Burnt edges?

Shotgun said:
There was a thread here that I read recently arguing against the idea that you can’t ruin the edge of a blade through machine/hi-rev sharpening.

I’ve experienced this. I had a fiskars made, gerber branded axe that had this problem. The consensus, whether true or not, was that the final sharpening ruined the temper at the edge through overheating. Like others I experienced chipping and came here all “WTF” and saw the advice of;

“The final sharpening burns the edge. Hand sharpen past the “burnt” part and it will go away.” -synopses. Not verbatim.

I did that and the blade singed with sharpness. It did not chip. My brother in law needed a root grubbing tool so I gave it to him knowing it will do the job with full confidence after chopping with it.

Is there any data on how much heat a small piece of steel(like an edge) experiences through sharpening? Any kind of sharpening. By machine or by hand.
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It's not cost effective to sharpen mass production by hand.

Just another reason on an overwhelming pile of reasons to sharpen your own knife.

My honest opinion is that for the average "non-geek" user/consumer/collector it's irrelevant.

If someone is a more discerning performance focused user they sharpen anyways to set the angles and finish to there specific needs and preferences.

It's like a graph and at some point between cost and performance there is a place on the graph that they intersect at "good enough"

Get a handmade custom knife/axe/tool at a premium if it's that big of a concern.
 
I've never experienced the problem, either with my own sharpening or with production knives.

As 42 said, using new, coarse grits will help a lot to keep the edge cool, along with smooth, steady movement of the blade. Personally, I use power belts only to seriously reprofile small blades or to sharpen machetes. The apex of the edge is very thin and you'd think it would build up heat fast, but it's also backed by a huge heat sink. Those two factors have to be considered together.

I had a 12-inch chopper sharpened on belts by a custom maker, and that edge was a thing of beauty that lasted a long time.

Oddly, I've never heard of the paper wheel guys complain of this problem, and I'd expect they'd suffer it the most because of the fine grits they use.
 
The thing missing in most of these discussions imo is it could happen but doesn't have to happen. Equating machine sharpening with burnt edges is false. It's good to explain the risk. And if someone knows that they would be wise to be careful. As with all methods a junker blade isn't a bad idea to start with.
 
Joker66 said:
Larrin just addressed this topic and posted this short video.

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Here is the research about Worksharp vs Benchstone that Larrin refers to in his video - New Zealand WAIKATO University website:
The Measurement of Knife Sharpness and the Impact of Sharpening Technique on Edge Durability

All this discussion is about "locating the fences", so we know just how much margin we actually have not to compromise the edge retention during sharpening and especially fine grit honing. Belt grinders and paper wheels are safe if used with the right blade feed rate. Too many, however, move the knife across the belt/wheel too slowly and do not allow the blade to cool between passes, so the risk is there. The shorter is the belt, the smaller is the wheel - the higher is the risk at the same RPM.
 
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jpm2 said:
Everything I've overheated on the grinder/sander got soft.
Did you quench it?
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It was a knife I made, heat treated and tempered.

The little bit of point/edge that turned color, I was able to bend back and forth till I broke it off. It was also too thin, even if not overheated, I would have had to fix it.

Handle already attached, I removed a good bit of edge including the soft spot. It has held up pretty well over the years of use. Including using as a chisel to remove wood, and aluminum. It was pretty fine at the point. I removed about 1/8 of the steel and reground the convex edge with more care.

Uglied it up, but it was for me, and my first attempt.

I also messed up the pretty/neat/ clean work I had done with files in the curves of the handle. 3000 rpm on a 1/30 is moving pretty fast. I'm better with it now....
 
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Bigfattyt said:
It was a knife I made, heat treated and tempered.
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So it sounds like yours softened up just like mine, except you bent it back and forth work hardening and making it brittle till it broke. That makes sense now.
 
jpm2 said:
So it sounds like yours softened up just like mine, except you bent it back and forth work hardening and making it brittle till it broke. That makes sense now.
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Yes... without the burn/heat softening it would have taken a good bit of effort to get the steel to snap (again, only talking about the very thin edge on the very tip... it was a square/chisel design with a point where the two edges met at 90° like a Graham Razel.... made by a blind troll on meth....
 
bucketstove said:
Writeup Does Sharpening with a Grinder Ruin Your Edge? - Knife Steel Nerds
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Thanks for that link. Good article that says that nearly all production knives suffer from edge softening, which amazes me.

"Roger Hamby of CATRA also tells me that in their testing of edge retention that over 75% of the knives they test suffer to some degree of edge softening due to the sharpening process. He reports that nearly all manufactured knives have the problem, while hand sharpened knives generally do not."​
 
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