A great degree of heat is generated by the violent friction of the steel-cutter on the cast-iron, during the operation of cutting off the heads of guns. The quantity of this heat has been estimated by Rum ford in one of his Essays on Heat.
After the head is taken off, the workmen proceed to bore the gun. This is done by exposing the re volving gun to the action of a steel-cutter, fixed on the end of a bar, which bar is placed on a carriage, and impelled continually towards the gun. The operation of boring is done on the same axis on which the head was cut off, if the power be suffi cient ; if not, the gun is removed, by means of the crane, to an axis, where it is made to revolve by a stronger power. • The boring-bar is fixed on a carriage, sliding in iron grooves, which are truest when made triangular. The carriage, which, in the apparatus represented at fig. 5. consists merely of the bar on which the rack is, is pressed forward by a pinion P, whose gud geons are on a fixed frame BB : this pinion works into a rack It. The axis of the pinion has mortise holes in it, through which one end of a lever L is passed ; the other end of this lever is loaded with a weight W, which causes the pinion to propel the carriage and boring-bar towards the gun. In many boring-machines there are two pinions on the same axis, acting or two racks ; in others, the carriage is propelled by two upright levers, on the end of one of which acts a weight, hanging from a rope, that passes over a pulley ; the lower end of the upper lever acts on the upper end of the lower, whilst the lower extremity of the lower lever presses forward the carriage. This method, which is free from any inequalities that may arise from the teeth of the rack, is figured by Smeaton in his Reports, Vol. I. 896. Another method of propelling the carriage of the -boring-bar, is by a screw acting on the end of the carriage. See Mey er in the Transactions of the Acad. of Stockholm, 1782, Tab. IX.
The boring-bar is a strong piece of wrought iron, of less diameter than the intended calibre of the piece, in order that the boring dust or shavings, de tached by the cutter, may be got out. The boring bar is increased in diameter near the end, for some inches ; see fig. 6. B ; in this part there is a superfi cial groove for receiving the sides of the steel-cutter or bit, which is to be firmly fixed in the bar. The
bit T, fig. 6. is made from a rectangular piece of a steel bar, in which the two diagonally opposite up per angles are cut of obliquely, so as to form two cutting edges like an obtuse angled drill ; the side of the rectangle, opposite to the point of the drill, is hollowed out in the form of a pigeon hole ; this hollow fits into, and embraces, the solid part of the boring-bar, whilst the sides of • the pigeon hole fit into the grooves of the bar. The point of this ob tuse angled bit is pressed against the revolving metal of the gun, by the force which propels the boring-bar; and the edges coming in contact with the revolving metal, a conical cavity is produced ; and by taking of successively a multitude of similar shells or shavings, the cylindrical bore, with a mai cal termination, is formed. The diameter of the pointed bit first used, must be less than the intend ed calibre of the piece, as the boring is to be re peated again at least once, in order to make the in ternal cylindrical surface as smooth as possible, by taking off any inequalities that have been left by the first. cutter. In finishing the bore, a cross bit may be employed. It is a rectangular piece of steel, ground to a cutting edge at each end, andput through a hole in the boring-bar, in which it is Ox ed. The edges of this cutter, in revolving, describe a cylindrical surface. After the cylindrical surface of the bore is made sufficiently true, and of the re quired calibre, a bit without a point, and rounded off to the desired curve, is used to form the bottom of the chamber.
Some recommend, that the boring bit for cast iron Should have its cutting edges brought tu an acute angle, by being filed hollow; but in this case the two edges cannot be brought into one point ; but the obtuse angled edge formed by the thickness of the metal of the bit, joins the two cutting edges crossways, and forces itself forwards by being near the centre, requiring, however, a considerable pressure. These hollow edged bits are not so well adapted to continuance of grinding, as the plain ones, but they make amends by their much less frequently wanting sharpening. It does not appear, however, that these hollow edged bits have been found advantageous in gun boring.