Recommendation? How thin is too thin? (Edge geometry)

[N.W. Gean]

I know, it's been talked about, it's opening up a can of worms, but I'm talking about it anyway

So I've got these blades made of 8670, hardened to 61.5 RC by JT.

I kind of want them to be a general purpose hunting/woods blade, but I'm not sure this is realistic based on my testing.

A zero edge with a 25 dps micro-bevel fails quickly when I stab the tip about 1/8" into wood. It just bends away and eventually snaps (my methodology is that the blade should be be stabbed as far in as the weight of the blade allows, hammering it into the wood is a guaranteed failure and therefore a waste).

Next I ground back to about .010, and applied the same angle micro-bevel. That does better, but the tip now breaks off in hardwood without bending (not softer wood, but that isn't to say that the soft wood hadn't already stressed it before I moved onto the oak).

Now, this is a really hard use/abuse of a knife, but I am going for something that my customer can trust in the woods not to fail. But the question is, what can the expectation really be? Prying with the tip of the knife is just asking for trouble, but what is realistic for me to shoot for in terms of having sharp geometry but being able to take some abuse from those who may not know how to properly use a knife. Obviously, if I made the knife totally indestructible it wouldn't cut very well, but where is the line?

 

[Joshua Fisher]

It would help to post a photo of the blade so we can see the profile as that will play a role, a fatter tip like on a blade with more belly will hold up better then a really sharp tip like a needle. Also what type of grind is on the blade? Zero edge doesn’t necessarily tell us much as a zero edge on a hollow ground blade will be insanely fragile versus a zero edge on a scandi or even convex grind that has more material supporting the edge. I can take my general Edc/hunting knives out of 8670 at about 62hrc and stab into wood with the weight of the knife and pry the tip out no problem, they are hollow ground to roughly .012-.015.

 

[N.W. Gean]

High saber grind. Grinding back changed the shape of the tip/belly a little. The 3 that don't have a micro-bevel are measuring less than .010. If I set a caliper to .010, they'll pass through by about 1/16" of an inch. Hard to say because calipers aren't that accurate. I figured they'd be close to .010 after sharpening.

The edge isn't ragged, that's just dirt from the wood.

 

[Joshua Fisher]

View attachment 1545965 View attachment 1545966

High saber grind. Grinding back changed the shape of the tip/belly a little. The 3 that don't have a micro-bevel are measuring less than .010. If I set a caliper to .010, they'll pass through by about 1/16" of an inch. Hard to say because calipers aren't that accurate. I figured they'd be close to .010 after sharpening.

The edge isn't ragged, that's just dirt from the wood.

I corrected a typo in the previous post, it should have read .012-.015. What I’d on the one where the tip broke up is refrains a tip in similar to the other knives then start thick and work thinner so take the entire edge back so it’s .03” behind the edge which is very thick for that size knife and test the tip and try chopping some antler for brass rods with the edge, then take it thinner by .005 at a time and repeat each test at edge thickness and record what happens then you can find the point at which the edge fails those tests and go just a bit thicker so if it fails at .01 maybe leave it .012 so there is just a bit more meat behind the edge, if it’s a saber grind the end user can always thin the primary grind as it gets thicker from normal sharpening.

 

[N.W. Gean]

Been a busy week for me, school wise.

Thanks for the suggestions. I think I'll try .015 tomorrow and see what happens. I bet it's the ticket.

I have to say I am a little disappointed in 8670 based on what I've heard from Fiddleback and Alpha Knife Supply, but I think they ran at 62-63 RC. I'm betting 61.5 is having trouble keeping up. I think I'm probably expecting too much as well. Everyone seems to love 8670 so I have to think that I overhyped it, especially when we know that the heat treat is bang on. It has to be user error.

 

[Joshua Fisher]

Where did you get the 8670 from? I’ve made probably 400 knives out of it at this point and been very impressed with it, edge geometry is almost just as important depending on what you want out of the knife, I take kitchen knives out of it thinner and I’ve found the thin edge geometry helps with edge retention and overall performance, and on heavier use blades like choppers I leave it a bit thicker and it does really well and is easy to sharpen, I make a wide range of types of knives and have found it performing well in a lot of different applications. I’ve been playing around with forging some wrapped eye tomahawks and had one with bad forge welds start to split during the final stages before quenching, decided to quench the edge anyway to do some testing, as quenched and not tempered I was able to hammer the axe head into 16P nails and chop them in half and hit mild steel under the nail, no chipping or rolling or anything to the edge and it was 8670, the head was a convex grind to zero. I’ve also taken 12” carving knives out of it and bent well past 90 to get them to snap, those were 1/16” at the spine.

 

[N.W. Gean]

I got it from pop's. That's what confuses me. Pop's steel and heat treated by JT. That's what's got me thinking I just can't expect ultra tough on a paper thin edge. That just isn't realistic.

 

[Joshua Fisher]

I got it from pop's. That's what confuses me. Pop's steel and heat treated by JT. That's what's got me thinking I just can't expect ultra tough on a paper thin edge. That just isn't realistic.

Yeah maybe just go a touch thicker for heavier use blades, like I said most of mine are between .012-.015 and hold up to that same test, I haven’t really tried a tip pry test on my kitchen knives as I would never recommend that at all, honestly I don’t recommend it for any knives but I have used my Edc to start a point for a hole and things like that. When you take it back thicker it should do just fine.

 

[CallumRD1]

I'm no expert, but it sounds like you're expecting more from this steel (or any steel) than is reasonable. There are large differences between steels, but edge geometry will impact durability far more than your steel choice. If your problem is the tip breaking when plunged into wood, then grind a thicker tip, maybe with a little more height behind it for support. That may be enough to allow you to run the edge as thin as you'd like without premature failure at the tip. Measure how thick the blade is right where it broke. Tweak your geometry to make that portion of the blade a little thicker and thus stronger. But when all is said and done, if you try to pry with the tip of any knife ground reasonably thin, you'll be able to break it. No steel will be able to make a 0.020" thick tip able to withstand much lateral force before breaking.

 

[N.W. Gean]

.015 was a big success. I even stabbed the tip into desert ironwood a few times. Once I missed and hit the vice anvil. Didn't hit too hard but that's a bad hit for any knife. The tip still shaved after that. The microbevel seems a little large now. I'll try to post a pic when I get home.

.015 seems too thick. I bet .020 would be like a club. But I agree, paper thin won't do the job on a hard user.

 

[N.W. Gean]

I did some more testing after I got back home. If you compare the shape, the tip is a little lower. I'm not sure how that affects the durability.

Hammered the edge and tip into some desert ironwood scrap. No damage. Still shaves.

Hammered the edge into a 1/4" brass rod but didn't chop through. No damage. Still shaves.

Chopped a small nail in half. Minor deflection as highlighted in image. Still shaves, cuts paper with a very minor snag where the deflection is. You almost couldn't tell if you weren't looking right at the knife.

I must revamp my opinion of 8670. At a zero edge with a micro-bevel, it's a disappointment. At .015 with a micro-bevel, it's everything I thought it would be and more. I'm not expert, but I don't think this knife's durability would be your issue if it was all you had out in the field.

 

[Richard Coyle]

You might try crowning the spine, or maybe just at the tip. I’ve seen it done on kitchen knives to increase tip strength. I have no idea to what extent it helps though. Just a suggestion... looks like you’ve pretty much figured it out

 

[N.W. Gean]

I've never heard that technique before. I wonder what the logic is.

I like having a square spine to strike a ferro rod with, call me one of those fad bush crafter guys

 

[Richard Coyle]

I've never heard that technique before. I wonder what the logic is.

I like having a square spine to strike a ferro rod with, call me one of those fad bush crafter guys

I think the idea is, it increases the surface area at the tip. I’ve mostly heard Jason Knight talk about it on his YouTube channel.

 

[Londinium Armoury]

You could do a compound grind, the knife I am working on now has one but it's blended by freehand filing, so you can't really tell unless you run your fingers over the blade and feel it. You don't have to have a uniform geometry along the whole edge, leave the belly and straight thin, and give the tip a bit of thickness.