Monday, February 26, 2007
Sharpening, a Beginning
As someone who works primarily with handtools, I've found that fully half of my understanding of the craft has come in the way of metal working. There is no possible way to cover the entire topic in one entry, so I'll start with basics and hope to build on it in later entries. The basic idea is to get a sharp piece of metal to cut a chosen piece from a chunk of wood. The wood basically sits there while I decide what shape of metal or metal housed in a jig (planes etc..) will take the desired cut. Now sharpening is as easy as sanding, you shape the metal with a rough grit (the grinder) and then follow through with finer grits until the two surfaces that make up the edge have a mirror finish. In the photo, you see a sharpened screwdriver and the mortise and paring cuts that I took with it. I did this to demonstrate that there are no magic tools. Yes, some work better than others, but with understanding, even a crusty old screwdriver can get the job done.
I started sharpening with an old hand cranked grinder and ran every chunk of metal I could find against it! I feel that a grinder of some sort is essential to fluent sharpening. Imagine the difference between removing 1/4" of wood with a drawknife or with 80 grit sandpaper. The grinder establishes the shape, the stones polish the surfaces and form the burr. The burr is simply the tiny unsupported bit of metal right at the edge of the tool that bends over as you abrade the surface. As the surface gets polished to a higher sheen, the tiny bit of metal connecting the burr to the edge gets smaller and smaller, until the burr falls off. Getting the burr to drop off cleanly is a huge part of establishing a fine edge. The edge is now sharp, there's just one question, is it the right shape to cut wood?
Many edges can shave hair, balance on your fingernail, cut flesh and whatever else method you choose to check for sharpness, but to cut wood, the geometry must be correct. The best way to learn to sharpen is to focus on getting two flat surfaces to meet, such as on a chisel. The bevel, concave from the grinder, will contact the stone in two places. At the edge and at the back of the bevel, this helps keep it from rocking on the stone. The type of cut that this edge can take, outside of a plane or spokeshave is limited to chopping and paring. The flatness of the surfaces directs the cut. Later I will talk about the slightly rounded surface used in carving tools and many freehand chair tools. There are many different tools out there that promise freedom and fun, I tend to try to find ways to achieve the results I need without buying a lot. Sometimes I cost myself in time while saving money! The one type of tool that I am always comfortable spending on is sharpening equipment. Some things to consider are: quality set of water stones, a piece of glass with sandpaper or drywall sanding screen (for keeping the stones flat), a slow speed grinder with aluminum oxide wheels, a quality tool rest (the one that comes with the grinder is generally poor), a piece of leather and honing compound for a strop, and my new favorite, the high speed buffer with a beartex wheel and a hard felt wheel. With a great sharpening setup, all else falls in place. And remember, the easier sharpening is in your shop, the more likely you are to actually do it.