Definitions of Sharp and Keen

Consider definitions from the Merriam-Webster dictionary:

sharp

adjective \shärp\
: having a thin edge that is able to cut things

keen

adjective \kēn\
: having a fine edge or point

These two adjectives will serve to describe

A)  The thinness of the edge (sharpness) as quantified by the edge width at 3 microns from the apex.

B)  The edge width or fineness of the edge  (keenness) as measured at the very apex of the bevel.

These two words are colloquially synonyms; however, we require more precise definitions reflecting the subtle differences in the meanings of the two words.

  4 comments for “Definitions of Sharp and Keen

  1. Dr. Matt
    December 30, 2015 at 10:28 pm

    I’m trying to visualize this but isn’t sharpness, according to the definition, a function of bevel angle? Won’t a 16° degree bevel angle have the same width 3 microns from the apex regardless of thickness of the steel (assuming no micro convexity and same hones used)? For any given bevel angle, the width at 3 microns should be the same… right?

    Like

    • January 1, 2016 at 1:01 pm

      For a triangular bevel, if we know two of; the edge width, the width at 3 microns or the bevel angle, we can calculate the third. For example, a 16 degree triangle has a thickness of 0.85 microns at 3 microns, and if the edge width is 0.15 microns, the thickness at 3 microns is 0.85+0.15=1.0 microns. Since we never have a perfect triangle – there is always finite edge width (or a radius of curvature) to the apex, the angle is not constant, and not straightforward to measure. For these reasons, I measure the apex width and the width at 3 microns. After extensive experimentation, I determined that 3 microns is a good predictor of shaving & shallow cutting behaviour. This quantity is representative of the “thickness behind the edge” for this type of cutting. For other applications, such as food preparation, thickness millimeters behind the edge may be more representative. The key point is that thickness behind the edge determines the volume of material that is displaced as the blade penetrates – thinner is better, provided the blade is not damaged.

      These measurements do not factor in the advantages of an irregular edge in enhancing draw-cutting performance; they are representative of push-cutting performance.

      Liked by 1 person

  2. Mike Shults
    April 7, 2019 at 3:58 am

    Great post. I have a question, and hope its not a silly one. If I hone with tape, do I leave the tape on when I go to a felt paddle strop? Or is there enough give in the felt to compensate for the missing tape? I understand that tape only changed the angle by a degree, but if I strop with tape would this dull my edge? Or if I don’t leave the tape on will the apex contact the substrate?

    Like

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.

%d bloggers like this: