03 January 2008

... and the bleat goes on ...

In the first installment of Sharpenology, I bored all y'all with my observations about what constitutes sharpness and a brief overview of tools that can be used to get things sharp.

This time, I'll be blathering on about grinding & edge geometry,
so be prepared for some more of my silly ranting and observations on what makes sharp things useful ... and perhaps even a little bit on how to keep them in a state of readiness.

If yer eyes are already rolling back in their sockets, just move
along to some gardening blog or other such nonesense ... or continue reading and kwitcherbitchin.

Ahem ...

As I was finishing up my last posting on things sharp, I was
beginning to think about some of the factors that make a sharp
thing useful for a particular task ... and while sharpness is
certainly an important factor, it isn't the only factor to be
considered when looking at an edged tool.

Take, for example, the humble wood-chopper's axe.

While a good axe can be made shaving-sharp [and a sharpening
tool manufacturer uses this as a marketing pitch for their
brand of gadget], the edge geometry of an axe isn't what you
would call ideal for cutting hair.

Why not ... ?

I'm glad you asked ... <grin> ...

The typical axe is ground to a convex cross-section, coming
down to a fairly thick edge ... while a typical razor is ground to a concave [or hollow] cross-section and comes down to a very thin edge.

What does this mean to the wood cutter &/or the barber ?

Imagine using a straight razor [open razor or cut-throat for
the Brits out there ... you know who you are ...<grin> ...]
to cut wood ... <shudder> ... you'd take a chop at the wood
and there you'd find your blade.  Stuck ... and probably
missing [or about to be missing] a chunk of that lovely, fine
edge you spent hours perfecting & polishing.

But it's sharp as a ... razor ... I hear you say, how can it
get stuck ?

It gets stuck because it is designed to cut without pushing
what it is cutting away from itself.

Huh ?

Think about it for a moment ... now remember those fancy ads
on teevee that show a razor gliding down a shave cream covered
face ?  Remember how that fine edge sliced through the thick
[relatively speaking] beard hair and pulled it along just a
bit ... so that the second [or third, or fourth ...] blade
could cut the hair off under the level of the skin surface
and create the perfect start for a painful ingrown whisker ?

That same effect will demonstrate itself on a much larger
scale in the wood you've tried to chop with that razor ...

The fibers of the wood will close back up against the blade
and trap it ... at best ... or break it ... and you won't have any firewood or any way to shave the next morning.

Ahem ...

So what happens when you cut wood with an axe ... ?

Basically, the edge is sharp enough to cut the fibers of the
wood ... and the convex shape of the blade behind the edge
pushes these cut fibers away from the blade, thus releasing
the axe for another swing ... instead of allowing them to
stay close and trap the blade.

Ok, but I'm not using a straight razor or an axe ... so what
does this all mean to me ?

The straight razor and the axe are two extremes of grinding
types used in modern cutting tools ... and as such, are good
examples of single or limited purpose tool construction.

Another example of this is the chisel [hi Kevin!], which
any sufficiently fanatical woodworker will polish to a level
that will bring tears to the eyes of the village barber.

Yes, like our friend the axe ... you could shave with a chisel.


But that isn't what it was designed to do, so we'll skip that
particular demonstration.

The chisel's grind, however is useful because it is very strong.

Unlike the hollow grind used for our straight razor, the chisel's
flat grind has left us lots of material behind the edge ... which
gives the chisel the rigidity and strength to push through tough
wood fibers, while the angle of the grind pushes those same tough
fibers out of the way to they don't impede the next cut.

Pretty good design, eh ?

But what has it got to do with knives ... ?

Ok, I guess it's time to bring all of this together and get on to
the more practical aspects of grinds & edge geometry.

All of the examples I've given so far have been at the extremes of
their respective geometries ... what you find in a typical modern
knife is usually a combination of one or more of these design ideals executed to a less extreme degree in order to make it more broadly useful in a variety of circumstances.

Here's an example ...

A certain American cutlery company, which shall remain nameless ... [but whose initials are Buck], used to advertise that their knives were 'sharp as a razor, strong as an axe' ... and used a graphic which depicted a knife cutting a nail to show this combination of strength and cutting ability.

They also took great pains to tell their customers that the actual performance of the nail cutting trick would void any warranties on their products, but that's another story.

How could they make this claim of sharpness AND strength ?

Better still, how could they back it up ... ?

Simply enough, really ...

They just developed [implemented, really] a grind which gave their knives a thin cutting edge [for excellent sharpness], yet a thick and strong spine for durability.

The semi-hollow grind.  While they didn't invent it, they certainly
put it to good use both in their products and their advertising.

Other effects of this particular grind are to make the blade lighter,
which can make it easier to control in certain applications and to
make the blade less likely to get stuck in whatever it is cutting by
moving the material away from the blade after it is cut.

Other examples of applying different grinds & edge geometries to knife design include such lovelies as Mr. Emerson's chisel ground tanto-like blades, which are superb for punching through tough materials and Mr. Nealy's combination blades which use a chisel type reinforced point, like a tanto, but are semi-hollow ground back from the point to provide a thinner edge for times when slicing cuts are more useful than thrusts.

What does all this mean to me, the general purpose knife user ?

More grist for the mill when you're looking at adding a knife to your collection [or arsenal ...].

Think about how you'll most likely be using the knife ... what materials you'll be cutting ... and what kind of maintenance the blade will need after some use.

Certain grinds are easier to maintain, yet may not be suited to the
type of cutting you'll be doing ... others may be a little more work
to keep in shape, but will require that maintenance less often when used in your particular environment.

These factors will also influence your choice of sharpening tools,
which is a subject near and dear to my heart ... and I know holds
some interest for at least a few other folks out in the ether ...

That's all for now, next time I think I'll blather on about blade shapes ... and what features I think make up a good everyday knife.