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 Post subject: 50/50 and double bevel
PostPosted: Tue Apr 30, 2013 2:26 am 

Joined: Thu Jan 03, 2013 11:01 pm
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Can someone please explain the difference (if any) between double bevel and 50/50 and how they relate to the different grinds?

I've been thinking of them as interchangeable, but now I'm thinking that may not be right.


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 Post subject: Re: 50/50 and double bevel
PostPosted: Tue Apr 30, 2013 3:51 am 
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RAY <> A "50/50" usually references an edge. So on the cutting edge, it is an even 50/50 "V". It can be 50/50 at 12 degrees or 50/50 @20, but each sides angle is equivocal.

A double bevel is a knife design created by grinding. So from the spine to the cutting edge, there is a blade face that has been ground (most of the time). It can be a flat grind, a convex grind, a hollow grind, and any of these grinds can be symmetric (50/50) or asymmetric. So for instance, my Ginsanko Hiromoto Western Deba has an asymmetrical semi convex grind. It has a flat ground left blade face at around 30% of the total included angle and the right side blade face has a distinctly convex grind that is the other 70% of the total included angle, but the actual cutting edge has an asymmetry, as well. The actual cutting edge does not look like a "V" as it is, in fact, a 60/40 right-handed bias. It's a particularly unique blade, but exemplifies your point, quite well. :mrgreen:



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 Post subject: Re: 50/50 and double bevel
PostPosted: Tue Apr 30, 2013 1:48 pm 
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Double bevel is less descriptive, 50/50 is more descriptive.

You could have a 50/50 double bevel, a 60/40 double bevel, a 70/30 double bevel, etc.

But a 50/50 is just a 50/50 grind.

Or, as previously said, 50/50 could be used to describe an edge condition



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 Post subject: Re: 50/50 and double bevel
PostPosted: Tue Apr 30, 2013 3:40 pm 
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Let's start with something which isn't in contention. Knife and sharpening terms are not consistent. They get used in many different ways, some of which are just ambiguous and some of which are contradictory. Even so, frequently each of the different usages will have an equal claim to being right. The most important thing is to nail the terms down, and get everyone on the same page before proceeding with the conversation.

Double bevel is one of the more problematic terms. It's predominantly used in one of two ways:
    1.
    It refers to any blade which is sharpened on both sides, and is distinguished from a blade which is only sharpened on one side.

    Because many traditional Japanese knives are sharpened on one side and so are nearly all straight razors, the need to differentiate between "single" and "double bevel" knives is important when you're talking to knife and razor enthusiasts.

    Worldwide though, the vast majority of knives are sharpened on both sides, and it's simply a non issue.

    Using the term "double bevel" as a way of differentiating the Japanese/razor edges from ordinary edges, double bevel edges come in three basic geometries. They are the flat bevel "V;" "multi-bevel" which could be could be two-stage or three stage -- aka "trizor;" and convex (aka hamaguri or hamaguri-ba).

    "Single bevel" edges are sometimes referred to as "chisel."

    2.
    The term "double bevel" is often used to describe any two-stage multi-bevel.

    The term "V edge" is often used to describe any edge which is sharpened on both sides (or at least significantly so), as opposed to only those double-sided edges which are flat beveled. So, "V" and "double-bevel" could mean the same thing... or not.

    Confusing ambiguity yet? QED.

I haven't talked about 50/50, which is not ambiguous -- at least I hadn't thought so until now -- though it can get complicated.

    Any time you see an edge described as fractional quotient (e.g., 50/50, 60/40, etc.) or as a numeric ratio (e.g., 2:1, 3:1, etc.), the numbers refer to the ratio of asymmetry, and only (well almost) apply to a knife sharpened on both sides.

    A 50/50 knife has bevels that are the same width on both sides. Since 50/50 edges are symmetric, calling it a ratio of asymmetry might be confusing... but just accept it. On an 80/20 knife the bevel on one side is four times wider than the bevel on the other.

    At some point, the ratio of asymmetry is so high that the edge might as well be "chisel." This happens at around 90/10. And because sharpeners often sharp the flat side of a chisel edged knife enough to "chase the burr," many chisel edges actually do have a discernible bevel on their back side, and probably are in the neighborhood of 90/10 or damn close.

    Sometimes the ratio numbers are reversed to express left handed asymmetry. That way, an 80/20 knife has the wider bevel on the right face of the knife (knife held handle towards the user, edge facing down). while a 20/80 knife would have the wider bevel on the left face.

    Conveniently, it might be said that right handed asymmetry favors right handed cutters, and left handed asymmetry favors left handed cutters. Inconveniently, that's not the case. It's truer but more awkward to say that right-handed asymmetry disfavors left handed cutters, and, of course, vice versa.

    Most of the inconvenience comes in the form of "steering." That is, asymmetry tends to seek its own path by straying away from the wider bevel either by racking or torquing, with wrong-handed asymmetry being particularly determined. However steering is also a matter of (bad) technique. The stronger the grip, the greater the tendency of the knife to steer; while a very soft grip can eliminate it almost entirely. A very sharp edge is prerequisite to a soft grip.

    The more asymmetry the narrower the edge just above its apex, the sharper the edge will seem, and the less it will tend to wedge. But asymmetry has it's problems. Everything else being equal, more symmetric the edges are more resistant to impact burring (i.e., getting bent out of true by contact with something hard), and are better able to hold up against the stresses of steeling.

    I'm throwing around numbers as though they mean something. It's almost impossible for an ordinary sharper to measure bevel widths more accurately than they can be measured by simple inspection. Consequently, as a practical matter, "60/40" and "70/30" are both a helluva lot more meaningful as 2:1 than their other expressions. And, obviously, if you're using the GI Mk IV eyeball as your primary reference, it's a lot easier to sharpen a knife with one bevel twice as large as the other than it is to try for 7:3.

    If you're going to walk away from this post with anything beyond a couple of definitions, it should be that sharpening always represents a tension between sharpness immediately off the stones and durability. Because the tension is usually best resolved with some sort of compromise, the 60/40 - 2:1 - 70/30 range is one of the most common and most useful ratios of asymmetry.

A lot to absorb, but I hope it helps. Ask lots of questions.

BDL


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 Post subject: Re: 50/50 and double bevel
PostPosted: Thu May 02, 2013 9:49 pm 
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BDL, that's one scary post :) Scary in the sense that I completely agree with all of it and it is so rare that this topic is correctly stated. Nicely done!

"A 50/50 knife has bevels that are the same width on both sides. Since 50/50 edges are symmetric, calling it a ratio of asymmetry might be confusing... but just accept it. On an 80/20 knife the bevel on one side is four times wider than the bevel on the other. " It is simply this and nothing more.

---
Ken



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 Post subject: Re: 50/50 and double bevel
PostPosted: Fri May 03, 2013 1:46 am 

Joined: Thu Jan 03, 2013 11:01 pm
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Good Lord, I can't breath. I think I'm actually beginning to understand some things.


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