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  • Standard for sharpness

    Discussion in 'The main mechanical design forum' started by Design4work, Jan 23, 2014.

    1. Design4work

      Design4work New Member

      May 2011
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      Hi All,

      We had a discussion about different standards around the world and were wondering if there is a standard for sharpness. I know there is a standard for measuring sharpness, but couldn't find any on sharpness.

    3. dynosor

      dynosor New Member

      Jun 2014
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      Patrice, you don't state sharpness of what; a knife, milling cuter or lathe tool?

      Why not develop your own sharpness metric if one does not seem to exist?

      If sharpness of a blade, it would be a question of:

      1. the angle approaching the cutting edge - the more acute, the sharper the edge.
      2. thickness of the blade - thinner blades are already sharp - think cutting yourself on a piece of foil or shimstock.
      3. edge radius - smaller radii are sharper unless the "face" has two sharp corners.
      4. quality of the edge - raggedness helps cut soft materials like tomatoes in the kitchen, but raggedness is bad for shaving your beard.
      5. Performance - if you apply a standard load to a standard length of the blade, it should generate a known pressure and sink into a known test medium a certain distance - much like Rockwell hardness testing standards work, only in reverse for sharpness.

      With bright steel blades, one way to evaluate sharpness at a glance is to see how much light is reflected off the edge - narrower edges are hard to see and are therefore sharper. Consistent level of reflection off the edge is a good indicator of sharpness when it is consistently low. A bright "knife edge" is usually dull, while a lot of variability in edge reflection may indicate high raggedness - good for the kitchen perhaps.

      The rule of thumb is if you can see the cutting edge of a knife with the naked eye it is not very sharp. If you use 100X magnification with a light source positioned to reflect off the cutting edge and a scale of some sort, you could certainly quantify the sharpness.

      If a blade will shave hair off your arm it has a very sharp and consistent cutting edge - that answers the question, how sharp is sharp?

      As a student intern working at a hospital workshop I sharpened osteotomes (chisels) made from 303 stainless steel so that they would shave arm hair. I did this by sanding on a surface plate up to 600 grit. The I used a buff loaded with metal polishing compound to achieve shaving sharp - all reusable surgical instruments have to have a high polish so that they are easy to clean and buffing them was standard practice.

      At first, my buffing would round the cutting edge of the chisels. Then I learnt to listen to the sound and turn the chisel into the buff only until the sound started becoming harsh. I would oscillate the chisel about that position on the buff to polish the chisel angle face to a mirror, right up to the cutting edge. I would flip the chisel over and buff the back every so often, shortening the duration between flips and reducing the harshness of the sound, as I approached what I thought would be sharp.

      This buffing action achieved what a barber does with a leather strop, but I did it with very soft blades and easily achieved shaving sharp with 20 degree chisels. I even buffed high speed steel lathe tools using sound as my guide, so they would cut perspex without the plastic going frosty due to melting and tearing.

      Hope that helps. If not; perhaps you can clarify your question.

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