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Blade geometry...am I playing with fire?

Discussion in 'Hammer & Tongs' started by kvehikite, Feb 1, 2015.

  1. kvehikite

    kvehikite

    202
    Jan 8, 2015
    I recently made a knife out of a Nichlson farrier rasp. Flat ground it to about 1mm. Heat treated, and procceded to finish grind it. In my hast I ground the primary bevel to pretty much zero so I went with it. I then sharpened it, and it is scary sharp. At least for me. The sharpest knife I've ever made.

    Question is: In the vast expereince found here, is grinding the primary bevel to almost zero pretty much relegating that knife to kitchen duty or can a man produce a field use knife with this geometry? (assuming that all other factors are in order, known steel, proper heat treat etc.)

    I've yet to really use this knife, although it just slides through paper (which I asume is because there is no real shoulder creating resistance), so I can't say that its tough enough to really do some work. I rolled the edge on a brass rod...seems good.
     
  2. Stacy E. Apelt - Bladesmith

    Stacy E. Apelt - Bladesmith ilmarinen - MODERATOR Moderator Knifemaker / Craftsman / Service Provider

    Aug 20, 2004
    I'll let some of the others chime in with more info, but I take almost all my blades to a near zero edge and then sharpen at the desired angle. Personally, I see way too mant thick edges on knives people make. Those are OK if all the knife will do is chop wood, but I have hatchets and axes for that. I like knives that cut.
     
  3. yepimonfire

    yepimonfire

    441
    Nov 7, 2013
    I'm no knife maker but I agree. Most knives are just way thicker then need be, even choppers. Machetes prove this point. Most are no thicker than 3mm, and zero ground. I've used them to cut down small trees before. Never had one break on me. Of course on choppers a thicker edge does a better job at chopping simply because it behaves more like a wedge but of course it's not necessary to be that thick for durability purposes. You could also argue a thin edge dulls faster, to a point that's true, but I've had thin kitchen knives that were dulled enough to see light reflect off the edge (ie no longer apexed) and some of the thinner ones would still cut paper to some varying degree, so a somewhat dull thin knife still has a better ability to cut. A few of my thicker knives any sort of dull and they fail to do any sort of effortless cutting and sure as hell can't cut paper. Just my two cents as a consumer.
     
  4. Daniel Fairly Knives

    Daniel Fairly Knives Full Time Knifemaker Moderator Knifemaker / Craftsman / Service Provider

    Jan 9, 2011
    When in doubt, test it out!

    I like to use some tests that are appropriate for the knife... on a chopper I'm usually going to test it on a pine 2x4 then some Oak... with a kitchen knife maybe some potatoes and onions depending on the style. Then again I've tested a kitchen knife on Oak just to see what would happen... :D

    I might need more coffee... :D But seriously that is the best thing you can do to feel confident about your process. I test just about everything, not just the grind. Worried about your epoxy work? Try to knock that scale off and see what it takes... I like to save destructive testing for the rare knife but it will leave you with little doubt.
     
  5. james terrio

    james terrio Sharpest Knife in the Light Socket

    Apr 15, 2010
    I'm with Stacy on this. With the exception of very heavy-duty choppers, I grind most of my blades so thin at the edge that they start to turn a burr. Then after final finishing, I put a micro-bevel on 'em.

    Thinner cuts better. There's just no way to avoid that fact.

    Depending on the blade and its intended use, overall geometry, alloy selection and HT will provide the required durability.
     
  6. james terrio

    james terrio Sharpest Knife in the Light Socket

    Apr 15, 2010
    Me too! And it's pleasantly surprising how much heavy cutting/chopping a light, thin knife like that can withstand, as long as it's high-quality steel with high-quality HT - and you don't torque on that thin edge or use it as a screwdriver/crowbar.

    There are ABS Journeyman and Mastersmiths who have demonstrated very thin, "delicate" chef's knives they made, passing all the ABS performance tests... including bending the heck out of them in vises (without breaking the blade) after chopping through 2x4's and slashing hanging ropes and still shaving hair. Many of them have videos of those tests on youtube and elsewhere.
     
  7. Mecha

    Mecha Madscienceforge.com Knifemaker / Craftsman / Service Provider

    Dec 27, 2013
    I've been thinking about this same subject.

    When looking at historical swords, it seems as if damn near every one for a thousand years is zero-ground. The first few little swords I made were thin, with a high mildly convex section, and zero. And they have been great for heavy machete duty, and definitely sharper/better cutting than the thicker ones with a secondary bevel!

    "Thin is in" even for swords, I say!
     
  8. GHEzell

    GHEzell

    354
    Nov 14, 2001
    I'm glad to hear I'm not the only one who does this.... I too take them down to a burr before refining the edge with a micro-bevel.
     
  9. SinePari

    SinePari

    906
    Oct 24, 2013
    I don't really know if this coincides with previous posts, because I'm still figuring out the jargon, but the few knives I've made so far have been exclusively convex edge grinds. I take the meaty football edge down a bit further than what I see as being defined as "convex"... less of an acorn/bullet edged profile, rather more of a distinct taper like a nice pointed round paintbrush... Then, when the meat of the blade can easily clear the accusharp sharpener, I clamp it in a vise and run the blade over it to put on an initial edge. I don't know if I'm cheating, it's just that I know of no better way to accomplish a more precise starting point for the sharpening stone... then the ceramic Sharp&Easy... Then strop...


    I might be doing it wrong but by the end of the process it's sharp... even after taking it to a pt fence post, it'll slice paper like a boss....
     
  10. chiral.grolim

    chiral.grolim Universal Kydex Sheath Extension Knifemaker / Craftsman / Service Provider

    Dec 2, 2008
    Like another poster above, i am just a consumer, but my questions would be a) what angle did you sharpen the edge at after bringing to a zero-edge? and b) how thick is the bevel behind that edge?
    Harder blades can take and hold a thin edge better than softer blades, but for a "field use knife" you need to take into account the stresses it will encounter at the hands of the user. In my own experience, a blade <12-dps will suffer significant damage readily in "field use", whereas a blade sharpened to ~15-dps even if left 0.005" at the shoulder will be more durable because the edge is stronger to prevent deformation and fracture from occurring. That said, 0.005" is awfully thin and easily bent/fractured under excessive stress. Remember that a blade's strength/stiffness is cubically related to its thickness, i.e. a blade 0.010" behind the edge is 8X stronger than one 0.005" behind the edge. How much strength does your blade require?

    ^This.

    Here is a schematic of a few knives I own - the Izula is ~0.025" at the shoulder, the CalyIII is ~0.015, and the timos- is 0.005":

    [​IMG]

    Here is the edge of the utility blade (~8 dps) after cutting some cardboard and then carving some wood:

    [​IMG]

    Compare that to the timos- that is ~15-dps (thicker edge angle) but thinner up to the edge:

    [​IMG]

    The thicker angle prevents deformation/fracture from starting, and the thinner geometry behind the edge makes it a much better cutter than the utility blade. I don't prefer that timos- for "field use" because it is so thin and I can feel the edge flex and hear it "twing", but it gives me a standard of reference for edge-angle and durability. I have other knives that are 0.010 - 0.020 behind the edge and use them with much more confidence and they still cut quite well. Knives that are 0.020-0.030 and 15-dps like the Izula are at the "chopper" level of stoutness - poor cutters but very strong.
     
  11. SinePari

    SinePari

    906
    Oct 24, 2013
    That was a great post chiral. There should definitely be more input from informed consumers throughout this section.
     
  12. samuraistuart

    samuraistuart KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Dec 21, 2006
    I second that. Great post, Chiral!!!! Great images, too. Like us all here, I am INTO thin knives! I think the .005" edges sharpened to 15dps on these nice, hard, lower alloy carbon steels is simply awesome stuff. I wonder about taking an already thin knife to a zero edge and then sharpening a micro bevel, exactly how much use an edge like that can withstand.

    Test it and see!!! That IS the answer.
     

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