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    Username Post: Asymmetric Bevels        (Topic#784639)
    bnovy
    Journeyman KnifeNut!
    *
    12-23-06 09:53.27 - Post#1113347    



    Over in the thread on Ken's new sharpening technique, I found something curious (to me) and didn't want to hijack the discussion, especially anticipating more questions. I've made a few attempts searching here for info on asymmetric bevels, and really haven't found much explanation.

    • nikoz Said:
    I then did my 70/30 bevels in the same manner by firstly setting up at 7* & following up with 17*.



    In my naivete, when I see people talking about 80/20 or 70/30 bevels, I've simply assumed that the angles on both sides are the same, but one side is ground more so that the width of the bevels is different. The edge ends up off center, but so what? You get much of the thinness benefit of a single bevel, but you can do so without having to make the back side concave, and for those who care, the knife would have less "handedness".

    Assuming for simplicity that you start with a flat piece of metal, and take off two right triangles of different sizes, but same angles, then any way you measure it comes out as 70/30 (for instance). The width of the bevels, measured either flat on the surface or parallel to the side of the blade, as well as the offset of the final edge, are all in the ratio of 70/30.

    Now, I didn't actually do the math, but if nikoz means that the bevel on one side is 7 and the other side is 17, it sure looks to me like that would end up centering the edge from side to side, as well as making the width of the bevels about 70/30.

    (I suppose the other possible interpretation is that on these particular knives he first does a 7 deg relief and then a 17 primary edge, but that seems pretty unlikely - I'm guessing the 70/30s are on better knives, where you could probably do better than an included angle of 34.)

    Maybe keeping the edge centered is important if you have a layered/clad construction, although I'm tempted to believe that by the time you get to the thinness of the bevels, you should be dealing with the good stuff anyway (again, maybe this is simplistic uneducated thinking on my part).

    So, what do people usually mean when they say 70/30?
    -Bob
     


    joe_c
    Master Member KnifeNut!
    *
    12-23-06 10:42.18 - Post#1113377    


        In response to bnovy

    An example would be where the main bevel is 21 degrees and the other is 9 degress giving a total of 30 degree bevel. This is what is generally meant by a 70/30 bevel.

    Joe


     
    bnovy
    Journeyman KnifeNut!
    *
    12-23-06 11:08.38 - Post#1113391    


        In response to joe_c

    Interesting, that's exactly the opposite of how I was interpreting it! I thought that the 70 referred to the bevel that was wider, which would be the 9 degree side in your example, but you're saying it's actually the ratio of the angles.

    Given the fairly small range of angles we care about, maybe it ends up pretty close anyway - a 70/30 ratio of angles results in a 30/70 ratio of bevel widths?

    But the main thing I was wondering was whether the two angles are generally different, and your answer is yes. If so, then what is the advantage of a 21/9, versus a 15/15 ground more on one side? I just drew a quick diagram, and other than keeping the edge in the middle, it's not immediately obvious why I would prefer the 21/9.
     
    nikoz
    Master Member KnifeNut!
    *
    12-23-06 11:20.24 - Post#1113404    


        In response to bnovy

    • bnovy Said:
    Over in the thread on Ken's new sharpening technique, I found something curious (to me) and didn't want to hijack the discussion, especially anticipating more questions. I've made a few attempts searching here for info on asymmetric bevels, and really haven't found much explanation.

    • nikoz Said:
    I then did my 70/30 bevels in the same manner by firstly setting up at 7* & following up with 17*.



    In my naivete, when I see people talking about 80/20 or 70/30 bevels, I've simply assumed that the angles on both sides are the same, but one side is ground more so that the width of the bevels is different. The edge ends up off center, but so what? You get much of the thinness benefit of a single bevel, but you can do so without having to make the back side concave, and for those who care, the knife would have less "handedness".

    I believe it adds to the "handedness" as it resembles a modified single bevel when you look at the cross-section

    Assuming for simplicity that you start with a flat piece of metal, and take off two right triangles of different sizes, but same angles, then any way you measure it comes out as 70/30 (for instance). The width of the bevels, measured either flat on the surface or parallel to the side of the blade, as well as the offset of the final edge, are all in the ratio of 70/30.

    Assymmetric bevel knives are usually flat (or very close) on the left side & convex on the right

    Now, I didn't actually do the math, but if nikoz means that the bevel on one side is 7 and the other side is 17, it sure looks to me like that would end up centering the edge from side to side, as well as making the width of the bevels about 70/30.

    Nope, it offsets the edge so that the edge is closer to the left side of the blade in relation to the spine

    (I suppose the other possible interpretation is that on these particular knives he first does a 7 deg relief and then a 17 primary edge, but that seems pretty unlikely (correct) - I'm guessing the 70/30s are on better knives, where you could probably do better than an included angle of 34.) 7 + 17 makes an included angle of 24*

    The Tojiro DP petty is an offset bevel as is the garasuki for sure &, I think the sujihiki. I do not have the honesuki. The gyuto & Western deba are 50/50.

    Maybe keeping the edge centered is important if you have a layered/clad construction,Note that the DP series is clad. It is because I think the core is also offset, that I don't change the bevel to 50/50 on clad knives although I'm tempted to believe that by the time you get to the thinness of the bevels, you should be dealing with the good stuff anyway (again, maybe this is simplistic uneducated thinking on my part).

    So, what do people usually mean when they say 70/30?

    I hope the Q's been answered, in part anyway
    -Bob



    Hopefully, one of our dudes good with graphics will post a drawing
    Nick

    Questions, questions, always questions.



     
    cbwx34
    Master Member KnifeNut!
    *
    12-23-06 13:47.57 - Post#1113494    


        In response to nikoz

    No graphics, but you may want to read this thread also....

    http://www.knifeforums.com/forums/showtopic.php?ti...

    cbw
    cbw

    Used Sharpening Equipment For Sale


     
    bnovy
    Journeyman KnifeNut!
    *
    12-28-06 09:06.35 - Post#1117161    


        In response to cbwx34

    • cbwx34 Said:
    No graphics, but you may want to read this thread also....

    http://www.knifeforums.com/forums/showtopic.php?ti...

    cbw



    Thanks, that was exactly the kind of stuff I was looking for (don't know how I missed it, but then the search mechanism here is not the best I've encountered...). Thanks also to Nick, for a nice set of answers.

    Reviewing that longer thread, my summary is that everyone's right, or at least no one is wrong. Some people, including manufacturers, use the same angles on both sides but grind one side further, and refer to the ratio of the bevel widths. Others use different angles on each side, expressing these as the ratio (the bevels here of course come out at different widths as well, but it seems in this case the ratio really does refer to the angles).

    Yes, some graphics would help...
     
    bnovy
    Journeyman KnifeNut!
    *
    12-28-06 09:11.09 - Post#1117164    


        In response to bnovy

    In the first diagram ("Same angles") I looked at using the same angle (15 deg) on both sides. The ratio here refers to the width of the bevels (or how far the edge is off center, which works out the same with identical angles).

    For simplicity, I assumed a straight sided blade. The numbers at the bottom show the amount of material removed in relative terms.

    To my eye, the 20/80 looks pretty appealing, quite a bit thinner than the 50/50, but still giving you an included angle of 30 at the edge for strength, compared to the single bevel case.

    In the second diagram ("Different angles"), I used the same total included angle of 30, but rotated such that the ratio of the angles was 80/20 or 70/30.

    The two on the left were done such that the edge is centered. It seems like this would not be the usual case (and they don't look that good to me), so for the third example I shifted the edge over so that it was 30% across the blade, left to right. I just picked this arbitrarily, but it looks fairly reasonable.

    Note that measuring the width of the bevels along the blade would give a drastically different ratio, since the angles are different - it works out to 15/85! - so it seems pretty important to be clear about what the ratio refers to.

    The fourth example was just to compare the same-angle 20/80, with the second example (edge-centered 70/30 angles). These two look like kind of a toss up to me, with a leaning toward the 20/80, but since apparently you wouldn't center the edge if using different angles, it's not a very useful comparison.

    Assuming I had a knife with construction appropriate for an off-center edge, if I wanted to use the same angles on both sides then the 20/80 bevel width ratio looks close to the sweet spot. But if using a ratio of angles with an off-center edge, that 70/30 case (third example) looks awfully nice, in spite of still having an included angle of 30.

    Might have to try this on one of my old Henckels just for fun...
    -Bob

       Attachment

     
    Louisianacook
    Master Member Knifenut
    *
    12-28-06 09:24.44 - Post#1117183    


        In response to bnovy

    Great work. I think this is sticky worthy.

    After seeing it all laid out, I know why the confusion was there.

    Great work!
    Lee

    Check out my knives!
    My Knives



     
    nikoz
    Master Member KnifeNut!
    *
    12-28-06 09:48.06 - Post#1117206    


        In response to Louisianacook

    • Louisianacook Said:
    Great work. I think this is sticky worthy.
    After seeing it all laid out, I know why the confusion was there.
    Great work!




    Yes, Lee, I totally agree with you. Now that I see all the variation/combinations, I am now wondering whether I selected & calculated the correct bevel angles on my knives. I had never performed the tip-the-edge trick nor measured my original factory bevel angles.

    Bob, that is a fine example of excellent research & reporting.
    Nick

    Questions, questions, always questions.



     
    Louisianacook
    Master Member Knifenut
    *
    12-28-06 09:53.03 - Post#1117212    


        In response to nikoz

    Due to the many questions we get on this subject I think it's definitely sticky worthy.

    Done.
    Lee

    Check out my knives!
    My Knives



     


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