Page:The American Cyclopædia (1879) Volume III.djvu/105

 BORING 99 tions are called plug centre bit, wine cooper's centre bit, and expanding centre bit. The tools in the form of a screw are the single-lip auger (fig. 12), made of a half-round bar wound l Fie. 11. Pio. 12. Fie. 18. FIG. 14. Fio. 15. FIG. 16. spirally around a cylinder ; the twisted gimlet, (fig. 13), made of a conical shaft, around which is cut a half-round spiral groove; the screw auger (figs. 14 and 15), formed of a flat band of steel twisted when red hot ; the American auger (fig. 16), made of a solid shaft, around which is a thin helical fin. The last much resembles a wood screw ; the cutting edge is removable, and resembles that of a centre bit. All these twisted tools are of American inven- tion, and were scarcely known in Europe 30 years ago. Another American tool is an auger for producing square holes or cutting mortices : it consists of a screw auger working in a tube, round inside and square outside ; the four cor- ners at the lower end of the tube are sharpened from inside, and proceed forward a short dis- tance behind the cutting edge of the auger, cutting through the wood as they advance, and making the round hole square. Several of these tools working side by side will cut an oblong hole. Boring tools for wood are work- ed by means either of a lathe, a car- penter's brace, a transverse han- dle, or a drilling machine. (See fig. 17.) Bor- F.G. 11-DriHtag Machine. drills, and are much less varied in shape than those for wood. The double-cutting drill, fig. 4, is made by flattening the end of a small bar of steel, catting it so as to form a point or pro- jecting angle of about 90 in the centre line of the tool, and grinding on both sides to trans- form the two flats, forming the angle into edges of about 60 sharpness. Another double-cut- ting drill, called the Swiss drill, is made of a wire filed on one side to the diameter, the end of the remaining half being ground in the shape of a half cone. The common single-cutting drill, fig. 3, is forged flat and cut pointed, so as to show at the end two small faces meeting at an angle of 90, and forming a point project- ing in the centre line of the tool. These two faces are ground so as to form angles of 60 with the flat sides of the tool ; the one face forming this angle with one side, the second face with the other. This drill is in universal use, the angles specified being slightly modified according to the nature of the metal to be bor- ed. It is very difficult to drill a hole in the ex- act place where it is designed to he, and the error is proportional to the size of the drill. For this reason, when exactness is required for a large hole, a small hole is drilled first, and this is enlarged by means of a pin drill. The shape of a pin drill is exactly represented by placing two carpenter's chisels side by side, the one presenting its face, the other its back, to the person holding them, and by letting the end of a wire project between them a little below the edges. In using the instrument, the centre pin must enter and fit the small hole previously bored, which acts as a guide. If the portion of the cutting edges nearest the centre pin is cut away, the tool will cut a circular groove ; such is the form adopted for cutting holes in the tube plates which receive the tubes in loco- motives. These drills are worked in various kinds of braces, in the lathe or in the drilling machine. After they are drilled, the holes of all carefully made machines, which are not tapped, are perfected by reaming. A large proportion of holes drilled are intended for screws, and are consequently tapped. Taps, master-taps, stocks, dies, and reamers are cost- ly tools ; hence it is the interest of machinists to devise and adopt a uniform system in drilling and making screws, so that a machine may be repaired in another shop than that of the maker, without the necessity of making a new set of tools for each particular case. Hard steel and glass are bored with the end of a rotating brass rod fed with oil and emery. Glass offers also this remarkable and little known peculiarity, that it is drilled through as easily as hard woods with a common metal drill, provided the drill is kept all the time moistened with turpentine. In boring rocks for blasting, the common hand drill and the jumper are more used than any other tools. (See BLASTING.) The situation of the place in which the holes are to be drilled is often very difficult of access with a machine, so that the time and expense employed in adjusting the apparatus would make it preferable to employ manual labor. When, however, large holes are desirable for the displacing of masses of rock, machines worked by compressed air fur- nished by steam power, when they can be placed in working position, are to be pre-