Normally you should grind the blade at the same angle as it was set. Usually the edge has merely become blunt, and you can take the angle from the face immediately behind the edge, called the sharpening bevel. You are grinding quite a large area, so it may take longer than you expect. Resist the temptation to grind only the extreme edge, which will make the edge progressively more obtuse, and develop a shoulder where it is not wanted.
After grinding as necessary, sharpen with a coarse file and then hone with a fine canoe stone. Sharpen with diagonal strokes of the file or stone, stroking away from the edge, and lifting it on the return stroke. The strokes should be such that the file or stone slides both across and along the edge, so that nicks in the edge are ironed out rather than perpetuated. This action also minimises scoring of the stone. Work with several strokes from one end of the blade to the other, and then back on the opposite diagonal motion. Then turn the blade over and repeat on the other side.
To check if the tool is sharp, hold it up to the light and look along the edge. Any points where light is reflected will indicate where the edge is still a little flat and needs further work. The edge is sharp when no points reflect light. Do not test by using your fingers.
When the edge is well honed, you may find that there is a burr, or very thin leaf of metal, which is left standing on the edge, because it is thin enough to bend and flow under the onslaught of the stone. It can be removed by taking alternate strokes each side several times, always driving the stone towards the edge only. Alternatively, rotate a well-wetted stone along the sharpening bevel, first on one side and then on the other. Do this sparingly, as each time it is done the shoulders are thickened slightly which hastens the need for regrinding. Taking a very slightly over obtuse angle on the final strokes with the fine stone also helps.
The bluntest part of a billhook is usually near the tip, mainly because this is the most used part. Don’t spend hours on the handle end and then skimp the tip. The blade should be sharp along its whole length, and particularly where it is most used.
Billhooks can be quickly blunted by improper use. Ensure that users are trained not to drive billhooks into the ground, nor into gritty surfaces, and that use of billhooks on dead wood is avoided as much as possible, as this dulls the edge.
