Sharpening tools on a belt sander or grinder.

A belt sander or belt grinder can put an edge on knives, chisels, plane irons, axes, lawnmower blades and garden tools quickly. Its speed is also the risk: a fast belt heats the thin edge of a tool in seconds, and steel that gets too hot loses its hardness. Sharpen slowly, keep the steel cool and keep the angle steady.

Heat and temper.

Tool steel is hardened and then tempered to the hardness its maker intended. Overheating the edge undoes that: the steel softens and won’t hold an edge. Coloured marks near the edge, straw, brown and then blue, are a sign the steel got hot; blue means it is likely to have softened, and the soft steel has to be ground away. To avoid it:

• Use light pressure and short passes.
• Keep a pot of water next to the machine and dip the tool often. If it is too hot to hold, it is too hot.
• Use a sharp belt. A dull belt rubs and heats; a fresh belt cuts cool.
• Consider a slower speed if your machine has one.

Belts for sharpening.

For shaping a damaged edge, a coarse belt removes steel quickly. For sharpening, move to medium and fine grits. Zirconia and ceramic belts cut steel quickly and stay sharp; aluminium oxide is fine for occasional use. See grits and abrasives. Never use a belt that has been used on aluminium for steel, or the other way round, and keep metal and wood dust separate.

Keeping the angle.

• Use the tool rest or a jig. A rest set to the bevel angle, or a simple holder, keeps every pass at the same angle.
• Grind on the platen for a flat bevel, as on chisels and plane irons; a contact wheel on a belt grinder gives a slightly hollow bevel.
• Move across the belt so the whole edge is ground evenly and the belt wears evenly.
• Mark the bevel with a felt pen; after a light pass, the mark shows where the belt is cutting.

Direction and finishing.

With the belt running away from the edge (edge trailing), there is less risk of the tool catching. Grinding with the edge leading can catch the belt and is best avoided unless your tool’s instructions say otherwise. After grinding, remove the burr and refine the edge on a stone, a strop or a fine belt. Very fine belts and leather belts for honing are available for some machines.