Over-Sharpening

[SteveG]

I've read a couple of times that continuing to sharpen past a certain optimal point reduces the sharpness of a blade.  This isn't something I've experienced with plane-blades or chisels, and my knife-sharpening skills are poor enough that I wouldn't notice (unless I measured a lot more often than I do).  Anyhow, it doesn't make sense to me, so I'm hoping somebody can explain why that would happen.  I can understand it if the sharpening is with a soft substrate, such as leather stropping, then it would make sense (and dubbing a perfectly good edge is something I've done all too often).  But I don't understand how/why it would happen with stones, unless it's simply due to burr formation.  I'm not disputing the claim, I simply don't understand the how/why.  If anyone has an explanation I'd love to hear it, and if it's something other than burr-formation, I've very, very curious, indeed.

[grepper]

I don’t understand that concept either Mr. Steve. Once the edge is ground to the edge, continuing to unnecessarily grind more just removes more steel than necessary.  If the knife is tapered from spine to edge, then removing more metal will take the bevel more toward the spine which may increase the width of the bevel.  But if the sharpening angle is the same, so will be the bevel angle.

Obviously more sharpening will continuously create burr, but as long as the edge is evenly ground from each side the burr should be no larger than with less sharpening.  The trick is to just keep flipping the blade so that newly formed burr is ground off as you go so as not to create some big gnarly burr that is difficult to remove.

If you keep grinding away, eventually the blade will be nothing but a toothpick with a sharp edge, but I don’t see how forming a completely new edge could possible make a blade less sharp.

If you really think about what is happening when grinding an edge, the idea of over sharpening makes little sense, other than it shortens the life of the blade by reducing the number of times it can be sharpened.

I think you are a far better sharpener than you give yourself credit for, or you wouldn't even be asking that question!

[KnifeGrinders]

The SEM image courtesy Todd Simpson scienceofsharp.wordpress.com

Imagine you've raised a burr grinding the right side, and now are grinding the left side. Green line is the right amount of sharpening, you get an edge of half a micron, while the red line is oversharpening and you get an edge of several microns. 
Thickness of the edge you get is shown by shorter horizontal lines; the material over these horizontal lines will be shifted to the right as a newly formed burr.

As you move to honing to remove the newly formed burr on the right side, in the end you get either a sharp edge under half a micron (green zone), or blunt end of a few microns (red overground zone).

Oversharpening is more of an issue with double-bevel edges

[grepper]

I understand what you are saying Mr. KG and good point. I don’t know if that is what Mr. Steve is asking or not. It might very well be. In the case you most magnificently show with wonderful SEM microscopy, then the bevel would be unevenly sharpened and the blade needs to be flipped and ground from the other side.

But even so, how does that differ from a chisel, or unequal bevel type of grind? As I’m sure you know not all bevels are ground evenly. In your image, the red line simply forms a bevel with a micro, almost chisel grind.

It’s completely possible I’m not understanding what you are saying, but as long as the burr is remove above the red line, there exists a sharp edge, albeit not the same secondary bevel angle as the green line.

[SteveG]

Mr. Grepper: Thanks for your words of encouragement; I'm ok as a theorist, as a practitioner not so much. As far as understanding this issue, you and I seem to be on pretty much the same page, and you clarified some of my thoughts on the subject. One are where I don't think we see things the same, though, is on single- vs double-bevel edges. To me there's no theoretical difference between them, the only difference is in practice, in that once a woodworker flattens and polishes the non-beveled side of a blade, they'll never touch them again with a low grit stone. I only use my last (finest) stone in the sharpening progression on the back, and then the strop if I go that far. All of the lower grit work is only done on the bevel. How much of a difference that makes, or whether it even does make a difference, I don't know.

Mr. KG: Thank you for explaining the theory, I know one of the postings I read about this was yours, so I'm glad to have your thoughts on this. I *think* I follow your reasoning, but perhaps my next question will determine whether I really do. When I sharpen knives, once I'm past the coarsest stones, I alternate sides on every stroke. My assumption is that at any given grit the burr created is about the smallest possible, and that I'm always then sharpening to your green line, regardless of how many strokes I take. Do you concur, or am I missing the point?

BTW, I love Todd's site, his approach, and especially his amazing images.

[grepper]

Mr. Steve, I agree with you.  Not sure how I suggested that a double bevel vs a single bevel is somehow different as far as an edge goes.  In fact, I’ve found it easier to get a single bevel edge sharper than a double bevel edge.   You mention in your reply that you are always on the green line.  

That’s exactly my view.  Only if you “over sharpen” one side does the red line come into play.  But even if the blade is sharpened to the red line, it’s just an uneven bevel, slightly less than a full single bevel grind.  Bevels sharpened at different bevel angels are not unheard of.  There are some very fine knives with that type of grind.

Observing Mr. KG’s SEM image, it’s difficult to tell where the “edge” ends and the burr begins.  At some point the edge is to thin that the burr just breaks off with honing.  It would be interesting to see a SEM image of the same edge with the blade sharpened to the red line.  What does the bur look like then?

For kitchen knives I like to keep it at about 150.  Sometimes they turn out sharper, but I think the extra sharpness is ephemeral at best.  Chopping one carrot will result in a pretty drastic reduction of sharpness, so for kitchen knives at least, 150 makes me happy. For general use, 150 is very sharp, and even 150 won’t last for long.  

I think it just depends on what you personally are going for.  For me at least, I’m just looking for a nice and sharp, durable and toothy edge and not a knife block full of razor blades.  As long as my blades are at about 150 and pretty well deburred I’m happy.  

I don’t think there is a right or wrong here.  That is simply what I am trying to achieve for the environment that I’m using my edges in.

[SteveG]

Mr. Grepper,

As a practical matter I'm in full agreement. In fact, it's rare that I can get a knife better than 150, but for my use that's good enough. But it doesn't stop me from wanting to know what's possible, and more importantly (to me) what sets the limits. KG has observed edges getting duller when honed too far, and I am deeply curious to understand the mechanism behind that.

I think KG has also observed that his knives (the ones his team sharpens) don't get dull as quickly as mine, and perhaps yours. I'm curious about that, too, and suspect it has to do with his ability to minimize, and maybe even eliminate, the burr. But, like you, I'm not aspiring to reproduce his results, only to understand them to the best of my ability.

Thanks for your input. I've read with interest your posts on a toothy edge, and will be rereading them when I start experimenting with that in the not-too-distant future.

[me2]

Could it just be edges sharpened aggressively and often thicken quickly and don't cut as well?

[Ken S]

The impression I get is that much of the lessening of sharpness with oversharpening actually occurs in the honing stage. I generally think in Tormek methodology. Since the set up is held constant with the grinding wheel, either graded just fine for most work or starting graded coarse for deeper grinding before fine grinding, the knife remains unmoved in the jig. Neither the grinding wheel nor the support bar are moved. 

At that point, if the support bar is carefully readjusted to maintain a proper bevel angle, the bevels should become more polished, but otherwise unaltered. This should apply with either the SJ 4000 grit wheel or the leather honing wheel.

Using the knife jig, if set up is careful, I don't see how rollover is possible. I can see it being very possible if honing is done freehand. 

Ken

[SteveG]

me2: I'm sure that's true, but KG is extremely precise in his honing process, so I would consider that a different issue than what I'm exploring.

Ken, that's an excellent point. If the discussion of the over-sharpening phenomenon is limited to high-precision Tormek sharpening, then it makes perfect sense to me, both in terms of burr and KG's red/green lines.