Brief Sharpening Notes
Though many woodworkers find the sharpening process a pleasant pre-work meditation, most of us would just as soon get it out of the way and get busy woodworking. There is much more to sharpening than I can cover here and I refer you to my book on the subject: The Perfect Edge: the Ultimate Guide to Sharpening for Woodworkers. What I offer here, in extremely condensed form, are some ideas and methods to help make the task less forbidding.
Ron’s Very Brief Sharpening Notes:
First, The Goal: A sharp edge only exists where two planes (i.e., the back and the bevel of a plane iron or chisel, or the two bevels of a knife) meet with zero radius. Of course, “zero radius” is a theoretical ideal that eludes us as we move to the next, more powerful microscope.
There will always be some radius to an edge but The Goal is to minimize it. (Our fine-grained steel helps you here; the hardened particles in our steel are very small, allowing a smaller radius to be sharpened.)
Next, Getting There: Any of the popular abrasive devices can and will sharpen your blade. The choice is yours. The venerable “Arkansas” oilstones are legendary and keep their shape and flatness with little maintenance. These are a natural, quarried product that will last a lifetime. Man-made waterstones were more recently introduced from Japan, having a long history there as a natural stone. These stones sharpen more quickly because they are softer and thus wear faster, exposing fresh, sharp particles as they wear. However, their softness requires they be flattened often to avoid their tendency to “dish,” which makes accurate blade flattening and honing impossible.
Many woodworkers use a series of sheets of sandpaper or honing film as their abrasive medium. A piece of glass serves as a flat base-plate and the sheets are simply switched as the blade is honed through successively finer grits. The low start-up expense, ease of use, and variety of grits (as fine as 0.3 micron) make this a great way to get started. Then there are diamond stones, lapping plates, ceramic stones, leather strops, and an overwhelming selection of powered machines all designed to make this task easier. Whew!
If you have a method that you like, that works for you, stick with it, use it. The following steps are mostly generic and you can follow along regardless of your abrasive proclivity. If you’re new here and “grit-less”, head to the store that sells automotive paints and related supplies and buy a sheet or two each of 180-grit (180X), 320X, 400X, 600X, 1200X, and 2000X. Some people use 3M’s “#77” spray adhesive to stick down the sandpaper sheets; they sell it where you buy the sandpaper. Or locate some pressure-sensitive-adhesive- backed honing film in a similar variety of grits. Next, to the glass shop for a piece of 1/4” glass to fit your abrasive sheets. A marble floor tile or scrap piece of monument or countertop granite, work well, too. Now go clear a spot on a workbench for the glass or tile. With a new blade, start with the 600X or 15 micron paper. If the back needs a lot of flattening, don’t be afraid to use a coarser grit to save time. When re-sharpening a blade, if the edge is chipped or horribly dull you may need to start coarser: 80 micron or 180X may be necessary.
Honing guides are useful things. If you have one, now is a good time to use it. Most block and bench plane blades are ground to 25° but some smart folks argue that there need only be clearance under the heel of the bevel. In other words, since the average bench plane blade is bedded at 45°, any bevel angle 10° or so less than that will provide the needed clearance. And a thicker bevel is stronger so the edge should last longer. Bench plane and block plane blades have traditionally been beveled to 25°. Our blades for the handmade wooden planes were specified by James Krenov to have a 30° bevel. Chisels get different bevel angles for different tasks: 25° or lower for paring, 30° or more for chopping. Experiment a bit with different angles to see which one works best for the wood and your style of work. A honing guide helps with all this by establishing an angle and sticking to it. It can also shorten the whole process by letting you raise the blade a degree or two so that you’re only honing the very edge. The angle of the bevel is determined by how far the blade sticks out of the honing guide.
At least one brand tells you right on it how far to extend the blade for a 25° or 30° angle. If your honing guide doesn’t tell you how far to extend the blade, you’ll have to experiment and measure to get what you want.
No honing guide? That’s okay, but you’ll have to exercise a bit more diligence and control while honing the bevel. It is important that the bevel angle be maintained throughout the sharpening process. If you rock the blade, the bevel will end up convex — “roundish” — and the actual angle at the sharp edge will be greater than you intended. Not the end of the world, but it makes apples-to-apples comparisons between woods, steels, tools and bevel angles impossible. You can cut an angle template from a piece of cardboard or whatever and use that to check the angle as you go.
Start by “grinding” the bevel until a burr forms on the back. It may not be very visible, and will get smaller as you move to finer abrasives, but the burr will catch your fingernail. If the edge radius is large (which is a fancy way of saying “if the edge is really dull”), it may take a while before the burr will appear but it must be there or you haven’t done enough work. It’s the burr that tells us when the two planes have met (that zero radius thing again.)
Now flip the blade over to do the back. Flattening the back is as important as honing the bevel. I repeat: Flattening the back is as important as honing the bevel. Think about it: the back of the blade is the cutting edge. So you have to make the back flat to insure that the edge is straight, smooth and sharp — without waves, valleys or “teeth.” Many woodworkers believe that the whole back, from the edge to the slot, should be flattened and honed. Others figure that a stripe about an eighth of an inch back from the edge is sufficient since the cap iron rarely exposes even that much. Your choice.
If you can leave the honing guide on the blade, just hang it over the edge of the stone or plate. If it’s in the way, you’ll have to measure and reset the blade extension from the guide each time you change grits. Or just sharpen and polish the back then start over to do the bevel. Start with the coarse abrasive you’ve been using and rub the back using even, firm, down-pressure and take even, steady strokes keeping the blade flat on the surface. Do this until the new scratches uniformly cover the area you want to hone. It’s quite common for a plane blade to have a slight “hollow” in the back and the early honing will reveal an arc of fresh metal along the edge and sides. You can expand this area as far as you want until the whole back is covered with the coarse scratches. When you’re down far enough, and the planar surface of the back meets the planar surface of the bevel (zero-radius!) you will raise a burr on the bevel side. You’re there.
Change to a finer grit and repeat the above process. Once the back has been ground flat with the first grit, it gets much easier and goes much faster. It’s a good idea to angle the blade slightly while working on the back and to change the angle with each successive grit. That way, you can readily see when you’ve honed off all the scratches left by the previous grit; another clue that it’s time to change to a finer grit.
Check the blade to be sure that it is staying square. If it’s not, push a little harder on the high corner while honing the bevel to bring it back square. Proceed through the grits until you run out of them. After a few, the honed surfaces will begin to act as mirrors; a sure sign of imminent sharpness.
For most efforts, the 2000X paper or 5 micron film is as fine as you need to go. But if you’re doing the final planing on a surface that you don’t want to degrade by sanding, you may want to go beyond the 2000X paper to a 8000X waterstone or astrop charged with chromium oxide compound (“green oxide” or “knifemaker’s green”). The 8000X waterstone cuts fast but can be tricky to use because the blade wants to stick to the fine surface. Slow strokes, plenty of water and patience are required. The strop can be leather, cardboard, or wood; a flat, fine textured surface that will take the crayon-like super-fine abrasive is what you want. It’s best to gently pull the blade across the strop or you risk cutting into it. Be careful to keep the back flat against the stone or strop and the bevel at the correct angle; you don’t want to round off the edge.
To test for sharpness, you can always shave the hairs on your arm (or wherever). A sharp edge will cut hairs with very little pressure. But if you’re running low on hair (or just hate that patchy look) there are other ways. A sharp edge will catch on the flat of a fingernail or plastic pen barrel while a dull edge will skid a bit. It’s really that simple; try it a few times to feel it but it takes only the lightest touch and if it skids, it’s dull.
Also, you can see if a blade is sharp. Closely examine the edge with good light and if the edge reflects at all, it’s dull. (Remember that zero-radius stuff? It’s the blade’s edge radius that reflects light and if there is no radius — The Goal — there will be no reflection.)
If you’ve just done a chisel, block-plane blade or other cap iron-less blade, you’re done! But if you’re working on a bench-plane iron, you’re not done until you’ve polished the cap iron. Make sure the cap iron, when tightened in position on the blade, makes complete contact along its edge with no daylight showing; no gaps at all where a shaving could catch. Now polish the cap iron ramp-surface. How much work is needed depends greatly on its condition, of course, and how smooth is enough is a matter of experience and performance. Use the same abrasives starting no coarser than you must. Cap irons usually aren’t hardened so the work should progress quickly. Rock and slide the cap iron along the different grits until all coarse scratches are gone and the ramp area looks and feels smooth. Now you’re done.
Resharpen often and lightly, no coarser than necessary, to insure good cutting performance and save time in the long run. Good luck, have fun, and… “Ommmm”