Well I did it this time:( 20" Kobra field report.

[MauiRob]

Just got back from a test run with my 20" Kobra.

Up till now I had only lopped off the tips of some saplings while showing it off to my brother. So today I decided to put it through its paces.

Started with small 1/4" and less branch tips on bushes that usually whack me in the face while mowing the lawn.

Moved up to a couple of 1/4-1/2" green saplings--no problem. Cut them like butter.

Then I started to limb some blue spruce branches that had died and dried awhile back. It had no trouble at first, chopping easily through 1/2" branches. Then while swinging at smaller branches I hit the very tip into some 1" branches that were clustered together=bent tip. Well, I know the tip is dead soft and I can hammer it back, so I kept going.

Now I'm cutting 1/2-close to 1" branches. Running into some hard knots near the trunk. cut maybe 10-12 of these and looked down at the edge. BAD NEWS!! The edge looked like a Lasagna noodle's edge! I knew the Kobra was a light chopper, but this surprised me a little.

I was able to hammer the edge back into more or less alignment with a hammer, and using a piece of hard steel as an anvil backing. The funny thing was I could do this with very light tapping. Maybe the thin edge profile explains this. I should be able to fine tune the edge back into good as new shape with a little elbow grease.

Does anyone think the hardness is off on this Kobra, or is it a classic case of operator error(my guess)? I have no intention of sending it back, but I would like to know what you all think so I can judge whether to get another Kobra(down the line) if I want a light chopper, or upgrade to a medium length sirupati for above chopping jobs.

Let me have it if I deserve it

 

[Yvsa]

Originally posted by MauiRob
Just got back from a test run with my 20" Kobra.

I'm cutting 1/2-close to 1" branches.
Running into some hard knots near the trunk. cut maybe 10-12 of these and looked down at the edge.

BAD NEWS!! The edge looked like a Lasagna noodle's edge! I knew the Kobra was a light chopper, but this surprised me a little.

I was able to hammer the edge back into more or less alignment with a hammer, and using a piece of hard steel as an anvil backing.
The funny thing was I could do this with very light tapping.
Maybe the thin edge profile explains this.
I should be able to fine tune the edge back into good as new shape with a little elbow grease.

Does anyone think the hardness is off on this Kobra, or is it a classic case of operator error(my guess)?

Hmmm? Lots of questions to solve this one.
First of all is the Kobra made from a spring or a file like the original?
How thick is the blade at an average?
Rob how does a really good file react on the edge of the Kobra?
If it cuts metal all the way down then it is probably softer than what it should be.
If a good file skates along the sweet spot without cutting then it's as hard as it should be.
Then again if it's as hard as it should be, the next question would be how far up the edge is it hard?
If it isn't hardened as far up the blade as it should be, basically just along the edge, that could account for the damage.

Running into some hard knots near the trunk. cut maybe 10-12 of these and looked down at the edge.
This is definitely a problem area for almost any knife. Those dayumed knots can be like steel and any deflection or cut that's not dead on can result in damageing even a Sirupati as one of our brothers in Germany discovered except he was cutting frozen birch.

There's so many things it could be that I really don't have an answer right now and may not even be able to form an opinion.
The best thing may to send it back to Uncle Bill and let him make the determination as this isn't at all like the Kobra.
Look at how Tom Holt's Kobra performed doing some really heavy chopping for such a thin bladed khukuri!!!!

 

[firkin]

Maui,

Don't have a Kobra, but your experience sure doesn't jibe with Tom Holt's description of his 25 incher that's about to get a second life. Poor hardening sounds likely to me. The harder sweet spot is quite noticeable to me when I sharpen most of my khuks. They almost aways are a little duller there, especially the side near the handle, and it takes a lot more work to raise a burr there.

I think I recall you having a chitlangi...how much thinner is the Kobra's edge than that? My chitlang's edge is fairly thin compared to the sirupati. And the YCS is even thinner. I have a log of dry hard fruitwood in the yard and I chop on it with all my knives when I get them sharpened and havent even slightly rolled an edge yet except right at the tip.

 

[not2sharp]

MauiRob,

What was the temperature like outside when you were doing this, and what time of day was it?

n2s

 

[Bill Martino]

Two possibilities -- blade too soft or knots too hard. Yvsa's file test will tell you the story. Every now and then for various reasons as all old shop hands understand things can go wrong.

Either case we'll be happy to exchange knives.

 

[MauiRob]

Rob how does a really good file react on the edge of the Kobra?

Good question. I will try a file on the edge asap. I seem to recall when I sharpened it that there really wasn't much difference in how the diamond steel cut, but those are really hard anyway. My 20" AK, "battle" tested on everything from bolts to hardwood has a VERY hard sweetspot that even the diamond steel will almost glide across! My 20" sirupati and my other ak's are similiar.

your experience sure doesn't jibe with Tom Holt's description of his 25 incher that's about to get a second life

Yeah, that's kind of what I thought. I assumed that in most chopping (within reason of course) the edge would behave similiar to other khuks with respect to its hardness. The degree of bending and warping was pretty amazing, especially when you consider that none of my swings was even close to as hard as I usually do with other khuks (because it was a Kobra and I didn't have the same toughness expectations). I guess like you guys said, Knots are a different animal altogether.

What was the temperature like outside when you were doing this, and what time of day was it?

it was about 3:30 IIRc and about 45-50deg F. Like I said earlier none the limbs was over about 1" thick, and generally spruce isn't very hard stuff.

Two possibilities -- blade too soft or knots too hard. Yvsa's file test will tell you the story. Every now and then for various reasons as all old shop hands understand things can go wrong

Either case we'll be happy to exchange knives

Thanks Uncle Bill I'll do the file test, and if that turns out to show that the blade just wasn't hardened at all like it should be then
maybe I'll exchange it for one that's properly hard. I don't care about the cosmetics of the blade as its a user anyway, and I can get it looking fine. But I really would like to be able to use the Kobra for light brush clearing jobs like the one I did today.

Thanks everyone for all the good advice and help I'll keep you posted on my results. probably won't get to check it til monday because tomorrow's Easter and I'll be at my folks house all day.

 

[SkagSig40]

Interesting Rob. I have several khuks I have not used yet and I always wonder if they would be ok in hard use. Most I will never use and will stay pretty! I have a few that are users and have really taken a "Cliff Stamp" beating and are just fine! Good luck and let us know what happens!

 

[Bill Martino]

Yep, keep us posted and remember you're covered.

 

[Tom Holt]

As possibly the biggest fan of the Kobra series in this group, I have to admit that they can be a bit variable... I bought a 30" while they were still being made, and I found that the edge wasn't perhaps as hard as it might have been. Also, I did manage to bend it perceptibly with hard use. Tests with a file showed that the edge wasn't significantly harder than the spine.

Maybe the Kobra is more difficult to make than most khuks. Let's consider the crucial hardening/tempering procedure. As I understand it from looking at the excellent pix on Uncle Bill's site, the kamis bring the blade up to full heat and then achieve hardening by dribbling water from a kettle or similar vessel down the cutting edge. The water quenches the edge, and the sudden reduction in temperature 'freezes' the molecules in their martensitic state. Then the heat remaining in the unquenched portion of the blade 'bleeds through' into the edge, reheating it sufficiently to achieve the tempering process.

Wonderfully simple, as compared to heat treatment methods used elsewhere; but clearly dependent on the kami's skill and experience to guage the extent to which the two processes have taken place. With the thinner, slenderer blades of the Kobra series, the variables must surely increase. Less steel in the unquenched portion of the blade means a smaller heat sink to bring about the 'self-tempering' function, thereby risking a blade that's too hard *and* hardened too far into the steel, thereby risking brittleness and a broken blade. The temptation would be to overcompensate, by pouring on less water, allowing more heat to bleed back into the edge; which would prevent brittleness but might tend to a softer cutting edge.

An essential part of the khukuri design philosophy is redundancy. Make 'em thick, and they won't break. Likewise, make 'em thick and they won't bend, even if the main body of the blade, as opposed to the edge, is to all intents and purposes annealed, unhardened steel. Spring steel, such as the kamis use, is far tougher and springier than mild steel even in its annealed state - it has an 'air-hardening' quality which means that it exhibits a fair degree of springiness and memory retention even when allowed to cool down unquenched and untempered. A thick khukuri made of spring steel will be practically unbreakable and unbendable in its annealed state, with only the cutting edge hardened and tempered. By contrast, a thin, slim blade depends entirely on its heat-treatment for its strength and toughness. Hence the superiority of the khukuri over most rival design profiles.

Make the khukuri thin and slim (like a Kobra) and you're sacrificing the margin of error built into the khukuri design. It's asking a heck of a lot of the smith to judge a complex differential heat-treatment by eye, sound and feel, particularly in a new design with which he probably isn't as familiar as he is with traditional profiles such as sirupatis and AKs. Very occasionally, as with my 30" and Rob's 20", things may not go quite right. It's the price you pay (IMHO) for the exceptional peformance of the Kobra series.

How many recorded instances are there of Kobras failing in this way? Two, out of Heaven only knows how many? I think that if we were discussing this question with the kamis themselves, they'd point out that it's us crazy Westerners who want light, slim khuks, and that left to themselves, they'd stick to making broad blades with half-inch spines, guaranteed to survive anything short of a direct hit from an asteroid.

 

[Bill Martino]

A smith who can write is a definite asset to this forum -- again, good to see you back in the fold, Tom. Great stuff and many thanks.

I think the fact that the kamis greatly resisted the notion of a 30 inch kobra lends a lot of credence to your observations. We ask them to walk into places which they KNOW they should not venture but do because we pay them well and don't blame them if things do not go as we would like.

 

[MauiRob]

. By contrast, a thin, slim blade depends entirely on its heat-treatment for its strength and toughness.

Thanks for the educational post Tom I think you hit the nail on the head. I borrowed a metal file from my dad (Sears "Kromedge" 8" Crosscut Saw File). To my inexperienced hand, there doesn't seem to be much, if any, difference in the amount of "grab" the file experiences when run along the blade (at the angle I would sharpen at). However, if the file is too hard, I imagine it would cut all the way along anyway. I need a file of known hardness I guess.

At any rate my gut feeling having used a fair number of khuks (although none quite as slimly profiled as the Kobra) is that Kesar slipped on the heat treat. The ease with which I was able to bend the edge back into place (i could do it with a kitchen steel and pressure applied without a vice), combined with the extent of the warping that resulted from moderate swing force, makes me think this also.

So I'm sorry to say I'm sending this one back to the Cantina

 

[Bill Martino]

We'll send you another and I'll check it myself just to make sure.

 

[MauiRob]

Many thanks Uncle Bill!

Here's proof to all newbies that HI stands behind their 100% guarantee A lot of other guys would have said it was my fault and told me to take a hike, but not Uncle Bill Its stuff like this(combined with the good it does in Nepal) that makes me choose HI in a heartbeat when I've got money to spend on Knives (ok, and often when I don't)

Rob

 

[Bill Martino]

And thanks for kind words, Rob.

We have so few failures the guarantee is not a tough one to honor. Educated guess is maybe one in 500. I'd rather see it 1 in a million but we'll never be able to make that many khukuris so I guess it's ok as is.

 

[foxjaw]

The only thing that I've run into that bends khuks like hitting steel is seasoned osage orange.