GUN EDU: Convex edged blades

Discussion in 'On Topic' started by psalm, Apr 12, 2008.

  1. psalm

    psalm Thanks niquesuave06 for the AV

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    Found this writeup on Koster Knives' website:

    [FONT=Verdana, Arial, Helvetica, sans-serif][FONT=Georgia, Times New Roman, Times, serif][SIZE=+1]The Convex Edge[/SIZE][/FONT][/FONT]
    [FONT=Georgia, Times New Roman, Times, serif]"What is a Convex Edge?"[/FONT]
    [FONT=Georgia, Times New Roman, Times, serif]This simple question seems to baffle even some of the most scientifically minded people - yet the principle is very easy to understand.[/FONT]
    [FONT=Georgia, Times New Roman, Times, serif]Look at the following diagram:[/FONT]

    [​IMG]

    [FONT=Georgia, Times New Roman, Times, serif]Simply put, the convex edge is a curve - an arc, more specifically. Seems easy enough....but now consider this:[/FONT]

    [​IMG]

    [FONT=Georgia, Times New Roman, Times, serif]Instead of a flat, fixed 20 degree bevel throughout, the convex edge angle changes across the entire bevel. Look at the tangent lines - it starts out more obtuse nearer the tip (shown in red) and becomes more and more acute the deeper it cuts. You get the durability of a wide bevel angle without wedging or sticking.[/FONT]
    [FONT=Georgia, Times New Roman, Times, serif]"It's all about material separation." - Jerry Fisk, Moran Hammer-In, 2003[/FONT]
    [FONT=Georgia, Times New Roman, Times, serif]It's been said that a "smaller bevel angle" can cut better, but is more fragile; while a "larger bevel angle" lasts longer, but won't slice as finely. This all becomes moot when utilizing the convex edge. Essentially, you get the best of both worlds - edge retention as well as fine cutting. Knifemakers have been putting convex edges on knives for hundreds, even thousands of years. Only since the advent of machine-made knives, has the convex edge dropped in popularity. But lately, it has been making a roaring comeback...[/FONT]

    [FONT=Georgia, Times New Roman, Times, serif]"What do these terms mean - full-convex, convex micro-bevel, convex grind, convex bevel, and so on?"[/FONT]
    [FONT=Georgia, Times New Roman, Times, serif]These terms make reference to the extant to which a certain knife has been convex ground. In my research, there are really three different kinds of convex edges:[/FONT]

    [​IMG]

    [FONT=Georgia, Times New Roman, Times, serif]The first (on the left) is called a Full Convex Grind. The entire face of the knife is one continuous and smooth curve. This is the most efficient use of the convex edge. However, for some, it is the most daunting to sharpen. Yet, if the knife is satin-finished, there is no worry of messing it up with scratches - it can be easily sharpened with fine grit sandpaper. There is much more room for creativity and freedom with this grind. It can be left thick as a cleaver, or tapered thinly as a slicer. It requires, however, carefully planning and grinding.[/FONT]
    [FONT=Georgia, Times New Roman, Times, serif]The second type (shown in middle two) is called a Convex Bevel. The bevel varies in size, but is noticeable at arms length. It is much easier to make than a full convex bevel, and has a little more flexibility in what "primary grind" it can be applied to - you'll find it primarily on flat-ground and saber-ground knives.[/FONT]
    [FONT=Georgia, Times New Roman, Times, serif]The third (shown in the last two) is called a Convex Micro-Bevel. Only the last 1/32-1/16" of the knife's final grind is given the convex edge. This is by far the easiest and more widely accepted application of the convex edge. You can apply it to any grind, any knife (the diagram shows a hollow-grind and flat-grind, respectively). This method takes less time, can be done by anybody with some free time, and with the right grind can be strong yet easy to sharpen and maintain.[/FONT]
    [FONT=Georgia, Times New Roman, Times, serif]The numbers you see below each example are the relative cross-sectional areas. For example, if a Full-Convex knife is 1.0, then a saber-ground convex beveled knife might have anywhere from 1.4 - 1.7 times as much "area". Since edge geometry does play a significant part in cutting tasks, it's important to understand how to choose the right grind to fit the specific application. [/FONT]
    [FONT=Georgia, Times New Roman, Times, serif]Note: The diagram above is a "real world" example. It represents actual knife cross sections - it is not merely "diagrammatic". I drew the outlines in CAD based on a knife ground out of a 1" wide bar of 1/8" thick steel. The same would apply to a 2" wide bar of 1/4" thick steel (both are common barstock sizes). The "base" for each was taken from a blade that tapers from the spine down to 1/32" near the edge. Then, I applied a typical full-convex, flat or hollow grind. The hollow grind is based on a 14" wheel to maximize cross sectional strength. What you see is what you get - except that the the diagram has been enlarged for clarity.[/FONT]

    [FONT=Georgia, Times New Roman, Times, serif]"Are all convex edges the same?"[/FONT]
    [FONT=Georgia, Times New Roman, Times, serif]Yes and No. It is a convex edge if it has an curved bevel, period. However, that's where the similarities end. It is up to the knifemaker to make a knife that fits the intended task at hand. Do not fall for the hype of the "Do-Everything" knife - it does not exist. In our day and age, we encounter specialized tasks at every corner. How many knife owners have just one knife? Would just one knife do everything demanded in the kitchen? Work as a cleaver, slicer, dicer as well as parer? Likely not. "Use the tool most appropriate to the task." That alone accounts for the hundreds of different knife designs, shapes, sizes and even grinds you can find at knife shows today. [/FONT]

    [FONT=Georgia, Times New Roman, Times, serif]"Is a Convex Edge sharper than other edges?"[/FONT]
    [FONT=Georgia, Times New Roman, Times, serif]Sharper for what? Cutting meat? what about bone? sushi? perhaps just vegetables? or maybe chopping wood? Again, the answer is both Yes and No. Let the knife perform the task for which it was intended. A highly polished edge works great for sushi, but bad for bone. A toothy edge works wonders on rope, but frustrates the wood-carver. Any edge can be made to have any of these characteristics. The convex edge, however, can be kept sharper longer if properly maintained - which is achieved by simply stropping after every other use. There is nothing "magical" about the convex edge, though - it will not cut your buddy's katana in half. The beauty of the convex edge is in the ease of acheiving and maintaining a durable, yet "scary sharp" edge. The same can be done for other edges, but it requires expensive sharpening equipment and more time "at the wheel".[/FONT]

    [FONT=Georgia, Times New Roman, Times, serif]"Are all Convex Edges polished? What if I want a "toothy" edge?"[/FONT]
    [FONT=Georgia, Times New Roman, Times, serif]To some extant:[/FONT]
    [FONT=Georgia, Times New Roman, Times, serif]All edges are polished
    All edges are toothy[/FONT] [FONT=Georgia, Times New Roman, Times, serif]Despite any hype you read about any of the edges, sharpeners, sharpening techniques, etc.....all edges have some degree of "toothiness" to them. Just look at an edge under a microscope - you will see scratches on even the most finely polished edges (10,000 grit).[/FONT]
    [FONT=Georgia, Times New Roman, Times, serif]Essentially, there are 2 steps to sharpening:[/FONT]
    [FONT=Georgia, Times New Roman, Times, serif]Grinding and Polishing[/FONT]
    [FONT=Georgia, Times New Roman, Times, serif]Grinding sets the edge bevel angle (do it once, only go back to repair dings/dents)

    Polishing refines the scratches on that edge to help it cut more smoothly.[/FONT]
    [FONT=Georgia, Times New Roman, Times, serif]I arrived at this conclusion after much study and thought on the subject. One day, I decided to test my theory by grinding out a Convex Bevel on a knife using only a 120 grit belt - it shaved hair on my arm! The reason? Despite the coarse scratches, the convex edge that had formed was being polished by the fast-moving slack belt. The uniformity of the edge allowed it to shave hair. There is no sharpener manufacturer that will stand behind a claim that their 120 grit stone will shave hair! This is by no means news for anybody who specializes in convex edges.[/FONT]
    [FONT=Georgia, Times New Roman, Times, serif]The more polished the edge - the better it "push-cuts". A push cut is where you push the knife through the material without any "back-or-forth" action - like how you would chop celery.[/FONT]
    [FONT=Georgia, Times New Roman, Times, serif]The more "toothy" the edge - the better it "draw cuts". A draw cut is a slicing, back-or-forth action - like how you would slice bread, or a tomato. The ultimate toothy edge is a serrated blade.[/FONT]
    [FONT=Georgia, Times New Roman, Times, serif]Decide for yourself what your application is, and tailor your edge to meet it. With the convex edge, this is achieved simply, using only sandpaper and a strop. Just stop at the grit level that gives you the desired balance of polish and toothines, and strop.[/FONT]
     
  2. Goat

    Goat That crack is really moreish

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    Bark River Knife & Tool > *
     
  3. psalm

    psalm Thanks niquesuave06 for the AV

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    QFT.

    I just can never decide which knife I want next :hs:

    So far I have a custom Patch

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    and a Boon II

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    the Aurora, Bravo-1 and Canadian special are all really high on my list.
     

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