Читать книгу The Handyman's Book of Tools, Materials, and Processes Employed in Woodworking - Paul N. Hasluck - Страница 137

The Forstner auger bit (Fig. 409) is a useful tool for smooth, round, oval, or square boring, scroll and twist work. Its speciality is that it is guided by its periphery instead of its centre, and consequently it will bore any arc of a circle, and can be guided in any direction, regardless of grain or knots, leaving a true, polished, cylindrical hollow. It is a great improvement in wood-boring tools. Hitherto, with the exception of the shell bit, which has a gouge-like cutting edge, and is in some respects very defective, the centre bit has been the type of wood-boring tools, for hand use. Some of the twist auger bits are really improved centre bits, and have a screw point in common. All have also a well-polished groove, along which the chips can pass out and relieve the bit, permitting it to cut without the frequent withdrawal that some bits require. With these twist bits and the centre bits as exceptions, all the rest of the wood-worker’s boring tools make a small hole first and gradually enlarge it, especially the spoon bits and the Norwegian twist-nose bits, so that to bore a comparatively large hole in a narrow strip of wood generally implies splitting it. Even the twist auger bits just mentioned are apt to do so by the wedge action of the taper screw. Then some bits bore well in only one way of the grain. Not one of them, however, is capable of boring a part only of a cylindrical hole, that is, they cannot form a semi-cylindrical groove on a piece of wood; every one of them has a tendency to split the wood, and some of them wander from the point at which they are started. The Forstner bit, on the other hand, can be placed close to the edge, or even with part projecting considerably beyond the boundary of the wood, and, with a little care in starting, it will bore its hole or its groove cleanly and well. Then, too, ordinary bits nearly always bore a hole larger than themselves, but the Forstner bit bores truly in the place in which it is set, and truly to the size of itself. It has no tendency to split the wood, and works entirely by the guidance of the cutter, which forms the periphery or circumference of the bit itself. Another cutter (or several, according to the size of the bit) is provided to remove the core, and the bottom of a hole bored by this peculiar bit is smooth and flat. The bit seems quite independent of the grain of the wood, boring equally well at any angle. But for deep holes the bit must be withdrawn frequently, or the chips will collect round the shaft and make progress or withdrawal somewhat difficult.

Fig. 406.—Anderson’s Expanding Bit.

Fig. 407.—Streers’ Expanding Bit.

Fig. 408.—Clark’s Expanding Bit.

Fig. 409.—Forstner Auger Bit.