Single side bevel edge sharper than double bevel edge?

[grepper]

In this post, http://bessex.com/forum/showthread.php?t...65#pid1265
 
I was talking about sharpening a single bevel chisel, and how easy it is to sharpen them to 100 gf sharpness.  I blurted, “I was surprised that it ended up @ 100 gf sharpness considering it is a 25° single bevel.  I would have thought that a more acute angle like a 15° dps knife would be easier to sharpen to 100 gf, but that was not the case.  The chisel sharpened to 100 gf without jumping through any hoops at all.”
 
Further in the thread, http://bessex.com/forum/showthread.php?t...78#pid1278
 
Mr. EOU stated, “To tell you the truth, we are very intrigued by the sharpness levels reported in single bevel edges. We do not doubt for a second that these edges are easily sharpened to that level because it is an edge commonly used in medical applications and we have seen reports of very similar successes before.”
 
I know that Mr. Ken routinely sharpens single side bevel chisels to 100 gf or sharper on the Tormek. 
 
I remember when I sharpened that chisel I thought the 100 gf sharpness without really trying was interesting, but for some reason didn’t give it further consideration.  But now I’m wondering if a single bevel edge can actually be sharper then a double bevel.  DE razor blades are double bevel.  Why not single bevel?
 
I know some knives are sharpened with one side bevel more acute than the other.  What is the reason for that?  If indeed a single side bevel can be sharper than an equal double side bevel, does it also follow that a 15° bevel on one side with a 20° bevel on the other sharper than a double bevel of equal angles simply because the bevels are not equal? 
 
I’m totally clueless!  Any ideas?

[Ken S]

Interesting topic, Grepper. 

I remember reading a recommendation somewhere about resharpening pocket knives to single bevel. (I think it was in Leonard Lee's book.) Woodworking marking knives are generally single bevel to eliminate having to correct for the second bevel. The blade rides directly on the straightedge.

My wife and I generally have a very light supper, usually apple, cheese, and nuts. She likes the apple sliced very thinly. I have found my santoku knife, reground to a single 15° bevel helps me cut very thinly. I make this conclusion by observation only, no supporting research or extensive expertise. I am left handed, and the only person who uses this particular knife. I wonder if the two bevel fashion was to have a knife function with either hand or direction.

Time to awaken my family.

Ken

[SteveG]

Isn't a single-bevel blade really just a double-bevel blade with one really long bevel? In woodworking, that long bevel, when using a chisel for instance, allows for a large reference surface when paring, providing a lot of control for the cut. I don't do much in the kitchen, but would imagine the same applies (I'm thinking fileting a fish would be a lot easier with a flat against the bones).

One thing I've noticed, which may be relevant, is that it seems to be a lot easier (for me) to debur a chisel or plane blade than a knife. When I sharpen a chisel, the back of the chisel (the long bevel) is already polished. I sharpen the (short) bevel only up through the grits, then remove the burr on the stones by repoliishing the back on the finest stone. Perhaps that large reference surface makes the deburring more effective (i.e., does a better job in abrading away the burr) than what is normally done sharpening a knife. From my own (limited) experience, I never feel like a chisel isn't completely sharpened/deburred when I'm done with it, but I always feel like a knife could use more work.

Just a though... if I keep barking up enough wrong trees, eventually I might find the right one.

[Ken S]

Good points, Steve.

I do a lot of testing for another forum. I wanted to keep the sharpening process as simple as possible. I realized that the key to this quest is to start with the most basic tool. For me, that tool is a 3/4" Irwin Blue Chip chisel. With some looking around, I have been able to locate them (new) for $8.00 US each. I don't remember how many I have. Last memory was eight. Incidentally, I only use them for sharpening tests. They are reasonably good carbon steel. The 3/4" single bevel is easy to shape, sharpen, and examine. The backs flatten easily. I find them a great learning tool, much better than inexpensive knives. Having several allows me to compare different grits or sharpening techniques.

When I was testing 180 and 80 grit CBN wheels, I found I could go from the 80 grit wheel (BESS just under 500) to a BESS 130 reading by just following the 80 grit with the Tormek leather honing wheel and compound. Not quite as good as the traditional Tormek technique, which read around 100, but quite respectable for a quick job.

Incidentally, I never use secondary bevels with the Tormek.

Ken

Ken

[Ken S]

Grepper,

Thanks for starting a new topic with this. I prefer many more focused topics to a few catch alls. Someday, the newbies and searchers of the future will thank you. Good moderating!

Ken

[SteveG]

Ken,

The Irwins are very good for the price, I agree, and have some myself. It makes sense to me that you can go from 80 to stropping and get those results, the Tormek strop is very effective (though I never use it on chisels, as it rounds the backs, which makes most chisels unusable for many woodworking applications). As a clarification, I also don't microbevel on the Tormek, only do that on stones. It sounds like the Tormek results match the results from stones as far as sharpness, so that's a nice confirmation of my benchmark (i.e., 100 and I'm done).Just need to figure out how to get that on a knife...

[Ken S]

Steve,

One of the questions still unanswered for me is is the rounding of the backs caused by the Tormek itself or by poor technique by the user. Flattening and polishing a flat surface does not seem a logical choice with a round grinding wheel, Tormek or otherwise. I tend to do any major flattening (a job done only with worthy chidels, like classic Bucks) on the Tormek and finish with an 8000 grit flattened water stone or wet or dry paper on glass.

Knives seem more difficult to me, too.

Ken

[SteveG]

Ken,

I think the rounding is a result of the Tormek, specifically soft leather on a round surface puts too much pressure on the edge. When I polish the backs of my woodworking chisels, I first do it on stones, making sure a substantial portion of the chisel stays on the stone (say 1/2 inch or more). Then, when I strop on leather, at least two inches are always against the leather, and I strop lightly. When doing a fine/precision cut with the back of the chisel against a reference surface, you want the edge to cut exactly on that surface. For my knock-around chisels for chores around the house and yard, I do them on the Tormek, since I don't care if they're precisely flat. For the initial flattening of the backs of chisels, the good ones don't need much, but for the Irwins I do them on stones (would do them on a belt sander if I had one of the nice ones that you guys seem to have).

Steve

[grepper]

Grepper,

Thanks for starting a new topic with this. I prefer many more focused topics to a few catch alls. Someday, the newbies and searchers of the future will thank you. Good moderating!

Ken

Yeah, that sure is how it goes with forums.  Conversation naturally ebbs and flows in various directions and soon threads bloat, end up having nothing to do with the beginning subject, and as you noted make it very difficult to ferret out information when reading a thread or doing a search.

An excellent example is: http://bessex.com/forum/showthread.php?t...78#pid1278 


That valuable info should be in the BESS or Edge Sharpness Testing forum, but is now buried in the All About Edges forum in a post called 150 gf sharpness.

I've seen stuff like that in many different sites, and good forum design can be ameliorative.  Too few or badly named forums leads to forum bloat and badly categorized threads.  Too many forums is just plain confusing and leads to the same issues.  I'm sure that good forum design can be a real hair pulling freak show.  I think the answer is not static, and that site design must be metamorphic and evolve with the forum as it grows and changes.

I guess it's up to us posters to try and control thread bloat as best we can, and start new threads when applicable as well as to provide useful feedback and suggestions to forum admins.  Sometimes however, conversational ebb and flow is just unavoidable and seems inherent in the nature of the beast.

[grepper]

It makes no intuitive sense to me why a chisel grind should be inherently sharper than a double bevel grind.  That said, in practice, for whatever reason, it sure is easier to get a chisel grind sharper.

Grabbing at straws, I keep thinking it must have something to do with the sharpening process or burr formation/removal in relation to the flat side.

Obviously I have absolutely no clue, and even to me, my speculation feels like unsatisfying, empty random babbling.  If was just me, I would simply accept my incompetence and move on, but other folks report the same findings and even similar sharpness test readings.

I'm not even sure there is there anything to this.  Is there?  Is there something to understand here?