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.
