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Knife Sharpening and Theory by Steve Bottorff


Knives that are used with any regularity need to be maintained. To function at its optimum efficiency a knife needs to be sharpened on a regular basis. This may seem like an obvious statement but it is quite common for this aspect of knife maintenance Knife Sharpening Information and Theory
by Steve Bottorff

Knives that are used with any regularity need to be maintained. To function at its optimum efficiency a knife needs to be sharpened on a regular basis. This may seem like an obvious statement but it is quite common for this aspect of knife maintenance to be neglected. This can not only decrease the level of your knives usability but can be dangerous as well. You have probably heard that a dull knife is more dangerous than a sharp one. This is certainly true. To maintain the best possible control and predictability of your knives operation as a percision tool you must maintain a sharp edge.

The following information was provided by Steve Bottorff and is used with his kind permission. His article describes some of the fundamentals of how to test sharpness and the various techniques and tools used in sharpening your valuable knives.

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 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 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

TESTING SHARPNESS
To be sure you are improving your sharpening; you need an objective way to test the results. Tests evaluating sharpness range from cutting silk to chopping trees. What you need is a test method that are useful in your workshop as you are sharpening.

Most people test an edge by rubbing their thumb lightly across the edge and feeling how the edge grabs as it tries to cut into the thumb pad. To keep your thumb calibrated, test a known sharp edge like a new razor blade periodically.

Shaving hair on your hand or arm is another common sharpness test. Shaving sharpness can be achieved even on heavy hunting knives or an axe. I own a hunting knife that will shave even though the edge angle is a rather blunt 30 degrees. I use the term shaving sharp to describe this degree of sharpness and razor sharp to describe even greater sharpness. Razor sharpness will literally pop the hairs off your hand or arm. Razor sharpness is only possible with both a polished edge and a small edge angle.

Testing by shaving can be misleading if the blade has a burr or wire edge. Steel naturally forms a burr - a thin bendable projection on the edge - during the sharpening process. A blade with a burr will shave but will not stand up to hard use. To test for a burr, slide your fingertips lightly from the side of the blade over the edge. You will feel the burr drag against your fingers. Test from both sides, because burrs are usually bent over one way or the other. As your sharpening improves you will be looking for smaller and smaller burrs.

The glint along this edge means a dull blade. Many good sharpeners, including my grandfather, have learned to see a dull edge. Hold the blade in front of you with the edge in line with a bright light. Move the blade around a bit. A dull edge will reflect a glint. Nicks and burrs will also cause glints. When the blade is sharp these glints will be gone.

Another test for sharpness is to press the edge lightly on your thumbnail at about a 30-degree angle. If it cuts into your nail it is sharp. If it slips it is dull. The sharper the blade, the smaller you can make the angle before it slips. Try this with a new razor blade to see how a really sharp blade feels. The down side of thumbnail testing is that the little cuts in your nail get dirty and look bad until the nail grows out. For this reason some people do this test using a plastic pen or pencil.

The only tool available for edge testing is the Edge Tester from Razor Edge Systems. The Edge Tester evaluates edges on a 100 point scale for sharpness and smoothness. The principle is similar to the thumbnail test, but the Edge Tester has a special material and shape for repeatable testing. If you're serious about sharp knives, get an Edge Tester.

THE EQUIPMENT SHARPENING STONES

No shop is complete without at least one bench stone, preferably two or more of different grits. I recommend you buy the largest sharpening stones you can afford. Stones for shop use should be as long as the longest knife you plan to sharpen. Remember that Momma probably owns the really big knives around the house, and you will be expected to sharpen her 8 or 10 inch butcher knives. Smaller stones are handy for field use. Large tool suppliers such as MSC or McMaster-Carr and restaurant suppliers are good sources for sharpening stones.

Natural sharpening stones include both stones found in nature and reconstructed stones. The original Washita and Arkansas stones were quarried natural stones, but now many stones sold by these names are reconstructed. The abrasive material is novaculite, a mineral related to flint and quartz containing mainly silicon dioxide. The relative hardness of novaculite is 6.5 on Mohs scale, just a bit harder than file steel. The original Japanese and Greek waterstones were also from natural sources. Natural abrasives work well on carbon steel knives, but they struggle with harder tool steels and tougher wear-resistent and stainless steels. For modern steels I recommend stones made with manufactured abrasives and industrial diamonds.

Aluminum oxide, which has a relative hardness of 9.2, is also bonded to form reconstructed stones, including Japanese water stones (resin bond) and India stones (vitrified bond). Originally this material was from natural sources (emery and corundum), but manufactured abrasives have dominated since the early 1900s.

Ceramic stones are made from alumina (aluminum oxide) or silicon carbide in a ceramic bond. Silicon dioxide has a hardness of 9.5 and will sharpen anything except carbide tools. Spyderco and others offer ceramic stones in a wide variety of sizes and grits.

Industrial diamonds are made into hones by bonding them to steel and are therefore also called diamond files. Diamond has a relative hardness of 10. Two very different types of diamonds are used in diamond hones. Monocrystalline diamond hones last longer because the diamonds do not fracture readily. Polycrystalline diamond is less expensive.

Diamond hones are made by DMT, Eze-Lap and others. DMT uses monocrystalline diamonds. EdgeCraft's unique answer to bench stones is the Chef'sChoice 400 series diamond file system. It consists of rather thin diamond hones that fit on a magnetic holder. It is a very good value. EdgeCraft has a good pamphlet on sharpening which you can request from the address at the end of this article.

An inexpensive alternative to stones is silicon carbide sandpaper. A piece of silicon carbide (also called wet or dry) sandpaper glued to a wooden block will work as well as a stone. Wet or dry sandpaper on plate glass is popular with woodworkers for sharpening plane irons and chisels, and for flattening the sole of planes. This method is called Scary Sharp by those who promote it.

GUIDES
You will also need a guide to control the sharpening angle. Guides are available for knives, chisels and plane irons. The drawback of most guides is that they waste about 3 inches of stone, so you would need a longer stone. If you mount your stone flush with your work surface, you can utilize the full stone length.

The Razor Edge Guide
The Razor Edge guide clamps on the blade with four Allen screws and I find it inconvenient to use. Also I managed to grind away some of this guide when I tried it on diamond hones. If you find a Buck HoneMaster, buy it. It is a good guide but no longer made.

ROD-GUIDED STONE SYSTEMS
The Lansky rod-guided sharpening system has been the industry standard for years, with good reasons.

Rod-guided systems have a rod on each stone that slides through a hole in the guide. This controls the angle and also prevents scratching the blade with the stone. Since the guide slides on the rod and not on the stone, a smaller stone is needed. Rod-guided systems sell in the $30 to $50 range, depending on the number of stones. A variety of stones are available, including ones for serrated blades. They will sharpen up to a 4 inch blade before you have to move the guide to a new position. Rod-guided systems are available from Lansky, Gatco, DMT, SKARB and others. I prefer the Gatco to the Lansky because of the former's larger stones. I have not tried the DMT or the SKARB.

The angles set by rod-guided systems aren't always what they are labeled and can vary a lot from blade to blade, so be sure to check the angle as described below. I found the Lansky to be off by 3 to 4 degrees on every setting. This is not a problem as long as you are aware of it.

The EdgePro Apex Sharpening System
The class act in rod-guided systems is the EdgePro Apex Sharpening System. Ben Dale, the owner of EdgePro, has spared no expense in his pursuit of excellence in hand sharpening. The Apex is rugged and uses relatively large 1 x 6 inch aluminum oxide waterstones. The angle guide is continuously adjustable for any angle from 10 degrees to over 25 degrees. My measurements confirmed that the angles were exactly as marked. The Apex comes with a good instruction book.

CROCK STICKS
Ceramic rod sharpeners, also known as crock sticks, are completely different than bench stones. The rods are held in a vee at a predetermined angle, and the blade is brought down against then in a slicing motion. You can manually make deviations from the set angle by tilting the blade.
Unfortunately most of these sharpeners come with only one grade of rods so they have limited use. An exception is the Tri-Sharpener from Spyderco. Their deluxe set comes with two pairs of ceramic rods, medium and fine, and a pair of medium diamond sleeves for pre-sharpening. A fishhook groove, a scissors position and a flat position extend the Tri-Sharpener for special uses. The Tri-Sharpener comes with a good instruction book.
Lansky makes a handy folding ceramic rod sharpener called Fold-A-Vee. It folds for easy carrying and features two angle settings for filet and hunting knives.

SLOT GADGETS
There are a whole lot of gadgets on the market that promise easy sharpening. I have tried a lot of them in my quest. Theoretically with slot-type gadgets you just draw the knife through a slot a few times and it will be sharp. Many are worthless gimmicks, but some are worth considering.

The most primitive type of slot gadgets uses a pair of tungsten carbide tool inserts set at an angle. A variation uses a set of overlapping carbide wheels. These literally scrape metal away from the edge, and leave a sharp, but somewhat ragged, edge. This type shapes the initial bevel but provides no way to hone or steel the edge. I bought one and it ended up in my junk box. Benefit from my experience and save your money.

More refined slot gadgets use two sets of ceramic wheels or rods, one medium and one fine. This type hones well but is limited in its ability to sharpen. In the TwinSharp from J. A. Henckels these wheels are in the same slot. In theory you might use both in one pass, but in practice I found that changing your hand position changes which set of wheels contacts the blade. Knowing this, you can sharpen and hone separately. The TwinSharp is handy for touch-ups and I used it between regular sharpenings until I gave it to a friend.

The FireStone 1302 Knife Sharpener from McGowan Manufacturing is also a two stage setup, but with sets of four interleaved medium and fine wheels, each set in its own slot. This makes it more convenient if the knife needs more sharpening than honing or vice versa. The Firestone would be especially handy if you own their electric sharpener, reviewed below. McGowan also makes a variety of other manual sharpeners marketed to fishermen and bow-and-arrow hunters. These feature additional tools like a broadhead wrench, fishhook sharpener and line cutter. Like golf spike tools and shotgun choke removers that are often featured on specialty knives, these tools are indispensable when you need one. I picked up a FireStone SharpPocket because it was winner of a product design competition. This is a single stage sharpener with only medium grit ceramic wheels. Instructions say to go light on the last few strokes to polish the edge. In my opinion the ceramic wheels are too coarse for a good edge, and they wobble. It has joined my junk sharpener collection.

If you are going to benefit from a slot gadget, it must hone at an equal or greater angle than your sharpening. The Chef'sChoice Model 450 uses diamond stones at the same angles (22.5 and 25 degrees) as the final two stages of their electric sharpeners. I keep one in a kitchen drawer for use between regular sharpenings on my Chef'sChoice Model 110.

For about $2, the Normark sharpener is a best buy.
The Normark knife sharpener is an inexpensive slot gadget that can be found at a sporting goods store next to Normark's filet knives. It has two sets of ceramic rods set at 20 degrees. The medium gray rods sharpen and the fine white rods hone. I have used it to restore a slightly dull blade to shaving sharpness. It costs about $2, so it surely is the Best Buy. The Normark's 20 degrees is perfect for touching up a filet knife where the initial edge was 17 or 18 degrees.

There is one class act in every category, and the Meyerco Sharpen-It is it for slot gadgets. Designed by Blackie Collins to be so simple that it could be used on horseback, the Sharpen-It features tungsten carbide wheels for the first stage and fine ceramic wheels for the second. The ceramic is so hard and fine-grained that it is more like using a steel. With this combination, the Sharpen-It performs well at both sharpening and honing.

Unlike other slot devices, the Sharpen-It adds a third wheel to each set, giving two slots, and shapes them so that they sharpen one side of the blade at a time. This setup allows you to vary the bevel angle somewhat. Drawing the knife through at an angle decreases the bevel angle and gives a more razor-like edge. Since it is assembled with tamper-proof screws, I could not measure the bevel angles, but this information is less important because you won't have to use it with another sharpener to get complete results.

Also unlike others, the wheel sets are centered in the body so that the Sharpen-It can be used equally well left-handed. It is so compact when telescoped closed that it can be carried in the watch pocket of your jeans. The unit well built and sturdy, and features a tapered hone for serrated blades. A less expensive model is available without the tapered hone.

ELECTRIC SHARPENING MACHINES FOR HOME USE
Electric knife sharpeners such as those found on can openers grind aggressively but with little control of angle or depth. I've seen many knives ruined by them and they have given electric sharpeners a bad reputation. Here are two power sharpeners worth considering for household use.

The FireStone Diamond Electric sharpener is a fast machine that produces a toothy, aggressive edge with just a hint of a burr. I prefer a more refined edge and the Firestone manual sharpener reviewed above is just the tool to refine it. The instructions don't say so, but the designer recommends pulling the knife through the wheels a few times to align the edge, using it as a manual sharpener. But this edge, right from the machine, will slice right through a ripe tomato while a fine edge may not.

The manufacturer specifies 23-degree bevels on all their FireStone sharpeners, but I measured 18 to 19 degrees on the electric sharpener and 21 to 22 degrees on the manual sharpeners. The18-degree hollow ground edge would be another reason for the aggressive cutting. The FireStone manual sharpener would hone it to a longer-lasting angle.

Note: there is no generally accepted method for measuring bevel angle of hollow ground blades. I like to measure the wheel diameters and spacing and calculate the angle at the edge by trigonometry. The manufacturer suggests measuring the average angle of the entire bevel, which varies with blade thickness. My low number is the angle at the edge, and my higher number is the average angle for a blade with 0.020 thickness at the back of the bevel, typical of a hunting knife.

The FireStone design features four interleaved, counter-rotating wheels like commercial machines, but without the adjustable angles that make the commercial machines so expensive. The wheels are 220 grit diamond-impregnated ceramic. I found it difficult to sharpen close to the bolster with the FireStone electric sharpener and, because it grinds so fast, you cannot play around much without grinding a swale into the blade. The instructions say you might need up to 10 passes on a new blade, but I found that every blade I tried was sharpened in a single pass and begin to show loss after only 3 passes. I suspect that repeated use of this sharpener would reduce knife life or require professional sharpening to re-shape the blade. I would prefer that this sharpener used finer stones and a slower speed.

EdgeCraft's Chef'sChoice Model 110 uses 3 sets of diamond hones and each sharpens at a different angle. The first stage is very aggressive, grinding even faster than the FireStone, but it is only used once to pre-shape the bevel. From then on you use the second and third stages (sharpening and honing) only. The final honing is at a very sturdy 25 degrees, which will give very long edge life. The Model 310 is similar, but with only the final two stages. The Chef'sChoice has a tendency to scratch the sides of a blade, so I can't recommend it for a collector, but it is great for working knives.

Please visit steve's homepage for more valuable information related to sharpening and other aspects of knife care. You can find him at: www.sharpeningmadeeasy.com

Don't forget to take care of your knife!

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