It is freezing here and I still do not have the slightest inclination to do something useful. But I need to heat the workshop occasionally to prevent it from completely freezing. Not that it would be super bad, except that maybe ice forming in the cooling receptacle near the grinder could damage it. Anyhoo, yesterday was a workshop heating day. I could not do anything super useful – it took me hours to raise the temperature to a bearable level and soon after I stopped feeding the stove, the temperature got down quickly again. But I could use the time to do something small and simple so I have made a device for measuring knife sharpness (I did not invent the concept, I saw the principle somewhere on the interwebs sometime ago).

© Charly, all rights reserved. Click for full size

It is simple and consists of two main parts. One part is a board with four legs and a 35 mm hole in the middle. A tiny table that can be put over my kitchen scales.

© Charly, all rights reserved. Click for full size

The second part is a small wooden cylinder with a cutout and two screws on each side of it.

© Charly, all rights reserved. Click for full size

I can span a thin thread between the two screws and when the cylinder is put on the scale through the hole in the middle of the tiny table, I can cut the thread safely for me, the scale, and the furniture.

The main downside is that my kitchen scale does not have a “Hold” or “Max” function so the measured values are not super precise. Another problem is the used thread – a very thin fishing line would be probably better since this one has a tendency to get damaged during spanning. Or perhaps a very thin copper wire – I might try to extract some strands from leftovers from speaker or ethernet cables. But when being very careful with spanning the thread and doing the cut slowly and carefully, the setup gives useable results and I did learn some things.

Here is a boxplot of 10 measurements with the three cutting implements in the photos – a fresh razor blade, a paper-cutting-but-not-shaving-sharp sharp knife, and a blunt table knife (I used the non-serrated part which is about 1 mm thick),

© Charly, all rights reserved. Click for full size

The razor had an average cutting force of ~6 g, the sharp-ish knife ~60 g, and the blunt knife ~ 1000 g. The less sharp the blade the bigger the spread but even with sometimes ridiculous outliers, there is a definitive and statistically significant difference between these three and it does give me some information and opens future opportunities. I would like for example to examine the relationship between sharpness and sharpening angle, to get some hard data to back up my opinion that anything between 10-30° works just fine. My prediction is that the relationship is not linear and as the angle gets steeper, the cutting ability gets exponentially worse.

I did learn one thing – my “Not a Masterpiece” was sharpened at a 20° angle and the only knife that actually scares me – the bigger knife from the two-knife applewood set – was sharpened at 15°, yet both measured within the same range as the razor. Although the applewood knife completely failed to register on the scale one time, thus my suspicion that it is the sharpest knife I made so far might be true.

Of course this only tests edge geometry, not blade geometry. I could use a similar setup to test the influence of blade geometry on cutting force too, but I do not have a reliable medium to do so yet. All things that I have thought of so far are either expensive (cork, rubber, silicone) or have highly inconsistent properties (fruit & veggies). But maybe I will think of something to test blade gomtry too.


  1. flex says

    That’s a pretty clever setup. I’ll have to remember that. Not that I make knives, but the idea may be useful.

    I wonder what variation you would get with different materials being cut? It’s possible that, for example, a fine copper wire has enough resistance that when it does part it does at a similar level of force across a number of different sharpnesses. I obviously don’t know. You’d want a material which has enough resistance to show good separation between the levels of sharpness, which means it isn’t too easy to cut but not too hard either because then it may break before cutting. You might find that the best material could be jute or coir.

    That’s an interesting question. If you investigate further I’m certain we’d like to hear about it.

  2. lochaber says

    damn, that’s pretty clever.

    I wonder if you could use something meltable/reusable like wax to test the edge geometry, so that expense would be less of an issue. I imagine common paraffin wax would be a bit too hard, but maybe if blended with something like petroleum jelly or mineral oil to soften it? Although, I guess their would be issues with heating and remolding, as the composition would likely change somewhat, and wouldn’t be as consistent from test to test… And then ambient temperature would likely cause issues as well…

  3. kenbakermn says

    That’s a pretty good test design. One thing I wondered, how sensitive are the resuts to the tension on the thread? Do you have a way to control that precisely?

    I designed something similar years ago when I needed a test to compare the sharpness of hypodermic needles. I used a block of silicone-ish substance -- don’t remember exactly what it was -- on a scale.

  4. says

    A thin thread will not take into account edge-binding from the flat of the blade. I’m not sure how much of an effect that has, but a lot of knifemakers now do weird S-grinds and textures to keep their sliced tomato from sticking.

  5. says

    @flex, I do not know about the variation regarding: different materials, but part of the wide variation in this series of measurements is that the thread is not homogenous. I have tried thicker linen thread and that had the same problem. That is why I think that old-fashioned silon fishing line would be better because in that case, the thread is a continuous piece of material of constant thickness and uniform properties. That is also why I think that a copper wire might work too. I do not angle fish and I do not know where to get a fishing line around here -- it is definitively not worth ordering it online just for tests.

    @lochaber using malleable/reusable material sounds interesting. Possibly ballistic gel, which can be easily made at home and eaten when done with. But I am not sure I would want to go through the hassle of preparing it, that sounds like too much work.

    @kenbakermn, tensioning of the thread has a huge impact, especially for blunter knives. It is in my opinion the main reason for the values being more and more spread out the blunter the blade gets, thus I have made ten measurements per blade. Plus, as mentioned in the article, sometimes the thread gets damaged during spanning. I had to discard about two measurements for each blade that were obviously botched (the thread either ripped way too easily or slipped). I am considering some modifications that would still hold the thread in place without tensioning it too much whilst avoiding slippage. And if I do not find a better-suited thread, I might try to infuse this one with wood glue to get it more sturdy and less prone to tearing or damage during spanning.

    @Marcus, exactly, thus the last paragraph. I do hope to come up with some ideas for how to test the blade geometry/crosssection too. Although for that a simple max value measurement probably will not be ideal, a complete force curve would be better. But that is not possible, for that I would need a force testing bench, which I do not have and do not have an intention to build because even building one would be a ridiculous amount of work and huge costs for very little use.

    I do miss my previous job sometimes, I like testing things.

  6. Tethys says

    Most hardware stores carry fishing line in the US, but maybe you could use 100% nylon upholstery thread from any sewing store? Most of my local crafting/sewing supply stores sell fishing line and beading strings in the jewelry making department.

    Our weather has also been gloomy and bitterly cold for about six weeks. I can confirm that it saps my energy, and makes me wish I could just hibernate until Spring.

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