BORING "MAentriz may be defined to be, any machine for working a borer, or tool, which, by a. rotatory motion on an axis, cuts out a hollow cylin der in any substance subjected to its action.
The carpenter's whimble or crank, the drill; pulley, and bow, are, in this sense, boring machines; but custom has confined the term to signify, the ap• paratus which is used for boring out larger cylinders more quickly and aocurately than can be performed by manual labour, but which requires the power of a water wheel, steam engine, or horse wheel, to give it motion. These machines are principally employed for two purposes ; for boring wooden pipes for the conveyance of water, and for boring out the metal= line cylinders used in hydraulics and in pneumatic en gines. In the first case, the whole cavity is removed by the machine, which will be described under the article PIPE BORING ; but in the latter, the machine is only used to smooth, and make true, the internal surface of the cylinder, which is cast hollow.
The accuracy of cylinders for pumps, steam en gines, blowing engines, &c. is an object of so much Importance in the construction of machinery, that many very expensive engines have been made for the purpose. The old and common • method is to have an horizontal axis turned sloirly round by the mill, at the end of which a borer is fixed, and the cylinder is fastened down upon a carriage, sliding in a direc tion parallel to its axis, and drawn forwards to the borer by the descent of a weight. The objection to this method is, that any deviation from a rectilineal motion in the carriage will be transferred to the cy linder, and cause it to be crooked ; and that the weight of the borer and its axis acting on the lower side only of the cylinder, causes it to cut away more at that part, and render the metal of the cylinder of unequal thickness. This evil, however, was in some measure obviated by a contrivance of Mr John Smeaton, which was a steel-yard mounted upon a moveable wheel carriage, running within the cylin der. By suspending the weight of the cutter and boring bar from it, the machine was much improved, though still very imperfect.
A boring machine, for metal cylinders, which is not liable to any of these sources of error, is construc ted in the manner shewn in Plate LXIV. Figure 5. is a perspective view of the machine in the action of boring out a cylinder for a steam engine ; the other Figures explain the construction of its parts, and are drawn to a scale. In, Fig. 5..AA denote two oak ground sills, which are firmly bolted down parallel to each other upon sleepers let into the ground. At each end of these a vertical iron frame, BB, is erect .
cd, to support the gudgeons at the end of a !Ong cy lindrical axis DD, which is turned round by the mill. The cylinder LL, which is to be bored, is fixed un moveably over this bar, and exactly concentric with it. A piece of cast iron, KK, LL, (Figs. 2, 3, and 4.) called a clutter head, slides upon the axis, and has fixed into it the knives or steelings, f, f, J, which perform the boring. This cutter head is moved along the bar by machinery, to be hereafter described ; by means of which it is drawn or forced through the cylinder, at the same time that it turns round with the axis D. The steel cutters will necessarily cut away and remove any protuberant metal which projects • within the cylinder, or the circle which they describe by their motion, but cannot possibly take any more.
The cylinder is held down upon an adjustable fra ming, which is readily adapted to receive a cylinder of any size within certain limits. Pieces of iron EE are bolted down to eie ground sills; having grooves through them to receive bolts, which fasten down two horizontal pieces of cast iron FF, at right angles to them. These horizontal pieces support four move able upright standards GG, which include the dia meter of the cylinder LL, which is supported upon blocks bb below, and held fast by iron bands aa, drawn down by screws in the ,top of the standards GG. The cylinder is adjusted to be concentric with the axis DD, and held firmly in its place by means of wedges driven under the blocks and the standards.