On the Bench

This is what I have on my workbench right now. I’m pretty pleased with how they came out because they’re all my metallurgy. There’s still plenty of time to ruin them, though.

The bars that I showed in my earlier post [stderr] have been hammered out into blade-shaped things, and profile-ground. Mostly, I designed the shapes in inkscape – it seems to me that knives like bezier curves – I print the design, transfer it to a piece of vinyl flooring, then transfer that to a piece of metal. The vinyl flooring trick is something I came up with for leather-working: you can bend and fold it about like real leather and it’s thick enough to make tracing a snap. Vinyl flooring is designed to last for a long time, and be flexible – so you can make a template and throw it on a shelf and years later it will still be perfectly usable. It also marks up great with a sharpie – I just scribble production notes right on the template.

I’ve also become a big fan of a diamond-edged cutoff blade, for turning a bar into a basic shape. Here’s another trick: I paint my billets with a quick shot of grey primer so I can see the outline better and so it doesn’t rust while it’s lying around the shop.

Remember this piece of steel?

I mashed it flat with the hydraulic press at about 2000F, then hammered it as flat as I could get it before I took it over to the surface grinder.

That was surprisingly little work: about 5 minutes of cutting. It’s still 1/8″ thick and needs a lot of thinning and more shaping. When you’re dealing with composite steel, you don’t get it very thin before you quench it – sometimes the inner tension created by the quench can cause pattern-welds to bend or even pop apart. If you’re familiar with a Prince Rupert’s Drop of glass, it’s a similar thing: the thinner part of the blade may have quenched much harder than the back, because there’s more metal on the back and therefore more thermal mass.

When I do the shaping, I just eyeball everything. The sweep of the little blade seemed to want to be a bit upturned, so I went with it.

That’s what’s sitting on my bench right now. When I get the forge re-assembled and can heat-treat them, I’ll mark and quench them then start seeing about putting bevels on them, and polishing the edges until they can cut. The scratch-marks and stuff like that will get polished out when the bevel is ground.

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All three blades are 15N20 and 1095 High Carbon steel, forge-welded with various transforms (mostly a good beating) from me. The middle – longish – blade is built around a core of Hitachi Blue Paper #25 steel with layered 1095 and 15N20 on the sides.

Question for the physicists in The Commentariat: What are the considerations about what makes a knife cut well?
I’ve seen a lot of claims that I think are questionable, including that the increased friction of a longer beveled edge will make the knife “stick” in the cut more.
It seems to me that what’s going on is that the knife cuts because it’s got a mechanical advantage – think of it as levering two pieces of stuff apart:

Those dimensions are approximate. But if I remember from high school (around 1975) this would have a mechanical advantage of about 3 (3:1). A shorter bevel would have less mechanical advantage and therefore would not be as good at separating two pieces of stuff. If the rise was about 1/4 of a unit, then the mechanical advantage is 12 (3:0.25) – the knife would cut much better.

Is that correct?

There are other factors: friction and whether there are significant surface material properties on the very edge of the blade. I.e.: a serrated edge or a micro-serrated edge are going to work better because they increase the length of the blade surface passing along the object being cut. Curved blades seem to cut better because they are slightly longer, but for some applications a curved blade does seem to work better – i.e.: slicing vegetables. Although I think that’s a factor of how the cutter uses the blade. There’s also the problem that some blades are shipped with a longish-seeming bevel, but the edge is ground at a sharper angle, reducing the mechanical advantage – I don’t know why anyone does that unless it’s that knife companies’ lawyers tell them “make it dull, like our customers.”

This seems to me to be one of those questions where the different sciences might give different answers. I assume a topologist would tell me that my knife is indistinguishable from a plate of pasta, or something. Knife-makers are full of stories about what makes an edge cut better or worse but to me it always seems to come back to mechanical advantage.

Unrelated: when I get some time I’m going to make some billet with 1095 that has cable damascus sides. I plan to make a pretty large chunk so if any of you are interested in a chunk of damascus billet to beat on, I’m tentatively willing to provide and even rough-cut it if you can send me a picture of what you’re looking for (please, no katanas…) When it comes to stuff like cable damascus I am now set up to produce it at nearly zero cost (I just let one billet soak in the forge while I am working on another)


  1. says

    The levering explanation sounds reasonable. However, when I think about it, I think the hardest part of cutting is not pushing the two pieces apart, but breaking the bonds. That is, you’d expect most of the work to be in the first few nanometers, and the rest hardly matters. So maybe what matters is the amount of pressure applied by the knife tip. A sharper knife applies the same force over a smaller surface area, so it applies greater pressure.

    I use knives to cut onions more than anything else, and in my experience the first failure mode of a dull knife is that it slips at the side of the onion. That seems to be a matter of how much force is needed to cut vs how much force is needed to make it slip. So I could see the friction explanation playing a role their too. The friction would probably depend on the pressure again, because applying a lot of pressure would make a small indentation in the onion, keeping the knife edge in position.

    This is just theorizing, I’m sure if I look it up I’ll find I said something ridiculous.

  2. Dunc says

    AFAIK, the definitive text on the subject of sharpening is The Razor Edge Book Of Sharpening by John Juranitch. Haven’t read it myself, but it’s the one everybody refers to.

    There’s always a trade-off between sharpness and durability. A hollow-ground straight razor is the sharpest thing you’re likely to encounter, but the edge is incredibly fragile and requires constant maintenance. The reason so many knives have a secondary bevel is to make the edge more durable while still leaving them easy to sharpen.

  3. cvoinescu says

    Not a physicist, so take this with a spoonful of salt.

    I agree with #1 and #2. It’s pressure that cuts most stuff, so you want the part of the knife that does the cutting to be very narrow. This is normally the very tip of the edge, so you want it very sharp. The shape is a compromise between keeping this edge sharp even after some wear (which would demand a very acute angle), and making this edge strong (which would demand a wider angle).

    However, there’s more. Once you’ve broken the bonds, the knife needs to be able to advance into the material. This requires the leverage you’re talking about — yes, pressure breaks the bonds at the cutting edge, but then you need to pry the material apart for the rest of the blade to follow through. Friction is important to consider, and I think the double-bevel sharpening helps with that too. This depends very much on the material you’re cutting — cheese, for instance, sticks to knives in a different way than onions. There’s also what happens after the cut: if you’re cutting slices of something, they often stick to the knife. Scalloped sides (like those on many Santoku knives) help detach the cut slices from the knife.

  4. dashdsrdash says

    You need to erode the material at the cut mark, separate the two sides so that they don’t rejoin, and keep the blade moving edgeward so that it can do more cutting.

    Material erosion comes from either levering stuff aside or applying mechanical force to destroy the bonds. Most organic things are largely held together by Van Der Waals forces — magnetism — or hydrogen bonds. Non-organics are often held together by much tougher bonds, and what you get are breaks in a crystal structure propagating out from your cutting edge. If there are existing discontinuities in the crystal, the break will stop at them.

    If you had an idealized cutting edge that was a one-dimensional wire, impractically stiff, strong and hard, you would see many materials self-heal right behind your blade edge. The body of the blade needs to support the force transmission into the edge, and wedge apart the material to prevent reform, and do so with little enough friction that you can continue to apply force.

    Moving parts have less friction than static parts, so having a cutting action that moves the blade body reduces friction of the cut material on the blade sides. So does having a smooth blade side, so polished blades cut better into deep material.

    Hollow-grinding is useful for getting sharp edges quickly with curved wheels, but it’s suboptimal for slicing thick material. A Moran grind (convex arcs meeting at the edge) is harder to get a sharp edge on but much better for reducing the friction with thick material. And a well polished flat-grind is pretty good at both thin and thick materials, but hard to sharpen evenly without forming a secondary bevel or becoming a Moran grind.

  5. says

    I’d never heard of the “moran grind” before. Interesting! I have lots of blades with that grind, and I’ve done it plenty of times.

    For aesthetic reasons, I love the chisel grind.

    In the Larry Niven Ringworld books there is a force blade that is basically a spool of molecules in a chain, held out with some kind of gravity generator. You wave it at things, and they fall apart. I was always a bit unconvinced – I thought some objects might simply let it flow through them.
    There was some book I read that was otherwise forgettable that had a memorable scene in which a boat going through a canal was attacked using monomolecular wire stretched at several spots across its path. Zing! The author imagined that the metal of the boat would heat up where the wire went through it, leaving glowing trails. It sounded mighty cool. In the case of the puppeteer force-blade, if you have focused antigravity, you don’t need a silly weapon like a blade.

  6. hackerguitar says

    >> There was some book I read that was otherwise forgettable that had a memorable scene in which a boat going through a canal was attacked using monomolecular wire stretched at several spots across its path. Zing! The author imagined that the metal of the boat would heat up where the wire went through it, leaving glowing trails.

    I think that’s in Cixin Liu’s “Dark Forest” trilogy. I remember wondering whether or not that had been copped from Larry Niven as well.

    I like the rightmost profile in the image above – makes for a nice tough edge even on marginal blades. That said, the hollow-grinds are perfect for Japanese nakiri….when those are sharp, they are the best on the planet for cooking (IMHO, YMMV).

  7. dashdsrdash says

    The variable-sword is even weirder than that — the wire is held rigid by a Slaver stasis field, which pauses the contents in time. Why they can be moved after that, I dunno — Asimov got an even more implausible murder weapon out of the same effect by concluding that the trapped matter would instantly remain in the same place of a universal field of reference and thus go blasting off at thousands of km/s relative to the motion of the Earth.

    Back to the variable-sword — I think Niven was trying to come up with a weapon that was usable in pressurized habitats, and failed. But he was also doing conservation of inventions, as he had previously written about the monomolecular wire and the stasis fields in other stories, and he had to remind readers about them in order to show off the shadow-square wire later on.

  8. says

    I’ve been partial to the Moran grind for many years now. Its my preferred. Its a good compromise between edge retention, strength, and maintainability.

    I especially love the shape of that second blade. Purty!

    That corner where the tang meets the blade, on the first one, seems a bit sharp. Ive had difficulties in the past with sharp corners during heat treating. Recommend caution on that one.

  9. says

    That corner where the tang meets the blade, on the first one, seems a bit sharp. Ive had difficulties in the past with sharp corners during heat treating. Recommend caution on that one.

    Hm, good point. So far I have been lucky. We’ll see what happens! (I’m quenching in matrix #50, which – as far as I can tell – is necromantic sorcery)

    You’re welcome to a chunk of damascus. If you have a rough idea what size you want, just let me know. Also, if you have suggestions about composition, I’m willing to listen. Right now I have a stack of 15n20 and 1095 that’s ready to chop chunks off of, and another on the way. My next project is to get my twisting jig built and test it on some cable. If the jig works I’ll make some billets that are 15n20 with cable sides. That ought to look dramatic. Also looking at raindrop pattern, depending on how long it takes me to get the milling machine up and operating.

  10. says

    I must confess, I haven’t done much non-cable Damascus, so on that front I cannot help much. I’ve made a few knives and sundries from other’s 15n20/1095 and they turned out quite nice.

    15n20 with cable sides? Sounds great! I’d love to see pics of the result.

    I’m not really ready to do any hot work yet. But soon I hope! When available, A piece big enough to forge a little drop-point hunter or paring knife would be awesome.

  11. chigau (違う) says

    A really, proper, chopping herbs on the cutting-board, kinda knife requires that some (fairly long) part of its length be flat and parallel to the cutting-board.
    I have the scars to “prove” it.

  12. nastes says

    Wow, that took me down an interesting rabbit hole, right down to fracturing forces in cutting gouda cheese and the forces involved when stabbing a person. Also, carbon diffuses surprisingly fast in steel.

    Otherwise dashdsrdash @ #4 already summed it up pretty nicely, you need to make a fracture in the material and force it apart. Slicing motion helps to initiate the initial fracturing [1].

    Just one correction: Van der Waals forces are not magnetic, they originate from static and/or induced dipole (or higher) moments of the electron clouds. Hydrogen bonds, dipole-dipole interaction, are a type of Van der Waals force. (at least that how I remember it from my university times and from what glanced over running down the rabbit hole).

    Interestingly, a smaller tip/edge diameter is more important for sharpness than the wedge angle [2-3]. I’m not sure how that would translate into durability though, which is a more important factor for me. I take a slightly less sharp knife that lasts a long time over a super sharp knife I have to sharpen after each use.

    To all the knife makers and users around here, what is your experience in that respect? Moran grind as mentioned by dashdsrdash?

    In case you want to read some of the articles here you go:
    [1] https://physics.aps.org/featured-article-pdf/10.1103/PhysRevLett.109.244301
    [2] https://www.sciencedirect.com/science/article/pii/S0013794406004073
    [3] https://www.sciencedirect.com/science/article/pii/S0013794409003191

    Not sure if those links for for you they might behind some subscription thingy, if not let me know, I should be able to send you the articles.

    Have fun,

    PS: Sorry for all the —
    How do I make paragraphs in the comments? using “p” or “div” tags does not seem to do anything for me.. Thanks!

  13. cvoinescu says

    nastes, just use any number of carriage returns (I use two, but a single one works too). Unhelpfully, they do not look like a paragraph in preview, but they do when you post.

    Marcus, I thought of you the other day and I was quite bummed out that I could not sneak a photo. I saw two Japanese swords that had been excavated from under a Buddhist temple (as well as a modern replica of the same type of sword). The blades were little more than hunks of rust — but what rust! You could see the structure: a stack of fine laminae, that made the rust look a lot like fossilized wood, but with a finer structure. Really, really impressive.

  14. says

    A really, proper, chopping herbs on the cutting-board, kinda knife requires that some (fairly long) part of its length be flat and parallel to the cutting-board.

    You mean for mashing stuff?

  15. says

    In case you want to read some of the articles here you go:

    Thank you. I could open the first one and it flew way over my head. The second two are behind a paywall but I suspect they would also fly over my head.

    It’s good to know that scientists are on the job regarding this matter. It seems like it ought to be an important topic, given the prevalence of Cutting Stuff in human history.

    Mostly, I’m just glad to see that there are theories that explain cutting – it’s a lot better than the “let me experiment with chopping frozen milk with a katana” youtube vifdeos.

  16. nastes says


    Yeah, it seems the food processing industry is quite interested in having decent blades to chop things and not having their machines go *clonk* too fast.

    Not surprisingly there is a whole bunch of journals out there on materials and material processing and somehow I like that the following is a real thing:
    “Wear” (An International Journal on the Science and Technology of Friction, Lubrication and Wear)

    Have fun,

  17. says

    “Wear” (An International Journal on the Science and Technology of Friction, Lubrication and Wear)

    Oh, man, I gotta see how much it costs to sign up for that.
    A bunch of years ago, I joined the Society of Forensic Toxicologists. Because they were having a conference at the same hotel as a security conference. Their journal was really interesting and they had awesome polo shirts, too… After that, my cat joined an international arms-dealer’s conference (I wanted to see what kind of mail she’d get) but then they started charging a lot of money for the membership so she decided to stop participating.