewhere, then it will not coincide with the chopping sweet spot- but it will show you where along the edge an impact will make the knife rotate around that point. For example on my khukri, if I had attached the wire directly at the pommel and tied the string there, it's corresponding impact node (or distal rotational center, or whatever you want to call it) would be about halfway down the blade. This would be the point where the pommel does no good, because the two centers cannot exert force on each other. Beyond that point, the pommel will help the blow.

 

[the possum]

I really don't think it will make much difference for clearing brush, because then I have enough time to make a good cut, and my hand/eye coordination ain't exactly stellar to begin with.

Dong some work recently and trying to pay attention to method and not just doing everything instinctively, I can feel a large difference made by snap cutting and thus how a blade responds to rotation makes a big difference. Plus you really want to work with the tip for such work for maximum speed but if the ideal impact point is next to the choil this is problematic.

I just realized we had a small misunderstanding here. When I said I didn't think it would make much difference, I was talking about the accuracy and tip precision. I do think it will indeed make a difference in regards to getting more tip speed for springy brush. I tried it out on honeysuckle vines and grass, and I was able to get clean cuts without over extending the cut.

 

[sodak]

This is starting to sound like a game I know a little about, which is golfing. If you can delay the wrist snap as long as possible, you will get the absolute maximum speed when you strike the target. That's also the secret to long drives in golf.

 

[Cliff Stamp]

I do think it will indeed make a difference in regards to getting more tip speed for springy brush.

Yes, that was what I was thinking of, so you increased both the heavy power chopping as well as work on lighter brush.

-Cliff

 

[the possum]

I increased blade speed overall- or at least the acceleration. I guess I could still get it to max speed before, but I had to really rear back so I had enough time/length of swing to get it going.

I have increased chopping power at the curve of the belly near the tip, but when I think of "power chopping" I still think of an area slightly further back yet. But, I can't even use this area unless I'm cutting standing wood or it's propped up off the ground, or the tip would hit the dirt. So overall I'd have to say the power would have gone up or stayed roughly the same depending on where I strike.