Mitchell Jones wrote:
In article ,
"Del Cecchi" wrote:


"Mitchell Jones" wrote in message
...

In article ,
"deowll" wrote:


"Del Cecchi" wrote in message
...

"Mitchell Jones" wrote in message
...

wrote:

can you imagine if the conventional wisdom for judging
size-tolerance
of fabricated-materials was along the lines of, "how does it feel
when
i shave hairs on my arm?"

I want to _measure_ knife sharpness. In a repeatible,
scientific
way. And preferably, non-destructive to the sharpened edge!


the floor is open for discussion.

***{You want to cut a homogeneous material that will not take the
edge
off of your blade. Use paraffin.

You want the conditions to always be the same, so that you can
compare
results. Clamp the blade to be tested so that it forms the
horizontal
member of an isosceles triangle, the equal sides of which point
downward. Hang a platform in the "V" and add weights until the
blades
cuts smoothly through the paraffin. The less weight required, the
sharper the blade. Always use identical blocks of paraffin.

The weight required, then, is the measure of dullness; its
reciprocal is
your measure of sharpness. Always include the weight of the "V" and
the
blade in the data you record.

That should work.

--Mitchell Jones}***

******************************************** *********************
If I seem to be ignoring you, consider the possibility
that you are in my killfile. --MJ

Scanning electron microscope.


Agreed but with the observation that this won't tell you how well the
edge
will cut any give material to many things come into play.

***{The original poster wanted a technique that would measure the
sharpness. The electron microscope would be utterly useless in that
regard because it would provide an image of the edge, not a measure of
sharpness.

The main difficulty with measuring the force required to cut paraffin,
on the other hand, is that force is required not merely to cut the
paraffin, but also to slide the flat part of the blade against the flat
part of the cut as the cutting edge moves through the material. All
that
is relevant to measure the sharpness is the part of the force which is
required to get the cut started, not the part required to slide the
blade along behind the cutting edge. Spraying all blades with WD40
before doing the test would eliminate a lot, but not all, of that.

Another approach would be to clamp the blade in a horizontal position
with its cutting edge upwards, and hang an empty, light-weight
container
from it by looping a piece of nylon fishing line of known quality
around
the blade. You could then pour water into the container slowly, until
the weight was enough so that the sharp edge cut through the line.
Catch
the container as it falls and weigh it, and use the reciprocal of the
weight as your measure of sharpness. That method would have the
advantage of eliminating the force of sliding friction/adhesion from
the
measurement.

While that method would provide a generalized measure of sharpness, it
would not change the fact that the force required to make a cut would
vary depending on the material, and, in particular, on the adhesive
forces between the flat sides of the blade and the sides of the cut, as
the blade moves along. It is simply a fact that the ease of cutting
depends not merely on the sharpness of the cutting edge, but also on
the
adhesive forces between the blade and the material being cut. (Not to
mention the fact that the cutting edge must hold its sharpness while
the
cut is being made. A copper blade will not hold its edge while cutting
a
steel wire, for example.)

--Mitchell Jones}***

*********************************************** ******************
If I seem to be ignoring you, consider the possibility
that you are in my killfile. --MJ

Fine, you don't like looking at the edge width and angle as a sharpness
measure, even though "a knife is just a wedge". So then you need to
write a definition of what the parameter you call "sharpness" is.
Ability to push cut flimsy material? slice meat? indent paraffin? If
you can define what you are looking to measure some genius in this group
can suggest a method, some of which might work.

But you can't measure what you haven't defined.


***{There is a difference between being able to define something in
words that will communicate the meaning to others, and understanding
what it is. In fact, you have to understand a concept before you can
convey it to others.

For example, most people who understand how to ride a bicycle would have
great difficulty conveying the technique in words, yet they can most
assuredly get on a bicycle and ride it. If I were to describe the
technique in words, I would simply tell the other person to turn in the
direction that will keep the bottom of the front wheel beneath the
center of the handle bars.

Measuring sharpness, like riding a bicycle, requires understanding, but
does not require the ability to communicate that understanding. Most
people have an understanding of what sharpness is, and, thus, could
competently attempt to measure it, but would have to struggle a bit to
define it. Since you want a definition, I would describe it as the ease
with which the edge of a blade separates the material being cut. That
concept relates directly to the measuring technique I described
yesterday because ease is inversely proportional to the force required,
and, since we are talking about the edge only, the force required to
move the rest of the blade through the cut is irrelevant. I would note,
however, that I came up with the measuring technique yesterday and the
definition today. Hence the ability to measure sharpness only required
understanding. I was able to measure what I had not defined.

Of course, the more you work with a concept, the better you understand
it, and putting it into words is an instance of working with it. Thus
defining something enhances one's understanding, even though it is not a
prerequisite of understanding.

--Mitchell Jones}***


del


************************************************** ***************
If I seem to be ignoring you, consider the possibility
that you are in my killfile. --MJ

By inventing the measurement process and using that to define the
parameter you haven't produced anything valid. You have merely asserted
a particular meaning for "sharpness". If I used a pork chop or a 2 by 4
instead of paraffin I would get different values and perhaps even
different ordering. And if I did slicing motion instead of push, they
would be different still. And why ignore the friction with the sides
of the blade? Isn't that an important component of cutting?

I don't know what group you are posting from but after this I will be
trimming followups and crossposting to rec.knives.

--
Del Cecchi
"This post is my own and doesn’t necessarily represent IBM’s positions,
strategies or opinions.”