Friday, July 23, 2010

Knife Sharpening Explained with Helpful Tips & Illustrations


SHARPENING PROCESS

COMMON MISTAKES

The mistakes commonly made in knife sharpening are uncontrolled edge angles, failure to establish a new edge, and leaving the edge too rough. The following methods address each of these mistakes.

The keys to success are:

1) Use an angle guide to control the edge angle,

2) Sharpen until you raise a burr, and

3) Hone or polish the edge smooth.

Some instructions refer to the sharpening motion as trying to slice a thin layer or a decal off the stone. This is bad advice, and here's why: most people won't hold a constant angle this way. Every different edge requires that you hold the blade at a different angle when slicing a thin layer. You instinctively raise the blade until you detect the edge working. This is almost a sixth sense, involving both feeling and hearing. The same thing happens when sharpening by hand. The duller the blade becomes the more you raise it more before you can sense the edge working against the stone. This creates larger edge angles as time goes on and the results gradually deteriorate. Skill and practice will overcome this problem, but the sure-fire way is to use a guide to maintain edge angle.

If you do not remove enough metal to create a new edge, you will leave some of the dull edge in place. The easiest way to determine that you have removed enough metal is to grind until you have raised a burr. Steel will naturally form a burr when one bevel is ground until it meets another. You can then remove the burr in the honing process and have a sharp edge every time.

A final honing and polishing will bring the edge to perfection.

CONVENTIONAL METHOD

For fast removal of the old edge, start with a coarse, fast cutting stone. Diamond stones are the fastest cutting manual stones, with Japanese waterstones second. The first step is where most of the work is, and you can benefit most from using a power sharpener.

Set the guide and take a light stroke with the stone. Check the angle against the old bevel. If the new scratch pattern is on the back edge of the old bevel, you are lowering the angle. If it is at the edge, the angle is being increased. When the scratch pattern is centered on the bevel you are duplicating the original angle. Keeping the original angle is a safe strategy until you gain more knowledge.

Black marker helps show the honing angle, here with a rod-guided system.

If you can't see the scratch pattern, try darkening the old bevel with a black felt tip marker, then stroke the stone again. The scratch pattern will stand out against the dark marking.

When the angle is set correctly, grind one side of the blade until you have removed the old edge. Grind until you have raised a burr. The burr will appear on the side opposite the one you are grinding. With experience you will learn how to stop with just a small burr in this step. If you are not sure, grind until you can feel the burr. Then turn the blade over and grind an equal amount off the second side.

A burr is a natural occurrence in steel when one bevel is ground until it meets another. When I was learning I would show my work to my grandfather, and he would often show me that I had a burr. It seemed sharp, but the burr would bend over and become dull. I tried to avoid ever raising a burr for years after that. As a result I never got anything quite sharp. Now I know that one of the secrets of sharpening is to raise a burr, then hone it away.

Ceramic knives and some very hard steel will not raise a burr. Here again experience will tell you when you have ground enough.

There are three basic strokes when you sharpen - sliding the stone onto the edge, sliding it off the edge, or circular or alternating strokes. At the first stage, any of the three is okay.

For the next step use a medium stone. Its purpose is not so much to remove material as to grind away the scratches made by the first stone. The medium stone should be about twice as fine as the first. If you started with a 180 grit stone, you can use 320 or 360 now. Use circular or alternating strokes until the old scratch pattern is gone. Then do an equal amount of grinding on the second side.

You might still be able to detect a small burr at this stage. Finish with a few light strokes sliding onto the edge to remove the burr. This is where slicing a decal off the stone is an accurate description. The blade should now be sharp with no burr. The edge now has 320 or 360 grit micro-serrations, which is good enough for many uses.

The micro-serrations are providing some of the apparent sharpness now but they will wear and bend. A steel or a touch-up stone will straighten them and bring back the sharpness. Continue to the next step if you want a longer lasting edge.

For the third step use a fine stone, 600 or 800 grit, and hone using only strokes going onto the edge. Alternate sides with every stroke. This will help prevent forming a new burr.

Your edge should now shave. Test it as described above. If there is roughness, go back to the medium stone. If there is no roughness but the edge doesn't have enough bite, continue with the fine stone.

When the blade becomes dull, repeat the medium and fine stones. Only when the blade becomes nicked or damaged will you need to go back to the coarse stone.

MULTI-BEVEL METHOD



This variation will give you a longer lasting edge than the conventional method described above. The multi-bevel edge that results is similar to the convex edge found on Moran and BlackJack knives and the Trizor edge on Chef's Choice knives. This method can be adapted to many types of sharpening equipment.

The first step is to grind an initial edge bevel about 5 degrees less than you want your final angle. This is sometimes referred to as pre-sharpening or thinning the blade. You will put a little more work into this step, but you will save some work later. Grind until the old edge is removed. As described above, the proof is that you have raised a burr.

Now change to a medium stone and set your guide for a few degrees greater angle. On a clamp-on type guide you increase the angle by moving the guide closer to the edge. On a rod type systems you can easily select another angle. Other systems have different ways to adjust the angle. See the section below for a method using the Lansky sharpener.

When you get to the fine stone increase the angle again another couple of degrees. Hone with strokes going onto the edge and alternate sides with every stroke. You are now grinding only a small area right at the edge, removing the burr and the scratches from the medium stone.

Since a finer stone cuts more slowly, it usually takes quite a bit of work to remove the previous step's scratches. By increasing the angle by a couple of degrees when you change stones, you focus this work on a smaller area near the edge and reduce the work needed.

MULTI-BEVEL WITH A LANSKY SHARPENER


A multi-bevel edge can be accomplished with a Lansky Sharpener
by fixing the rod at different positions with each stone.

Here is an easy way to do a multi-bevel with the Lansky:

1. Push the rod into the coarse stone as far as it will go and still have the screw tighten against the flat. This decreases the angle by a degree or so. Do extra-coarse stones the same if used.

2. Mount the rod on the medium stones in the center of the flat per the instructions.

3. Push the rod into the fine stone only far enough to tighten the screw against the flat. This increases the angle by a degree or so. Do the ultra-fine the same if you have one.

Now, when using these stones you will automatically create a three bevel edge.

Tip: Replacing the thumb screws with flat head screws will give you another 1/2 inch or so of useful stone.

HONING

You can further improve the edge by honing the edge on an ultra fine Japanese, Arkansas or ceramic stone, 1000 grit or better. Maintain the same angle as the final step above.

USING OILS AND WATER ON STONES

In North America we usually use oil on sharpening stones; in the rest of the world they use water. Tests by John Juranitch show that because oil carries the dross against the edge, better results are obtained with a dry stone. However, natural stones tend to clog without oil. I prefer ceramic and diamond stones used dry, and my second choice is Japanese waterstones.

I'll leave this up to your personal preference, with the following guidance. With India and bonded Arkansas stones you can use oil or use them dry. Clean them with paint thinner. Use and clean Japanese waterstones only with water, but store them dry and soak them before using. Ceramic and diamond stones can be used dry or with water. Clean them with water and scouring powder when necessary. Washita and natural Arkansas stones can be used with oil, water or dry, and cleaned accordingly.

If you have used water on a stone and want to change to oil, let it dry thoroughly, and then oil it. Once you have used oil on a stone, it is difficult to change back.

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.

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.

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.

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

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.

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.

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

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.


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