Regaining The Cutting Edge

by Mike Callihan

Chisels and gouges .ire indispensable tools for most woodworkers. So when we destroy a tool's hardness, or temper, by burning it on the grinder, we're likely to lose our own tempers as well. Although burned cutting edges can usually be prevented by frequent water-cooling while sharpening, the occasional overheated edge is inevitable.

You'll know you've burned the metal if it turns blue. If the damage seems very slight, you can try grinding back the cutting edge about 1/8 in. and resharpening. But if the tool won't hold a sharp cutting edge, it needs retempering.

II 2 AMtKICAN WOODWORKER A DECEMBER 1996

Fortunately, it's not difficult to rctempcr the high-carbon tool steel from which most chisels and carving tools are made. Retempering requires a good heat source and a little patience. I he process involves three hear treatments: annealing, hardening and tempering. Annealing softens the metal and removes internal stresses in preparation for hardening. Hardening restores the metal's hardness but makes it too hard and undesirably brittle. Tempering reduces the brittleness and restores the hardness to a more desirable level, toughening the hard- : ened metal and increasing its shock resistance. i

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Recempering doesn't require a lot of equipment. You'll need a propane torch, seme dry sand, and a vise to hold your work. Eye protection and gloves arc also important for this work.

Annealing

To properly anneal steel, you heat it to a cherry-red, then let it cool very slowly.

Before annealing, remove the tool's handle if you can. To remove a wooden handle, clamp the tool's shank in a vise and drive the handle off with a punch. Protect a non-removable plastic handle from heat by wrapping the upper section of the shank in a very wet rag.

Clamp the tool in a vise and direct a strong flame an inch or two down from the cutting edge, letting the heat conduct itself upward toward the edge. Work in low light so you can see the steel begin to glow as it heats. It should reach cherry-red in less than five minutes. (See lead photo.) If it takes longer, you can speed the process with another torch.

If you're unsure if the tool is hot enough, check it with a magnet while it's still cherry-red. Properly heated steel won't attract a magnet. But don't keep heating after it has turned cherry-red, or you might permanently damage it.

When the tool is hot enough, quickly bury the blade in dry sand to cool, as shown in the photo at top right.

Once cooled, the metal should be soft enough to sharpen easily on the grinder. (Moderate overheating now won't affect the final tempering.) Your goal is to prepare the tool so that final honing can be done right after tempering.

Hardening

The next step is to rcharden the steel by heating it to a chcrry-rcd color again. This time, though, you want to cool it rapidly by dipping, or quenching, it in oil or water.

Oil works best for smaller tools; it's a less severe quench than water. Too severe a quench can induce stress cracks. Almost any kind of oil will do. I use 30W motor oil, but even cooking oil will work.

Heat the metal as you did when annealing it, then plunge the cutting edge about 2 in. into the oil. (See center photo, right.) Don't quench the upper section of the shank; it should cool very slowly to remain soft and shock resistant. Stir in a sweeping figure-eight pattern to keep fresh oil in constant contact with the steel. Otherwise, a gas pocket might form next to the steel, causing uneven cooling. After the bubbling and smoke subside, set the tool aside to cool.

Caution: When oil-quenching, wear eye protection and gloves, and hold the oil in a lidded metal container. Oil can ignite during quenching. If it does, smother the ft re with the metal lid.

Now, try to cut the steel with a good file. If the file skates off the metal, your tool is properly hardened. If the file cuts, your tool probably requires a more severe quench. Repeat the hardening process, using water this rime. For an even more severe quench, use a saturated saline solution, adding salt to water until no more will dissolve after it's stood for an hour. Whatever your quenching medium, plunge and stir the tool in it as you did with the oil.

Tempering

For proper tempering, you'll need to observe subtle changes in the steel's oxidation color, so work in a brightly lit area. Prepare for tempering by removing all the oxidation and combustion residue from the steel. Use progressively finer Carborundum finishing paper to polish the hardened steel to a bright shine. The brighter the shine, the easier it will be to read the color changes for tempering. Finish up with at least 600 grit—preferably finer. I like to finish with 2,000-grit paper from the auto supply store. Clean the surface with soapy water or denatured alcohol, and avoid touching the metal afterward.

Adjust your torch for a soft flame and very slowly heat the steel, playing the flame over the entire surface. The metal will turn pale yellow, then light yellow, then a pale straw color and finally straw-yellow. Done correctly, tempering even the smallest tool should take at least five minutes, preferably more. Remove the heat immediately when the straw-yellow color reaches the cutting edge. (See bottom photo, right.) Let the steel air-cool.

The cutting edge should now be hard enough to hold a sharp edge, but tough enough to resist chipping. Hone it with sharpening stones, and you're ready to go back to work. â–²

Woodworking Tools and Installation Tips

Woodworking Tools and Installation Tips

There are a lot of things that either needs to be repaired, or put together when youre a homeowner. If youre a new homeowner, and have just gotten out of apartment style living, you might want to take this list with you to the hardware store. From remolding jobs to putting together furniture you can use these 5 power tools to get your stuff together. Dont forget too that youll need a few extra tools for other jobs around the house.

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