How to measure the thickness behind the edge???

Discussion in 'Knife Reviews & Testing' started by miso2, Jun 1, 2020.

  1. miso2

    miso2 Gold Member Gold Member

    Nov 19, 2014
    I am looking for a good method to measure the thickness behind the edge.
    It is because I am planning to investigate the relationship between the blade geometry and sliceyness myself.

    It is not hard to measure it with a micrometer, when the edge is thick.
    But below 0.01", it becomes quite difficult to put the micrometer to the right place, partly due to my presbyopia.

    So, I tried an alternative method.
    I took pictures of the edge from above along with a ruler using my iPhone at the highest zoom.
    (I first verified the accuracy of the ruler using a certified microscope reticule slide.)

    The ruler was lifted to a similar height to the edge bevel.
    The focus and exposure were adjusted to the ruler and fixed.
    I used the burst mode to take 10 consecutive images.
    Then, on the computer, I aligned the images and sum them to intensify the signal (using ImageJ freeware).

    With right lighting and focusing, I could clearly see the transition points from the edge bevel to the primary grind.

    [​IMG]


    Then, I determined the length of a millimeter (or a few) in the image and computed the "thickness" of the edge bevel.
    The values I got were quite consistent with those I previously measured using a micrometer, at least for thick edge knives.
    With this method, I can measure very thin edges at 0.003".
    You just need a camera, a holder to keep the camera above a blade, a ruler, and a platform to lift it.

    I think it works for now.
    But I have seen microscopic measurements of edge thickness using some kind of molding.
    That looked great and must be more accurate.

    Anyway, if you know a better way to measure the thickness behind the edge, please let me know.
     
    bucketstove likes this.
  2. JimMD

    JimMD

    575
    Dec 6, 2017
    It looks like you're measuring the length of the edge bevel. If you presume your edge bevel length is correct, you could calculate the thickness behind the edge from this and the edge bevel angle if you know that, although if there's a microlevel that's more complicated.

    One method I suggest considering is to set a fixed reading on the micrometer and apply it to the edge by pushing it onto the edge as far as it will go at that setting. Keep changing the set width on the micrometer and repeating this until you see the calipers extend to immediately past the edge. That should give you the thickness. The hard part is seeing the edges of the calipers in relation to the start of the edge bevel, but magnification can help with that if you need it.
     
  3. miso2

    miso2 Gold Member Gold Member

    Nov 19, 2014
    It is actually the distance between the transition points, where the primary bevel meets the edge bevel.
    That is the thickness behind the edge.
    The images were taken from the above the edge as shown in the illustration.
    The red line indicates what was measured.

    Untitled-1.jpg
     
  4. JimMD

    JimMD

    575
    Dec 6, 2017
    Yep, that illustration's correct.
     

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