General Tool Sharpening Tips
Whether you’re sharpening up a chisel, a knife, a plane or just looking on some background information about which sharpening stones to use, look no further.
First of all, make sure that the tools you are sharpening are equal to your efforts. Top quality steel is essential for a true, long lasting edge. An edge of lesser quality material will literally crumble over time in use.
To get an edge tool really sharp, the meeting surfaces must both be honed. With many high-quality tools, this should be done before the first use. Most woodworking edge tools do not come pre-sharpened. (Japanese chisels and plane blades can be an exception and Lie-Nielsen tools are furnished with a good edge. But we recommend that you rehone all edge tools before first use, even if it is claimed that they are “sharp”.)
Woodworking edge tools all have a basic bevel, usually machine-ground, with serrations in the steel from the grinder. Similarly, the back of the chisel or plane iron will have grinding marks on it. These must be removed to create a perfect edge. If you only hone the bevel of the tool, you will find that the edge still has serrations, because of the grind pattern on the back of the tool. When you first get a chisel or plane iron, you must lap the back of the stone or steel plate to get it as flat and as smooth as you can. Water stones are ideal for this, because they cut so quickly.
Once the back is flat and polished, turn your attention to the bevel. Most chisels come with a factory bevel angle of 25 degrees. For regular use, these should be honed with a supplementary short bevel at 30 degrees if you are working mostly with hard woods. This strengthens the edge and allows more rapid subsequent honing, because you do not have to work on the entire bevel surface, just a short portion near the edge. After repeated honing, a larger portion of the bevel is at a 30 degree angle. The chisel should then be reground, to restore the basic bevel angle and make honing easier. This also makes the chisel easier to use, since it restores the slim taper to a point, which penetrates wood more easily, since it has less of a wedging action.
Maintaining a bevel is extremely important, and, when doing so, consistency is more important than a particular precise angle. Use a Honing Guide if you feel it will help. Get into the habit of honing with your stone always in the same place, and at the same height from the floor, while making a conscious effort to adopt the same stance and body position each time.
If a cutting tool is poorly sharpened, either from failure to lap the back or from using too coarse a stone, it may cut well initially, but will dull quickly. The small serrations leave a number of sharp but very weak projections on the blade edge. These projections can heat up, lose their temper, and bend, leaving a blunted hook; or they may simply break off because of inherent weakness, leaving a tiny flat spot on the edge. When two smooth surfaces meet, the resulting edge has no weak spots, and any heat generated is dissipated into the main body of the chisel or plane blade (if you doubt that a blade heats up in use, just check your woodworking plane after strenuous planning of hardwood).
Not all edges should be honed at 30 degrees. Proper honing angle depends on intended use, and on the nature of the steel in the blades. For light paring work, it is often useful to have a chisel honed at a low angle of 20 to 25 degrees. The low angle (like a low-angle block plane) minimizes tear-out in cross grain work, and takes less effort to use.
For mortise chisels, the honing angle should be 35 to 40 degrees, depending upon the wood. Hardwoods such as oak put tremendous stress on an edge. If the bevel angle is too shallow, the edge will chip. In softwoods, too blunt a bevel angle can cause excessive fibre crushing and tear-out along the edge of the mortise. A razor-sharp edge allows you to increase the bevel angle and make the tool usable in a range of woods.
Japanese chisels, because they have a very hard layer of steel bonded to a layer of iron, need steeper bevel angles. A Japanese mortise chisel, for example, normally comes with a factory grind of 45 degrees. This should never be reduced, as the steel is simply too brittle to support a shallow bevel (the hard steel, though brittle, takes a better edge, and holds it longer)
Most woodworkers will be well served by just wide, flat bench stones. But because of the shape of carving tool edges, carvers frequently also need to have small shaped stones with curved edges. These are typically called slipstones.