Engineering Question: How To Measure Sharpness

I want to measure the sharpness of a razor blade, for reasons. I have two ideas so far:

  1. Spring based force measurement device that somehow is attached to weights or the razor blade and pushes the blade into rayon thread. Jot down the number of Newtons, or whatever, it takes to finally slice through that thread. The lower the force required, the sharper the blade.
  2. Use a printer to push paper in to a clamped blade and under a microscope see how clean the cut was on the paper. Sadly this does not result with a number, which would be more ideal.

Neither of these ideas are great. #1 is bad because it would be hard to use for comparing different blades. I would need to apply the exact same amount of force, in the exact same increments, and keep blade/thread at the same angle. It's not a realistic experiment without precision machining. Meanwhile, the printer idea although easy to setup for consistency, does not provide a numeric outcome.

So ... I am wondering if any of you actual engineer/science people have thoughts on this.




  • HeroHero formerly known as XGPHero

    not an engineer, but im thinking that with a powerful microscope and a proper cross section of the edge in question, one would be able to measure the thickness of the point where the edge starts to become "blunt"

  • HeroHero formerly known as XGPHero

    something like this maybe?

  • midgamidga "There's so much hot dog in Rome" ~digi (> ^.(> O_o)>

    I would actually recommend looking into hardness testing (like a Rockwell test) but utilizing the blade you want to test instead of a hardness testing tool. Basically, you're going to want something that can lower a blade onto a fixed material at a consistent force, or something that can lower a material onto a fixed blade at a consistent force. The issue you're going to run into is equipment cost. If I know whether you're looking into serious equipment or just doing stuff for shiggles, I can make much better recommendations.

  • HeroHero formerly known as XGPHero

    @midga said:
    I would actually recommend looking into hardness testing (like a Rockwell test) but utilizing the blade you want to test >instead of a hardness testing tool. Basically, you're going to want something that can lower a blade onto a fixed >material at a consistent force, or something that can lower a material onto a fixed blade at a consistent force. The >issue you're going to run into is equipment cost. If I know whether you're looking into serious equipment or just doing >stuff for shiggles, I can make much better recommendations.

    I think that would probably be the simplest method for field testing, but the answer would be a relative index value, rather than a subjective numerical value (those are made up terms, but i think appropriate) so i think you would have two methods for different requirements.

    adding to my suggestion, you would also need to factor in the angle of the 2 edge surfaces somehow.

  • @midga I'm in it for the shiggles, I already spent my budget on this (see below). I could probably spare another $50 at most.

    @XGPHero Yeah I am buying a usb microscope that comes with measuring software. However, I was hoping to measure sharpness as well as blade size in order to properly establish the relationship between the two.

  • HeroHero formerly known as XGPHero

    then i think a more appropriate term might be effectiveness, but thats me. i would use both midga and my method together for that. you can measure angle and width of the edge as i described to quantify the edge, and then the material test as midga suggested to qualify the edge...hope that makes sense

  • I think your string theory can be useful if you can add some consistency. Use the same ball of thread. Use a reference set of blades and establish that they have consistent results. Like, say, 20 samples from each reference blade, and each new blade you are testing. You should normalize the results based on the reference set then.

    If you find a force measurement device that can record the maximum it reads before reset, I think you can set this up.

    Its not ideal from a pure materials stand point, but if you are looking for an indication of something actually noticeable, this may find it for you. Thread matches hair more closely than paper.

    Most indicators of sharpness I have seen in industry are microscope measurements of the tip radius like @XGPHero mentioned. The setup I mentioned is a "bare bones" style engineering test I would use to evaluate a product. I can go into more detail of how I'd set that up to be meaningful (ish) if you'd like.

  • HeroHero formerly known as XGPHero

    perhaps a homemade ballistic gel or something, ensuring to only push and not slice to eliminate the added variable of serration...

    PS: i love stuff like this...really stretches out the grey matter

  • @drasnor said:
    What's the objective? From a purely materials point of view, the ability to cut with "sharpness" alone is dependent on the area of the blade in contact with the medium to be cut through the mechanism of contact pressure causing local stresses in the material in excess of its yield strength.

    I want to be able to buy razor blades that I shave my face with, and find some meaningful measurements of quality besides "this one feels nice, this one cut me" because that is useless opinion. I can easily perform tests to measure blade durability with rusting agents and some time, and spatially measure them under a microscope. But acquiring a numeric measure of actual sharpness is difficult.

    The larger objective is an experiment in which I see if I can create a useful business out of differentiating commodities that traditionally consumers find impossible to actually develop measurable differences in. This is something even consumer reports sucks terribly at (in my opinion), on top of them using an antiquated business model. Wirecutter does an OK job, but I believe I can do it better because they are still ultimately just trying to get affiliate revenue and have ulterior objectives besides honest data regardless of what they market themselves as.

    Razor blades will be my first test commodity.

  • The issue I am having with push tests is finding a way to apply the exact same force increments. I don't have a precision machine to do this for me. I thought about using a vice or a printer, but neither of those quite fit the bill. For the material I want to push on, I was going to buy thick rayon thread and use it throughout. I believe it would be a good material because it is consistently woven unlike more organic materials (cotton ish stuff).

  • HeroHero formerly known as XGPHero
    edited February 2015

    then might i suggest meauring sharpness in every way you can think of, since different applications will make some aspects more imortant than others... parameters i think would be important both separately and together:

    angle of edge
    tip radius
    hardness of material (for durability)
    quality of cut (clean or rough)

    im sure there are others.

    PS: for razor blades specifically, i have usually heard people refer to the angle and the tip radius as being most crucial to a good shave, though the best combination varies by skin type.

  • I think you could make a pull test that uses a threaded rod. Requires a bit of doing.

    Here's my 2 minute sketch of such a device.

  • MyrmidonMyrmidon Baron von Puttenham California

    I'm in favor of the microscope approach, followed by the string approach.

    Cheaper than buying a USB scope:

    For the string approach, buy a wig and do the test statistically to rule out hair inconsistency. Use multiple hairs, if you like.

    There are cheap ways to create a scale; namely find a spring and a ruler, then record the experiment on your smartphone and retroactively make the measurement. I like Ryanfodder's drawing, personally, but I imagine you can rig something using hanging weights as well.

    There are, however, other ways to measure effectiveness of a razor that you will be less able to explain scientifically (e.g. with a 'sharpness' or 'serration' measurement) but possibly more applicable to your situation - done by simulating the application. We commonly do these types of tests where I work because the stuff we play with is so hand-wavy and complex that it's prohibitively expensive to evaluate every variable, and because we usually only use each 'part' for one-and-only-one application. For instance:

    Map out your leg, chest, belly, ass, whatever in discrete 2x4 inch areas. Use as uniform skin as possible. Count the number of hairs in each area. Scrape each area with a separate razor blade and record the number of hairs removed per scrape. This metric will give you exactly no information about the inherent 'goodness' or 'sharpness' of the razor (because the data is still intertwined with your shaving technique, coarseness of your hair, etc), but will give you an idea of the number of scrapes per razor required to get your manfuzz down to a manageable level, which is a hand-wavy measure of a situational efficiency.

    After three scrapes or so, use your microscope and a micrometer (or a bright directed light and a little math to determine the length of the shadow) to find the height of five or so (possibly more; I doubt you'll hit statistical significance but that's what you should aim for if you're furry enough) separate hair stubs still in the shaving area to give you an 'initial height' of the remaining hairs. Scrape again. Re-record the height. Continue until the heights no longer decrease and record the %decrease for each scrape. Again, gives you no information about the blade, but gives you information regarding the effectiveness of the blade for your given application.

    Now, I'm just sitting on my ass belting this out as fast as I think of it, so this is a pretty rough experiment (don't crucify me @drasnor). I'm sure with a little consideration, you could refine this very rough experiment to eliminate variability (hair length, different parts of the body, error in length measurement) and actually create a good simulation... but yeah, the only reason I mention it is because simulated metrics can be easier-to-obtain without sacrificing too much viability compared to empirical metrics.

  • MyrmidonMyrmidon Baron von Puttenham California

    I would note that simulation is also usually done in the field of avionics, where lawsuits can come from failed product - Rockwell-Collins chucks its radios in environmental chambers that cycle from absurdly low to absurdly high temperatures in varying humidities... and lets them sit for hundreds of days at a time. Low maintenance test.

  • Creeperbane2Creeperbane2 Victorian Scoundrel Indianapolis, IN

    Given a razor would most likely be used on a draw cut/pull (Depending on if it is to be used for cutting or shaving) have you considered using a cutting medium and just slicing into varying thicknesses of it, the thickness that it struggles with would be it's upper limit.

  • @Creeperbane2 good idea but I think that is a better measure of friction and it would also require solving the complex mechanics mentioned above, regarding equal incremental force application to every single blade I test.

    @Myrmidon Good thought. I used to be quite handy with extendsim and discrete event simulation, but even with a simple data set + usage of Excel I'd be getting too far in to this. My target market will likely not comprehend simulation results well enough to give them confidence in the data they are looking at. However, if I was producing this dataset for engineers this would be a good path to investigate.

  • Given the feedback I'm settling in to an idea/device that I am capable of putting together and is cheap. A basic drug dealer scale, synthetic hair, and a pully. I'll basically pull the hair down towards the scale where the razor sits facing up and have a camera recording video as I pull. I can then stop recording when the hair cuts, and review the exact moment on my computer to get the force reading from the scale. It should cost like $20. Scientists could rip this apart, but for my purposes I think it will do. I'll do about 3 runs per blade to get a simple mean of the force required. This along with the microscopic pictures and measurements should cover my mental cost of entry in to the consumer's mind as a legit source of data.

  • MyrmidonMyrmidon Baron von Puttenham California

    Whoops! Somehow I thought you were just doing this for the hell of it.

  • I think that is a pretty decent method. Simple is good. You can easily to a lot of runs and get some good statistics to prove to yourself that it is consistent.

  • @Myrmidon said:
    Whoops! Somehow I thought you were just doing this for the hell of it.

    I am, but with hopes that maybe I can turn it in to something. Most of all I need a hobby besides work and take care of baby for my own mental well being.

  • drasnordrasnor Starship Operator Hawthorne, CA

    @CannonFodder said:
    I think that is a pretty decent method. Simple is good. You can easily to a lot of runs and get some good statistics to prove to yourself that it is consistent.

    Agreed. Consistency is key and you'll be able to do enough trials to get a respectable sample size for your statistical analysis if everything is cheap (including time).

  • d3k0yd3k0y Loveland, OH

    Some of the Modern Marvels shows I have seen usually use something along the line of a stack of thin paper strips where the knife/blade is run along the top by a set distance with a set amount of force, and the blade needs to cleanly cut through a certain number of pages for the blade to pass sharpening tests.

    Mount the razor in a jig, use a springboard under a stack of paper strips pushing it up into the blade with a certain force, then drag the paper/razor jig about an inch or so, see how much paper it cuts.

  • I ordered all the parts to do this last night. I'll keep you guys updated :buck:

  • HeroHero formerly known as XGPHero

    what happened with this?

  • edited May 2017

    Hey all!
    I found a lot of interest in this as well and recently have been putting something together. There is a lot to explain so you can check out the following site and follow the links for more information and even a video demonstration.

    (link removed until you come to Detroit and spend the weekend at ICHQ or crash on the couch)

  • primesuspectprimesuspect Beepin n' Boopin Detroit, MI


  • Phew, yeah. I aced this project didn't I?

    Don't have kids.

  • primesuspectprimesuspect Beepin n' Boopin Detroit, MI

    Is that a vibrator?

  • edited May 2017

    Showing your age, wired vibrators.
    (edit: usb microscope, digital scale, some razor stuff. I threw out the other materials I got for this project maybe a year ago.)[I suck.]

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