In entering upon this subject, which performs so important and indispensable a part in every branch of mechanical industry, it is proposed first to notice some of the general methods pursued, commencing with the heaviest works, and gradually proceeding to those of the smallest proportions. This arrangement is principally adopted that the apparatus, which undergo a corresponding change in their kind and dimensions, may be adverted to.
After this, the management of the fire, and the degrees of heat required for various purposes, will be described; and then the elementary practice of forging will be attempted: those works made principally in one piece will be first treated of, and afterwards such as are composed of two or more parts, and which require the operation of welding.
The heaviest works of all, are generally heated in air furnaces of various descriptions, some of which resemble but greatly exceed in size those employed in the works where iron is manufactured, and in which the process of forging may be truly considered to commence with the very first blow given upon the ball, as it leaves the puddling furnace for being converted into a bloom.
At these works, in addition to the ordinary manufactures of bar, plate, and hoop iron in all their varieties, the hammer-men are employed in preparing masses, technically called "uses," which mean pieces to be used in the construction of certain large works, by the combination or welding of several of these masses. A square shaft, to be used at an iron-works in Wales, was made by laying together sixteen square pieces, measuring collectively about twenty-six inches square, and six feet long. These were bound together, and put into a powerful air furnace, and the ends of the group were welded into a solid mass under the heavy hammer weighing five tons; the weld was afterwards extended throughout the length. The paddle-shafts of the largest steamships are wrought by successive additions at the one end, as follows. A slab or use is welded on one side close to the end, and when drawn down to the common thickness, the additional matter becomes thrown into the length; the next use is then placed on the adjoining side of the as yet square shaft, and also drawn into the length, and so on until the full measure is attained.*
These ponderous masses are managed with far more facility than might be expected by those who have never witnessed such interesting proceedings. First, the "heat" has a long iron rod attached to it in continuation of its axis, to serve as a "porter" or guide rod; the mass is suspended under a traversing crane at that point where it is nearly equipoised, the crane not only serves to swing it round from the fire to the hammer, but the traverse motion also moves the work endways upon the anvil, and small changes of elevation are sometimes effected by a screw adjustment in the suspending chain. The circular form is obtained by shifting the work round upon its axis by means of a cross lever fixed upon the porter, and moved by one or two men, so as to expose each part of the circumference to the action of the helve; this is readily done as the crane terminates in a pulley, around which an endless band of chain is placed, and the work lies within the chain, which shifts round when the work is turned upon the anvil: the precision of the forgings produced by these means is very surprising.
A similar mode of work is adopted on a smaller scale for many of the spindles, shafts, and other parts of ordinary mechanism, which are forged under the great hammer, often of several bars piled together and faygoted; a suitable term, as they are frequently made of a round bar in the center, and a group of bars of angular section, called mitre iron, around the same, which are temporarily wedged within a hoop, somewhat after the manner of a faggot of wood. Such works arc likewise made of scrap-iron, which consists of a strange heterogeneous medley of odd scraps and refuse from a thousand works, scarcely two pieces of which are alike.
* The above remarks refer to the paddle shafts of engines, such as those built by Messrs Maudslays for the "Great Western" steam-ship, consisting of three pieces connected as usual by drag links. The center piece, or middle length, was 12 feet long, the two outer, or the paddle shafts, were 22 feet long each, the largest or the central diameter was 18 inches, and they tapered off to 12 inches at the smallest or the external parts, the bearings being 16 and 15 inches diameter. The collective weight of the three pieces was near 20 tons, and their value upwards of a thousand pounds before they left the forge of Messrs. Acramans of Bristol at which they were made.
The reader is here referred to the account of Nasmyth's Direct Action Steam Hammer, now principally used in forging heavy works, and to his Pile Driving Engine. See Notes Q and R, in the Appendix to vol. ii. pages 958 - 961.
A number of these fragments are enveloped in an old piece of sheet iron, and held together by a hoop, the mass is raised to the welding heat in a blast or air furnace, and the whole is conolidated and drawn down under the tilt-hammer; one long bar that serves as the porter being welded on by the first blow. The mingling of the fibres in the scrap-iron is considered highly favourable to the strength of the bar produced. The scrap-iron is sometimes twisted during the process of manufacture, to lay all the filaments like a rope, and prevent the formation of spills, or the longitudinal dirty seams found on the surface of inferior iron.
Sometimes the formation of the scrap-iron is immediately followed by the production of the shafts and other heavy works for which it is required; at other times the masses are elongated into bars sold under the name of scrap-iron, although it is very questionable if all the iron that is so named is produced in the manner implied.
The long furnaces are particularly well suited to straight works and bars, but when the objects get shorter and of more complex figures, the open fire or ordinary smith's hearth is employed. This, when of the largest kind, is a trough or pit of brickwork about six feet square, elevated only about six inches from the ground; the one side of the hearth is extended into a vertical wall leading to the chimney, the lower end of which terminates in a hood usually of stout plate iron, which serves to collect the smoke from the fire. The back wall of the forge is fitted with a large cast-iron plate, or a back, in the center of which is a very thick projecting nozzle also of iron, perforated for admitting the wind used to urge the fire; the aperture is called the tuyere.