Over-Sharpening

[grepper]

Mr. Steve- Yup.  I’m impressed by Mr. KG’s edges and also find it very interesting.  For my use, I’m not trying to do that, but nonetheless I find his effort and expertise commendable, informative and enlightening.
 
If an abrasive is used, honing really is sharpening only with a very fine abrasive.  I agree that it is possible to dull a blade when honing.  I’ve done it often.  I think what is happening is that either the edge is bending, or the edge get’s ground away.  When using leather to hone on, I think both can happen as the soft surface can actually wrap around the apex and/or rollover happens if the honing angle is steeper than the sharpening angle. 
 
Mr. Me2-  Yes.  I agree. The further up the taper, the bevel becomes longer and therefore thicker the further up the bevel you go, making the blade thicker overall.  A thicker blade might not cut as easily as a thinner blade.  What I was trying to say is simply that, referring to Mr. KG’s SEM image, if the sharpening angle remains the same, the bevel angle right at the apex is the same.

Gotta love those SEM images.  If you guys all want to get together and buy me one, I promise to post lots of images to the Exchange for your enjoyment!

[SteveG]

I'll chip in $5, when you get the other 100k or so subscriber commitments, let me know!

[grepper]

Actually, you'd be surprised:
http://www.ebay.com/bhp/scanning-electron-microscope

Nothing like a handy table-top SEM in your shop!

[KnifeGrinders]

...
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?
...

Your are right when grind on both sides of the edge is symmetric.
Keeping to our promise to the customers of sharpness under 100 BESS for mainstream knives, and <=50 for high-end, I've noticed that I cannot get under 100 BESS when the edge apex is not centered in the jig.

Below is an oversimplified modelling of what is happening during sharpening.
The first picture is for an ideally centered edge and even grind on each side, where you get the ultimate sharpness


1 - the burr is raised by grinding one side, at this stage you even out the bevel
2 - you start grinding the other side and evening out the bevel
3 - the burr is raised on the other side
4 - you minimise the burr by alternating sides and grinding with minimal pressure
Then deburr


The next picture shows an asymmetric grind - you eventually get a less sharp thicker edge.
This happens every time when you overgrind one side and/or where the edge is not centered in the jig, resulting in bevels of uneven height.
You may overgrind one side when you apply a little more pressure, or spend more time on that side.



I hear all the time that "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."
In real life you have to even out the bevel on each side before you can work on the edge as such, and in the process lots of metal gets not only ground off but also shifted from side to side - and this is when the undesired asymmetry comes into play.

[SteveG]

I am surprised, especially if they take images like Todd’s. Well, in that case, let’s get one for each of us (not that I have any space to put one)!

[KnifeGrinders]

If i am not mistaken, the edge specimen for SEM can't exceed 20mm by 9mm - Todd had sacrificed lots of straight razors and a couple of knives to get those images.

[SteveG]

Mr. KG, I thank you for the further explanation, of course it was more complex and subtle than I had suspected, and you clarified it beautifully. I can't tell you how much I appreciate the time you spent clearing that muddle from my mind; I now have new respect for the subtleties that I haven't even encountered yet. Very nicely done!

[KnifeGrinders]

No worries. Happy it makes sense.

[grepper]

That handy little guy has a "large" specimen stage:

Specimen stage Eucentric large-specimen stage
X: 80 mm, Y: 40 mm, Z: 5 mm to 48 mm,
Tilt: −10° to 90°, Rotation: 360°
Maximum specimen 150 mm diameter

There is one of those on eBay for $1,500. I have no idea what it takes to use one, but it's interesting how cheap used ones are. Hmmm... I wonder what repair costs would be if the electron gun went out. Seems like one of those ubiquitous Square Trade extended warranties might be a good idea.

[Mark Reich]

I've heard "oversharpening" described in ancient Japanese culture differently, but still well related to freehand sharpening.

Basically, each stroke carries the chance of error. Every mis-stroke needs many perfect strokes to fix.

Totally relevant to minuscule error in guided sharpening such as Edge Pro as well.

This is a tricky subject for honing straight razors too.