Burnt Edges

J

James Mayo

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In your experience, is there any truth to what people say about sharpening a knife when you get it because the steel at the edge of new knives is no good? "Burnt" is a word I hear a lot.

I asked David Mary David Mary this earlier today and he gave me his input and now I pose this question to the forum. I’d love to hear from any and all.

Thanks, guys.
 
I'll just reiterate what we discussed on the phone briefly. It depends on how it was sharpened. An attentive craftsman can avoid burning an edge when sharpening on the belt as long as he or she understands heat management, and to avoid building up enough heat to ruin the heat treatment. Sharpening on a slack belt is one way to minimize the risk because it does not impart as much force into the edge as it would if you were sharpening with a platen underneath the belt, and is thus slower to heat up the workpiece.


Also, a lot of custom makers will sharpen while grinding with a spray mister, which also goes a long way towards preventing a burnt edge.

The finer the grit on a belt the easier it is, in my experience to generate heat quickly.

I slack belt sharpen with quick passes only up to 220 grit on flexible ceramic belts (they get honed/microbeveled by hand afterwards too), and I find my edges perform as they should.

That process may not work for steels using certain high low (I misspoke) temperature tempering protocols like Delta 3V. To date I have not used such protocols with any of my steels.
 
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All that being said, I wouldn’t really know if any production knives I’ve owned have benefits from sharpening past the initial factory edge, due to burnt steel, because all my production knives get significant regrinds not necessarily affecting the edge (but sometimes) but removing material behind it, which always improves cutting performance regardless of edge condition.
 
I’ve only had one truly burnt edge. It was a case limited edition. To get it right, I would have to reprofile the blade quite a bit, not worth the headache. Sharpened it up and when the burnt part starts rolling, I’ll touch it up as needed.

I’ve received some knives with burrs that need to be touched up, a truly burnt blade is few and far between for me.
 
Delgadonine said:
Actually, I've never heard the phrase 'burnt edges' before. Apparently, I've spent too much time away from planet Earth.

Reading this with interest.
Click to expand...
first-time-meme-first-time.gif

Buckle up
 
I often see people hesitating to make modifications to a knife they own because they wonder if they’ll ruin heat treatment. Someone worried recently about cutting off the wave on an Emerson.

Let me show you an angle grinder cut I performed on some hardened blanks before cleaning up the profile on my belt grinder:

74830056311__B3DA8363-2E21-4AA0-96B8-FA5D2D04177C.jpeg
IMG_8889.jpeg
74830052714__6EE8C51B-F966-43E6-B54C-A642145C9B5C.jpeg

That is the same blank from two different sides. As you can see there is a rainbow that goes from white to blue to brown to straw. The white and blue parts are demolished, and would never hold a proper edge (if we assumed that part of the profile wasn’t going to be removed during profile clean up, which it was).

The brown and straw, as I understand it, have little to no affect. So when grinding (or sharpening) any blade, I go the overprotective route and take steps not to build up enough heat to change color at all.

Now that pic was with a careful but quick angle grinder cut. You can see that the heat that would affect the microstructure transferred only a mm or two into the profile.

So cutting a wave off an Emerson isn’t going to do anything to affect the heat treatment of the edge, and if anything you’ll end up with a small spot on the spine that’s a little softer.
 
The apex of a blade edge is so small that it can heat up very quickly. That heating can soften the edge, reducing its ability to hold an edge. Most manufactured knives have this problem.

Competent knife makers can avoid this problem, but it's not easy for manufacturers.
 
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I have had a couple factory knives that had a bit of a burnt edge, specifically a Benchmade Griptillion. That knife in particular felt nicely sharp as I received it but that edge was very quickly filled and once sharpened again it held the edge for quite a bit longer. The issue I had was after two sharpenings it still never held the edge I was expecting from that steel so I sold it. I will never know if that knife had a deeply bad edge or an overall poor heat treat. I definitely soured my opinion of Benchmade.
 
Delgadonine said:
What's the #1 Rule here?

Post pictures!

Thank you Dude Mary for those pics. Not sure what I'm seeing, but it's a start.
Click to expand...
I’m in the midst of a move but I’ll try to pull out the knife I’m talking about and take a few pics. The tip/last 1/4” was heated to the point that it was blue. I’ve got it down to a straw color now.

I’ve seen others call burrs, burnt edges. Or if the factory edge isn’t up to snuff and needs a touch up. My problem was that the actually edge was heated to a point, the color changed.
 
Delgadonine said:
Can a burnt edge happen with routine sharpening?
Click to expand...

You didn't define "routine sharpening".

Burning something means you transferred too much heat into it, as implicitly stated. So the answer is easy to determine based on what you define as routine sharpening.
 
David Mary said:
I often see people hesitating to make modifications to a knife they own because they wonder if they’ll ruin heat treatment. Someone worried recently about cutting off the wave on an Emerson.

Let me show you and angle grinder cut I performed on some hardened blanks before cleaning up the profile on my belt grinder:

View attachment 2664173
View attachment 2664174
View attachment 2664185

That is the same blank from two different sides. As you can see there is a rainbow that goes from white to blue to brown to straw. The white and blue parts are demolished, and would never hold a proper edge (if we assumed that part of the profile wasn’t going to be removed during profile clean up, which it was).

The brown and straw, as I understand it, have little to no affect. So when grinding (or sharpening) any blade, I go the overprotective route and take steps not to build up enough heat to change color at all.

Now that pic was with a careful bit quick angle grinder cut. You can see that the heat that would affect the microstructure transferred only a mm or two into the profile.

So cutting a wave off an Emerson isn’t going to do anything to affect the heat treatment of the edge, and if anything you’ll end up with a small spot on the spine that’s a little softer.
Click to expand...


Thank you for posting this. 👍

While I've never been motivated ENOUGH to "mod" one of my production knives, the thought has crossed my mind on occasion...and I've ALWAYS WONDERED about this. Admittedly, I know ZERO about tempering, heat-treating, etc.
 
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