The first thing you’ll need is a piece of 1/4-in.-thick glass. The exact size isn’t important. A glass shelf, which you can buy at a home center for about $10, works well. Glass is extremely flat, so it’s ideal for sharpening.
Next comes the sandpaper. Use high-quality sandpaper—pick the most expensive paper at the home center. The cheap stuff cuts too slow and wears out fast. Look for a combo pack that includes 80-, 150- and 220-grit paper.
You’ll also need spray adhesive for sticking the sandpaper to the glass. A low-tack adhesive is best (it’s available at art supply and craft stores), but the standard spray you’ll find at home centers is also OK—just follow the directions to make a “temporary bond.” When you change paper, use a solvent to remove glue residue from the glass (lacquer thinner works great). Low-tack sprays leave much less adhesive behind than standard sprays.
Cut a sheet of 80-grit sandpaper to fit your glass, then spray a small amount of adhesive onto the sandpaper and press the sandpaper onto the glass (Photo 1). Repeat with 150- and 220-grit sheets. Apply the sandpaper to both sides of the glass—this will prevent the glass from sliding around.
Keep Your Tool Cool
Dunk the blade in water every two passes (three to four seconds) to keep the blade from burning. A straw or bluish color means you've overheated your chisel and ruined the temper. Practice with your least-favorite chisels first.
If you’re used to semi-sharp chisels, be extra careful with your freshly sharpened chisel. It will slice through wood faster and easier than you expect. Keep your fingers out of its path!
It’s easy to forget that every edge tool has two sides that need maintenance. They’re equally important, but let’s talk about the back of your chisel first. The back should be dead flat, which enables the tool to cut straight, and as smooth as the beveled side.
The process of making a flat, smooth back is called “lapping.” Fortunately, you only have to lap a chisel once. Almost every chisel needs to be lapped, even one right out of the package, but some require much more work than others. In any case, you don’t have to lap the entire back—just the first 2 in. or so.
The first steps in lapping are to find out how much metal you must remove and which grit to start on. Pressing hard, rub the back of the chisel a few times on 220 grit (Photo 2). Go in a diagonal direction. Turn over the chisel and look at the back. If it’s nearly covered with scratches, continue lapping on 220 grit. If there are scratches only on a few high spots, you’ll need to start with a coarser paper. First try 150 grit; if that goes too slowly, use 80 grit. No matter which grit you start on, keep at it until the scratches extend across the full width of the back.
Let’s assume that you started with 80 grit. The next step is to make finer scratches with 150 grit. This time, push the chisel in the opposite diagonal direction. Keep going until you can’t see any of the 80-grit scratches. Repeat the process on 220 grit. Push the chisel in a new direction—in and out—until the 150-grit scratches disappear.
Not All Grinding Wheels Are Created Equal
You can use two kinds of aluminum-oxide wheels to sharpen your chisels; one is blue-gray and the other white. We used the darker-color wheel, which is harder and will keep its shape longer. The drawback, however, is that it grinds hotter than the softer, white wheel. Too much heat will weaken the steel. The soft wheel will need more frequent shaping with a dressing tool, but you'll be less likely to burn the edge of your chisel while grinding. For best results, use a 100-grit wheel to shape your chisel blades.
The most efficient way to sharpen an edge is to use a honing guide. The guide holds the tool at a fixed angle, so that every stroke counts. With most guides, that angle is determined by the distance the blade projects from the guide. You could use a ruler to measure this distance (30 mm, or 1-3/16 in. on the guide shown here), but it’s awkward. Here’s an easier way: Draw a line 1-3/16 in. from the edge of a piece of wood, then extend the chisel to the line (Photo 3).
Start by sharpening on 80 grit (Photo 4). Keep going until you can feel a ridge of metal (a “wire edge”) start to form across the back of the blade (Photo 5). This indicates that the bevel has come to a point.
Remove the wire edge by rubbing the back of the chisel on 220 grit (Photo 6). A few strokes should do it. Next, hone the bevel on 150 grit until you develop a new wire edge. Remove the wire edge on 220 grit again.
The last step is a bit different: Hone the bevel six strokes on 220 grit, then flip the chisel over and rub the back six strokes. Repeat this four or five times and you’ll have a sharp chisel—probably the sharpest chisel you’ve ever used.
If you need a super-sharp edge, continue honing with finer sandpaper or buffing compound (Photo 7). Buffing compounds come in different grits. They all work, but I recommend Ryobi “D”. First, pull back the chisel about 1/16 in. in your honing guide so you’ll be sharpening only right at the chisel’s tip. Rub the compound on a piece of plywood, then pull the chisel and honing guide across the compound a dozen times or so. (Don’t push, or the edge will dig in.) The buffing compound creates a mirror polish, resulting in an edge that’s wicked sharp!
If your chisel is in really bad shape, like the one shown in Photo 8, sharpening on sandpaper would take forever. To remove a lot of metal quickly, you need a power tool—preferably, a bench grinder.
Before we get going on the grinder, remember this: Heat kills. Grinding will naturally heat up your chisel, but too much heat will kill the steel’s temper, rendering it unable to hold an edge. If the edge suddenly turns blue as you grind, you’re cooked. The only remedy is to keep grinding and remove the soft, blued steel.
Here’s how to prevent overheating. First, use the coarsest wheel on your grinder. (Fine wheels create more friction and heat than coarse wheels.) Second, set the grinder’s tool rest to about 90 degrees and renew the wheel’s surface with a dressing stick (Photo 9). This simple step is crucial: To cut fast and cool, your wheel must be clean. Keep at it until any shiny bits of metal embedded in the wheel are gone.
Next, draw a “stop” line across the back of your chisel with a square and felt-tip marker to indicate how much metal you need to remove. Place the chisel on the tool rest, bevel side down, and gently press its end into the grinding wheel (Photo 10). Don’t grind any farther than you have to.
Blunting the chisel like this allows you to grind a new bevel faster. Here’s why: Heat builds up rapidly on a thin edge. You must stop frequently to cool the edge by dipping it in water. Heat builds up slower on a blunt edge, so you don’t have to quench it as often.
Which Honing Guide is Right for You?
Some experts master the art of sharpening entirely by hand. But for the rest of us, a honing guide is essential. There are basically two types of guides: Some clamp a chisel from the sides, while others clamp top to bottom.
The side-clamping guide works only with chisels whose sides taper to an edge that’s less than 1/16 in. thick. If your chisels have thicker sides, you’ll need a guide that clamps top to bottom.
The side-clamping guide shown here is made by many manufacturers and is widely available in woodworking stores and online. Inexpensive top-to-bottom clamping guides, made by General Tools and Stanley, are harder to find in stores but are also available online.
Use a protractor to draw a 25-degree angle on a piece of cardboard, then cut the cardboard with a pair of scissors. Adjust the grinder’s tool rest to this angle (Photo 11).
Grind the chisel until its end comes to a point (Photo 12). To prevent overheating, dip the chisel in a cup of water every five seconds or so. (If the water sizzles, reduce the grinding time to four seconds or less.)
The end of the chisel doesn’t have to be perfectly square. Close is good enough. But it really needs to be straight—keep grinding if the edge is uneven.
Two Bevels Are Better Than One
A new chisel has just one bevel, usually 25 degrees. But the tool should be sharpened at 30 degrees, which creates a new bevel. Why have two bevels? It’s all about saving you time and energy.
The 25-degree bevel is called the “grinding bevel”; you’ll use a bench grinder to renew it. The 30-degree bevel is called the “honing bevel”; you’ll use a honing stone or sandpaper to create it.
Grinding goes fast, but honing can be slow. If your chisel had only one bevel—the 30-degree one—you’d have to hone its entire surface to get a sharp edge. Creating a second bevel at a lower angle—the grinding bevel—reduces the amount of steel you must hone. When the 30-degree bevel extends three-quarters of the way up the chisel, it’s time to go to the grinder and create a new 25-degree bevel.