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Geometry: It Matters Most

Discussion in 'General Knife Discussion' started by Eli Chaps, Apr 21, 2019.

  1. CanadaKnifeGuy

    CanadaKnifeGuy Basic Member Basic Member

    212
    Jan 27, 2019
    The steel used determines the thinness / sliciness and geometry that is even possible on a given blade.

    For folders, it's all moot because they aren't (or shouldn't be) heaviy used and are all thin... but a fixed blade that can be 2mm or 2.5mm thick with a modern steel vs 4mm for something more pedestrian makes a difference.

    Geometry determines cutting performance, but material determines which geometries are available / possible.
     
    B Griffin, Natlek and Eli Chaps like this.
  2. B Griffin

    B Griffin Gold Member Gold Member

    Dec 22, 2007
    LOL Oh yes!! I grew up here in the Tennessee hills and know the black walnuts very well and what a horse of a different color they are by comparison. They are one of my favorite nuts. When they have dried and mellowed in flavor a little bit, I love them in brownies and banana nut bread. They, and my fondness of them, have been the reason some of my knife designs have been 3/16 and 1/4 inch thick, so I can use the spine like a hammer to crack them. I don't necessarily suggest everyone else try this, but my first times were many years ago when I had started carrying a large bowie knife and had stopped carrying my hatchet. The first time I did this was with my W-49, because the sand stones got grit in the nuts and the wooden bludgeon had to be so big around it was dangerous to my fingers. If you try this I can tell you from experience that missing the nut can smart a good bit, but with the mass of the steel and the PSI in a small space means you really don't have to swing all that hard, and if you have good aim it works really well. But sometimes I have to switch to the awl on my Swiza or whittle other tools to get some of the meat out of the shell with ...
    [​IMG]

    [​IMG]

    [​IMG]
     
    Last edited: Apr 24, 2019
  3. marchone

    marchone Gold Member Gold Member

    Mar 13, 2013
    Well said! Here's a man who know his nuts! (Pun not intended. Sorry! :oops:)
     
    B Griffin likes this.
  4. Danke42

    Danke42

    Feb 10, 2015
    This makes me laugh, unrelated to knives but there are 2 big walnut trees next door and during the fall the crows and ravens like your avatar here are dropping them on the roof and all over the yard to crack them open.
     
    marchone and B Griffin like this.
  5. B Griffin

    B Griffin Gold Member Gold Member

    Dec 22, 2007
    LOLOLOL I don't think I want to go into how well I know them, but I have known them for fifty-three years... :D

    Yes the crows, which I have have seen as my "medicine animal" if I have one ever since childhood, are very smart little critters. I like watching how they solve problems like that. Without having, or knowing how to make, the proper tooling a person could starve in a wood full of black walnut and hickory trees just because they are so hard to get into.
     
    Last edited: Apr 24, 2019
    Danke42 and marchone like this.
  6. HwangJino

    HwangJino

    Dec 2, 2012
    Geometry is very important to me.
    As a person who uses knives daily as income, I've fiddled with geometry for my uses in the kitchen and put some time into splitting wood with my choppers.

    I have plenty of stones so as long as the steel is somewhat hard, they do tend to last longer than the softer steels.
     
    CanadaKnifeGuy and Eli Chaps like this.
  7. chiral.grolim

    chiral.grolim Universal Kydex Sheath Extension Gold Member

    Dec 2, 2008
    Cutting is a combination of edge-strength (and stiffness) and mechanical advantage. The latter is what you mean by "slicey-ness".

    Beginning at the very apex of the edge, every edge ends at ~90' per-side, i.e. flat blunt (for any who think "tangent" is appropriate for establishing the edge-angle on convex edges, let that sink in). Microscopic measurements of edge-thickness at the apex are given an assumed diameter, a small diameter = "sharper apex". However, the apex is only where the cutting begins, that apex must be pushed to some depth to complete a cut of material with thickness 'x'.
    To maintain a straight cut, the material behind the very thin apex-diameter, i.e. the blade, must be sufficiently strong/stiff to withstand forces that could compress it or turn it aside. To resist compression, the blade may be made of very hard material (perhaps measured by Rockwell). But to resist bending, the blade must be "stiff" which is determined almost entirely by blade thickness and indeed the relationship between stiffness and thickness is cubic and can be simplified as such for our purposes. To maintain the mechanical advantage in cutting, the blade must remain thin. Mechanical advantage for a wedge like a knife blade is calculated simply as the ratio of the "slope length" to the maximum thickness of the amount of blade inserted into the cut. If we were to plot these values relative to edge angle (I used degrees-per-side), we can visualize the different profiles of knife edges as seen below. You will note that I described the mechanical advantage as "cutting efficiency" and resistance to side-bending as "strength" which is not entirely accurate but is more colloquial.

    [​IMG]

    While these charts describe first-and-foremost the edge-bevel of a blade, they can also be used to describe the primary bevel. To make sure we are on the same page, it must be understood that the "primary bevel" describes the main bevel of the blade, usually ground before a secondary or edge-bevel that establishes the final geometry leading to the apex.
    On most knives, that secondary bevel should be very very small, as small as is required to achieve sufficient durability at the edge. Most chainsaws, axes, chisels, planes, and knives of all sorts recommend an edge bevel ~15' per side or 30'-inclusive to maintain sufficient durability, but this of course depends on your use. You can see where 15-dps falls in the charts above.
    For the primary bevel behind that edge, "strength" and "cutting efficiency" only matter once the cut has progressed deep enough to reach that section. Here, the blade usually continues to get thicker although this can be mitigated in large part by a very thin or even hollow (concave) grind that keeps the cutting efficiency as high as possible (why Bucks are often amazing slicers). In most knives, the primary bevel averages <5-dps.

    I hope this is helpful to y'all.

    ETA: "Slicey-ness" of a blade with bevel-angle X = 1/(2*Tangent(X)) Please note, this comparison assumes the same blade height at the measured thickness or angle. If comparing knives with different blade-height, the taller blade is disadvantaged if the depth of the cut progresses beyond the height of the shorter blade.
    Also please note that this calculation takes no account of frictional forces encountered with increasing wedge-thickness or with various surface features of a blade that may increase the force required to complete a cut. The reduced friction of a smooth bevel may outweigh differences in geometry compared to a thinner blade with a very rough grind.
     
    Last edited: Apr 23, 2019

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