Woodworker's Journal 1985-9-2, страница 19

Woodworkers often disagree as to which is better — flat or hollow ground. The hollow ground edge is sharper, but it is also more likely to chip in heavy work. Taken on balance, though, it is our opinion that hollow ground is better, if for no other reason than convenience. Using a grinding wheel is quick and easy, permitting blade tune-ups in a fraction of the time hand grinding requires.

There are several types of grinding wheels in general use. Old-fashioned crank or treadle wheels, which revolve in a trough of water, are expensive and difficult to find, but have several advantages over motor driven units. Because of the large wheel diameter and the low r.p.m.'s a fine, long lasting bevel may be produced with no danger of overheating the steel, which makes it brittle. Most woodworkers use a commercially made motor-driven grinder with both coarse and fine wheels. The wheel diameter should be a minimum of 5 inches, and the grinder should be equipped with adjustable tool rests so the proper bevel angle can be achieved.

The beginning woodworker can improvise an inexpensive and handy grinder by utilizing a simple pully-driven grinder in conjunction with a V* or Vi horse power 1750 r.p.m. motor. The motor may be salvaged from an old washing machine or other large appliances. Although the grinder may be permanently mounted on a table or workbench, we recommend mounting the system on a length of 2 x 10 plank. This way the unit is portable. When in use it is clamped securely to the workbench top, and when not in use it may be easily stored where it will not take up valuable floor or bench space. As shown in Figure 5, the motor is mounted on several hinges so that the motor's weight serves to apply tension to the drive belt.

Whichever grinder system you choose you may wish to build a simple hardwood tool rest, as illustrated in

Figure 6. By cutting the rest at the precise bevel angle you wish to achieve, you will always be assured of the correct bevel on chisel blades and plane irons.

Protective glasses or goggles should always be worn while grinding. Eye injuries from grinding steel are extremely painful, and may result in loss of sight.

The best grinding procedure is to steadily move the tool from side to side, maintaining a constant even pressure. Do not bear down hard on the blade or the steel will overheat and loose its "temper." You know this has happened when the blade begins to smoke and turns blue near the edge. Should this happen the blue area must be ground away. To prevent this condition keep a can of water close at hand and frequently dip the blade to cool it. Check the blade with the try square after several passes to determine that it is square, and then continue grinding to establish the desired bevel.

Grinding tools on the side of the wheel is not recommended. Although many woodworkers use this surface because it is wide and flat, (ideal for removing a wire edge, for instance) there is a very real danger of the grinding wheel breaking apart and causing serious injury if it is used in this manner.

The next step in sharpening — called "whetting" — is one of the most discussed, and rarely agreed on aspects of woodworking. Everyone it seems, has his own technique. The bottom line of course is this: Whichever method you prefer, use it often. There is no substitute for a sharp tool.

Whetting involves smoothing the edge on an oilstone to remove grinding marks and further sharpen the blade. Start with a coarse grit and work up to the finest grit stone. This is also the stage where a slight secondary bevel (usually about 5 degrees) is added (Fig. 7).

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There are two types of stone: natural and manufactured. The natural stones are generally considered a superior choice, and it follows that they are the most expensive. Natural stones come in three varieties. The pinkish-white Washita stones are fast cutting, and should be used first. Next, use a soft Arkansas stone to bring the edge to a more finished state, and last, use the famous hard "black" Arkansas stone to apply the final edge.

Manufactured stones are generally made with silicon carbide abrasives, which are bonded into a block. The most popular man-made stones have two surfaces, one side coarse and the other fine. For a still finer edge, use an India stone, which is made of aluminum oxide.

There is a great variety of sharpening stones available, including Diamond Hone, Cyrstolon, and the Japanese water stones, which many woodworkers now consider to be the very best choice. Whichever type you select, the important things to remember are to protect the stones from breakage and to keep them clean. With oilstones, a few drops of light machine oil or kerosene spread across the stone will both help to reduce heat from friction and serve to prevent particles from building up, a situation which will eventually clog and glaze the stone's surface.

The blade may be whetted free hand, or with one of the many commercial guides, such as is illustrated in Figure 8. These devices fix the blade at the proper angle and have a roller so the blade may travel smoothly back and forth.

BY BOLTING, f^OTDRTO TWO HEAVY STTV^P HINGIS

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