The bench grinder is to the workshop as the trainer is to the team. It doesn't run out on the field when a tool goes down, of course, but if it weren't for the grinder in my shop, there would be a lot more dull tools (and probably more injuries).
The bench grinder isn't very expensive, is roughly the size of the box in which a pair of work boots arrive from the shoe store, and requires only to be secured firmly to a workbench or other mount. It'll grind smooth the rough cutting edge on wood and cold chisels, plane irons, drill bits, scissors, and knives: it'll repair screwdrivers and punches, and can be used for smoothing welded joints or other imper-fections, and even grinding off rivets. With a buffer wheel or wire wheel, it also cleans and polishes many different tools and objects.
The bench grinder has two grinding wheels, one on either side of the motor housing. Most of each wheel is covered by a guard, but roughly a ninety-degree arc of each wheel's perimeter is exposed at the front of the grinder. An eye shield is mounted above the opening in the guard; below is a tool rest.
Most home workshops will never need a high-powered, heavy-duty grinder. One powered by a one-quarter to one-half horsepower motor is probably adequate, with half-inch or inch-wide wheels of five or six inches in diameter. Larger grinders, with more powerful motors and wheels eight inches or more in diameter are also available. Typically, the speed at which the wheels rotate is between 3,000 and 3,600 revo-lutions per minute.
Grinding Wheels. A wide variety of wheels is available. They are manmade stones of grit bonded together at high temperatures. The abra-sive varies depending upon the use to which the grinder is to be put. Vitrified aluminum oxide wheels are best suited for grinding steel; sili-con carbide (white quartz sand) is best for cast iron, as well as brass, aluminum, or copper and other nonferrous materials.
The abrasive particles, or grit, do the work of the grinding wheel. They act like countless minute knives that cut away tiny pieces of the metal being ground. As the individual particles of grit become dull, they break off, exposing new, sharp particles that continue the work.
The usual arrangement is to mount one wheel that is coarser than the other, typically, wheels in the midrange, say one medium-coarse, one medium-fine. Coarse stones are used for rough shaping or for removing deep nicks in a blade. Finer gritstones suit sharpening or honing tasks.
Since grinding wheels are easily broken or cracked, set up your grinder in a quiet corner of your shop where it's less likely to get banged or whacked by incoming materials or workpieces in motion.
Take the precaution of checking the wheels, as well, by doing a wheel test periodically. Tap the wheel with a rubber-faced mallet or a wooden screwdriver handle. Listen for a ringing sound when you strike it about midway between its outside edge and the mounting hole at center. A ring means that the wheel is sound; a dull thud means you must replace it. Do not use the wheel if it's cracked, because it might shatter.
Remember, too, that as handy as a bench grinder is, it isn't the whole story as far as sharpening is concerned. Final honing is always best done by hand, probably on a naturally occurring stone like Arkan-sas.