infamous "rolled edge"?

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BobinCA

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OK...please explain to me exactly what is a "rolled edge"?
How is it formed......by use of knife, or poor sharpening technique?
And, how do you detect it and remove it?
I had read in the Sharpening forum, that edge trailing strokes cause rolled edges, and I thought that edge trailing strokes (stropping) removed burrs.
Confused....
 
BobinCA said:
OK...please explain to me exactly what is a "rolled edge"?
How is it formed......by use of knife, or poor sharpening technique?
And, how do you detect it and remove it?
I had read in the Sharpening forum, that edge trailing strokes cause rolled edges, and I thought that edge trailing strokes (stropping) removed burrs.
Confused....
Click to expand...

Rolled or rounded? I find that soft knives get "rolls" or dents in the edge as they dull and or with poor technique. Whereas harder steels tend to chip.

A rounded edge is when you have poor sharpening technique and can not hold an angle. You take what was once a sharp edge and go over your angle and leave it blunt or at least not as sharp as it was. This can also be done with stropping. Your edge simply isn't as crisp as it should be. Or at least that is how I look at it. I find that when people say that their edge has no "bite" that it is often rounded over. My brother has this issue now that he is learning. Sharp on the 1k and dull on the 6k because he is not holding the angle.
 
"rolled edge" feels like a longer rough burr. The steel is fatigued and should to be remove by sharpening.

Edge trailing passes should prevent rolling or damaging the edge from my experience.
 
Think of it this way.... rounded edges are like when you are sanding crisps edges on a piece of timber and you aren't careful the edge gets rounded over instead of that crisp 90 degree.

Rolled edges refer to how steel with lower hardness will deform rather than chip. In both circumstances the edge has failed... the lower hardness steel does as is suggested and the edge essentially rolls, or bends, over meaning you aren't cutting with the edge really anymore.
 
Razor said:
As the link indicates it is a scienceofsharp image.
Click to expand...
On the app, I could only see the picture, not the link, so I wasn't sure.

I found SoS to be a fascinating series of experiments. They were performed on razors, so probably not 100% applicable to kitchen knives but lots of useful information that can inform refinements to your sharpening techinique.
 
Would a folded burr be easier to "snap off " with edge trailing strokes? IE: if it "caught " the stone and broke off rather than grinding it off?
 
nevrknow said:
Would a folded burr be easier to "snap off " with edge trailing strokes? IE: if it "caught " the stone and broke off rather than grinding it off?
Click to expand...
A folded burr as I've seen with soft steel has to get abraded by a complete new sharpening. As it is folded you can't catch it any longer.
 
Please remember the steel must be fatigued as it has failed. Better abrade it than reconstructing some even weaker edge out of it.
 
Rolled Edge HOWTO:
  1. Start with stainless laser and sharpen without microbevel
  2. Cook one pound of thick cut bacon
  3. Place stack of bacon on polypropylene cutting board
  4. Place palm of hand on spine of knife while chopping stack of bacon

Result: Perfect rolled edge.
 
Benuser said:
Please remember the steel must be fatigued as it has failed. Better abrade it than reconstructing some even weaker edge out of it.
Click to expand...
This. It seems there is some misconception out there. In terms of failure chip=rolled edge. Both have yeilded and been exposed to similar strain levels they are just different failure mechanisms as a result of the material hardness.
 
Benuser said:
Please remember the steel must be fatigued as it has failed. Better abrade it than reconstructing some even weaker edge out of it.
Click to expand...

Does this mean that even on soft stainless, using a steel to straighten out a rolled edge is a suboptimal or a best a temporising solution?
 
Nemo said:
Does this mean that even on soft stainless, using a steel to straighten out a rolled edge is a suboptimal or a best a temporising solution?
Click to expand...

That rolled edge will never be as strong as a newly sharpened edge. You can straighten the edge with a steel. But the edge's steel is already fatigued so it is prone to rolling to one side or the other.
 
I am confused by the obsession with the creation of a burr. I avoid it and never have to remove it. Simply alternating sides with circular strokes seems to eliminate the burr. I focus on creating a bevel that is flat sharp and durable with an acute angle on the outside of the blade and less acute on the inside. Finishing on muddy natural stones inherently diminishes burr creation. Of course good steel helps. I finish with the stone in hand rather than on a platform. This emphasizes visual, tactile and audible feedback that I don't get with the stone on a platform.
 
Razor said:
I am confused by the obsession with the creation of a burr. I avoid it and never have to remove it. Simply alternating sides with circular strokes seems to eliminate the burr. I focus on creating a bevel that is flat sharp and durable with an acute angle on the outside of the blade and less acute on the inside. Finishing on muddy natural stones inherently diminishes burr creation. Of course good steel helps. I finish with the stone in hand rather than on a platform. This emphasizes visual, tactile and audible feedback that I don't get with the stone on a platform.
Click to expand...
Very interesting points here. Open a new thread.
 
unprofessional_chef said:
That rolled edge will never be as strong as a newly sharpened edge. You can straighten the edge with a steel. But the edge's steel is already fatigued so it is prone to rolling to one side or the other.
Click to expand...

I would like to throw a spanner in the works and say that this is not true.

Moving a piece of metal back and forth can actually work harden it - that is to add dislocations to the orderly array of atoms in the crystals, strengthening it.
When we create martensite (the stuff we tend to make knives out of) we're packing as many dislocations into the steel as possible and spreading them out as evenly as we can. Tempering allows some of these to disappear. We use measured amounts of time and temperature to let the atoms move (only a little bit) and they line themselves up a little better.

Physically bending hardened steel beyond its elasticity will cause the dislocations to group up. This results in a concentrated weakness, where the steel will crack and the burr will fall off.

PzcDr.jpg


Folding a burr over on itself doesn't allow any of the above to happen.
 
Once you straighten the edge (on a honing steel) and it gets work hardened is it then less tough so more likely to chip instead of roll next time the edge fails? Is this what is meant by fatigued steel? Or do I have completely the wrong idea?
 
I am going to disagree with Kippington here. The theory is sound, to a degree, but not all work does worm hardening and given the thickness and type of deformation rolling and the steeling does, i think at best you may be creating some work hardening but also introducing stress fractures within the steel (aka fatiguing the steel).

Hence why steeling never lasts long. The edge just rolls back over.
 
I think I may have given the wrong impression.

Let me explain it this way: When you take thin steel wire, say a straightened paperclip, and bend it back and forth in the same spot, what happens?
First it work-hardens. We don't really feel it in our fingers/hands because the steel is so thin, but the hardness in the wire goes up and the toughness drops. Knives have already been hardened to a degree so this part may not apply... but what happens next certainly does.

If we continue to bend the now hardened steel, the dislocations in it move around and group up. This happens in martensitic and work hardened steels, which are both packed full of dislocations (more dislocations = more hardness and less toughness).

Dislocations collect and grow into a big one (mostly around the grain boundaries), this concentration becomes a weak spot and the thing breaks. We see this in both the wire paperclip and in the wire edge.

Clear as mud? :)
 
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