Workshop
See What Happens When You Sharpen a Knife Under a Microscope
pocketknife_full
Our subject: A knife ready for a new edge.

Anyone who has worked in a kitchen, or any other job that regularly interacts with knives, knows that sharpening is a very important skill. The ability to put a fine edge on a knife allows you to have better working tools at your disposal. Not only is it pleasant to be able to slice and dice easily, it is also safer. Sharp knives are more predictable and there’s less of a chance of you losing control of your blade when overexerting in an attempt to push a dull edge through something.

Typically, sharpening your blade with a kit involves three major steps. Each involves rubbing the blade along a stone meant to grind off imperfections on the surface, which are often called burrs. The steps begin with a very rough stone and work toward a finer and smoother stone.

Knowing the details and results of a process can often help you become better at that process. To help with that, we’ve taken some images with a microscope to show the effects each step has on the blade.

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A fairly standard sharpening kit

Here is the raw blade. Though this knife has been sharpened in the past, it is ready for a fresh edge.

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Typical usage marks

See how there are lines going in many different directions? This is typical after use. We ultimately want to end up with a nice uniform series of marks on the blade.

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Note the nearly flat ridge

Here, you can see the light catching the leading edge of the blade. It is nearly flat, and reflecting light back to the microscope.

The Rough Stone

The first step involves the roughest of the sharpening stones. We’re basically shaping the entire cutting area of the blade here. This ensures there is a uniform angle along the entire blade, which will help in the next steps. This also saves considerable time. You could skip this and go directly to the next stone but shaping the blade would take much longer.

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The shaping stone

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The Honing Stone

The actual sharpening of the cutting edge begins here. Our bevel, which leads to the cutting edge, is getting smoother. The edge itself is being reduced down to a foil thin point. This extremely thin edge curls a bit since it is thin and fragile.

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The honing stone
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Note the thin curl of metal at the edge of the blade

The Polishing Stone

The final step smooths out the blade and removes that tiny curl on the cutting edge. What you’re left with here is a nice sharp edge and visibly aligned marks on the blade.

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The polishing stone

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6 thoughts on “See What Happens When You Sharpen a Knife Under a Microscope

  1. I’ve often wondered how useful it is to go the last mile in sharpening. Toward that end, I would like to see what happens under a microscope to a super sharp blade and just a sharp blade after one use, say five passes with a plane or whittling a piece of wood for a minute.

  2. Look up Experiments on Knife Sharpening by John D. Verhoeven for some nice scanning electron microscope micrographs of blade edges in various states of sharpness and great information on sharpening.

  3. What an excellent idea to share this insight with this community – very bloody interesting, thank you!

  4. Another thing you can do which will help the edge stay longer, and it may sound ridiculous, is to polish the edge. Consider what these micrographs show, though: an unpolished edge is essentially a microscopically serrated blade, and serrations are easy to break down. If the edge has no scratches in it, practically speaking, damage is less easily done.

    Also, after sharpening (polished or not) steeling or stropping before each use will also keep the edge longer.

  5. Interesting photos/videos on knife sharpening. I am on
    bit of a makers mission to develop my own device to sharpen most ‘edge’ tools –
    knives, chisels, plane blades, drill bits etc. At the moment it is a belt
    sander with VSD speed and direction control with a growing assortment of
    attachments.

    I felt there was a need to work out what sharp is then I
    would probably be able to achieve it. I eventually found the
    definition on a knife sharpeners website (lost the link – sorry). It is
    the best I could find and contains the absolutely crucial and often
    misunderstood aspect that there are always two surfaces involved! With
    this in mind sharpening things like hand saws, chainsaws, drill bits becomes so
    much easier to understand and be successful at.

    Definition of sharp:

    “Two flat, smooth surfaces that meet at zero radius that have an intersecting angle
    and edge thickness suitable for the intended purpose”.

    Looking more closely:

    There are always two surfaces involved. On a knife this is pretty obvious, however on scissors, a chisel or plane blade there are also two surfaces, it’s just that some people don’t recognise the back of a scissor, chisel or plane blade as being part of
    the edge, just the bevel. No cutting tool will be truly sharp unless both
    surfaces are dealt with equally.

    The surfaces must be flat and smooth. Flat and smooth surfaces produce less damage when cutting, they also give a more accurate cut. Flatness is a function of the sharpening process and the method used to move the edge against the abrasive, the more accurate this is the flatter the surface. Smoothness is a function of the abrasive grit size which can be measured in grit (ANSI or JIS) or microns.
    Grits of 1600 ANSI grit (about 5000 JIS grit or 2.8 micron) will produce very smooth surfaces and finer grits will produce a mirror like finish and an extremely sharp edge.

    The surfaces must meet at zero radius. This means that there is not the slightest rounding where the surfaces meet. Any sort of radius indicates a degree of bluntness and the larger the radius the blunter the edge is. This is what happens as edges wear, the meeting point that was an apex becomes rounded, that is blunt.

    The angle and edge thickness are suitable for the intended purpose. Angle and edge thickness work together and both must be correct if the edge will perform its intended task.

    A very acutely angled blade will be very sharp however it
    will not be very strong and may chip depending on the angle and blade
    composition. The ideal is to use the most acute angle that will not fail in
    normal use.

    A 40 degree included angle on a 1.5mm and 4mm thick blade
    could be equally sharp in terms of angle and surface smoothness. However, the
    1.5mm blade will be able to easily cut a tomato whereas the 4mm blade will be
    more like a wedge. Edge thickness is often never considered when sharpening
    knives leading to “the more I sharpen the blunter it gets”. This phenomenon
    is due to repeated sharpening dealing only with the cutting bevel which
    effectively increases edge thickness over time. Edge thinning is an important
    part of sharpening. John Juranitch in his book “The Razor Edge Book of
    Sharpening” says that for knives the edge thickness should be .02 inch
    (.5mm) .25 inches (6mm) behind the cutting edge.

    Happy sharpening.

    Paul Saunders

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Senior Editor for Make: I get ridiculously excited seeing people make things. I just want to revel in the creativity of the masses! My favorite thing in the world is sharing the hard work of a maker.

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Adam Spurgin

I'm a software engineer with too many hobbies and a firm believer that there's not enough hours in the day.

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