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.


02 January 2008

First, a bit of history ...

There's been lots of talk recently about sharpening knives and other toys with edges ... what to use, how to do the deed, etc. ... but we haven't really touched on what sharpness *is*, and how an edge can [for some purposes] be *too sharp* ...

If yer eyes are already rolling back in their sockets, just move along to another blog or continue on and kwitcherbitchin.

Ahem ...

What is sharpness ?

Let's start with this:

Sharpness is the condition of an edged tool which allows it to be used for it's intended purpose.

Vague, huh ? To an extent, it needs to be vague ... because what is sharp to someone using a knife as a utility tool for camping isn't the same as sharp to someone using a chisel or plane to carve or shape wood ... or the same as sharp to a butcher ... or the same as sharp to a barber using a straight razor to give a customer a super smooth shave.

Some folks have referred to 'slicing' sharp and 'pushing' sharp, to distinguish between the two primary types of cutting methods used with edged tools.

Both of these cutting actions test a different kind of sharp on an edged tool ... slicing tests the bevel or grind angle of the edge and pushing tests bevel as well, but adds a focus on smoothness of the edge.

Here's where it becomes really fun: the kind of edge you want for one test is in many ways the opposite of the kind of edge you want for the other test.

Let me illustrate a bit:

Say you're getting ready to carve the Thanksgiving turkey [or any other turkey with whom you have a grudge]. You want an edge with a moderately fine bevel [say 19 to 21 degrees], which will force the slices apart as you're moving the cutting edge through them ... and you want the edge to have just a bit of 'tooth' in order to slice through tissue membranes and not just mash them.

Hold that thought while we look at the other extreme ...

If you're a woodworking fiend [and I mean that in the kindest possible way], you need to have chisels & planes which are sharp ... as defined by the original bevel angle, and smooth as possible so as not to tear out chips or chunks of delicate and EXPENSIVE wood that you're carving or planing down to the correct size for your project.

Now let's look at what would happen if we tried to use a woodworker's dream edge to carve a turkey. The slices would tear and fall apart.

Huh ? Why would that happen ... ?

Because the edge is so polished that it will actually create a bit of suction as it cuts through the meat and that, coupled with the mashing effect it has on membranes because it is 'toothless' will give you terrible results for slicing tissue.

The flip-side situation is equally bad, because the fine toothed edge that serves us well for slicing through tissue will rip the heck out of those very expensive wood fibers, pulling chips & chunks out and causing the woodworker to scream blue murder and begin looking for fresh pieces of wood to finish his/her project.

So how do we solve these problems ?

By asking what the intended purpose of the tool is before we begin sharpening ... and using the appropriate sharpening tools &/or methods to do the job ... and by knowing when to quit sharpening the tool.

Assuming the your edged tools are in reasonable shape [MacYoung, this is NOT a safe assumption for you to make. EVER!], you won't need power tools or extremely coarse hones to begin the job. For kitchen knives, utility camping / outdoor use you'll want to start around 500 grit and finish at about 2000 to 2400 grit.

If your knives need a lot of work, you might start at 200 grit ... but be careful, grits that coarse take off A LOT OF METAL VERY QUICKLY. Let me repeat that, just in case it didn't come through clearly the first time:

... you might start at 200 grit ... but be careful, grits that
coarse take off A LOT OF METAL VERY QUICKLY.

Another factor which may come in to play with modern knives is the hardness of the steel. This isn't as big a factor as some folks would like to make it, but it can have an effect on how quickly you'll get the results you want.

Typical abrasives [hones, sharpening stones, etc.] are made from things like silicon carbide, aluminum oxide and various ceramic or clay binders with these [or similar] abrasive materials mixed in and fused with them.

These are great for steels hardened to about 60 on the Rockwell scale, but will be less effective on steels much harder than about 65 or so.

For steels hardened to those rarified levels [you won't find many, and in my not-so-humble-opinion, they're not worth the trouble] you'll want a different type of abrasive material: industrial diamond or cubic boron nitride.

Don't worry about these for now, just be aware that the folks who want to sell sharpening gear to you will push these little darlings at you because they can ... and because they're EXPENSIVE.

Ok, back to the real world. Good quality silicon carbide [Norton Crystollon stones] or aluminum oxide [Norton India stones] are fine for most applications ... and if you want to get fancy, you can use waterstones [ceramic or clay binders with these same abrasives] in the appropriate grit ranges to do the work.

How to choose ?

How much sharpening do you do ?

How much fussing are you willing to do to get ready for sharpening ?

Norton [or similar] stones should be used with light mineral oil as a lubricant while sharpening ... and gently rinsed & lightly re-oiled after use before you put them away.

Waterstones need to be soaked in water before use, and use water as a lubricant during sharpening. They also required gentle rinsing before they are air dried and put away ... AND they will require periodic re-flattening of their surfaces in order to maintain a useful working area.

The reason for this is that the binding materials which hold the abrasives in these stones wears away during the sharpening process.

This wear is responsible for two of the better qualities of waterstones: they cut quickly, because fresh abrasive is constantly being exposed; and they polish the edge while they are sharpening, because the binder material which is wearing off during the sharpening process mixes with water and forms a paste which lubricates and also contains small amounts of the abrasives which are being released by the wearing of the binder.

Which one is right for you ?

If you just want sharp edges and don't want to spend too much time maintaining your hones, go for oilstones.

If you feel the need to become somewhat fanatical about your sharpening gear, try waterstones ...

Either way, you'll have the basic tools required to get edged things sharp.

But what about other tools ... ? Ceramic hones, diamond hones, strops, ScarySharp, etc. ... ?

Ceramic hones can be very good, and I've recently been told about some which sound excellent ... but they won't do a 'better' job than the oil or waterstones I've already talked about.

Some folks like them because they've been told they can be used dry. True enough, but they work better with water as a lubricant ... and they still need periodic gentle cleaning to get the ground off metal out of their surfaces, so it all balances out in the end.

Diamond hones are lovely, and I own and use several for specialized tasks [like fixing serrations] ... but unless you're working on this kind of stuff all the time or have knives / tools made out of the previously mentioned super hard steels, save your money for buying more knives.

Strops are lovely, and are an absolute necessity for putting the final polish on straight razors and some superfine woodworking tools ... but again, save your money unless you use these tools or plan to make a go of sharpening as a business [not something I would try, but you might be able to do it].

ScarySharp ... a great concept, but it's primarily good for woodworkers who have tools with rectilinear edge geometry and large edge bevel area.

It was designed for woodworking tools and not for knives, and when one attempts to apply it to knives ... well, it just seems to fall short.

At least for me ...

Ok, I've spewed long enough ... chew on this and I'll just wait for the questions [and comments] to start coming in.