Modifying my Khukri's balance for improved performance.

[the possum]

I also posted this in the reviews & testing section, but figured it would be appropriate here too since it's all about an HI Khukri.

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 its 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.

 

[Daniel Koster]

looks great to me!


:thumbup:



really nice job on the new handle.

You already know this by now...but it would have been nicer to arc the tang so that the nut goes through the middle of the buttplate. This is kind of hard the first time you try it, then it gets easier once you figure it out. Fitting the handle can be a pain.

What I do is this - drill the handle holes from both ends, rather than all the way through. Then open it up and "round" it throughout with a round file chucked into a drill press (be careful !)

Great work on the handle ring, by the way.

what did you do to finish out the fullers?

 

[the possum]

I hadn't thought of chucking a file in the drill press. Thanks for the tip!

I really didn't "finish" any of the surfaces at all, which is why I'm not showing any close-ups. Once I botched the grinds, I figured there was no further point in trying to make it pretty and perfect. So I used one of those flexible emery cloth "flap" wheels to remove the coarse grinding scratches, and left it at that. Note I didn't grind on the whole blade- in my last picture you can see the difference between the highly buffed origional finish and my rougher finish on the bottom fuller, about halfway up the sword of shiva. Also still some sap stains and minor rust on the edge grind.

The new handle is a bit thinner and taller to help keep it from twisting in my hand. I should have made it longer, too.

 

[Berkley]

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.

That is really good to know. The one thing I've worried about ever since the first HI version arrived, so much heavier than the original, is that the handle could break up and injure somebody who was used to the full length tang models. I'd say you've largely laid that issue to rest!:thumbup:
Berk

 

[munk]

Very very interesting and bold project.
I think the final product is a khuk, and is handsome.



munk

 

[Big Bob]

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 don't care what the "traditionalists" or anybody else would say; IMO you've earned the right to call it whatever the hell you want. Great, great work! :thumbup: Does it still have a tendency to stick when chopping or has that problem been fixed?

Bob

 

[the possum]

Does it still have a tendency to stick when chopping or has that problem been fixed?

I didn't modify the edge geometry at all yet because I wanted a direct comparison of the before & after performance based on balance alone. If I had changed the edge geometry, that might have thrown off my experiment. I would still expect it to bind the same as of now. But now that I'm satisfied things worked fairly predictably with the balance, I may regrind a convex edge on. Guess it depends on whether I end up using it more for heavy wood or other targets that don't have a problem with binding. Will see.

 

[lefthandblack]

Great job, looks awesome. Don't worry, it is still a Khuk.

 

[Madmanamus]

Very nice, very cool.

Take care.

 

[Hang_them_later]

Nice work! Very interesting. I realise that I need some more tools

 

[cndrm]

Wow. That's a lot of work. Great job on the new handle. The epoxy is called laha, and it is a real PITA. I think the easiest way to get a handle off is to boil it in a pot of water. It stinks "REALLY" bad, so if you can do it outside, that would be best.

All in all, a really neat "experiment". IMHO, it's still a kukri, but as stated earlier, you can call it whatever you like.

"FWIW, the balance point started out 6 inches ahead of the bolster." That's crazy! I've never even heard of kukris having balance points that far out.

I've got several ahada paro (partial tang) kukris, and IMHO, they're more plesant to work with. (usually have a lot less shock) When I got my first one, I was aprehensive about the handle giving out durring use, but I've since gotten over that. I got a couple from AC, both are well over 100 yrs. old, both have partial tangs, and both are just as tight as my recent production partial tang kukris. I don't worry about it any more.

Did I say great job on the new handle? :thumbup: :thumbup: :thumbup:

Great work. You did way more than most anyone I know. Most people would probably complain about the poor balance, and just let the kukri sit and collect dust. But you took a proactive approach, fixed the problem, and learned a bunch about kukris. Great job. My hat's off to you.:thumbup:

mike

 

[the possum]

Actually, I'm pretty sure it wasn't laha, just based on descriptions of the stuff here. This was gray instead of brown in color. I did try boiling it first, for at least 15 minutes, and it did not soften at all. Some of the glue had oozed out of the gap between the bolster when it was made, and I couldn't scratch it with my fingernail even when hot. Likewise, the blowtorch did nothing to soften it.

I once asked Berkley on another forum about the balance point on his origional. He said it's "about 1/4" forward of the end of the Sword of Shiva." If the dimensions on my copy are similar, then the origional balances closer to 7" out.
Here's a picture of an antique from another gentleman- the pencil is pointing out the center of mass.

If I were really striving for historical accuracy, then I would have removed more steel from the portion of the blade just forward of the handle rather than from the last half. This would have still decreased the moment of inertia like I was wanting, but I don't think I would have been able to move the sweet spot as much with a pommel. (Or then again, maybe I wouldn't have had to. Hmmmmm..... Damn. This does require some more thought.)

 

[cndrm]

If it didn't soften with a good boiling, then it wasn't laha. That's interesting. Maybe it was something like JB Weld. Curious.

That's a nice old kukri. Is it ang khola or dui chirra? The POB looks right on that one.

Either way, your HI Dui Chirra kukri seemed to come out just fine. As long as you're happy with the results, then that's all that matters.

mike

 

[spiraltwista]

Nice work, & definatly still a kukri! Perhaps a step nearer Berks original in fact.

Those fullers look deep!

Spiral

 

[Yvsa]

If it didn't soften with a good boiling, then it wasn't laha. That's interesting. Maybe it was something like JB Weld. Curious.

That's a nice old kukri. Is it ang khola or dui chirra? The POB looks right on that one.

mike

Possum you done good!!!!:thumbup: Will it be a coon getter now or do you think the Bowie will still out cut the khukuri?

Mike the kamis have taken to using both the laha and JB Weld. I don't know if JB Weld is available in Nepal but Pala took a bunch of it back with him the last couple, three times he was here.

When I disassembled one of my full sized Foxy Folly's I was worried that I wouldn't get the handle off without destroying it.
When I finally got the butt cap pried off I discovered that it was the only thing held on with the JB Weld, the handle itself was poured full of laha.
I doubt that a partial tanged khuk would ever come apart if it were assembled with JB Weld!
On the other hand I think some of the laha can become brittle over time depending upon what it was made from.
I think some recipes are better than others.

 

[the possum]

I dunno Yvsa. But I definitely think now I'd be comfortable grabbing either tool, especially if a buddy comes along and borrows my bowie.

Yes, the fullers are quite a bit deeper now. Seven ounces deeper! Again, if I didn't want to add the pommel, I may have done them differently.

 

[Svashtar]

Possum, what cndrm said! :thumbup: My hat's off to you as well. You really did a beautiful job. I have an older 22" DC at 36 oz., and it balances about the same as yours did. I have a newer one at 21" and just 29 oz. with a pinned chandan handle, that seems to handle much better. I like the workmanship on the older model a bit better though. I have chopped with mine with good results, but have not tried striking very near the tip with it.

The pinned handle was supposed to make it stronger, but based on your info it looks as if I have nothing to fear about using the older unpinned model despite its heavier weight.

A new full tang Dui Chirra sold the other day (7/2) on the DOTD, and it would be interesting to compare its balance and handling with your new improved model.

BTW, what wood did you use for the new handle? And did you reuse the original bolster?

Thanks for the great post.

Norm

 

[the possum]

A full tang DC, eh? Dang, that would've been lots easier to work with. I could have also left it much wider for stiffness since adding the pommel.

I used black walnut for the new handle, since we have a great deal of it, but I actually think whatever wood the kamis origionally used was actually better suited to the purpose. The walnut seems to split along the grain much easier- the stuff my origional handle was made of still wanted to hold itself together with fibers even after split. I honestly believe that I could have still used the khuk even after splintering the origional handle with a hatchet, by just wrapping it with some duct tape.

Yeah, the bolster is the origional. The project was already way more involved than I wanted to get; especially after I had already messed up the blade.

 

[sodak]

You folks in the Cantina crack me up! Anywhere else, people would have been calling for the Possum's hide (pun intended). The only thing going on here is "Good job, we all learned something"! Talk about a bunch of people secure in their khuk's!

I'll have to start popping in more often. This place is quite refreshing!