Chris Pariso

Advice on heat treating a 2H machete/chopper in 5160

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Hey all, 

I'm planning on forging a kukri-like blade out of 5160, which I want to mount onto the end of an axe handle. I've used something similar in the past, and it's a great tool for clearing small to medium brush, limbing branches, and otherwise chopping through things up to 3"-4" thick. The stock I'm starting with with .25" thick and 1.5" wide, so the final blade will be close to those dimensions, and approximately 8" long (not including tang).

The question is, what do you all think the best heat treating process would be for it? Should it be more like a knife, with the entire blade hardened and tempered, or more like an axe/hatchet, where only the edge is hardened, and the spine is kept softer? Under the harshest conditions, the blade will be swung two-handed at the end of a 30" axe handle into fresh wood as thick as your forearm. Most common use will be more like speed cuts into brush and small vegetation, to clear hiking trails.

Logically speaking, I would guess that edge hardening and keeping the rest of the blade relatively soft would be best - worst case scenario, I hit something, the blade bends, and I can pound it back into shape. If the blade is fully hardened, on the other hand, it might straight up snap on me, and there's not much coming back from that (outside of reforging the pieces into smaller blades). But, I don't know how likely quarter inch thick 5160 is to snap, especially when the blade is 1.5" wide. So... thoughts? Ideas? Suggestions? And of course, thanks!

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For starters, I’d suggest thinking less “kukri” and more “billhook”. That way, you can copy an existing and appropriate design rather than adapting something originally designed for a different use. 

Otherwise, have you read the heat treating threads, especially the pinned introduction?

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You've answered most of your own questions.  You want the blade heat treated for the harshest use/abuse it will likely encounter.  Edge quench or differential tempering would be my suggestions.  Even at the cutting edge I'd probably go a little bit softer than you would for a general use knife.  Better to roll an edge than to have chips coming off the edge imho.  With a 30 inch handle I don't think it would be too difficult to generate enough force to snap a fully hardened blade of 5160, especially if you had any weak spots or stress risers from forging or grinding.  The longer the blade the easier it would be to break it.

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You could quench as normal and then go closer to a spring temper after.  Go low to mid 500’s with your temperature (purplish).  5160 is tough stuff.  I don’t think it was ever considered a high edge retention steel, so you gave up on that when you picked it. 

If you really want that differential hardness you can edge quench or manually temper the spine with the edge resting in water.  Both are a little more risky, in my opinion, than just tempering it to purple.  Just give it a nice beefy edge geometry.

Have fun however you choose to do it.

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of course there is the problem of risking separation with a differential hardening...

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So, Steve, are you saying that if someone wanted a harder edge and a softer spine, you’d recommend hardening the entire blade and then drawing the temper from the spine with a torch or tempering tongs?

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JHCC - I had originally considered a kukri-like blade for a few reasons. First, I have used a tool like that in the past, and really liked how it performed. Most of the cuts I made were quick, one-handed swings while walking the trail, and the forward cant of both the blade and handle lent itself well to that. When thicker branches or limbs were encountered, a good two-handed swing took care of it. If anything was too big to chop, I usually carried a folding saw in my pack. Also, I've trained with kukris for many years now (Kali and Silat), so it's a blade type that I'm very comfortable and familiar with. That said, given what the tool will be used for, the final shape and geometry of the blade I forge will probably fall somewhere between kukri and billhook - thank you for the suggestion about that. As for the stickies, I've read them several times, as well as most of the posts in the heat treating forum, and they've been very helpful. 

Buzz and Lou - I appreciate the second opinions. Logically, that seemed like the way to go, but the logic of a novice is sometimes flawed. 

Steve - Could you elaborate on that risk a bit? Are you saying that the blade could essentially split somewhere along the line between the harder and softer tempered steel?

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22 hours ago, JHCC said:

So, Steve, are you saying that if someone wanted a harder edge and a softer spine, you’d recommend hardening the entire blade and then drawing the temper from the spine with a torch or tempering tongs?

definitely, to Chris comment also. Martinsite is much stronger that pearlite and ferrite anyway.

some Japanese cutting arts refuse to allow dif hardened blades in competition due to accidents.. just a FYI

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Interesting. Even though that’s the traditional hardening method for katana?

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To be fair, traditional doesn't mean best... bladesmiths and metallurgists have learned a lot in the last 700 years.

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True.  My knowledge of Japanese swordmaking is limited compared to that of many here...but what I do know is that their incredible skills developed out of necessity because their steel was lower quality.  That, likely, is why they got away with edge quenching in water.  Modern steels would fail given the same treatment (except, of course, those designed to be treated that way).

I wasn’t aware that edge quenching was THAT risky, thanks for the info Steve.

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I second what Lou said - much of the lore surrounding Japanese swordsmithing (folding steel, etc) comes from the fact that they were often starting with pig iron, and had to refine it any way they could. Speaking as someone that has trained in a variety of martial arts for 20+ years, I've found the myth and legend surrounding katanas has far outmatched their actual performance. Traditionally forged, they can be fantastic blades in the context of feudal Japan, but I think they are far outclassed by many European designs (long sword, gladius, etc), and especially by modern blades. I think traditional katanas are a perfect example of making the best of a bad situation. 

(Admittedly, I might be slightly biased from years of dealing with mall ninjas in the dojo.)

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Note there are a large number of billhook variations that were used for centuries in England; every area had a version they thought worked best for their conditions. (google old billhooks images for some of the variations---including a picture of Roman ones...)

It's modern times when we start getting factory standardization with no guarantee that the one they chose is appropriate for YOUR uses!

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Thomas - That's an excellent point, and actually one of the reasons that I got into forging. If you can't find exactly what you're looking for, or exactly the right tool for the job, why not make it yourself? 

Whenever I get the blade hammered out and profiled, I'll be sure to post a shot of it on here.

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There is a technology progression that goes: everything is custom made as it's all made by hand to suit a customer (stage 1) Factories take over and make a lot of just one design (stage 2)  Automated factories and computers tie in to where everything is custom again (stage 3)

Full stage 3 is currently a science fiction thing but we are getting there!

More power to your kuk-bill!

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Also, Stage 1 and 2 are cyclical:

Thoughts during Stage 1 - Man, I wish there was a faster, more efficient way of doing this. Production takes too long!

Thoughts during Stage 2 - Man, I wish there was more variety out there. Everything looks the same!

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Stage 2 has a fork into: hand made custom work at a high prices and how can we modify the factory process to allow changes in design but still do production---which will hopefully end up in stage 3.

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So, after sketching out the profile that I wanted to make, and doing a cold run on a block of plasticine, I realized that I was punching above my weight a bit. To date, I've only made two knives, a pair of tongs, and a handful of hooks and spikes. The design I want to make is going to take a bit more control/skill than I currently have, so I instead hammered out a simpler sickle-like blade, about the width and curve of a banana (for lack of a better comparison). Still plan to mount it onto a hatchet handle and carve through some brush with it, the former hopefully being done this weekend. 

You win this round, kuk-bill... but your day will come!

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Take a look at this one for mounting:

ae119eb0df00f3bcb8ea72e5f3421a9b.jpg

I have one like this at home and it works quite well for brush maintenance.  Slotted handle, sheet metal reinforcement and rivets.  Please be careful with anything that you mount on a longer handle.  The force multiplication is significant and your heat treatment needs to be on target (especially tempering).  The antique samples are quite thin (more machete like than axe like) so can be used one handed.  I wouldn't go at anything branchlike in excess of 1/2" diameter, though weeds are fair game.

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Thanks for the advice Latticino! I had planned on slitting the top of the handle and epoxying/bolting the blade in, but I like the idea of adding in a band of sheet metal for a little extra reinforcement. The handle won't be quite long enough for a decent 2-hand swing, but even still, I'd like to secure that section as much as possible.

Given that this will only be the third sharp thing I've heat treated, I do eventually want to torture test it a bit (including the ABS tests), to try and get an idea if the heat treating is indeed on target. I'm happy to destroy a few blades to see if I'm doing things right or not. 

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I once forged an O Kama Yari for a martial arts instructor, he chopped up oak trees with it. (At my last contact with him he was training Marines on Okinawa.) 

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heavy duty leafspring and if you look at images for the O Kama Yari many of them were full tanged; so mine was too.  (not like the flimsy sheet metal kama sold for MA folks who don't know better.)

I personally thought it was way overbuilt; but it was built to customer's spec and he liked it---perhaps as a training aid so when he switched to lighter ones they would feel like nothing?

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Could be an intentionally heavy training aid - I've got a 30" x 3/4" steel pipe filled with sand that I use sometimes for training Kali stick work. When I switch back over to an ironwood stick, its like swinging a plastic straw. 

Then again, if the guy is off training Marines in Okinawa, he way have just wanted something that could stand up to whatever punishment he threw at it (or threw it at, as the case may be)

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