Page:The American Cyclopædia (1879) Volume III.djvu/104

 98 BORING for another cut. Boring machines were made to avoid the bulging of the sides of cylinders when placed horizontally, as this was the main impediment to good boring; they also avoid the deflection of the boring bar. They require much less power than lathes to do the same work, and have several other minor advantages. Messrs. Nasmyth, Gaskell and company con- structed the boring mill represented in fig. 1 for the purpose of boring the large cylinders, 10 ft. in diameter, for the Great Western steamship navigation company's vessel the Mammoth, at their works at Bristol. The mo- tion is communicated by the driving pulley c to a bevel pinion working the bevel wheel -d. The shaft on which this wheel is fixed has on its opposite end a worm for communicating motion to the upright shaft/" and boring bar a. This boring bar has vertical grooves a', in which the cutter head J is movable, sliding up and down according to the progress of the work ; k is a tool-carrier, fixed to the cutter head, by which the boring is effected. The foundation plate h forms a bearing for the upright shaft, the lower end of which rests in the step <7, while the cylinder I is secured by the clamps _; j to the supports i i, which are fixed to the foundation plate. Two strong pieces of masonry, m ', support the entablature m, for carrying the self-acting apparatus for raising and lowering the cutter head b. The entablature is secured to the masonry by strong holding down bolts. This self-acting ap- paratus consists of a rack, n, worked by a pin- ion, the motion being transmitted by trunnion wheels through two spur wheels and pinions, oo. The whole of this upper machinery revolves with the boring bar, with the exception of the ringjp, upon which the trunnion wheels rest and revolve. The motion thus produced is communicated to the rack, which is either raised or lowered according to the direction in which the boring bar revolves. Smaller hollow cylinders are bored in a similar man- ner, except that they are usually placed in a horizontal position. The cutter head may be made to revolve in the cylinder, or the cyl- inder may revolve about the cutter head. The barrels of muskets and other small firearms, being forged hollow, are bored upon a similar principle. The barrel is screwed on a carriage which moves in iron grooves, and is propelled toward the boring bar by a rope which passes over pulleys and has a weight hanging from the end. (For the boring of cannon and rifle guns, see CANNON, and RIFLE.) In the boring of solid substances various questions require to be taken into consideration. If the tools had only to cut away a portion of matter, as is done in cutting, planing, and turning, the directions given for cutting tools as to the angles of the faces of the edge with the work, the velocity, and the lubricating liquid proper for the sub- stance to be cut, would have to be strictly ap- plied. Such is not the case, however; a drill has not only to turn off the bottom of the hole, but also to pare its sides, to guide itself in a straight line, and, for wood and some other substances, to eject the shavings. Moreover, the velocity is unavoidably different at all points from the centre to the circumference. In consequence, the rules given for cutting tools are observed in boring tools only as far as they accord with other important requisites ; but they must never be lost sight of. Drills are made', in general, to bore straight holes, by providing them with a centre point or pin pro- jecting beyond the cutting edge just in the centre of the hole, or by tapering the cutting edges to a point. They are made to bore clean holes, by providing them with a shearing point on the side, that cats like the point of a knife ; FIG. 2. FIG. 8. I A FIG. 4. or by prolonging the cutting edge along the side ; or, for metal, by making a reamer with the stem of the drill (figs. 2, 3, and 4). Boring tools are made to eject the material cut away, by shaping the stem in the form of a screw, or by making it hollow. The various tools used for boring wood are as follows : The brad awl (fig. 5) is a cylindrical wire, with a chisel edge ; it packs the material around the hole. The awl Fio. 5. Fie. 6. FIG. 7. Fie. 8. Fio. 9. FIG. 10. (fig. 6) is a square bar tapering to a point. A great number of tools are fluted, that is, have the shape of the half of a tube. Such are the gouge bit (fig. 7), the spoon bit (fig. 8) and its varieties, the table bit and the cooper's dowel bit, and the nose bit or auger bit (fig. 9). The gimlet (fig. 10) is fluted, but terminates in a screw, which drives it into the wood. The cen- tre bit (fig. 11), an instrument of English in- vention, consists of a centre point, a shearing point, and a broad inclined cutter. Its varia-