Single side bevel edge sharper than double bevel edge?

[SteveG]

All other things being equal, I believe any differences could only be due to the sharpening process, which is much easier to control with a chisel-grind. When I sharpen a chisel or plane blade, I can tell right off the stones whether I'm going to have a good reading or not, by how the sharpening session "felt." With knives I have no clue.

[wadebevan]

Interesting topic.
One thing that I have noticed in kitchen use of a single bevel is that if the bevel is on the correct side for the handedness of the user, the slice/chop is quite different than a double bevel.
For instance, I am right handed, and having the flat on the left side (looking down as you would use the knife) seems to work well when slicing something like a potato.
And I have also experienced a sharper edge more easily obtained than the double bevel.
My big issue with the single bevel is that I cut myself more often!
Obviously because there is less distance from the flat side to my left hand digits.

[grepper]

"And I have also experienced a sharper edge more easily obtained than the double bevel."

It surely is a curiosity, and yet seems to be a fairly common experience.  No doubt there is an explanation, but I'm at a loss as to what is going on.  

"My big issue with the single bevel is that I cut myself more often!"

Don't you just hate it when that happens?!

[Ken S]

With a single bevel knife, the back of the blade (non beveled side) rests directly against the surface of whatever is being cut directly at the cut. With a double bevel knife, that contact is not made until the bevel intersects with the main body of the knife. That's why we must tilt a double bevel knife for marking.

I reground a santoku knife with a single left handed bevel to make thinner slices of apples and cheese.

Ken

[Edgepal]

In my opinion...

A straight edge means, for me, that the cutting edge are 100% a straight line seen from above. I am now talking very small variations on the edge

A flat edge is flat = plain.

If the edge is not straight, the edge work like a saw, not as a knife.

To get a flat edge both flat and straight are difficult. A edge can be flat - but the edge is not straight.

Chissels edges have often a long flat support for the sharpener. This support give chisseledges a higher quality, they get both flat and straight becouse of this support - when you sharpen by freehand.

If you sharpen both the flat edge on a knife and a chissel edge in a good quality sharpening tool there is no difference between the edges.

Thomas

[Mark Reich]

Actually this is pretty simple, and Mr Ken and Mr SteveG are correct in posts 7 and 8, about maintaining the perfect flatness of the back of chisels and plane blades. That is the crux of single bevel sharpening, and the scientific advantage of single bevels. The entire advantage of a single bevel is eliminating the vast majority of the human error. If one side of the bevel is *perfectly* flat, it would eliminate 50% of the human error. 

Next, the width of the bevel, twice as wide as a double bevel of the same inclusive angle, allows one to place the whole bevel on the stone. You can rock back and forth slightly, then "lock on" to the bevel. You can easily feel when you are at the exact angle, no matter what it is. That eliminates about half the remaining potential human error. 

I am super fortunate and blessed to have my Grandad pass along his passion for crafting of all sorts, with an emphasis on woodworking. Chisels and plane blades were always sharp or freshened up quickly. Grandad used an ordinary oilstone, as did another early woodworking mentor. Both hollow ground the bevel on bench grinders with good tool rests and trued wheels, and they both used oil stones that were probably around 325 grit. Many edges have been sharpened, lots of wood has been pared, and many critters have been cut up with well maintained medium grit carborundum oil stones.

By the time I was old enough to start looking for my own stones, I immediately fell in love with a sexy blue 2x8 DMT the second I laid eyes on her. With this paradigm shift in technology, I proceeded to sharpen everything within an inch of it's life, on that perfectly flat, "lifetime" stone. It's lifetime was probably 20 years as the go to shop stone, which I thought was pretty remarkable.

I had picked up plenty of DMTs by then, but I preferred "Coarse" (45u or about 325 grit) for practically everything. I Rarely use Fine, because C works Way more efficiently, and everything cuts like a champ. I still use an industrial 10" pedestal grinder to hollow grind the bevel a few degrees less than original. I still finish on 325 grit, and skew the scratch pattern 45*.

After 30 years of professional experience with furniture, cabinetry, high end finish work in residential and commercial construction, I can honestly say I haven't met anyone who can cut smoother or sharpen more efficiently than I can. I'm sure they are out there, especially the diehard forum type weirdos.   

I see how deburring on leather could defeat the purpose of a single bevel edge by removing too much material on the back side, so the edge wouldn't touch a precisely flat surface of a diamond plate. To check this, use a Sharpie to color 1/8" from the edge on the back side. Put a couple inches of the blade flat on a flat surface. Pull straight back 1/2" and you should be able to tell if you're hitting the edge. If not, you are pretty much defeating the purpose of the single bevel.

As this forum is learning, there seems to be very little difference in sharpness in relation to refinement, to an astonishing degree. The accuracy and angle of the apex make a much greater difference in sharpness. We also find that low grit/high tooth affects the performance and useful longevity of the edge in a way that is counterintuitive, or simply not recognized by most standard theorists. That is why I don't need more than 325 grit to finish. Single bevels can be very precise, and thereby, very sharp.  

It's very easy to deburr carbon steel, which makes up the vast majority of woodworking blades. Actually, I don't remember Any stainless steel woodworking blades, so that's another huge bonus for woodworkers. I still deburr and distress on hard plastic when I can, or a wad of paper, wood or cardboard. I *never* need a finer stone or leather to get a clean edge on single bevels. If the apex of an edge is formed precisely enough, burr removal is virtually eliminated. 

JMHO  YMMV

[grepper]

As sad as it is, the blades we sharpen do not come to a one atom thick actual point at the apex cutting edge, but rather, a rounded dome of some thickness.  That thickness determines the sharpness of the edge.
 
If we bisect the blade through the center creating a single bevel edge, the apex is ½ the thickness of a double bevel edge and therefore sharper.
 
Is there any truth to that, or is it just a ridiculous oversimplification?

   

[SteveG]

From my point-of-view, if you turn the image on the right clockwise 15 degrees, the comparison is more realistic; i.e., your just comparing a 30 degree (inclusive) edge to a 60 degree edge. I do believe that the more acute angle makes the edge sharper (or feel sharper, at least) if the apex thickness is the same, but I don't think the apex thickness is determined by the angle, or by the single-vs-double bevel, but rather by the sharpening process (or by the limits of the steel).