By: Dick Anderson of Savage Forge
Well, what if you screwed up and lost the temper by overzealous grinding? Shame on you, but don’t commit suicide. We can fix it…chances are. Notice how the file is dragging at the edge now. At least we have a tool with a decent high-carbon spark and the tool had been working well enough. Or maybe it never did perform well and there’s nothing to lose by trying a repair. Or maybe your shop burned down and you’ve got a couple dozen ruined edge tools. We’ll want to restore the temper.
You need a heat source capable of heating the business end of the tool to the critical temperature. I recommend propane or Mapp gas brazing torches, two positioned opposite each other, if necessary. Weller, Turner and Bernzomatic are acceptable brands. Specifications should indicate a “target heat” of 3600º F. (I prefer simple torches without push-button ignition, often available from hardware stores only by special order. Maybe Americans have gotten too lazy to use a spark striker.) Have a small magnet handy. Also required: the quench medium in sufficient volume to chill the tool speedily. This fluid delivers a sudden shock to the steel at the point when it is extremely vulnerable, its molecular structure highly excited. Warpage can occur (there’s always some deformation) and cracking is pretty much fatal. Just the same, in over thirty years, I’ve heat-treated many hundreds, if not thousands, of factory-made tools successfully and cracked only one. Take heart.
But first, remove the tool’s handle. Not always necessary, just usually. You might want to snub the edge to a slight dullness less likely to cause the thin metal at the edge to overheat, compared to the thicker body of the tool. In new factory-made tools the 45º “factory edge” serves that purpose. Don’t worry; just slightly dulled will suffice. We’re being much more controlled than factories and their ovens. Grasp the tool by its tang with a vise-grips pliers or a reasonable facsimile. If it’s a long-bladed tool maybe the handle can stay on, or you can hold it safely in hand by the tang. You guess.
The best medium for quenching while reducing the amount of warping is vegetable oil. You might try the spent, cloudy stuff in your deep fryer, just don’t keep cooking with it afterward. Really massive pieces of steel can cause the oil to ignite (gonna stink some at least anyway) so there needs to be enough depth in the slack tub (quench tank) to submerge the heated part of the blade well below the surface. This avoids cavitation around the piece, which would otherwise allow oxygen into the equation, permitting ignition. No cavitation, no oxygen, no fire. All the same, keep a cover at hand to smother any possible combustion; a fire extinguisher might be advisable, at least for your peace of mind. The main thing is to plunge the yellow-hot tool deeply and not withdraw it until quenched hard and relatively cooled.
The goal here is to control warpage as much as possible by cooling the steel at the slowest rate at which it will harden fully. That condition is called 100% martensite and reflects the ultimate hardness achievable for a given kind of tool steel. Good high-carbon steel fully hardened is harder than glass and a sharp corner will scratch your window if you don’t believe me. That’s too hard for cutting wood, however. The steel is brittle and a fine edge may crumble if the blade doesn’t break. We need to mitigate the situation. Call that tempering. Proper heat-treating must be done in at least two steps.
Now, more on hardening. This is the crux of toolsmithing. All reasonable effort must be applied to raise the heat to harden as uniformly as possible. Heat the front couple inches of the tool slowly, turning it over frequently (even with opposed torches) to where it loses its magnetic attraction. You’ll observe a sudden brightening to incandescence. Be careful touching anything to 1450º steel. Put a grip or handle on the magnet or touch the steel to a magnet fixed in position. Hold the steel at the critical temperature at least a few seconds, longer for thick sections. Quench immediately, plunging the blade swiftly straight (edge first) into the medium. Swirl it around in a figure-8 motion to help prevent cavitation and minimize warpage. Withdraw the tool after the medium has calmed from its agitation by the heat (there will be bubbling and steaming) but while the piece is still hot to the touch, about 200-300º Fahrenheit. Test now or later with the file, or scratch your window. Hard as the hubs of Hell, I hope.
If not, we better try a faster quenching medium. Next in line is water heavily laced with soap. This medium is less drastic than water unadulterated. I use a 20% (four-to-one) solution of water and dishwashing detergent. This formula seems to approximate the performance of commercially available “tempering fluids.” More sudden is pure water, preferably stale water. (An ancient prescription called for the urine of a red-haired boy for tempering. Less air in the water, less cavitation.) All in all, fine edge tools should harden completely in oil. Big, thick poorly forged items or steels lacking the optimal carbon content may require water quenching. And then there’s brine quenching. If you fail to harden in these mediums and really need that tool, try water in solution with 20% salt. It’s desperate; the last resort next to mercury (not recommended) but it could help in a pinch. Odds of failure increase, however, with each successive hardening attempt. Cracking and warping due to induced stress.
Suppose we’ve somehow gotten the tool hard as the proverbial hubs. The file won’t touch it, eh? Congrats! Now polish at least one side of the blade shiny enough to see if there are any cracks (and later, to display the oxide spectrum colors). We need to remove the gray oxide left after hardening. Go with the satin finish compound on rag wheels; otherwise, fine sandpaper. Be sure to expose all the hardened part. Examine. Okay? Not too warped? On we go.
The second step in proper heat-treating is tempering, reducing the extreme hardness of the hardened metal. Turn the torch or torches down to the lowest level it or they’ll stay lit. You may be better off with a regular plumber’s sort of soldering torch for especially small blades. We want to reheat the tool now as gradually as we can stand to wait for. Start from well behind the hardened zone. Play the flame(s) gently on the steel, turning the piece often as it heats. You’ll see the polished metal begin to take color, initially a pale yellow. This is the first color in the “oxide spectrum,” the most accurate measure of temperature currently possible. You’re at about 400º now, where tempering commences. The file was tempered to this temperature. At this point much integrity has been returned to the altered internal structure of the steel. It is much stronger already than it was as hardened. Keep going patiently: these changes of state benefit by slow adjustment. Also, slow equals uniform; go too fast and you grossly multiply the risk of cracking the brittle hardened steel due to uneven heating. Toolsmith’s Russian roulette with six bullets.
Once the first pale straw color is drawn, a progression to darker colors continues marching forward toward the cutting edge. You want to stop the process at the hardness most appropriate for the design and function of the particular tool. Judge that by the color at the edge. Darker colors show behind, giving a gradient beneficial for structural integrity—strength and resistance to impact. It’s best to stop the progression by air-cooling, but quenching may be necessary, preferably in the same medium as used to harden. Hard-to-sharpen items like veiners and parting tools can be left pale straw the whole length of their blades because their shapes are strong. So too deep gouges, sweeps seven to nine. Drawknives, scorps and plane irons used for hours on end to delicately finish whole square miles of surface area should be left file hard or just a bit less. That old slick planes away for hours, days, maybe weeks between licks from a strop, doesn’t it? Shallow gouges, chisels, knives and any delicate blades or tools likely to flex in use should be tempered to darker colors, in roughly that order, from medium straw to dark straw, peacock (dark straw mottled with greenish purple) and then solid purple. Blue is the final tempering color, best used for springs, much-flexed edge tools and to impart wear resistance to machine parts. Lots of cold-cutting blacksmithing tools, like cape and cold chisels, are tempered to blue. Heat the steel still more and it turns gray. Start over.
In the manufacture of truly fine edges, all the above-mentioned processes are greatly enhanced if the steel is prepared for the experience by proper forging. Moreover, thinner yet stronger blades are possible only with thorough forging, ideally by many blows of relatively small power hammers or by hand forging. Thinness of blades is crucial to the performance of deep gouges, veiners and parting tools, which penetrate far into the wood and travel along more readily in the cuts than would thicker blades. Thin-bladed shallow gouges and chisels can more often be pushed by hand through wood, leaving long, smooth toolmarks, compared to thick tools requiring use of mallets, leaving ridged, crude-looking toolmarks. Well-forged steel also results in edges less likely to fail due to chipping or bending when too low a bevel angle is applied.
One standard procedure in pro tools shops and some small tool factories involves annealing tools prior to hardening them. Supposedly this treatment “relaxes” the internal stresses induced in the steel by grinding, machining or improper forging. You heat the item to the critical temperature then slowly cool it in a cycle-controlled oven or by burying it in some insulating medium; I use sifted wood ash, but lime or dry sand are effective. Just as good or better in my experience, is to merely heat the piece to critical temp then let it cool in still air until it no longer glows. This is called “normalizing” and may be performed just before final heating-to-harden. In extreme cases, where the tool’s shape is wildly asymmetrical and unlikely to quench (or take heat) evenly, it may help relieve stress further by tapping on the steel with a light hammer as it cools in normalizing. Old trick of the trade.
Another advantageous procedure is to quench with the thickest part of the tool entering the medium first. That is generally impractical with gouges and chisels, but can be applied with knives, drawknives, scorps, etc. It slightly improves one’s chances for success. Some knife shapes may suffer less warpage quenched edge first; only doing production-size batches both ways and comparing the results can conclusively determine the best treatment. In any case, it’s probably always best to make sure the blade is horizontal over the surface of the medium as it enters. No certain exceptions come to mind. Once in the tub, change the angle to roughly vertical as you swirl in the figure-8 motion. The warp given to long, thin blades can be corrected if the tool is withdrawn with 200-300º of heat remaining in the steel. Holding the blade in hands protected by thick gloves or rags, gently apply pressure opposite the direction of the bend. Straighten it. Inspect. Once corrected, it usually stays straight. To be safe, stay with it until cool, repeating the pressure as needed. That figures to be one of the cutlery trade’s oldest and best-kept secrets. Doesn’t work with thick or stiff blades; you’re stuck with that warpage, pardon the expression.
Here’s another great insider’s gambit: oven pre-tempering. Use it for all tools having critical shapes, like those with wide or thin, asymmetrical blades. Your kitchen oven should work fine. Mind you, not all have reliable, accurate temperature sensors and controls. Some may not heat evenly throughout. (The cheap little oven thermometer you bought at the supermarket may or may not be calibrated. Calibrate it yourself or don’t use it.) Nevertheless, any oven should work for this operation because we won’t trust it to do the whole job, just to cut our chances of failure later, when we finish the heat-treating. We seek to heat the tool(s) just enough to help resist cracking when we return to the torch(es) to temper. Remember, just the slightest temper increases the strength of tool steel. As before, we’ll rely on the good old oxidation color spectrum to give us the temperature.
Set the partially polished hardened tool or tools onto the oven rack roughly in the center of the oven. Turn the dial(s) to 425º. Don’t bother pre-heating; we’re in no hurry. Set the timer for ten to twelve minutes. Turn on the interior light. Wait nearby until the buzzer goes off then check the tool(s) inside. Use an oven mitt if you can’t readily see the surface of the tool(s). We’re being cautious; pre-tempering is rarely accomplished so soon. Reset the timer for eight minutes. Check again when you hear the call. We want to see the first pale straw color. It may take another 8-minute period, and another, even. If it saves a good tool, it’s worth it. Pull out the tool(s) when you see that first tinge of color. Once cool enough to handle, you may proceed to safely temper the rest of the way with torch or torches.
Tempering knife blades by amateurs can best be done using a glowing electric hotplate or stove element. High heat. [This was described in Fine Woodworking magazine, issue #44 and in Fine Woodworking on Handtools, the Taunton Press, 1986.] Holding the piece by the tang with vise-grips pliers lay the back of the blade upon the element with the tip well beyond the heat source. Draw the straw color at the rear of the blade, then chase it forward toward the tip and up toward the edge slowly and carefully until the final desired color is reached, usually in the dark straw to purple range, as judged at the edge. Darker at the back than at the edge is best, but if the knife is properly forged and fully hardened, it will perform well cutting wood if drawn to uniform peacock or purple. Remove the tool from the heat to halt the process or quench in oil or water, if necessary.
Long, thin blades are extremely prone to warping. It is advisable to employ as many such “tricks” as will logically benefit the process. I suggest also you avoid extremes in tool design, especially at the edge. A blade with a bevel angle of only one or two degrees is apt to warp wavy at the edge—like a potato chip—when hardened, requiring much careful regrinding to fix. Easy to burn the temper out in the process of reshaping the tool to what it should have been in the first place. Best to get it right the first time. Besides, a knife with a 5º or greater angle will perform better, giving longer-lasting sharpness while still being easy to sharpen. It will resist damage to the internal structure caused by overheating when hardened and avoid subsequent failure in use.
Respect the limits of tool steel; its properties are remarkable, proven for centuries, doubtless near perfect for cutting wood. I remain convinced nothing has surpassed plain high-carbon steel in edge-tool smithing. Maybe nothing ever will. No alloying element has improved it. Alloys have been formulated merely to mitigate problems in factory forging and heat-treating, especially those created by drop hammers. Virtually all factory practices developed after the Second World War have favored production goals, not performance standards.
A couple generations ago, state and county fairs (even the world’s fairs) fostered competition among tool companies of every stripe. Medals were cherished and represented the finest achievements of tool companies everywhere. Criticism was harsh and bragging rights well earned, widely used in advertising, even to the point where images of the medals were stamped hot into the blades of champion tools. Today competition has degenerated to market warfare aimed at making the most tools as cheaply as possible. Quality is barely an issue. This benefits only casual tool users—hobbyists and home handypersons—those least able to discern real value. Obviously, it does not serve the interests of professionals and other serious woodworkers and carvers.
Those crafters in wood intent on mastering their art simply must learn to sharpen their tools to perfection. There can be no excuse to not refine such a fundamental skill. It facilitates many operations in fine woodworking starting with cutting the wood itself. It may allow the crafter to eliminate sanding to finish the artwork or design features. Who likes to sand for hours on end? Then there’s western red cedar, which disagrees with sandpaper; only thin blades with perfect edges will produce acceptable results—shiny toolmarks—the marks of a master artist.
Practice the tests described above. Use them to evaluate your arsenal of tools. Experiment. Those Japanese chisels and plane irons that chip at the edge and drive you nuts show the qualities of laminated blades. Mild steel welded to tool steel, another standard by which to judge sparks and hardness. Unfortunately, I have consistently failed to improve such weapons by re-tempering them, even by totally re-hardening and tempering them. That proved to be the case for five or six different brands. All had the same faults. I surmise the internal structure of the tool steel has been fatally compromised by the welding process. Then, too, the finest Japanese tools probably never leave Japan. Most exports function only marginally better than run-of-the-mill Yankee hardware store butt chisels, at best. Besides that, they seem ill designed for the typical uses and woods Americans require. In former times, wrought iron (now essentially extinct) made the forge-welding process practicable, even advantageous in some cases. The proof lies in the exquisite performance of so many old laminated tools of American and European execution.
There you have it: on these subjects, the culmination of over thirty years experience. Three decades of discovery by trial and error experimentation. Hopefully, those whom I’ve charged hundreds or thousands of dollars for that information will not be offended by my divulging it. On the other hand, I assume I have saved them a great deal of time and effort over the years. I hope the knowledge serves you as well.
White Eagle Studios would like to thank Dick for his indepth and comprehensive discussion On Heat Treating. Dick can be contacted at firstname.lastname@example.org, or (360) 856-4018 – PO Box 75, Clearlake, Washington 98235.
FOR FINE EDGES
By Guest Contributor Dick Anderson, Savage Forge
Since 1977 Savage Forge has specialized in manufacturing high-performance edge tools, primarily for professional woodworkers and sculptors. From the beginning, our tools have been delivered with what I call “perfect edges.” That may have constituted the best initial point-of-purchase selling feature; better than the careful hand forging, individual hardening and tempering or the air-dried hardwood handles. Several of our outlet stores reported their customers cutting themselves while testing the edges with their thumbs and then saying, “I’ll take this one.” But the best aspect of perfect edges was that it gave users a way to judge the edge-holding capabilities of the tools. They must have been satisfied—we’ve worked for most of them for decades.
No other single issue is more important to cutting wood than how the tool is sharpened. Nothing frustrates beginners more than their own inexpert sharpening. Many otherwise talented carvers give up the art because of inability to create perfect edges. They assume lack of talent led to their failure. One master artist/woodworker told me it took him ten years as a professional sculptor and cabinetmaker to teach himself truly masterful sharpening. And he’s a fine scientist and a fast learner. Not a few “successful” woodworkers who think they have mastered their art survive in the market only because of the buying public’s limited ability to perceive quality. In previous generations, artists in wood were obliged to present perfect, shiny tool marks on carved surfaces. Nothing less was acceptable. Nowadays, most people can’t distinguish carved details from embossed ones, or solid wood from veneer. In the past, hand tools offered real versatility and design freedom. Nowadays, mortises and dovetails are almost universally made by routers. Hand planing has been replaced by power planers and sanders. Antique furniture and old tools show how standards of quality have been lost.
Not all tools are created equal so don’t expect them all to perform equally. A file test can tell you much about the quality of your weaponry. Take a new, unused file for testing, use it only for that purpose, and change to fresh ones occasionally. Triangular (“taper”) files and round (“rattail”) ones seem to work best, and they are cheap. A light touch is usually sufficient. Scrape the surface of the tool near or at its edge. The file should almost skate across, resisting little. Try now at the shank or well back from the edge. That part should be softer, allowing the file to cut deeply and to drag somewhat. This is not always apparent with some factory-made tools, but it should be. These may be heat-treated in ovens to uniform hardness—not the best practice. Sometimes the file drags anywhere you test, suggesting the tool wasn’t hardened in the first place. But with most tools you’ll find a hardness gradient decreasing rearward from the edge. More on that later.
With luck, you may find you own an old laminated blade. Hatchets, axes, chisels, slicks and drawknives forged before the First World War are still to be found in garage sales and junk stores. Treasure them; they were made by some of the greatest smiths who ever swung a hammer. These were forge-welded of wrought iron and so-called “blister steel,” technology necessary before entire pieces of fine tool steel (called “hard steel” in the trade) became available. That it was a handicap I’ll probably never face leads me to consider those predecessors my superiors. You’ll find the file biting deeply into the top face of the tool, even near the edge, but skating off at the edge, which may be virtually as hard as the file, but only a small fraction of an inch thick. Often, the weld is visible on the back or bevel or both. Another test involves a grinder or disc sander. Medium grit, any speed over a couple thousand rpm’s. Again with a light touch, take some slight amount of metal from just behind the edge itself, the hard steel part of the bevel. Notice the bright, white sparks exploding near the point of contact with the abrasive. Tool steel, likely very near the carbon content of your file. Go ahead, put the end of the file to the grind. Same spark? Look for that in the future. Now bring the shoulder (top) of the tool’s bevel to the wheel or disc. Observe the duller, redder sparks bursting slightly, if at all. Wrought iron, the almost pure element, practically devoid of carbon. Obviously, you have in one tool the extremes of hardness by which to calibrate your sense of touch with the file. Most fine modern tools will test within this range. Furthermore, the specimen displays a wide range of carbon content and corresponding sparks. Japanese laminate tools are also good examples of hardness and carbon range, if not much else. More on this later.
No simple tests prove the edge-holding ability of a given tool. Not the Rockwell C-scale, nor the similar Brinnel system, nor the Shore Scleroscope. These procedures indicate only the relative hardness of steel, and do it no better than an experienced hand with a file, in my opinion. All are essentially irrelevant to the performance of a tool cutting wood. With time and practice, you’ll find there is no direct correlation between hardness and edge holding. Only diligent, regular use with perfect edges can reveal the quality of edge tools. Some of the tools that perform best may be softer than some of the worst. There are other factors.
One variable is the chemical composition of the steel. True tool steel is plain high-carbon or ultra-high-carbon steel. Carbon content of the most common grade—1095—is around .90%. Ultra-high-carbon content is above 1% carbon. W-1 is the grade most widely available, having about 1.20-1.25% carbon. Files typically have upwards of 1.35%, often 1.5%. At least nine out of ten Savage Forge tools are forged from W-1 drill rod; only some adze blades and hatchets are forged from 1095. Besides carbon, minute traces of phosphorus, sulfur and silicon will be found in the analysis along with around .5% manganese. These elements are basically impossible to entirely eliminate from the crucible’s charge of ore, especially silicon and manganese. No matter, they have little to do with the far greater influence of carbon on iron. Those two elements unite to make steel. Even just .15% carbon turns iron into mild steel, stronger and more rigid than the parent metal, iron. Good stuff for lots of things. No good for edge tools.
Most pertinent is the molecular structure of the steel. No competent metallurgist will dare pretend to minimize the benefits of forging ferrous metals. Mechanical working of steel refines its internal structure, strengthening and toughening it, making it more rigid and improving the property we appreciate most in woodworking tools—edge holding. Well-forged steel blades harden more thoroughly and at a significantly lower temperature than poorly forged ones. No treatment is more beneficial than extensive forging by repeated impact: gradual pounding to shape with hammers. This alters the spheroidal (theoretically more cubic) molecular structure of unworked steel to become a flattened, interwoven network of longitudinal, lozenge or disc-shape molecules. The high-pressure rollers of steel mills achieve this to some extent, but cannot mimic real forging.
The effects of forging border on magical. It may only be performed successfully above the “critical temperature,” the point at which steel loses its attraction to a magnet. This is the temperature at which high-carbon tool steel becomes plastic—forgeable—and can be hardened when quenched. That much is observably true. This change of state may be realized after a few seconds of “heat soaking” in small, thin pieces, or some minutes with thicker sections, prior to the shock of quenching, which theoretically freezes the internal structure at the condition to which it is brought by heating. Forging should be accomplished at heat well above the critical temperature; hardening preformed at nearest proximity to the critical temperature. Tool steel will literally burn, like a child’s sparkler, if heated much beyond 2000 degrees Fahrenheit, so the range useful to forgers lies from 1450 to 2000 degrees, generally speaking. (Many alloys withstand or require forging at higher temperatures.) Striking high carbon steel at too high a heat fatally destroys the molecular structure, because the grain of the steel has had its carbon content and condition of solution compromised. So too, quenching steel above the critical temperature results in enlarged grain, weakened integrity and poor edge holding due to friable structure throughout. Steel’s structural integrity is also forfeited by forging at too low a temperature, causing extreme induced stresses.
I call it all magic since these theories are principally nominal in nature: scientists name the progressive physical states of the ferrous medium and the resulting chemical forms but fail to explain why particular phenomena occur, vaguely describing the processes and results. The relationship between magnetism and plasticity and hardenability is wildly debatable. There are some great theories, although the reality of the science figures to be unknowable. But then, you don’t need to know why it happens, anyway. You only need to know what happens and how to make it happen. More on that later.
Let us assume for now that the tool you aim to sharpen is worthy of a perfect edge. The file test was promising and the spark looks proper. Or maybe it has always performed well cutting wood in the past. If the edge is nicked or otherwise damaged we’ll need to treat it more severely than if it is merely dulled by ordinary use. The side of the tool opposite the bevel (the inside of an in-cannel gouge or the flat back side of a single bevel chisel) needs attention first. This is the surface to which the fine edge will be drawn. Hopefully it is already polished to perfection. If not, we grind and then polish with whatever abrasives we have. Let’s do a chisel first. Fine whetstones or waterstones will perfect the chisel’s back. (Best in my opinion is to tack fine sandpaper to glass plates half a sheet wide with disc cement. Modern glass is always dead flat.) A good belt sander can sometimes be employed. Once the back is flattened and nearly polished, turn the beveled side to the abrasive and grind until a burr is drawn beyond the damaged part of the edge. Try to keep the bevel at the same angle, assuming it was appropriate in the first place and the tool was performing well. Otherwise, now is the time to make alterations. For work in most hardwoods the bevel angle should be within 20 to 30 degrees, less for softer woods like cedar and poplar. Depending on how coarse were the abrasives used so far, further dressing with finer stones or sandpaper may be necessary. Something 400 grit or finer. Do both back and bevel. You should find a thin false edge of metal flexing back and forth, visible and palpable to the stroke of a finger. Sometimes called the “wire edge,” it’s known to toolsmiths as the “burr” or “burr edge.” The final edge will be drawn now with a fairly stiff leather (preferably latigo) strop or high-speed buffer with felt or hard-sewn cotton wheels loaded with emery (gray or black) compound. Also suitable are the old “razor hones” found in junk stores, often virtually unused, at giveaway prices. Highly recommended for chisels and plane irons. Keep polishing both back and bevel until the burr parts off the edge. Voila! You should have a perfect edge. No light should be visible reflecting from that edge.
Now for the Toolsmith’s Acid Test. Hold the chisel in the horizontal plane in front of you with one hand. With your thumb, pull back the flesh of your index or middle finger of the other hand to expose the greatest length of fingernail. With that nail in the vertical plane, draw its tip carefully across and along the length of the edge of the tool. It should travel the distance without dragging (indicating dullness or a remaining burr) or stopping (locating a remaining nick). If it fails to slide easily you may have to go back a step or two in sharpening on the bevel side, even to return to the coarse stone if necessary. A standard by which to judge a perfect edge comes in every fresh package of razor blades, as long as we can still buy blades not encased in plastic. Learn the feel of it, do it every time you hone, but be careful not to slip and cut yourself. As with the file and spark tests, the more you practice, the more revealing these procedures will be.
The value of such tests lies in gaining more information by which to reckon your success. If my techniques don’t suit you, develop your own. Pick the brains of peers or anyone else who claims to know how to sharpen tools. Judge by the results—that perfect edge, those shiny toolmarks. It doesn’t matter how you get there.
I haven’t mentioned hollow ground bevels for several reasons. First, most tool users don’t have the right equipment to do the job safely and properly. One can ruin the temper of a tool in a second if not careful, especially with a high-speed grinder reducing the mass of the tool’s bevel right at the point of friction. The tool dresser’s Russian roulette. Second, that practice was unheard of in the days when almost all handtools were righteous; it came into vogue with drop-forged tools and alloy steels. Some such tools won’t even get sharp without the expedience of the hollow grind. Third, the edge is weakened and may not stand up to extreme use, such as being driven by mallet through knots and very hard woods. The only excuse I find is when the user must change the bevel angle frequently, as a cabinetmaker should for chopping the pins out of dovetails in, say, pine drawer sides one day, the pins in the rosewood drawer fronts the next. Do the former with a low angle bevel then steepen the angle with a so-called “micro-bevel” for the latter. Hollow ground bevels simply alter more easily. If you must grind hollow, try the roller of your belt sander. Good luck with that rosewood all the same.
The inside of the gouge’s channel corresponds to the back of a chisel or plane iron. Fine slipstones or ceramic rods will refine the inside of a dinged gouge. In a pro tool smithy you’ll find every conceivable shape and size of abrasive burrs and felt wheels to fit the contours of carving tools. My shop has several hundred for use in three machine heads. Of course, woodworkers don’t need such production equipment. If you are blessed with a nearby resident toolsmith and you’ve got a mess of damaged tools or have bought new tools with crude “factory edges,” get ye there pronto! Ol’ Smitty can do in minutes what may take you many hours, even days. Many factory-made carving tools have coarse grinder scratches on the inside channel, or “cannel” of the blades. All but unforgivable. Have the smith polish them smooth. You’ll be glad you did ever after. Prefect edges are not possible otherwise. At least, you must refine the inside surface near the edge with slips or ceramic rods.
To dress the bevel on carving tools I use a flexible disc sander for repairing damage and felt or hard-sewn cotton buffs loaded with satin finish compound for regular sharpening. (I buy my stones, slips and compounds from a gunsmith’s supply catalog.) Most carvers should find that Arkansas whetstones do a good job. Being harder than waterstones, they resist the wear from sharpening that results in a “saddle” on the surface. Like waterstones, they can be readily resurfaced from time to time by carefully stroking them across sandpaper tacked temporarily to glass or a machine table. I’ve been impressed with ceramic stones, less so with the diamond ones. Neither will ever lose its shape. Rock the gouge’s bevel side-to-side on the stone. Maybe you want to rub the corners in circles on the stone in order to round them slightly—better for finesse cuts in sculpture. This is an especially valuable attribute in carver’s chisels and shallow gouges, where the corners can be used like knives for detail work. Bevel angles should be kept in the same range as for chisels, relative to the hardness of the wood and severity of use.
Since the inside of the gouge is ground and polished to near perfection we may draw the fine wire-edge burr, again beyond the dulled or damaged part of the edge. A hard-sewn cotton wheel charged with emery (gray or black) polishing compound can deburr the tool by pressing alternately the inside and bevel side to the wheel’s top face. Contrary to what it may say in the instruction page that came with your buffer or polishing head, the wheel or wheels should turn opposite the direction for a grinder. You want to work at or near the top of the wheel as it spins up and away from you. If you adapt a conventional grinder for use as a polishing head, turn it around backwards. That will probably put the switch inconveniently behind the machine, but that’s a small price to pay for safety. Do otherwise at your peril. Another safety precaution is to be careful who or what lies in “the line of fire” should you snag a wheel with a tool. Something may go flying past the bench. Keep kibitzers beside or behind you.
Nearly as fast and effective for final honing are leather strops dressed with the same abrasive compound. Latigo leather is best. The belts that hold up your britches are most often latigo. A single short strap of such material can be shaped in the hand semi-circular (convex) to strop the inside contour of a given gouge, then laid flat or bent concave to strop the bevel. Stroke only away off the edge, never into the edge or into the leather. Thinner leather can be glued to a wooden dowel of appropriate diameter. In any case, keep the hair side out to avoid the fuzzy knap of the flesh side. If the smooth side resists loading with compound, rough up the surface slightly with 100-grit sandpaper. Shaped strops may be necessary or desirable if you hope to rely entirely on strops for honing. Carve wooden paddles about six inches long with handles carved or attached about that same length. Make the paddle ends convex on one side slightly smaller than the contours of your gouges, flat or slightly concave on the other and affix leather to both sides with good wood glue. In the hands of experts strops perform as well as buffers. Use of either vastly extends the life expectancy of your tools while improving the appearance of your products.
Hardest to sharpen are the radical shapes of U-veiners and V-parting tools, along with knives having recumbent-bent blades. Truth to tell, it’s hard to beat shaped felt wheels and high-speed abrasion for these. Three of my polishing heads have drill chucks mounted on the left side of the arbors. Thankfully, the machines came with ½”x20 right-hand threads. The chucks hold everything from 2-to-8-inch sanding discs, aluminum oxide grinding burrs in all shapes and sizes as well as ⅛-inch to ½-inch mandrels with small buffs or shaped felt wheels. I wouldn’t want to practice my trade without them. Again, have your local smith or pro sharpening shop dress the inside surfaces of your tools if equipped to do so. The trick to maintaining the symmetry of the tools’ shape lies in treating the straight sides as if they were chisels. Dress each side separately before tackling the middle part of a veiner or the intersection of the angle of a parting tool. Address those as if they were wee gouges, rocking them carefully on the face of the whetstone of choice. Just be careful not to go too far here. Losing the profile of the edge may compromise the tools’ use in some applications. For these, the only effective strop design I’ve come up with involves V- and U-shaped wooden imitations of slipstones. Rub into them as much compound as possible. This is easier if the “strop” is made of soft wood having fine grain—alder, birch, basswood and poplar work well.
The easiest tools to sharpen are straight-bladed knives, probably because we all have the most experience sharpening them. As before, choose a whetstone, sanding disc or belt appropriate to the task. One’s accuracy in such operations depends to some degree on how rapidly the material is removed from the tool being sharpened. If steel is torn off too quickly, it’s difficult to control the shaping of the edge and may threaten the temper. For this reason I generally recommend sharpening by hand. If, however, the process becomes tedious, one is tempted to take a shortcut, like tipping the blade to attack the bevel or bevels more aggressively thus losing the desired bevel angle. If power sanding is applied, use fresh, sharp discs or belts to keep down the friction coefficient and hold close to the edge with your thumb or fingers. If the blade is getting dangerously close to being “blue-burned,” it will probably be too hot to hold. In any case, use a light touch and a measure of patience. Doesn’t hurt to have a can of water close by to quench the blade when necessary. Next, you want to remove the scratches left by whatever abrasive you used. Do so with a finer stone or hard-sewn buffs loaded with satin finish compound. Felt buffs may also be used but the machine must turn true and balanced to avoid bouncing the tool wildly; cotton buffs are more forgiving; they are also less likely to overheat the steel than are felt wheels. Make sure you purchase the “hard-sewn” variety having spiral stitching at ¼” separation. Most are only about ½” thick but can be stacked on the arbor if desired. Loose-sewn wheels are not recommended for sharpening. Soft or medium-hard felt buffing wheels perform most tool-dressing operations best, while hard felt buffs are best for maintaining V and U shapes. Satin finish compound can be made into a paste with warm water, smeared into the buffs and then dried for a longer-lasting surface. You may see sparks flying from the tool, showing how expedient this system can be. Mind you, it is possible to alter the shapes of tools with such aggressive abrasives. Finally, strop or buff to deburr with the emery polish. Give it the old acid test. Shave your forearm. Actually, the ultimate test comes with cutting wood. You be the judge.
White Eagle Studios would like to thank Dick for his indepth and comprehensive discussion of creating a fine edge. Dick can be contacted at email@example.com, or (360) 856-4018 – PO Box 75, Clearlake, Washington 98235.
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