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What is Sharpness?

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  • #54564
    Modernflame
    Participant
    • Topics: 5
    • Replies: 35

    How do we know what we think we know about knife sharpness?  Spend a couple of hours on YouTube and you will be exposed to many contradictory assertions, usually based on anecdotal evidence.

    Let’s assume normal cutting tasks. No chopping, whacking, or otherwise wrecking things. Mike Stewart of Bark River Knives insists that a convex edge will hold an edge “four times longer” than a V edge. Nero knives says the opposite. The Apostle P famously adds a micro bevel in order to increase edge retention and stability. DBK claims that a micro bevel decreases both. So does Knife Grinders in Australia, who also says that coarse toothy edges decrease edge retention, while increasing user fatigue. On the other hand, Rick Hinderer believes that a coarse “field edge” performs best. Some people think that a more obtuse angle will hold its edge longer, while Cedric and Ada has accumulated rope cutting data which contradicts that.

    One might be tempted to say that the ideal edge type depends on the intended cutting task, but I think the differences here are larger than that. For example, I’ve seen a video of Clay putting a coarse micro bevel on a mirror polished edge. One school of thought would say that this edge is well suited to cutting fibrous material. Another school says that you’ve only made it more difficult to cut and have shortened the useful life of the edge. I don’t want to find myself parroting unfounded assertions. I want science! At the very least I want to discuss your observations based on use, rather than received knowledge.

    • This topic was modified 4 weeks, 1 day ago by Modernflame.
    #54567
    MarcH
    Moderator
    • Topics: 61
    • Replies: 2127

    One might be tempted to say that the ideal edge type depends on the intended cutting task

    “The art of sharpening”.  I think it’s as much practical as philosophical.

    It’s like everything… one version is never perfect for every need.

    Cars/trucks, grills, computers, guns, etc, etc, etc.

     

    Marc
    (MarcH's Rack-It)

    2 users thanked author for this post.
    #54569
    Modernflame
    Participant
    • Topics: 5
    • Replies: 35

    Fair enough. I suspect, however, that there are myths among these accepted truths. They cannot all be true simultaneously.

    #54570
    airscapes
    Participant
    • Topics: 13
    • Replies: 277

    Well for me, when the wife says, cuts good, I know I did it right! 🙂 It is going to get dull if you use it, that has been happening since the first stone was sharpened..

    4 users thanked author for this post.
    #54571
    MarcH
    Moderator
    • Topics: 61
    • Replies: 2127

    I suggest you start with the basic “V” grind knife edge our W.E.  sharpeners allows us to apply well.  Pick one knife, like an edc, or a kitchen knife. Then use that knife and sharpen it only as needed.  After some time through using that specific knife, as you normally use it, again and again, you should be able to get a read on it’s shortcomings and areas it excels.   Only then would I mess with the applied edge.

    IMO, that’s the only way to experientially correlate edge with function, (i.e., longevity, durability, sharpness, cutting ability, slicing ability, etc.).

    Otherwise,  like you said, you’re simply taking someone else’s word on it.  I’m willing to emulate the edge sharpening of a knife smith or  a knife forger. These guys have a lot of experience with their own knives and have adopted the edge they apply over years of learning.

    Marc
    (MarcH's Rack-It)

    #54580
    000Robert
    Participant
    • Topics: 4
    • Replies: 73

    How do we know what we think we know about knife sharpness? Spend a couple of hours on YouTube and you will be exposed to many contradictory assertions, usually based on anecdotal evidence. Let’s assume normal cutting tasks. No chopping, whacking, or otherwise wrecking things. Mike Stewart of Bark River Knives insists that a convex edge will hold an edge “four times longer” than a V edge. Nero knives says the opposite. The Apostle P famously adds a micro bevel in order to increase edge retention and stability. DBK claims that a micro bevel decreases both. So does Knife Grinders in Australia, who also says that coarse toothy edges decrease edge retention, while increasing user fatigue. On the other hand, Rick Hinderer believes that a coarse “field edge” performs best. Some people think that a more obtuse angle will hold its edge longer, while Cedric and Ada has accumulated rope cutting data which contradicts that. One might be tempted to say that the ideal edge type depends on the intended cutting task, but I think the differences here are larger than that. For example, I’ve seen a video of Clay putting a coarse micro bevel on a mirror polished edge. One school of thought would say that this edge is well suited to cutting fibrous material. Another school says that you’ve only made it more difficult to cut and have shortened the useful life of the edge. I don’t want to find myself parroting unfounded assertions. I want science! At the very least I want to discuss your observations based on use, rather than received knowledge.

    Here’s an article that you might enjoy from Knife Steel Nerds.

    2 users thanked author for this post.
    #54582
    Brewbear
    Participant
    • Topics: 7
    • Replies: 146

    Well for me, when the wife says, cuts good, I know I did it right! 🙂 It is going to get dull if you use it, that has been happening since the first stone was sharpened..

    I’m with you on that one! If my better half says it cuts well (on occasion she will say it is too sharp – go figure) then I’m happy.

    3 users thanked author for this post.
    #54583
    tcmeyer
    Participant
    • Topics: 37
    • Replies: 1939

    I think there’s too many variables to say categorically that a knife is sharp.  Sharp is what the user perceives when cutting a particular object with a particular knife under a particular set of circumstances.

    I will concur that a convex grind will cut more easily than a standard V grind at the same apex angle.  A polished edge will cut more easily than a coarser grind at a given angle, once the cut is initiated, simply because of the reduced friction.  Some materials – particularly those which are slippery – may cut more easily with a toothy edge.  Hence, a polished bevel set with a coarser micro-bevel is usually a better option for all-around use.  That might be in a kitchen environment or as an EDC, where it might be called on to cut hemp or sisal rope or to slice open an envelope on the same day.

    Like almost everyone here, I enjoy showing off the highly polished edge on my Delica 4, but do not like to be seen having to apply an enormous amount of force to cut a 3/4″ manila rope.  I’ll admit that I’ve had the experience of trying and having to give up on cutting through a rope with my freshly sharpened Buck 110.  An extremely small, 1000-grit microbevel is all it takes.  The micro-bevel takes care of the fibers at the apex, while the polished bevels reduce the friction.

    How long an edge lasts is another subject entirely.  Profile, angles and steel types all add to the variables.

     

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    #54584
    000Robert
    Participant
    • Topics: 4
    • Replies: 73

    Your knife is sharp when their knife will not cut it, but your knife will. LOL!

    #54592
    Dwight Glass
    Participant
    • Topics: 0
    • Replies: 64

    TcMeyer said: I will admit that I have had the experience of trying and having to give up on cutting through A rope with my freshly sharpened Buck 110.

    I have also experienced “failure” with a sharp smooth edge when I was digging post holes and needed to cut small tree roots out of the way, where if the edge was toothy there wood be no problem.

     

    #54593
    Modernflame
    Participant
    • Topics: 5
    • Replies: 35

    I think part of this discussion is the proper use of serrated knives. As a rule, I don’t really like them, but I recognize their utility. Sometimes a plain edge knife is not the right tool for the job. As a suburb dwelling office drone, I admit that I don’t often encounter rope or tree roots in my daily cutting tasks, so I’m planning to get a few different types of rope for experimental purposes. I will also admit that I’m skeptical of the deficiency of plain edge knives for rope, but I’m fully prepared to be wrong.

    Consider this document. This knife testing data comes from Pete of Cedric and Ada Outdoors. He’s used various sharpening methods over the years, but in recent times his edges are 17 dps mirror polished KME edges with no micro bevel. He goes up to 0.1 micron CBN strops and then cuts twisted sisal rope literally hundreds of times until the knife fails his sharpness test. On the other hand, he’s using a cutting board. Maybe that is what makes the difference?
    I also wonder if thickness behind the edge is the culprit. I’ve seen video of a Grimsmo Norseman failing to cut rope with its tanto edge, but that part of the knife is 50 thou bte. I may just be reinventing the wheel, but I am determined to understand these things for myself. I am indebted to all for your feedback.
    #54597
    wickededge
    Keymaster
    • Topics: 122
    • Replies: 2933

    There is a lot of good info here, especially about edge shapes and finishes. I’ll add that we’ve done a fair amount of testing for edge retention at different angles. In general, we’ve found that if you’re cutting materials that are not excessively abrasive, and you’re keeping the edge angle at or above the minimum angle the steel can support, an edge with a lower angle will stay sharp longer than an edge with a higher angle. The hypothesis is that force is the major factor (ignoring for now cutting very abrasive materials) that dulls a knife by deforming the edge. The edge is either chipped, rolled or flattened by the application of force from the edge to the material being cut. An edge with a higher angle requires more force to rupture the surface of a material, so edge deformation happens more quickly and accelerates as the knife dulls, requiring increasingly even more force to complete the cut. An edge with a lower angle requires much less force to rupture the surface of a material, so edge deformation happens more slowly and the rate of deterioration is lower. There are exceptions for extremely abrasive materials, very soft steels or knives that are sharpened too acutely for the structure of the steel e.g. a very hard steel that is sharpened too acutely can be prone to chipping with minimal force or a very soft steel will roll or deform if the angle is too low.

    -Clay

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