Thread: Why such a small angle?

The other day one of my coworkers showed up to work with a new handy little tool. He had purchased a handheld jig which he could clamp his knife into. The jig allowed him to more precisely sharpen his knife by utilizing calibrated holes on either side of the knife jig and one of several different grit hones. He successfully sharpened his knife with it to a small angle and proved that he could shave his arm hair with it since it was so sharp. I told him that the only problem with sharpening it to such a small degree was that the edge would be damaged much more easily even by just using it since the edge doesn't have as much geometric structure below it to provide strength. I told him that his knife would be much more usable and reliable with a higher angle.

I should note that that was just my opinion and I could be completely wrong, so please let me know if you believe I was incorrect.

This instance at work got me thinking about the different angles of a blade and it's sharpness. I read on one of the forums where someone defined sharpness as the attribute of a blade where the two sides of an edge meet at an infinitesimally small width line. If this definition is accurate, which logically it sounds, then it could be said that one could attain an equally sharp blade at 45 degrees as opposed to 10 degrees (just numbers from the air). But a 45 degree edge would be more resilient since it has more supporting material below it.

I should state that I don't believe this to be the case because of experience, however, I wanted to state the contradiction I see so people will better understand my question. Then again maybe it is true and I am some genius that has uncovered a truth that will make humanity better. Nobel Prize anyone J/K. I guess my question is why do we choose one edge angle over another? It will probably be answered real quick by someone telling me that the definition I read, however elegantly correct it sounds, is incorrect. I appreciate any insight.

Jeremy

Ok,

Am I understanding you correctly? you think a fat short cut is better than a long thin cut on a knife correct? In otherwords the angle of the bevel as in relation to the center of the blade?

Consider the picture below. If I'm understanding you correctly you're saying blade A has a wider angle than blade B and thus better structure. Is this correct?



2ndly are you referring to the angle as being angle 1 or angle 2? I'd assume angle 2.

Strength of the blade depends on multiple things. First and formost the metal itself (including forging/proccessing) and equally important what you're doing with the blade. Consider a maul-splitter. It's a thick, heavy wedge metal on the end of a metal pole and you use it to split logs. it's thick and wide because of the work you're doing. While a regular axe will cut into the log much faster, it doesn't force the log apart properly and gets stuck. Both items are sturdy. The axe has to be sharpened more often than the splitter.

The splitter would be wider than blade A and the axe would be closer to blade b.

I wouldn't take a fillet knife and try to cut a frozen roast with it. I wouldn't shave my face with a meat cleaver and I wouldn't chop steak with my razor.

All that being said, two blades being equal save the angle of the bevel, Blade B will require stropping more frequently because it will have more of a tendancy to fold at the cutting edge. However, if blade A and B were both razors, Blade B would tend to cut the whiskers more smoothly versus "splitting" them. I think this is why hollow grounds would be considered a smoother shave than a thick full wedge.

Is this what you're talking about?


All of the above is purely my opinion based on my experiences and conversations with locals.

Glen F

The sharpening jig he showed you sounds like a lansky, I use them myself on my smaller knives and they work quite well.

Taking your edge angle example to its extreme, an edge of 179.999...9 degrees would be as sharp as a 1 degree angle, only *really* strong.

The fallacy in this reasoning occurs in at least two misperceptions:

1) It is possible to get an atomic-thickness edge.

Because of grit and imperfections in the hone and the crystalline structure in steel, the edge is actually quite raggedly serrated. For thin angles, this doesn't hurt too badly because the steel at the back of a serration is still thin enough to cut easily. For thicker angles, the steel at the back of a serration is too thick to cut without pressure and some slicing.

2) The things being cut are also atomic-thickness.

Once the edge has penetrated the thing being cut, there is still the issue of friction and pressure as the edge pushes through. Thicker edges have to spread the cut farther to allow the blade to enter, and the friction is greater because as well. For a hair, there is obviously the problem that the only thing pushing the hair back against the blade is your delicate skin. This applies at both the micro and macro levels -- Swiss army knives don't use great steel, but tend to feel subjectively "sharp" long after their fin is gone partly because their wedge-ground blades are pretty thin.

If you were cutting atom-thickness wires, and could achieve an atom-thickness edge, then yeah, a very obtuse edge would work just fine; once that atom at the tip of the fin had cut the atom-thick wire, it wouldn't matter that the next level of the fin is hundreds of atoms thick.

fat blade:

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thin wire:

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thick wire:

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thin blade:

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You can see that either works for a thin wire, but imagine that wire as 5 or six atoms thick and things change quickly. In the real world we're talking practically about the difference between 17 degrees and 25 degrees (which I think is the range of a lansky), so it's not quite so extreme. Even so, a good compromise is to use a narrow angle for the primary bevel to minimise spreading force, and a wider angle for the cutting bevel for strength. You can also use a continuous curve instead of a pair of angles; this is usually done on a hanging strop or a slack continuous belt with charged with abrasive pastes. If you've ever seen high-magnification photos of the feather blades, they use a two-angle system.

Originally Posted by 13th_stage

someone defined sharpness as the attribute of a blade where the two sides of an edge meet at an infinitesimally small width line.

That definition must be right, but it doesn't mean much because it depends on your definition of "infinitesimal." In real life, they assign values to those small dimensions, and the actual value tells you how sharp it is.

why do we choose one edge angle over another?

There's a tradeoff between sharpness and durability, with the smaller angle providing more sharpness. A razor has a bevel angle of about 15 degrees, while knives don't tend to go below 30. Theoretically, both can be made equally sharp, but with wear the smaller angle stays sharp longer under the same use. The larger angle just stands up to rougher use.

Originally Posted by FUD

I think this is why hollow grounds would be considered a smoother shave than a thick full wedge.

I agree with everything you said but this. Remember that the bevel angle of a razor is determined by the width of the blade and the thickness of the spine. If these are the same for both razors the bevel angle will be the same. Everything else and the sharpening being equal, the shaves should be the same. Hollow grounds were created to make the blade lighter and easier to hone.

Originally Posted by Joe Lerch

Hollow grounds were created to make the blade lighter and easier to hone.

And to use less steel, in an age where good steel was more expensive than labor.

I have a lansky sharpening system and you have a choice of several different angles to sharpen you knife with. The instruction manual is pretty clear that as you lower the angle the sharpness will increase but the durability of the edge becomes more delicate while the higher angle won't be as sharp as say...a straight but the edge will be more durable. They also give suggestions for the proper angle for different types of knives ie. kitchen, hunting, pocket knives etc and no you can't use it for sharpening straights.