Sharpening and Caring for Metal Cutting Chisels

Why edge care pays off

A cold chisel with a tired edge is not just slow, it is unsafe. You hit it harder to compensate, the head mushrooms, the blade skates across the work instead of biting, and sooner or later a chip of hardened steel goes somewhere it shouldn't. Five minutes at the grinder and a few strokes on a stone will restore the edge, and the difference is immediate: the chisel cuts with fewer blows, tracks where you aim it, and leaves a cleaner shoulder. The same goes for a hot cut, where a sharp edge means the metal parts cleanly rather than tearing and dragging.

Sharpening is also cheap. A chisel is a simple piece of steel, and there is nothing about it a hobbyist cannot maintain with a bench grinder, a stone and a little patience.

Angles that suit the job

The angle at the tip is everything. Too thin and the edge chips or curls over on the first hard blow; too thick and it refuses to cut, so you beat it harder and the metal goes nowhere. As a rule of thumb:

• Cold chisels (flat, cross cut, cape, diamond point and round nose) generally run at an included angle of about 60–70°. Harder, larger work wants the blunter end of that range.
• Hot cuts can be thinner, around 40–50°, because hot steel is soft. Some smiths go as fine as 30° for small work, accepting that the edge will need touching up more often.
• Cape and cross cut chisels for keyways and grooves usually sit near 60°, with a little relief behind the edge so the body doesn't rub.

Measure the included angle across both bevels, not one side. A simple gauge, a cardboard template or an angle finder will keep you honest, and once you have a good angle for a particular chisel, keep to it.

Reading the edge — and the head

Look closely before you pick a chisel up. A good edge is even, bright and free of nicks; it should catch your thumbnail lightly but never shave. Watch for:

• Chipped or rounded tips, usually caused by too fine an angle, hitting hard steel, or using the chisel on the wrong material.
• Bluing near the tip, a sign the steel has been overheated at the grinder and the temper drawn out. Grind past it.
• A mushroomed head. This is the serious one. Metal that has flowed over the top of the shank can spall off like shrapnel. Dress it back with a grinder to a neat chamfer, or retire the chisel if the shank is bent or the mushroom is deep.

Sharpening: grinder and stone

Start with a bench grinder fitted with a dressed, medium-grit aluminium oxide wheel. True the wheel with a dressing stick first — an out-of-round wheel bounces and gives you a wavy bevel.

• Set the tool rest so the chisel meets the wheel at your chosen angle, and lock it down.
• Grind with light, steady pressure, alternating sides to keep the bevels even and the tip centred. Dip in water every few seconds and never let the tip change colour.
• Stop just short of a wire edge, then finish on an oilstone or a diamond plate of around 400–1000 grit.
• Hone at the same angle, five or six strokes per side, alternating, until the burr falls away.

A finished chisel should have two clean, matching bevels meeting in a straight, unbroken line. Wipe it over with a little oil before it goes back on the rack.

Hot cuts and handled chisels

A handled hot cut lives a harder life than a cold chisel, so treat it with a bit more respect. Cut hot metal over the edge of the anvil or on a cutting plate — never straight down onto the anvil face, or you will mark both the anvil and the chisel. Keep the work hot; forcing a hot cut through cooling steel overloads the edge.

Cool the chisel in water as you work, and re-dress the edge when it stops slicing cleanly. If the blade takes on a temper colour, it needs re-tempering or at least a light re-grind. Handles of ash or hickory should be tight, wedged and free of splits; a loose head is an accident waiting to happen, and a handle that has dried out can be revived with a coat of boiled linseed oil.

Storage, use and safety

Keep chisels in a rack or a roll where the edges don't knock against each other, lightly oiled to keep rust at bay. Never use a chisel as a pry bar, a punch or a wedge — that is how edges get wrecked and shanks get bent.

Match the hammer to the tool: a 1 lb ball pein for fine work, 2–4 lb for heavier cutting, and never a claw hammer. Wear safety glasses every time, keep your fingers behind the blade, and make sure the workpiece sits on something solid. Sharp, well-dressed chisels cut cleanly and predictably, and that predictability is what keeps you safe.