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Sharpening Made Easy - Knife Sharpening Information and Equipment


A PRIMER ON KNIFE SHARPENING - Chapter 4

by Steve Bottorff



 

Contents

Knife Steels
Sharpening Theory
Stropping
Using Steels
Power Sharpening Machines - continued
Paper Wheels
Sharpening ceramic knives
TABLES
Mohs hardness scale
Abrasive grits
Suppliers
To Read Further

BOOKS
front cover

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 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 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 beaverís tooth. And donít 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 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.

steel use

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.

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.

Delta and Makita sell horizontal waterstone grinders for about $200. AMT sells a version for about $100. 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 www.sharpeningwheels.com. 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 $35 for the wheels, plus another $30 to $50 if you have to buy a bench grinder.

Most paper wheel sets are 3/4" wide.  Koval Knives sells its 1" wide 8" paper wheel set for only $25. Their 6" set is also 1" wide and sells for $20.  I also found a cheap 1/2" set made from gray composition board instead of laminated paper.  Avoid it, look for the white or brown 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.

paper wheels marked with angles

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

MOHS HARDNESS SCALE
  This chart was moved to its own page. Click this link to jump to Mohs

  STONES AND ABRASIVE GRADES - FIVE DIFFERENT SYSTEMS COMPARED

This chart was moved to its own page. Click this link to jump to GRITS

SUPPLIERS

This chart was moved to its own page. Click this link to jump to SUPPLIERS
 

TO READ FURTHER

Click this link to jump to BOOKS

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.  

Knife making catalogs are good sources of information.  Blades N' Stuff, whose catalog was an encyclopedia of knife making, is no longer in business since the death of Bob Engnath, but their information is on the WWW at www.engnath.com

"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.

This is the end of Chapter 4
Click here for Chapter 5


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