H O N I N G G U I D E
S
The first trick to getting a tool sharp is to shape the edge to the correct angle of sharpness. For a carving gouge, this might mean an edge ground at an angle of 17.5°, while for a woodturning gouge, the angle might be 60°. The exact sharpening angle depends on the qualities of the alloy from which the cutting edge is made, the way in which the tool is intended to cut into the wood, and the types (or hardness) of wood that will be cut - but that's another discussion. For now, I'll just keep the discussion to this: a blunter angle holds a more durable edge, but also requires more force to cut the wood. If the current angle doesn't work for you for whatever reason, make an adjustment to the tool's edge angle next time you sharpen. I recommend making adjustments of only 2.5° to 5° at a time, as it's easy to overshoot the ideal angle. A protractor can help you to find the same angle each time, but a shop-made jig that lets you set a tool into a honing guide in exactly the same position each time can greatly speed up the process.
In any case, the first step, after laying out and preparing your sharpening gear, is usually to place the tool into a honing guide, which will hold the tool at a fixed angle to the stone while it is ground. The tool is then ground on a coarse or medium stone to remove the damaged or dull edge. Once the old edge has been ground away, a thin flap, or burr, of metal will form at the cutting edge. This is very similar to the phenomenon of wood tearing out on the exit side of a crosscut; the metal on the surface of the tool that is opposite the grinding stone bends away from the cutting surface rather than being cleanly ground away. Once the burr has formed, you know that the first grinding step is done, and it's time to move on to honing, or refining the edge.
Honing is where a lot of woodworkers diverge in opinion as to exactly how the sharpening process should continue. There are many schools of thought, each with its own advantages and disadvantages with respect to particular tools and the techniques with which they are used to work the wood. Whether a hollow grind or a microbevel or a flat grind is used, the end result is to produce a very smooth edge with a very thin, small burr and only microscopic scratches. That's all detailed elswhere, so continuing on with the nickel tour...
The last step of sharpening most woodworking tools is to polish the fine scratches away. Polishing is most often done with a leather strop and a polishing compound, but there are other methods as well. Whatever the method, the concept is to leave a cutting edge that is a perfectly straight line or smooth curve, formed by two surfaces meeting at an edge that is only one atom wide. That's the concept, anyway. The reality is a bit rougher when viewed under an electron microscope, but to the human eye, as an experienced woodcarver once told me, "If it's not a mirror, it's not sharp."
That's as near to a full description of the process of sharpening any given woodworking tool as I can get, without making it into a whole book of its own, that is. The next step after learning to correctly sharpen a tool is to learn to recognize when it's time to repeat the process. It's always much easier to re-sharpen a slightly dull tool than it is to recondition and sharpen a badly blunted tool, so it's best to get in the habit of regularly checking and sharpening the edges on your tools. Many carvers keep a strop handy while they carve, and will touch up the edge of a gouge every fifteen minutes or so while they carve. By maintaining the edge on their tools in this way, carvers can often go for days, or even weeks without having to go back to a stone and grind the edge.
H O N I N G G U I D E
S
A honing guide is a simple sharpening aid which holds the tool at a fixed angle to the stone (or to whatever will be doing the grinding). It makes sharpening easier for those who do not wish to spend the time required to learn to hold the tool freehand at a consistent angle and avoid digging it into the stone. By making it easy to control the grinding angle, it allows the woodworker to remove less metal in order to get to a sharp edge, thus prolonging the life of the tool.
There are two important things to consider when purchasing a honing guide for your tools: First, is the honing guide the correct size and type for your sharpening stones? I prefer the precision of a honing guide that rides on the stone's surface, but for smaller stones and some diamond pads, a guide that rides on the bench top next to the stone might be more suitable. Second, is the honing guide big enough and the correct type for your tools? The variety of honing guides available for all different types of hand and power sharpening systems is endless. If you make furniture, you'll need a guide that is wide enough to accommodate your widest chisel or plane iron. If you carve, you'll want a honing guide that allows the tool to be rocked from side to side as you sharpen the curved bevels on the various gouges. Some honing guides also feature devices that make it easy to form a microbevel during the honing process, which is a very handy feature if you sharpen without the aid of power grinding equipment.
In any type of sharpening guide, there are a few features that I look for before I buy. First, I look for a design where I can lock a tool into the guide so that it is held rock-solid, without the need for a wrench or other tool to tighten the clamping screw. If I can twist or wrench the tool loose, I look for another guide: after all, if it's that easy for the tool to wriggle loose, it will surely do so once I start applying the repetitive stress of grinding. Next, any tool with a good fit and finish on its parts tends to perform better and last longer in use than a tool that is shoddily produced, as evidenced by the poor fit and rough finish of its parts (not to mention the thick, sloppy paint job). In any case, don't be afraid to spend a little extra for truly good sharpening equipment. Remember that dull tools just aren't worth the effort of using, and a crummy honing guide won't encourage you to sharpen when you should, therefore the tools won't be sharp, therefore… I think you get the point, right?
Varieties of guides include those that ride on the stone or those that ride on the bench next to the stone, and types which clamp on to the sides of the chisel or plane iron, or those that reach around it and clamp on it from the top. I find the top-clamping styles are usually easier to tighten properly and set the sharpening angle than the side-clamping ones. I've also found that side-clamping honing guides "automatically" square the tool to the stone, so that the tool's edge is ground at exactly 90° across as most chisels should be, but you'll have to beware: such guides are seldom actually made perfectly square.
I prefer honing guides that ride on top of the stone with the tool, since that tends to give a more accurate and consistent angle than a guide that rides somewhere on the uneven bench top next to the stone. However, for smaller stones, the bench-riding guides make it possible to use the entire surface of the stone, as long as a smooth flat surface is available on which to sharpen. It is easy to produce a microbevel by doing the final honing with the guide riding on a piece of cardboard or thin plywood, thus increasing the angle at which the guide holds the tool. For the larger stones, guides that ride on top of the stone are simpler to use, and some models mount the roller on a cam mechanism which allows you to produce a microbevel easily.
A feature that I look for in a honing guide for chisels and planes is that the guide provides a firm stance when the tool is moved on the stone. Many guides have a narrow roller to ride on the stone, which makes it especially easy for narrow chisels to tip, rounding their corners or gouging the stone in the process.
There are a number of specialty jigs for sharpening carving gouges, V-tools, and skew chisels. Varieties can include bench and stone riding models, with the same advantages as listed above. For the most part, though, carvers tend to sharpen their tools either freehand or with power grinders designed especially for sharpening woodworking and carving tools. One popular honing guide for sharpening gouges is called the Oar Jig. It holds the tool by the shank, and features a curved foot to ride on the bench and facilitate the rocking motion necessary to grind along the entirety of the curved bevel. The freehand grinding technique usually involves placing the wrist on the bench top to act as an anchor point, while the rest of the hand moves the tool in a sweeping motion that rocks the tool as it is moved along the length of the stone, thus grinding the length of the curved edge.
There are specialty jigs for sharpening hand scrapers, including guides for filing them accurately, and special burnishing jigs with dial adjustments for the burnishing angle. Whether to use the jigs or sharpen freehand is up to you, and your results with any method will depend upon practice and good quality tools.
There aren't many guides, per se, for sharpening knives, but there are many jigs and specialty knife sharpeners that are designed to get the right angle on a knife blade. Some have stone or ceramic rods or wheels that cross at the desired angle for sharpening the edge, so if the knife is held perfectly vertical and drawn through the jig, the edge will be beveled at the correct angle on both sides.
Saws are generally sharpened by filing or by grinding with a power grinding stone and honing is not really part of the process. The teeth of a handsaw do need to be jointed flat so the tips are all in the same plane before they are filed or ground for sharpness. For that job, there's a jig rather like a honing guide that holds a file perfectly square to the blade for jointing. The jointing jig is rather like a scraper filing jig, in that it holds the file in a slot in a wood block so that the face of the block, when placed flat to the side of the blade, will set the file at 90° to the saw for jointing. The difference is that there is usually a notch in the face of the block next to the file so the saw teeth won't rub directly against the block.
O I L S T O N E S
Oil stones include natural stones such as the famed Washita and Arkansas stones. Similar stones are or have been quarried in Turkey, Crete, France, the Charnwood Forest in the U.K., and elsewhere around the world. Many reportedly work equally as well with water as with oil, but oil is often preferred for its greater ability to prevent these stones from clogging as they are used.
Among Arkansas stones there are several grades of fineness. Washita (Ouachita) stones are the coarsest, and are used for faster cutting to restore a heavily dulled edge, and for sharpening tools that don't need a highly refined edge. Soft Arkansas and Hard Arkansas stones get progressively finer, and are used for more refined cutting edges, such as kitchen knives. The finest Arkansas stones are Black Hard Arkansas (Surgical Black) and Translucent Hard Arkansas. These are almost porcelain-like in texture, and are used to produce the finest edges for medical instruments, woodworking tools and tools for leatherwork. All of the Arkansas stones are quarried from a novaculite deposit (a sort of flint) found in the Ouachita Mountains of Arkansas, and their color varies from white to tan to reddish to bluish-black. Color does not indicate the grade of the stone; rather the texture and density of the sediments in the deposit are used to grade the quarried stones. Obviously, since the stones are quarried from a natural deposit of stone, the quality and texture of every stone is unique, so not every stone of a given grade will produce the same surface finish or cut with the same speed. In general, older Arkansas stones are of better quality than the newer ones, and truly natural stones are better than the reconstituted Arkansas Perfect stones.
Manufactured Oil stones are usually made from alumina or silicon carbide abrasives that are bonded to produce India stones and Carborundum stones, respectively. Since they are manufactured from synthetically-produced abrasives, these stones offer greater consistency of cut from one stone to the next. The down side is that the way the abrasive particles are bonded to make the stone causes them to cut more slowly than natural stones. Also, the denser structure and smaller pore spaces on synthetic Oil stones cause the stones to glaze more easily. The end result of the various pros and cons is that most woodworkers use coarser grades of synthetic stones and finer grades of natural stones when sharpening, in order to get the best features of both.
W A T E R S T O N E S
Water stones have been around just as long as oil stones; it was found early on that some stones needed oil to keep them from clogging in use, but most worked just fine with only water. One advantage to water is that it cleans up more easily, since once a stone is soaked in oil, it will always tend to seep oil onto whatever container it is kept in. The best-known natural water stones include Japanese stones from the Honyama quarry near Kyoto, Belgian coticules from the Ardennes region, and Slate honing stones from Wales. Most commonly available stones these days are synthetically produced in such a way as to try to mimic the natural stones as closely as possible.
Japanese Water stones
Water stones are the traditional Japanese sharpening method, and they have two advantages over traditional oil stones. The first is that they use water as the lubricant, not oil. Water is cheaper than oil, and using water reduces the mess involved in sharpening, since the water can simply evaporate, while an oil stone will always leave an oily mess wherever it is stored. The second advantage to water stones is that they remove material quickly and can produce a mirror finish without the need for polishing on a strop. This is because water stones wear away relatively quickly as they are used, continuously exposing fresh abrasive particles and producing a slurry on the surface of the stone. This slurry aids in the sharpening and, on very coarse, porous stones, it also holds water at the surface of the stone.
Water stones should be soaked in water for several minutes before use, so the porous stone will become saturated with water. To prepare the stone for use, it is customary to rub it with a nagura, which is used to rub down any high spots on the water stone and produce an abrasive slurry with the swoof. It is this abrasive slurry that does much of the cutting during the sharpening process, reducing wear and hollowing of the stone to a minimum. Periodically, during prolonged sharpening sessions, the stone may need to be cleaned and rubbed doen again with the nagura to keep its surface level and revive the slurry. When the sharpening is finished, the stone should be rinsed clean to prevent any swarf from rusting in the pores of the stone. Water stones may be stored wet, in a "stone pond," or patted dry and left on edge on a cloth or paper towel and allowed to dry out.
It should be noted that oil should never be used on water stones, nor should they be kept in a stone pond with antifreeze. Most water stones that are readily available on the North American market are manufactured stones, rather than natural, quarried stones. The resin bond on these stones will often dissolve if oil or antifreeze are used on the stone. Water stones should also be kept from freezing unless they are absolutely dry, as the expanding ice crystals would easily shatter the stone. When using a water stone, remember that it is a rather soft stone, and avoid digging sharp corners or cutting edges of tools into it. Care should be taken when sharpening curved edges, such as carving gouges and V-gouges, since even moderate pressure, when concentrated on the very small area of metal in actual contact with the stone, can result in crushing the porous stone and leaving a nasty gouge or groove. As stones wear with use, they will naturally become hollowed in the center, where they are worked the most. Flattening a water stone is simple: just tape some wet-or-dry sandpaper to a flat surface, such as plate glass, and wet-sand the stone flat. For stones around 800 grit or finer, sandpaper in the 220 to 320 grit range is fine. For coarser stones, use 100 grit paper if it is available.
Nagura Stone
An accessory for Japanese water stones is a natural stone called a nagura. It is used as a sort of dressing stick to help level the stone's surface if it becomes warped or hollowed. It is also used to remove any irregularities in the stone that would scratch the blade being sharpened. Nagura stones also find use in preparing finer finishing stones for use. The nagura is rubbed over the stone just prior to sharpening to produce a slurry, which helps the stone to wear more evenly and also speeds up the honing process, as the slurry will often be able to cut more quickly than the grit that is still imbedded in the stone's surface.
Belgian Coticule & Belgian Blue Water Stones
These are natural, quarried stones from the Ardennes region, and are highly prized for sharpening knives, straight razors, and woodworking tools. The abrasive material in these stones is spessartine garnet in a soft schist matrix, so these stones wear similarly to Japanese water stones. The rounded crystals of garnet cut slower than many aluminum oxides, but generally leave a smoother surface. The yellowish Coticule stones are considered to be of higher quality since they contain a greater percentage of abrasive and most of that is the type with sharper grains. The Belgian Blue Water stones contain a little less garnet, but in slightly larger, smoother-shaped crystals. As with Japanese water stones, the sharpening process often involves first rubbing the stone with another to produce a slurry paste. An already sharp edge can be finished by drawing it backwards over a clean stone with water alone. The final polish is put on the edge with a strop and polishing compound.
Welsh Slate
Little information is forthcoming about Welsh (or Scottish, or any others) slate honing stones, but they seem to be a fairly fine abrasive grade. Slate is not usually composed of very abrasive minerals, but it can contain certain amounts of garnet and silica; both of which are softer, "milder" abrasives than aluminum oxides.
D I A M O N D S T O N E S
Diamond "stones," really diamond plates, use a layer of industrial (synthetic) diamonds bonded to the surface of a metal base, usually by using a heavy plating of nickel. Since diamond is the hardest material known to mankind, these stones have the potential to outlast all others, and will never need flattening. They don't need a lubricant, although they can be used with water or oil when desired. Diamond plates can also be used to flatten other kinds of sharpening stones, although this may lead to reduced life for the diamonds, since the abrasive grit removed from the other stone can wear away the plating that bonds the diamonds to the base metal.
Since manufactured diamonds are usually used as the abrasive, the process used to produce the diamonds has a tremendous effect on the quality of the final sharpening plate. The detonation process uses explosives to create the heat and pressure necessary to make diamonds from carbon. Due to the brief, explosive nature of the process, the diamonds that are produced are composed of many sub-microscopic crystals of diamond that are usually clumped together, making what is called a polycrystalline diamond. As they are used, polycrystalline diamonds tend to break apart into smaller clumps and eventually into individual, sub-microscopic crystals. This kind of fragmentation makes for a cutting action that starts out very fast, and progresses to a slow cutting abrasive that leaves a very fine finish: perfect for polishing and lapping compounds, but lousy for a sharpening plate that only has one layer of abrasive over a steel base.
Better quality diamond plates use monocrystalline diamonds that are produced using some variation of the high pressure, high temperature (HPHT) process--originally developed by General Electric--which uses much longer periods of heat and pressure to grow larger crystals. These larger, single crystals behave a lot more like natural industrial diamonds in the way they cut and wear down over time. The result is a more expensive, but much longer lasting abrasive.
Since diamond is the hardest material known to science, it tends to make an excellent abrasive. There are limits, however, to what jobs diamonds can do. Since the diamonds on a diamond plate are held in place by a heavy nickel plating, these plates are not necessarily the best thing to use to flatten other sharpening stones, as the grit produced would tend to wear away the plating from around the diamond grit, thus reducing the life of the diamond "stone."
Another job diamonds have difficulty with is high-speed grinding of ferrous metals. Diamond crystals are only stable within a certain range of temperatures and pressures: heat up a diamond too much and it will vaporize. The issue with grinding is that the high heat and pressure created at the grinding surface allows the iron atoms from the tool that is being ground to "rob" the diamond crystals of carbon atoms, thus wearing the diamonds prematurely. A better alternative for this use is CBN, or cubic boron nitride, which is nearly as hard as diamond, and is more stable under high-speed grinding conditions.
C E R A M I C S T O N E S
Ceramic stones are a relatively new trend in sharpening. They often don't need a lubricant, although they can be used with water. Ceramic stones usually cost less than comparable diamond plates, and they come in finer grits than do most diamonds. The abrasive used in ceramic stones is alumina (synthetically made aluminum oxide) - the same type of abrasive used in Japanese water stones - which is mixed with varying amounts of ceramic bonding agent and "welded" together by sintering in a furnace. Depending on how hard a bond is used to form the stone, a ceramic stone may wear very slowly or almost not at all, which means that it will not need regular flattening as Japanese water stones do.
S A N D P A P E R &
G L A S
S
Also known as the scary-sharp system, sharpening with wet-or-dry sandpaper on a glass plate is essentially like sharpening on a lapping plate. The advantages are that the "stone" never wears hollow in the middle, and the system is inexpensive to get into, since the only costs are a piece of scrap glass and some sandpaper (plus a good honing guide, of course). The disadvantage of the system is that when used regularly for sharpening, the sandpaper wears out very quickly and more time is spent working on worn out paper or in replacing worn sandpaper than would be spent flattening a good waterstone. One of the best uses for this system is as a stepping stone when first putting together a proper sharpening system, since it can be used both for lapping and preparation of new tools (or reconditioning of older ones) as well as for sharpening them, at least until a better sharpening system can be fit into the budget. In my own shop, I prefer to use Japanese water stones or power sharpening systems for regular sharpening, and keep the glass and sandpaper handy primarily for use as a lapping plate.
F I L E S
File Grades
Steel files are generally not used to sharpen most edge tools such as chisels or plane irons, since the steel used in these is very hard, and the file does not produce a smooth enough surface. Tools which are sharpened with files include scrapers, saws, axes, adzes, certain drills, etc. Steel files come in several grades, as follows:
Double Cut Files
Fast cutting and long lived, but leaves a rough surface. Best suited for softer metals, and not normally used for sharpening.
Single Cut Files
Slow cutting and tends to wear out more quickly, but produces a smooth surface. Used on harder metals and for final shaping of the edge. Available in smooth (finer) and bastard (coarser) cuts.
Diamond "Files"
Not really a traditional steel file. Consists of a handle (usually plastic) that holds a metal plate or rod with diamonds bonded to its surface. Found in grits from ~220 to ~1200. Useful for finely finishing a filed edge, touching up the edges on router bits and other carbide tools that cannot be filed with steel files, and touching up knives and woodturning tools between sharpenings.
File Types
Files also come in a variety of shapes, according to the tools to be sharpened, as follows:
Mill File
This is the flat, single-cut file that is most commonly seen and also the most useful for any number of tasks. Used for jointing the teeth of many saws, sharpening axes and scrapers, easing sharp corners on new cast iron machinery, and innumerable other jobs.
Triangular or Saw File
A file with a cross-section in the shape of an equilateral triangle. Used almost exclusively for sharpening Western-style saws. Available in a range of sizes; the best size for any given job has faces a little more than twice as wide as the height of the teeth on the saw to be sharpened. Since the corners and adjacent faces of the file is used for cutting the space between the saw teeth, saw teeth that rub more than half the width of the faces will result in the middle of each face being dulled twice as quickly as the rest of the file.
Feather or Feather-Edge File
These are the sharp-edged triangular files developed for sharpening Japanese saws, which have very acute angles between the teeth that no other file could ever fit. Available in full-cut and safe-face versions, in which one face is left uncut (safe) to avoid accidental damage to surfaces adjoining the one being filed.
Round or Chainsaw File
A round file that, as its name implies, was developed for sharpening chainsaws. These come in a variety of diameters to fit various sizes of chainsaw teeth.
Crosscut File
A specialty triangular file that combines the features of a feather file on two faces with a half-round file for the third. The round face is for cleaning out the gullets between the teeth on larger crosscut and bucksaws.
Auger File
These files are made double-ended, and are flat in cross section but highly tapered along their length. Meant for sharpening augers, they feature milled faces with smooth edges on one end, and milled edges with smooth faces on the other end. Because of its combination of cut and safe faces, this file is very useful in fitting and deburring parts.
S L I P S T O N E S
Slip stones are simply contoured sharpening stones, usually of finer grits. They are used to maintain the unusually-shaped bevels on carving gouges, and sometimes on woodturning gouges. Slips can be oil stones, water stones, ceramic or diamond, and come in rounded, triangular, square, cylindrical, and roughly conical shapes, as well as multiform stones that combine several shapes into one stone.
S T R O P
S
A strop consists of a piece of leather or (occasionally) hard felt mounted on a backing board, and usually loaded with a polishing compound such as chromium oxide. A strop may also be made from fiberboard that has been shaped to fit a particular tool's edge and loaded with a polishing compound. The purpose of the strop is to polish the cutting edge and to remove the fine burr which is left on the edge after honing on the finest stone. If this burr is not completely removed, it will quickly break off in the wood and leave a dull edge. Other advantages to producing a mirror edge are a noticeable reduction in cutting friction, and a polished cut. If the strop is mounted on a board, then the tool can be stropped while still in a honing guide. Simply draw the tool over the strop, being careful to only move the cutting edge backwards over the strop; otherwise the sharp edge will cut into the leather.
S T R O P P I N G & P O L I S H I N
G
This is a listing of some compounds that are commonly used to achieve the final polished cutting edge on tools. Compounds are usually composed of an abrasive (or a blend of abrasives) in a wax base. The wax serves to hold the abrasive to the strop or to a polishing wheel, and the hardness of the wax varies depending on whether a softer, stickier compound or a harder, low residue compound is desired. Many of the same abrasives are available in paste or powdered form as well. Compounds are listed in approximate order from coarsest to finest, and the color and composition of the compound are listed in parenthesis. Please note that color alone does not indicate which compound a particular sample might be, since the wax base is often dyed to produce the colors.
P O W E R S H A R P E N I N G
S Y S T E M
S
Power sharpening systems for woodworkers have proliferated on the market over the last few years. They can be roughly grouped into two categories: dry grinders and wet grinders. Dry systems tend to be less messy, but also tend to have more problems with heat buildup due to the friction of grinding hardened steel. Wet systems run at lower speed to avoid flinging water about, so they keep the tool nice and cool, but there is the issue of cleaning up the splashes and drips, and avoiding problems with rust.
Dry grinders include bench, or pedestal grinders as well as powered lapping systems. Lapping systems usually run at a lower speed and offer much finer grit abrasives, so they can produce a finished, polished cutting edge right from the grinder. In woodworking, bench grinders are used for grinding primary bevels on tools and for restoring nicked cutting edges, or any other job where a bulk of metal must be removed.
There are two basic types of bench grinders; regular and slow speed. Slow speed grinders (~1725 rpm) are preferred for tool sharpening because they minimize grinding friction, thus producing less heat. High grinding temperatures (above 500°F) are the enemy of tempered steel. There are also a number of types of grinding wheels: gray, blue, pink, and white. Most bench grinders come with gray wheels on them having grits of 60 and 80. These are useless for sharpening, as they are very coarse, and will quickly heat tools. "Softer" wheels, from blue to white, which is softest, continually expose fresh abrasive, reducing heat buildup and removing metal more efficiently. Blue and pink wheels are used mostly for sharpening lathe tools, and white wheels are used for all edge tools.
The small, flimsy tool rests that come with most bench grinders should be removed and replaced with a rest designed specifically for sharpening. These have a larger surface for the tool to rest on and can be adjusted for the individual tool's bevel angle. Specialty grinding rest systems are available for woodworking tools such as chisels and plane irons, for drill bits, and for woodturning tools, which are often ground exclusively on a slow-speed bench grinder and then taken straight back to the lathe. Advantages to having a good tool rest include wasting less time and metal each time you grind, since without a proper tool rest, it is impossible to match the existing angle accurately. Speed and repeatability are other major advantages to a good sharpening system, especially for woodturners, who sharpen frequently.
Lapping systems that are designed especially for sharpening woodworking tools can take much of the tedium out of keeping your tools sharp while minimizing the heat produced when grinding, plus they often cost less than wet-wheel grinding systems. A number of lapping systems use a revolving glass disk to which adhesive-backed abrasives are applied. The glass ensures a perfectly flat surface, and the abrasives grade from coarse (~180 grit CAMI) to polishing abrasives (>6000 grit JIS).
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Water cooled grinding systems prevent overheating of the tool by bathing both the grindstone and the tool's edge in a water bath that also serves to lubricate the grinding action and carry away the swarf. These systems run at low speeds (typically from 90 up to 500 RPM) to avoid flinging water about, which also means that they tend to remove material more slowly than a bench grinder can. Wet grinding systems can be of either the horizontal or vertical wheel types.
Horizontal wheel grinders use a wheel set on its side and use the top face to do the grinding, much like a millstone is used to grind grains into flour. The advantage to this system is that the grinding surface produces a flat bevel rather than a hollow grind. The main disadvantages are that it is easy to wear a groove into the face of the wheel and the water supply is usually of the drip type, and must be refilled periodically to keep the wheel wet.
Vertical wheel grinders use the edge of a vertically mounted wheel to do the grinding, and have the bottom of the wheel immersed in a water trough. The wheel's rotation causes it to carry a film of water around with it, which keeps the grinding action cool and lubricated, and since the water tends to naturally drain back into the trough, there's much less need to periodically refill it. The main disadvantage to vertical wheel grinders is that since they use the curved edge of the wheel, they produce a hollow grind on the tool. Many woodworkers feel that any hollow grind weakens the shape of the tool's edge, although in purely practical terms, the degree of weakening of the edge depends greatly on the degree of hollowness that is ground into the tool. Larger wheels, such as is used on the larger of the Tormek machines, produce such a shallow hollow on the tool that any weakening of the edge is laughable. One need only calculate the change to the effective cutting angle at the tool's edge (the main bone of contention with flat-grind advocates) to find that the change from a 25° flat grind to a 25° average angle with a hollow from the 10" diameter wheel is miniscule, and any amount of honing on a fine bench stone would remove it entirely if it were still a concern.
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