A PRIMER ON KNIFE SHARPENING -
Chapter 4
by Steve Bottorff
Contents
KNIFE
STEELS
Traditionally knife blades are made
from steel, an alloy of iron with carbon and other elements. The steels
used in knives are called high carbon steels and typically have a carbon
content of 0.5 to 1%. This steel in its unhardened or tempered state is
easy to shape by forging or grinding. It can then be heat-treated to
hardnesses suitable for knives. High carbon steel takes an excellent edge,
but it has no corrosion resistance.
Most knives today are made from some
form of stainless steel. Stainless steel is made by adding 12% or more
chromium to the alloy. It is a little harder to work with and sharpen, but
it has the advantage of corrosion resistance. Because of this, it will
hold an edge longer in wet conditions. The term surgical stainless steel
is meaningless, because there is more than one stainless steel used for
surgical instruments. Other elements added to steel to improve hardness,
toughness and wear resistance are cobalt, manganese, molybdenum, nickel
and vanadium.
The most popular stainless steels in
use today are the 420 and 440 families. A typical kitchen knife will be
made from one of these steels or a close relative. They are easy to
sharpen and have moderately good edge retention. 440C is an excellent
compromise of price and performance and is used by many custom and
production makers. 440C is slightly more difficult to sharpen than the
others, but has better edge retention.
If you buy specialty cutlery or a
custom made knife you will have more steels to choose from. ATS-34 is used
by custom makers and by a few production makers, notably Benchmade. Among
steels, CPM-440V is the edge retention champion, but it is difficult to
sharpen. BG-42 challenges CPM-440V in edge retention, and is as easy to
sharpen as 440C. Only a few custom makers are using CPM-440V and BG-42 at
this time.
Powdered metal technology makes it
possible to incorporate higher percentages of alloying elements than will
stay in solution in molten steel. Increasing desirable elements like
carbon and vanadium has created a whole new family of steels like CPM-440V
mentioned above. CPM stands for Crucible Particle Metallurgy, and CPM is
the pioneer in this area. Look for new knives to be produced with steels
that start with CPM and end with a V.
Steel is heat treated to control its
hardness. Steel hardness is measured on a Rockwell hardness tester. Knife
blades may vary from about 55 to 62 on the Rockwell C scale. Blades over
60 Rc are difficult to sharpen and chip easily.
Using combination of tempering and
annealing, the maker tries to get the perfect balance between hardness and
strength. You want enough hardness for wear resistance without being
brittle. Cryogenic treatment, freezing with liquid nitrogen, is used to
quickly complete any further transitions in the steel that would normally
take place over time, creating a more stable blade.
Sometimes differential heat-treating
is used to combine a hard edge with a tough spine. Mechanical methods can
be used to create this same effect. Laminated steel with a hard core that
becomes the edge and tough outer layers is available in both regular and
stainless. The samurai sword is a well-known example of both differential
heat treatment and mechanical layering.
Laminated steel is different than
Damascus steel. Laminated steel has only three layers, stronger outer
layers for strength and an inner layer for its hardness. Damascus steel
has many layers, and is often chosen for decorative effect.
Another approach to knife steels has
been taken by knifemaker David Boye. His knives are made from cast
stainless steel. His steel has a matrix of carbide dendrites that are
exposed to form a micro-saw when sharpened. These carbides are highly wear
resistant.
The search for edge retention led
knifemakers to try the wear resistant materials like Vascowear. It is used
in industrial knives subject to high wear. Vascowear is a high vanadium
steel that has great wear resistance.
Another wear resistant material is
Stellite, a cobalt alloy with about 30% chromium, 3% or less iron and 1 to
3% of other elements. Since it contains so little iron, it is technically
not steel but a cobalt-chromium alloy. Stellite tests at a low Rockwell C
hardness, about 38 to 40, but it contains harder carbides that do the
cutting and retain the edge. Stellite will tie or beat CPM-440V for
edge-retention, but it is very difficult to sharpen. A
cobalt-chromium-tungsten alloy named Talonite is similar to Stellite. Both
alloys cannot be heat-treated and are non-magnetic.
Titanium is known mainly for making
lightweight, high-strength fasteners for aerospace use, but it is also
used for knife blades. Titanium is favored for salt water diving because
of its excellent corrosion resistance. Like Stellite and Talonite,
titanium has a low Rockwell C hardness, but it has good wear resistance
and requires diamond hones to sharpen. Titanium is also non-magnetic.
Ceramic materials exhibit very high
hardness and wear resistance. Boker, Kyocera and others make knives with
ceramic blades.
In the future expect to see surface
coatings take a greater role in blade technology. It is now possible to
coat extremely hard materials like carbides, nitrides ceramics and even
diamond onto steel. This can dramatically improve edge retention, and
application of these materials to only one side can result in a blade that
is self sharpening like a beaver's tooth. And don't think the underlying
material will always be steel. Carbon and ceramic fibers have some
superior characteristics that I would love to see incorporated into knife
blades.
The materials used for grinding are
measured on another scale intended for minerals. It is called Mohs' scale
after its inventor, Friedrich Mohs. The original Mohs' scale runs
from 1 for talc to 10 for diamond. Scientists introduced a new Mohs'
scale that spreads out the scale between silica and diamond to make it
more closely equal to physical hardness, but it never caught on. Because
the Mohs and Rockwell scales use different methods they cannot be compared
exactly, but knife steel is roughly 5.5 on Mohs' scale and files are
roughly 6. A chart at the end of this article compares these scales and
the new Mohs' scale.
SHARPENING
THEORY
A knife edge 
Several things - blade thickness,
blade shape, edge angle, edge thickness and edge smoothness, determine
cutting ability.
Blade thickness is set by the
manufacturer and has a great effect of slicing ability. Your hunting knife
will never slice like a fillet knife or a kitchen knife, no matter what
you do to the edge. It is possible to change blade thickness a little near
the edge, but that can make a big difference in cutting ability.
Blade shape likewise is set when the
blade is made and is determined by the usage. For instance, more belly or
curve helps skinning and fillet knives slice, while a reverse curve is
needed on a linoleum knife. Blade shapes like serrations and reverse
curves give an aggressive look to fantasy knives.
Serrations help with some cutting
chores by letting the edge attack repeatedly from different angles, always
slicing the material a different point. This lets you cut with less
pressure. In my opinion serrated edges are desirable for three common
cutting tasks - slicing tomatoes, slicing bread, and cutting rope. Rescue
workers like them for cutting rubber and Kevlar. All other tasks are
done as well or better with a plain edge (sometimes called a fine edge). A
plain edge is also easier to maintain.
Sharpening is about the remaining
three items - edge angle, edge thickness and edge smoothness. Edge angle
is measured between the center of the blade and the bevel or flat cut by
the stone. Most Western knives are double bevel, so the total angle at the
edge is twice this angle. Asian knives and woodworking tools are single
bevel, and the resulting smaller angle can make them aggressive cutters.
That is why sashimi knifes seem so sharp.
Edge angles can vary from 10 degrees
to 40 degrees, but most are between 15 degrees (fillet knives) and 30
degrees (survival knives). Different angles are suited for different
tasks. What's suitable in the kitchen will not do for camping. Twenty
degrees is about right for kitchen knives, twenty two degrees is good for
pocket knives, and twenty five degrees gives a long lasting edge to a camp
knife. A good starting point is to duplicate the angle the maker put on
the blade. Edge angle is difficult to measure after the fact, but is
fairly easy to control when sharpening by controlling the angle between
the stone and the blade.
Any edge thickness under a few
thousandths of an inch may be considered sharp. Paper is about 2 to 3
thousands thick and will cut you if conditions are right. Edge thickness
naturally increases with wear.
Ideally the flats cut by the stone
would come together to make a perfect edge with zero edge thickness, but
edge thickness is limited by several factors. First is malleability, or
the tendency for steel to move when it is pushed. The yield strength of
steel is thousands of pounds per square inch, but as the edge thickness
approaches zero, it takes only a fraction of an ounce to move it. The
force of your hand with a stone or steel can move enough steel to create
or smooth a burr.
The second limit to edge thickness is
edge smoothness. You can't have a 1/10,000-inch edge if you have scratches
1/1000 inch deep. The grit of the cutting stone determines scratch pattern
or smoothness. Good edge smoothness requires careful work with your finest
stone.
STROPPING
Stropping the edge to a mirror finish
on a leather strop or a buffing wheel charged with a fine abrasive can
improve an edge beyond where the hone leaves off. When stropping or
buffing you always stroke off the edge to prevent cutting into the strop
or buff.
STEELS
A butcher's steel is a round file
with the teeth running the long way. They are intended for mild steel
knifes that are steeled several times a day, but are not suitable for
today's tougher and harder steels. I know a knife shop owner and
knifemaker that disagrees, but in my opinion they belong in a knife museum
along with natural stones.
A meat packer's steel is a smooth,
polished steel rod designed for straightening a turned edge. It is also
useful for burnishing a newly finished edge. Because steels have a small
diameter they exert high local pressure. Therefore they affect the metal
in a knife when used with very little force.
The secret of using a steel is to use
an angle about 10 degrees larger than the final honed edge, and use light
force. I am not aware of any guide for use with steels. The Raz-R-Steel
from Razor's Edge is marked for the proper angle. It's use is similar to
crock sticks.
A variation on the steel is the
ceramic steel, where the steel rod is replaced by a ceramic one. Since
ceramic is an abrasive, it can polish as well as burnish. Ceramic steels
are available from many suppliers.
I prefer to use a steel in the
vertical position as pictured, instead of the in-the-air method.
Small ceramic steels are available in
several grades, and are useful for sharpening serrated knives, or carrying
in the field for quick touchups. Ceramic sticks without handles are
available very cheaply at pottery shops if you want to make your own.
There are people that swear by burned out quartz lamps for sharpening
rods. They are textured at about 500 grit, and are harder than natural
stones.
POWER
SHARPENING MACHINES - continued
While hand sharpening meets the needs
of most of us, a machine is the way to get the work done. Here are some
power sharpeners worth considering if you do a lot of sharpening.
A wet wheel machine is very useful if
you have to remove a lot of material, like re-grinding a broken tip. The
water prevents over heating the blade and ruining the temper. Sears and
Wen sell small wet wheel grinders for about $30. They are suitable for
light use. The Sears Home Sharpener rest is easy to adjust and can
be set from about 10 degrees to 90 degrees. It is reversible so that
you can grind on of off the edge from the same rest setting. There
are several 10" wet wheels available for $150 to $800.
Delta and Makita sell horizontal
waterstone grinders for about $200. The advantage of these is the flat
bevel they put on knives. They are popular with woodworkers.
Larger wet grinders for professional use cost from $400 to several
thousand dollars.
The wet wheel machines mentioned
above have a limited number of guides or fixtures available, mostly for
planer and joiner knives and other woodworking tools. The only
wet wheel grinding system with guides and fixtures for all sharpening
needs is the expensive Tormek. I will review the Tormek in Part
five.
Woodworking catalogs offer a variety
of rubberized, nylon and composite buffing wheels for sharpening. These
are usually sold industrially for deburring and polishing. They
require skill and practice, and they are expensive. I think paper wheels
are the best choice for the home knife sharpener.
PAPER
WHEELS
If you are comfortable using power
tools, try a paper wheel system. Paper wheels are safer than buffing
wheels and less likely to catch and throw a knife, but you still work with
the wheels moving off the edge, like stropping, for safety.
I use the paper wheel set from Razor
Sharp Edgemaking Systems. They are often seen demonstrated at gun and
knife shows, and are also available from knife making supply shops and
woodworking tool stores. These wheels mount on a grinder or buffer. The
sharpening wheel is coated with silicon carbide, and grease is used to
cool the blade. Buffing compound is used on the other wheel for honing.
Cost is about $80 for the wheels, plus another $50 to $100 if you have to
buy a bench grinder.
Most paper wheel sets are 3/4"
wide. I also found a cheap 1/2" set made from
gray composition board instead of laminated paper. Avoid it, look
for the white paper wheels.
I've had good luck with this system.
The sharpening wheel raises a burr quickly. The honing wheel polishes the
burr off and leaves a mirror finish comparable to stropping by hand. Both
operations are done with the wheels moving off the edge for safety.
Using paper wheels requires a little
skill, but once you get the hang of it, it is very fast. I sharpen twenty
knives at a time for my church's kitchen, and I can do them in less than
30 minutes with this system.
The most difficult knives I ever
tried to sharpen was an old set of Gerber kitchen knives. They were so
hard that natural stones hardly touched them. Diamonds would grind them,
but I don't have a diamond stone fine enough for a shaving edge. Paper
wheels is the only system that has ever brought these knives to a razor
edge.
I use paper wheels a little
differently than recommended by the manufacturer. Normally a grinder wheel
turns toward the user, and grinding is done on the front, where debris is
thrown downward. The instructions for paper wheels say to use this same
rotation but sharpen on top, where debris is thrown toward you. This seems
inherently unsafe to me.
Here is how to modify a grinder for
safer use of paper wheels.
I recommend you buy a dedicated
grinder motor for this purpose. Changing the wheels too often can
introduce wobble in them. When you buy a grinder make sure it has
removable guards, because you are going to take them off. Put a good light
over the grinder so you can see the burr as it develops then polishes
away.
Mount the grinder so the top of the
wheels moves away from you, and sharpen and hone on top of the wheel with
the edge away from you. This lets you see better, and debris or anything
caught by the wheel is thrown away from you. Hold the blade level
and work near the top for a small angle, down the wheel closer to you for
a larger angle.
If you thought trigonometry was
something you learned in school but never thought you'd use, think about
this. When the blade is horizontal the angle between the blade and the
wheel is equal to the angle between the point of contact and vertical
(identical triangles). I've marked angles of 0, 15, 20 and 25 degrees on
my wheel. I put zero at the top and position the blade at the angle
mark I want to grind before I start the motor. Then I turn it on and
hold the angle steady as I move the knife lengthwise. Practice a little
and you will learn to see the burr and where to hold the blade to get the
proper angle.
SHARPENING
CERAMIC KNIVES
Diamond stones will sharpen a ceramic
knife, but you must remove all scratches caused by the diamonds. Scratches
act as stress risers and can cause the brittle ceramic blade to fracture.
Silicon carbide wheels or stones can
be used to sharpen ceramic knives, which are made of relatively softer
aluminum oxide. Since paper wheels use silicon carbide abrasive, they too
can sharpen ceramic knives. SC wheels can also remove the scratches from
sharpening with diamonds.
Ceramic blades will not raise a burr.
You have to use the other tests to determine if you have created a new
edge.
TABLES
STONES AND ABRASIVE GRADES -
FIVE DIFFERENT SYSTEMS COMPARED
SUPPLIERS
TO READ FURTHER
Woodworking catalogs have lots of
sharpening equipment for hand and power tools, and most of it can be used
for knives.
The best book on tool sharpening is
"The Complete Guide to Sharpening" by Leonard Lee. It has a chapter
devoted to sharpening knives.
"Sharpening Basics" by Patrick
Spielman covers sharpening knives as well as tools.
"The Gun Digest Book of Knifemaking"
by Jack Lewis and Roger Combs has a good chapter on sharpening.
"Step by Step Knifemaking" by David
Boye covers sharpening with a belt grinder and buffer, as well as manual
sharpening and stropping.
Company and product names are trademarks of their companies.
