Keeping your chisels and plane irons
sharp is not the chore that most woodworkers make it out to be. Think of it as just another setup procedure. Just as you would set up any machine before you use it (i.e., install the proper cutter, set the depth of cut, position the fence), you should prep your hand tools before you start cutting wood. If you make it a habit to touch up a tool every time you use it, the time involved will be minimal.
The Shape of the Edge
- You know the old adage about using the right tool for the job; it’s also important for the tool you grab to have the right type of cutting edge. Chisels and plane irons are shaped like wedges to provide the mass needed to withstand the resistance against the wood.
- The cutting edge is the apex of the wedge, and the slope of the two surfaces that meet at the apex is the included angle. A steeper angle means a sturdier edge, but also increases the resistance. The ideal shape for a cutting tool is a wedge that is as thin as possible, but thick enough to support the edge.
- The best way to achieve this happy compromise is to have two different bevel angles on the tool: a primary bevel angle that is low enough to allow for minimal resistance as the blade severs the wood fibers, and a steeper secondary bevel right at the edge to give it additional strength.
- The secondary bevel needn’t be very wide; therefore it will only need a few strokes on a whetstone to quickly restore the blade to razor sharpness.
These “ideal” wedge shapes will vary according to which direction you will be cutting relative to the grain. It’s easiest to cut across the grain; parallel to the grain requires almost twice as much force and cutting end grain takes nearly six times. Obviously, you can’t be grinding a new included angle for every cut. To get around this, most woodworkers find it useful to have two sets of chisels: paring chisels with a primary bevel of 20 degrees to 25 degrees for cutting parallel and across the grain, and thicker-bladed mortising chisels with steeper bevel angles of 30 degrees to 35 degrees for end grain cuts. Both of these tools may have a secondary bevel that is as much as 5 degrees steeper.
The Sharpening Process
Even a tool that’s fresh out of the package will need a bit of fine-tuning. Most manufacturers grind a general-purpose bevel on the steel but leave it up to you to make it sharp. If you’ve never done this, or have otherwise neglected your edge tools, you can still bring them up to cutting speed.
When dealing with a new chisel or plane iron, first strip off any lacquer that has been applied to the steel with mineral spirits or lacquer thinner. Be careful not to remove any finish from the handle. On a rusted old tool, use naval jelly and a stiff wire brush to remove any corrosion. Rust will forever eat away at your tool, creating pits that will ruin its performance.
Once you’re down to the bare steel, you can proceed with the more routine tool maintenance practice: the tasks of grinding, honing, and polishing.
Nose to the Grindstone
If you hone and polish regularly, you will only occasionally find it necessary to regrind. However, if the edge is badly nicked or out of square, you’ll need to grind the blade before you do anything else. Although you could grind the bevel on a coarse whetstone, it’s a slow process, especially if you’ve nicked the tool on a nail or dropped it onto a cement shop floor. The job will go faster with a hand-powered grinding wheel or, better yet, a bench grinder. See “Grinders, Friction, and Steel” on page 95 for additional information on getting good results with this machine. To remove nicks and square the end of the blade, set the tool rest so that it is perpendicular to the grinding wheel. You will be blunting the edge drastically, but at this point what matters most is grinding away the chipped-out areas and creating a square, true edge. Use a small square to check that the end of the blade is perpendicular to the side. For best results, clamp the tool in a guide that will hold it at 90 degrees. I have a tool rest made by Veritas that secures the tool in a holder, which slides in a slot in the tool rest, allowing you to pass the tool back and forth across the wheel with perfect control.
Once the nicks are gone, reposition the tool rest so that the tool meets the wheel at the appropriate primary bevel angle. Grind the bevel until it meets the face of the tool, eliminating the flat you created in the previous step. Once again, make sure the end is square to the sides.
Many woodworkers prefer the concave bevel created by the arc of the grinding wheel. They simply hone a secondary bevel on this hollow ground surface. Others feel that a hollow grind weakens the edge, particularly on mortise chisels and especially if the wheel is less than eight inches in diameter. I like a hollow ground bevel on my paring chisels, but on plane irons and mortise chisels I hone the bevel flat on a coarse whetstone after grinding it to rough shape.
Look Sharp So far we’ve just been shaping the blade. Honing and polishing really start to make the edge sharp. Think of an edge as the beginning of nothingness. It’s the point where two surfaces converge and disappear, where two planes intersect and, because of their relative angles, cancel each other out. Any scratches or pits in these planes will interrupt their smooth encounter, ending it abruptly. The meeting of the bevel and the face of the tool steel will be, to put it bluntly, dull. This is why honing and polishing are so essential to maintaining sharp tools.
Your objective is to remove scratches from those two intersecting surfaces. Start with the face of the tool, the side without the bevel. Most chisels and plane irons come from the manufacturer with visible grinding marks on the face. Deal with these as you would with scratches on a wood surface; remove them using progressively finer grits of abrasive, only use whetstones instead of sandpaper.
There are two basic types of whetstones: oilstones and waterstones. Oilstones require three-in-one oil, kerosene, or mineral oil to float the metal particles above the surface and prevent clogging the pores. I prefer waterstones because they cut faster and are easier to flatten than oilstones. However, they’re easier to gouge and must be protected from freezing.
You’ll need stones of several grits, starting with 220 and progressing in three or four stages up to 8,000-grit. Double-sided combination stones with two grits are a good, economical investment.
Don’t lap the whole face of the tool, just the area nearest the cutting edge. Start with your coarsest stone and stick with it until you’ve removed all the grinding marks, replacing them with scratches from the whetstone. Progress through each grit, using it to remove the scratches left by the previous stone. Move the tool about, using the whole surface of the stone so you don’t wear it unevenly.
No matter how careful you are, though, you will eventually need to flatten a stone; regular honing will wear a hollow in the center. Flatten the stone by abrading it on a truly flat surface. To do this, affix a piece of 120-grit wet-or-dry sandpaper to a machined surface such as a table saw or jointer table using double-sided carpet tape, or use Carborundum powder and water on a piece of 1/4″ or thicker plate glass. Rub the stone in a circular motion, then check the surface. The high spots will be scratched a different color; when the whole surface is evenly scratched, the stone will be flat.
Getting a Grip
Honing the bevel without a guide takes patience and practice. Grip the blade in the palm of your hand with your first two fingers behind the bevel and your other fingers wrapped around it. Set the bevel down on the stone and rock it somewhat to get a feel for how it sits on the stone. The tricky part is stroking the steel on the stone without rocking it as you go back and forth. It helps to listen to the scraping sound; the tone will be harsher and more pronounced when the heel of the bevel or the tip is rubbing. When the bevel is in full contact with the surface of the stone, it glides. You’ll feel less resistance and the sound will be softer.
The job is easier and the results more dependable when you use a honing guide. These devices roll on the whetstone and grip the chisel or plane iron at an angle determined by how far the tool projects from the guide. Try not to put a lot of pressure on the roller; it’s just there to support the blade at the proper angle. Instead put pressure directly on the bevel, especially during the pull stroke.
Use your coarsest stone to flatten the primary bevel. When you proceed to the next grit, tip the tool up a few degrees to create the secondary bevel. This is the part of the blade you will concentrate on whenever you touch up the edge. It needn’t be wider than 1/16″. Just a few strokes with each grit should be enough to polish it to a mirror finish. When it becomes as wide as one-third of the primary bevel, it’s time to reshape the primary bevel.
As you hone both sides of the tool, smoothing each surface into nothingness, a wire edge will develop. Don’t break it oil,, it might tear back into the steel, creating a jagged edge. Instead, polish it off on a wooden strop (a flattened block of hardwood charged with chromium oxide honing compound). Draw the tool toward you for just a few strokes to remove the thin line of steel and put on that final shine. Now your tool will cut cleanly and effortlessly.
Taking the time to hone that cutting edge just as soon as you lift a tool out of the drawer will not only make it cut better but also make it easier to control and therefore safer to use. When you consider that chisels are the number two injury-causing tool in the woodshop (after the table saw), it makes you want to work with a tool that will do what you expect it to, rather than fight–and possibly bite–you. What’s more, honing an edge each time you use it will keep the process short and simple. You won’t have to spend so much time and energy grinding and polishing if you just do a little at a time–every time.
For a complete selection of whetstones, honing and grinding guides, honing compounds, and other sharpening accessories, contact Garrett Wade at 800-221-2942.
I recommend The Complete Guide to Sharpening by Leonard Lee (Taunton Press, 1995) for the most comprehensive reading on the subject of sharpening all woodworking tools, including carving and turning tools, handsaws, axes, drills, scrapers, and power saw blades.
Grinders, Friction, and Steel
A spinning grindstone on a bench grinder generates a lot of friction very quickly, which is manifested as heat in your tool steel, but applying too much heat to a blade can easily ruin it.
Chisels and plane irons are made of carbon steel–iron with a small percentage of carbon added to it. Iron by itself is relatively soft. By adding carbon while it is melted, the resulting alloy is much more durable. Heat is also applied at every other stage of the process. The results are determined by controlling the temperature and the rate of cooling. A carbon steel tool blank is heated to a critical temperature and then cooled very gradually in a process known as annealing. Annealing transforms the steel, softening it so it can be shaped by forging. Heating it again to critical temperature and then cooling it a bit faster, called normalizing, relieves the stresses of stretching, bending, and compressing that were introduced during forging and hardens the steel enough that it will retain its shape. This is the point in the manufacturing process where the maker refines the shape and grinds the basic bevel. Applying heat a third time and then cooling it relatively quickly tempers the tool, making it hard enough to retain its edge.
Reshaping a tool or grinding a new bevel with patience and a coarse whetstone or a hand-powered grinding wheel will not generate enough friction to overheat the steel. But if you use a bench grinder, you run the risk of heating it to the point where you draw the temper. Once you’ve done this, the steel will be too soft to retain an edge. No matter how diligently you hone and polish the face and bevel, you’ll dull the cutting edge as soon as you use the tool.
Grind with a light touch, and remove material slowly. Hold the tool close to the edge, and when you feel it getting warm, pull it away from the wheel and let it cool. Some woodworkers dunk their blades in water as they grind, but this can cool too quickly, introducing small cracks in the steel. It’s better to lay the steel on another metal surface and let this draw out the heat.
If you have gotten the tool too hot, you’ll know you’ve drawn the temper if the steel turns blue. This blue area is annealed steel; to fix it, grind away the blue steel, being certain you don’t just keep softening it as you go. Once you’ve gone through this process and re-established the bevel, you’ll have the grinding expertise necessary to avoid creating the problem again in the future.