The electrician's knife has three parts: a frame, which also acts as the handle, a cutting blade, and a screwdriver blade. When in use, the screwdriver blade is locked into place. This prevents the blade from shutting suddenly against the fingers and from giving way and sliding off the screw head.
The screwdriver blade and the cutting blade should never be open at the same time. The old-type electrician's knife has a screwdriver blade that is unlocked by pressing on the back of the cutting blade when that blade is closed. In the new-type electrician's knife there is a different type of locking mechanism. When the screwdriver blade opens, a spring pushes over and catches under a part projecting from the front of the screwdriver blade.
Since the grip on the handle may increase as pressure is applied to the screwdriver, this newer type is much safer than the old type; on the old type, increased pressure on the handle can release the lock and cause serious injury to the worker.
Care of the knife
Oil and rust
The same general care should be given the knife as is given any other tool. The metal should always be covered with a thin layer of oil as a protection against rusting. This is easily done by wiping with oiled cotton waste or rag. If rust does get on any part of the knife, it should be quickly removed by rubbing with fine emery cloth and then wiping with an oiled rag. Occasionally a drop of oil should be placed on the pivot on which the knife and screwdriver blades move.
Shaping the blade
The cutting blade of the knife is a double, beveled tool; that is, both sides of the blade slope to form the cutting edge. The woodworker's chisel, for example, is a single beveled tool; only one side slopes to form the cutting edge, and the other side comes straight down to join the sloping side to form the cutting edge.
The knife is made of low-temper steel; that is, it will lose its temper or hardness if exposed to too great a heat. Never grind the knife on a carborundum wheel to remove nicks or re-form broken point, for a carborundum wheel generates a high temperature. This can be easily seen by grinding a thin piece of low-temper steel on such a wheel. If the metal is not frequently removed and dipped into water to reduce the temperature, the steel will quickly lose its temper and burn, or turn black.
The grindstone should be used to remove nicks and reshape the cutting blade and the screwdriver blade. With this stone the blade cannot be burned. The water on the grindstone serves to reduce the heat generated by the friction of grinding.
Sharpening the blade
After all the nicks have been removed and the blade is reshaped, hone (whet) the knife on an oilstone. Be sure that there is a light oil on the stone, for this prevents the burrs removed from the metal from sinking into the stone. If the oil becomes muddy, wipe it off with a rag and replace it with clean oil.
Keep the blade at an angle of 20° with the stone to obtain the proper bevel — a long bevel, never a short blunt bevel — and hone until the required degree of keenness is obtained. A test for this is to place the edge of the blade carefully on the thumbnail. Press just a little and draw the blade carefully along the nail. If it slides over the nail, it is not sufficiently sharpened; if it seems to ding or grab, it is sufficiently keen.
The method of sharpening and shaping the screwdriver blade is similar to that for any screwdriver. One long side of the screwdriver blade is fairly sharp ; it should never be made keener than it was when it was new.
After using the electrician's knife, wipe it clean with an oily rag. If it is not to be used for some time, store it with a protective coating of heavy grease.
Emergency knife care
If a knife must be sharpened when there is no oilstone available, a temporary (and less satisfactory) edge can be put on it by honing it on any flat, smooth stone. Follow as closely as possible the instructions given above. Lacking an oily rag, a knife can be cleaned reasonably well by plunging it several times into the earth.
Use of the knife
The electrician's knife can be used for a variety of purposes, but chief among these are:
Safety in using the knife
The general rules of safety for using any hand tool apply in using the electrician's knife. However, special attention is called to the warning that when using any cutting blade, the direction should always be away from the body. A little thought will show how practical this warning is.
If the knife slips while cutting away from the body, injury to the operator is unlikely ; but if the knife is used for cutting toward the body, the slightest slip may cause a serious injury. When someone is working with you or near by, extra care must be exercised to avoid injuring him in case of a slip.
If you use the older type of electrician's knife, be careful that pressure on the handle does not allow the screwdriver blade to get unlocked and close. And while using the cutting blade, be sure that there is no chance of the blade's folding up suddenly and catching a finger.