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Thread: This might interest serious sharpeners - free SEM imaging

  1. #1

    This might interest serious sharpeners - free SEM imaging

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    It looks like ASPEX is now offering free SEM imagery of whatever you send to them (limit of 2 samples). It might be really cool to see a very refined edge under a SEM.

  2. #2
    Join Date
    Nov 2009
    Mapleton Illinois
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    That is cool!

    My company has an electron microscope in the local metals lab - it is a Hitachi unit. When I hit the lotto, I'll have one!!

    The cool thing (to me ) is that you can put a piece of metal in the scope, then focus in on a spot, and with the software the scope will show you the base elements of the material and the percentages etc - for a knife nut, that is just too much for words!

    Thanks for the link - I hope some of the folks on here send some samples in (not sure how you would send a sample of a knife in - but I am sure someone will figure that out)

    Now - back to that web site to browse the images -



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  3. #3
    Quote Originally Posted by Magnaminous_G View Post
    It looks like ASPEX is now offering free SEM imagery of whatever you send to them (limit of 2 samples). It might be really cool to see a very refined edge under a SEM.
    Wow...!! Thanks for the heads-up.

    If they are able to take SEM pictures looking directly into the edge (the same as Prof. John D. Verhoeven), then they may be able to measure the actual sharpness of your knife edge. That would be amazingly cool... With modern ultra-fine abrasives, do you think your edge is sharper than 0.4 microns? Now you can find out!

    Last edited by Lagrangian; 07-21-2012 at 10:26 AM.

  4. #4
    Join Date
    Jun 2010
    Upstate, NY
    The father in law came up with a method for measuring edge "sharpness" using a SEM, a microtome, and some silver solder. Make a fresh cut across the end of the solder with a microtome, press your edge into the solder, making a wedge shaped indentation. Make another skim cut across the end of the solder with a fresh stretch of mictrotome blade, cleaning up the distortions from the indentation. Now you have a near perfect negative of your cutting edge that can be micrographed and measured very accurately.

    I'm actually a little more curious how a very coarse but well finished edge might look. You can get a very good look at a refined edge using an optical microscope - the limited depth of focus makes it impossible to do the same with a coarse edge.

    I'm also pretty sure Verhoeven used very obtuse edge angles for his images, a more acute edge might not take as nice a picture edge on.

  5. #5
    Hi HeavyHanded,

    I guess the point is solder is very soft and ductile, but has almost zero elasticity? I don't know, but I would be concerned about the following: if the solder has any degree of elasticity, then it may "spring back" slightly after the knife edge is removed. If you want to measure sub-micron features on the knife edge, it would not take much for this to affect the results. I don't know if this is an issue. But if it is, I imagine it would also affect the surface texture, such as scratches etc. The other thing I don't know is how pure the solder is; if it has various inclusions and impurities which might show up, but not actually represent the knife's surface. In practice, it could be neither of these are problems, but I think they would have to be tested for.

    As for Verhoeven's electron microscope pictures: On page 5 of his tech report, Verhoeven says,"The majority of the experiments" in the report were sharpened to a included angle of 44 degrees. Most of us here use something around 30 degrees, but I don't think the 7 degrees per side affect his images too much. One incredible strong-point of the SEM is a fantastic depth-of-field, which is why most of his images can look directly into the knife edge. If you look at his pictures, even parts of the knife more than 10-20 microns are in focus; and this distance is in the plane of the photograph. If you do the trig for 22 degrees, then that represents a depth-of-field which is at least 24.8-49.5 microns, if not much larger.

    By contrast, for an optical telescope, the depth-of-field is a fraction of a micron; for an objective at 100x, it might be as small as 0.2 microns. See the column labeled "Depth of Field" in this table from Nikon (this is for the magnification of just the microscope objective):

    Here's another chart for the depth-of-focus as a function of Numerical Aperature of the objective. For high magnification, the numerical aperature will approach 0.75 to 1.0 (or higher if you use immersion) for an optical telescope. At those ranges, the depth-of-field is around a micron or less. If the sharpness of the knife is already 0.4 microns, then your depth of focus is not much bigger than the roudness of the "ridge" you want to measure, which can make things difficult.

    Verhoeven mentions on page 2, that "One of its [the SEM's] outstanding features is that the depth of field is much improved over the optical microscope, on the order of 300 times better. Hence, the SEM is capable of providing clear images of the edge of sharpened knives at magnifications up to 10,000x." (Brackets mine.) So SEM's have a huge advantage over optical microscopes in depth-of-field. This is why the SEM can look directly into the knife edge and give useful images.


    P.S. If you unfamiliar with Verhoeven's electron microscope pictures, here is a random example of one. If you're confused, just note that there are three images. In the first image you are staring directly into the knife edge, which runs diagonally from top-left to bottom-right. This means both of the two bevels of the edge are visible and on the two sides of the edge. You can think of this as looking straight down on a mountain ridge. The next two images are from two the sides of the knife, so in each image you only see one bevel of the edge.

    From these types of images (and others) Verhoeven was able to measure the sharpness of a modern razor blade at about 0.4 microns or so. Please note that this is a Gillette razor blade, so the included angle is relatively small (maybe 15-19 degrees inclusive), so within the plane of the image, the depth of focus appears to extend less than 10 microns to each side of the edge.

    Prof. Verhoven's tech report:

    Nikon's microscopy web page:
    Last edited by Lagrangian; 07-23-2012 at 12:50 AM. Reason: P.S. and P.P.S

  6. #6
    Join Date
    Jun 2010
    Upstate, NY
    From the FinL's point of view, the problem is that deciding exactly where to make your lines on an image is somewhat subjective. Stylus type measuring devices in his days weren't capable of measuring that small tho they might be today - I don't know. You need a cross section of the blade to be able to make a truely accurate measurement, hence his solution. Software advances etc might render such methods unnecessary, I honestly don't know.

  7. #7
    Current atomic-force-microscopes (AFMs) and scanning-tunneling-electron-microscopes (STEMs) can surely see the knife edge because they literally have atomic and/or sub-atomic resolutions.

    In both cases, a needle is moved over the surface, where the needle is so sharp it literally has a single atom at the tip. Basically, the needle is adjusted over the surface or just rides on top the surface and takes a bunch of height measurements. You could think of it as an airplane which is continuously bouncing radar off the ground to get altitude measurements. These measurements are then compiled together by computer to form a 3d topographic map of the surface. For AFM's they mostly just ride the needle over the surface, sort of like a phonograph. To measure the tiny deflections of the AFM needle, they bounce a laser off of the needle to create a very long "optical lever". The optical lever effectively magnifies the deflection and then the reflected laser spot be tracked using standard light detection methods, such as CCD, CMOS, photo-multiplier, etc.

    For STEMs, they use quantum-tunneling of electrons which is extremely (exponentially) sensitive to the gap between the needle and the surface. The STEM needle is typically positioned using piezo-electric actuators, and then the tunneling current is measured. Both the piezo-electric actuators and the tunneling current are controlled/measured with extremely precise electrical instruments.

    The AFM and STEM needles can even be used to pick-up and drop-off individual atoms, which is what IBM and many others did for fun.

    Outside of stuff like AFMs, and STEMs, there are now Coordinate Measuring Machines and Optical Micrometers with sub-micron accuracy. I believe research-grade versions of these could measure the sharpness of a knife edge, although I'm not completely sure. Similarly, modern profilometers probably can measure it as well, is my guess. I am very impressed, in recent years, by the insanely progress of rapid and high precision measurement in all areas (imaging, length, time, electrical properties, etc.).

    Current computer vision software has gotten very good. Not perfect, but very good. It is being applied to measure things such as carbide distributions in metallography, and things like distributions of blood-vessels and tumor size in medicine. In manufacturing, it is used to check that parts are within specification by optical measurements, and to establish alignment in pick-and-place machines for electronics.

    Last edited by Lagrangian; 07-23-2012 at 12:49 AM. Reason: added some explanation to AFM and STEM

  8. #8
    Join Date
    Apr 2012
    SB, CA
    I wonder if a researcher can modify an ultraviolet phase doppler or dynamic light scattering to measure a knife edge instead of particles/proteins. Perhaps using multiple receivers and sampling angles, a 3D image can be splice together.

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