TABLES
Mohs hardness scale

KNIFE STEELS

Just a few words about blade materials before we continue sharpening. Traditionally knives were made with carbon steel. Carbon steel is easy to shape by forging or grinding, and can be heat treated to hardness suitable for knives. It takes an excellent edge.

Most knives today are made from some form of stainless steel. Stainless is a little harder to work and sharpen, but it has the advantage of corrosion resistance. Because of this, it will hold an edge longer in some conditions.

Two of the best stainless steels used today are ATS-34 and 44OV. ATS-34 is used by custom makers and by a few production makers, notably Buck pocket knives. Only a few custom makers are using 44OV. 440C is an excellent compromise of price and performance and is used by many custom and production makers. The term surgical steel is meaningless, because there is no particular steel used for surgical instruments.

Steel is heat treated to control its hardness. Using a combination of tempering and annealing, the maker tries to get the perfect balance between hardness and strength. You want hardness for wear resistance without being brittle or too hard to sharpen. Steel hardness is measured on a Rockwell hardness tester. Knife blades vary from about 55 to 65 on the Rockwell C scale. (Yes, there are A and B scales.)

Cryogenic treatment, freezing with liquid nitrogen, is being used on better knives to further improve the steel's characteristics. Some Bowie Survival Knives use this method of treatment.

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 the best known example of both differential heat treatment and mechanical layering.

Laminated steel is different than Damascus steel. Laminated steel has all the layers parallel to the edge for strength and hardness. Damascus steel has the layers at various angles, and is often chosen for decorative effect.

Another approach has been taken by knifemaker David Boye. His knives are made from cast stainless steel. This steel has a matrix of carbide dendrites that are exposed to form a micro-saw when sharpened. These carbides are highly wear resistant. My Boye Basic was shaving sharp right out of the box.

The search for edge retention led knifemakers to try the wear resistant materials like Haynes Stellite and Vascowear.  Both are used in industrial knives subject to high wear. Vascowear is a high vanadium steel that has great wear resistance. Stellite is a non-ferrous alloy that tests at a lower hardness than steel but contains harder carbides.  It is non-magnetic and cannot be heat treated.  The newest wear resistant material being used in knife blades is Talonite, which is similar to Stellite.

Ceramic materials exhibit very high hardness and wear resistance. Boker and Kyocera make knives with ceramic 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, like the metric system, 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. [Top]
 

SHARPENING THEORY

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 filet 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 filet knives slice, while a reverse curve is needed on a linoleum knife. Blade shapes like serration's and reverse curves give an aggressive look to fantasy knives.

Serration's 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 cutting tasks - slicing tomatoes, slicing bread, and cutting rope. 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 (filet 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. [Top]
 

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. [Top]
 

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.

Small ceramic steels are sometimes called zip-zaps. They 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. [Top]

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 $400.

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. [Top]
 

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 The Kutter's Edge in Kent, Ohio. 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 $20 to $30 for the wheels, plus another $60 to $80 if you have to buy a bench grinder.

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 chirch'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. [Top]

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. [Top]

TABLES

MOHS HARDNESS SCALE
Hardness is measured on the Mohs Scale, identified numerically by standard minerals, from 1 being the softest to 10 being hardest. Note that the newer Mohs scale goes up to 15. A mineral of a given hardness will scratch a mineral of a lower number.
 

Woodworking catalogs have lots of sharpening equipment for hand and power tools, and most of it can be used for knives. [Top]

Updated January 30, 2001
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