The reader will now be able to understand that although the process we have just described is the one of all others that would recommend itself to a practical mechanic, on account of its simplicity and the efficient manner in which it attains the object in view, it also meets the requirements which are referred to under the second heading into which the subject has been divided. The pressure of the coiling-machine is exercised to a great extent in the same direction as that which is exerted by the rolling-mill, and of the many ways in which malleable iron can be and is welded together into large masses, this is pre-eminently the one which combines economy, solidity, and strength in the direction required to resist excessive strains.
When the coil has been completed and allowed to cool, it takes its turn with others which have been previously constructed to go to the great steam-hammer. It is conveyed by rail from one part of the works to the other, and now having acquired great dimensions and weight, presents an appearance altogether different from anything to be seen in the earlier stages of its development. It is now that a new set of difficulties present themselves, in consequence of this excessive unwieldiness. The removal from one department to another when cold, is a comparatively simple operation, as any one can understand who is acquainted with the every-day appliances of cranes and trucks; but when the mass is raised to a white heat, the obstacles to safe and rapid transit are enormously increased. Among the many masterly mechanical details that have been necessary to the development of Mr. Fraser’s system, there is perhaps none in which he takes a greater or more justifiable pride than those appliances which come under our third heading. Although the furnace in which the coils are heated is necessarily of great dimensions, and is made correspondingly strong, yet being constructed of brick-work, it would soon be brought to pieces by the accidental blows of large masses of iron as they were being placed in its recesses or removed from its doorway. And yet of all the appliances, the one that has been adopted is by no means that which would have recommended itself at first sight. A pair of tongs is more associated with the domestic hearth or the blacksmith’s anvil than with any idea of capacity for dealing with the weight of many tons. In the ordinary operations of the forge, they are seldom used upon a large scale, the masses of iron in these cases being generally welded upon a shaft that serves to convey them from the furnace to the hammer, and yet not only are tongs the apparatus which has been adopted at Woolwich, but any one who sees them at work would find it difficult to suggest more efficient or simpler tools. But there are tongs and tongs, and as those at Woolwich are sixty or seventy feet in length, and twenty-five tons in weight, the reader must extend his notions of the ordinary pattern before he can form an idea of what they are. The advantages that were offered by the adoption of these appliances are so far apparent. By being suspended towards the middle from the chains of a steam crane, an apparatus was at once provided, not only for passing into the recess of a furnace, but also for placing a mass of iron upon the anvil of a steam-hammer without coming into contact with its framing. Both conditions were of the nature of necessities, and as a pair of tongs embraced them both, it is not so wonderful after all that they should have been preferred to more elaborate contrivances. Like other tongs, these mammoths consist of two limbs and a joint or hinge upon which they move, and which renders them capable of adjustment so as to embrace objects of different size. The manner in which the coil is clinched or grasped after the tongs are placed in a suitable position for doing so, is the familiar appliance of a screw placed towards the opposite extremity of the limbs. In order to enable the workmen to slue the heated coil backwards and forwards and round in the direction of the steam-hammer, a large counterweight is added to balance the weight of the heated mass. This is adjusted in such a way that the additional weight of a few-workmen springing upon the extremity of the tongs is sufficient to raise the coil from the bed of the furnace and support it in the same place while being brought to the hammer. There is no operation of the iron trade, not even in the foundry where the metal is in a molten state, that is more impressive to the unaccustomed spectator than the withdrawal of a Woolwich infant coil from the furnace, and there is none that appears to be more easy of accomplishment.
From the description of the coiling process the reader will have no difficulty in understanding that the object of this heating is to weld the coil into a solid mass. As it is necessary that this should be done by pressure exerted upon the top and bottom of the cylinder of the coil by the blow of a steam-hammer, it follows that in this part of the process there is something which must occur inconsistent with the principles that have hitherto been adhered to in the construction of the gun. We refer to pressure in a direction opposite to that which had been given by the rolling-mill, and which, as we have already repeatedly pointed out, is essential to the structure preserving the utmost amount of strength to resist explosion. There is, no doubt, a disadvantage in this detail having to be carried out in the way that it is done at Woolwich; but it is not only questionable if there is any other practical method known to engineers that is capable of performing the operation, but it also becomes evident to any one who has witnessed the process, that the objection can be hardly more than theoretical, and that little or no practical harm can result from it. The heating of the coil is carried out with much care, and the temperature necessary for the ready adhesion and combining of its edges is so perfectly obtained that one cannot but be astonished by the lightness of the blows that are found to be sufficient for the completion of the welding. In relation to the huge dimensions of the mass, they appear to the spectator to be more of the nature of taps than of blows, and it is certain that nothing is added beyond what is barely necessary to attain the object in view. It is certain that any mismanagement or clumsiness in the manipulation which required very heavy blows from the hammer in order to effect the welding, would have an injurious result upon the fibrous structure of the gun; but so much dexterity has been acquired, and the apparatus itself is so handy, that there is no reason why such contingencies should happen, and quite as little reason for supposing that they ever do.
Several pages would be found insufficient to do justice to the other appliances which came under our third heading; but as these are distinguished more for their difference in degree(*) than in kind, from similar contrivances elsewhere, we must now go on to the last part of the subject, namely, the processes required for finishing the work of the forge, and rendering the masses of iron practically serviceable for the duties of the field or the fortress.
And first, something must be said of the steel tube which forms the internal lining of the gun. This has been adopted, not because it is a necessarily factor in the strength of the gun, but because its substance is better suited for withstanding the action of the gunpowder than malleable iron, and also because it affords a more enduring material for the rifling, and for resisting the wear and tear of the shot. It runs the whole length of the interior of the gun, and being a solid mass bored out so as to form a chamber at the breech, provides the means of resisting the explosion in that direction. When it has been finished internally and turned on its outer surface to a diameter that is measured with the utmost exactness, it is tempered by being carefully heated throughout, and then dipped into a well containing oil, where it is allowed to remain for a certain length of time, and then cooled very gradually. After this operation it is in a fit state to become the foundation of the gun. Accordingly, one of the great coils having been bored with a similar nicety to the outside of the tube, is placed in a vertical position, so as to form a receptacle for faggots of wood. These being set on fire afford the means of expanding the coil by the heat, and in this way the bore is sufficiently increased to enable it to be slipped over the steel tube, and also over a slight shoulder or projection with a corresponding recess. When the coil has been cooled it is then found to be shrunk upon the tube with a result that is practically the equivalent of a solid mass. It is in the same manner as this that other coils are embodied with the rest of the structure, and they are so placed that their largest diameter is disposed of in relation to the violence of the explosion, affording a great resistance in the immediate neighbourhood of the charge, and less towards the muzzle of the gun, where the pressure of the gases becomes greatly reduced by their expansion. Practically the turning-lathe is the tool par excellence which is used for the manufacture of big guns after their different parts have come from the forge. It is not only by means of these that the different coils are bored and turned, but the same species of tools afford the means of shaping the trunnions and giving the last finish to the gun itself.
Of the rifling it is unnecessary to say that it is a process which, of all others, requires the utmost exactness. It is performed by means of a great shaft, or bar, which has a diameter equal to about half the diameter of the gun, and which is provided with cutting tools that are carried by its means from the muzzle to the neighbourhood of the breech, cutting out the recesses of the rifling as they go. It is necessary that the interior of the gun itself should have been finished with scrupulous exactness before it is placed in the rifling-machine, because its interior surface affords the only support for the rifling-tool, which has to project so far from any outside means of support. A series of rings fit exactly into the bore of the gun, and through these the rifling bar is passed. By an ingenious arrangement of projections upon its surface, as it passes inwards through the bore it takes along with it, at different intervals necessary for its support, one of these rings, which remains in that position until the rifling bar has reached the extremity of its travel. Upon its being withdrawn, it takes these supports along with it one after the other, and is then placed in a position for again re-entering the gun and taking another “cut.” The required twist is given by the bar being forced to follow the guidance of an apparatus which is previously adjusted.
There still remains much to be said with regard to details that are somewhat beyond the scope of this chapter. For instance, there is the breech-piece which is screwed into the largest of the coils, so as to form the breech-piece of the gun behind that which is afforded by the solid back of the steel tube. These, like the other parts of the gun, require to be finished and fitted with the greatest exactness, and are of themselves a splendid specimen of the art of turning in iron. Then of the carriage department, we have only to point out the endless variety of patterns, from the light and portable apparatus that is supplied to our field artillery, to the immense structures that are required for the armour-clad war ship, to show how far the subject is beyond the limits of a cursory description.
(*) The great crane which has been in process of construction at Woolwich for several years, may be said to differ both in kind and degree from any that have hitherto been employed in this country. It is intended to play an important part in the construction of ordnance far in excess of anything attempted previously, and will probably be the means, along with other gigantic tools - notably the new turning-lathe - of placing Woolwich many years in advance of all its competitors.
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