Part 12

The “bore” is the hole extending through the center of the gun, from the rear face of the liner to its forward end.

The “powder chamber” is the rear part of the bore, and extends from the face of the breech plug when closed to the point where the “rifling” begins. The powder chamber is slightly larger in diameter than the rest of the bore.

The “rifling” is the name given to the spiral grooves which are cut into the surface of the bore of the gun, and give to the projectile its rotary motion when the gun is fired.

With the advent of “iron-clads” and heavily armored fortresses, it became necessary to increase the power of the guns in use, until to-day a 14-inch gun of 45 calibres fires a projectile weighing 1400 pounds, with an initial velocity of 2600 feet per second. An idea of this initial velocity may be better obtained by comparison when you realize that a train going sixty miles an hour is only traveling at the rate of 88 feet per second. Now, in order to produce such wonderful power in a gun, great pressure must be generated in the bore, and it was soon found that a one-piece gun, whether cast or forged, could not withstand such pressures.

To begin with, we may consider this one-piece gun, or any gun, as a tube which must withstand a great pressure from within, so that when a gun is designed care must be taken to see that the material from which it is constructed is strong enough to withstand this pressure. And not only must the gun be sufficiently strong, but it must not be too heavy, so that you see you cannot go on forever increasing the thickness of the walls of this tube. Besides, it is generally acknowledged that a simple tube or cylinder cannot be made with walls of sufficient thickness to withstand from within a continued pressure per square inch greater than the tenacity of a square-inch bar of the same material; in other words, if the tensile strength of a metal is only twelve tons per square inch, no gun of that metal, however thick its walls, could withstand a pressure of twenty tons per square inch, and the modern big guns are tested at that great a pressure. And if we look further into this matter of pressures we find that when a gun is fired the pressure exerts itself in two ways; it tends to burst the gun longitudinally or down the middle, and it tends to pull the gun apart in the direction of its length. Of course, some method of strengthening this one-piece gun was sought after, with the result that to-day guns are either “built-up” or “wire-wound.”

A “built-up” gun is one made of several layers, each layer being separately constructed and then assembled together. The order of assemblage differs somewhat with the different calibres, but the method of assemblage is essentially the same, that is, the outside layers are heated and shrunk on the inner ones. This question will be treated at greater length later on.

A “wire-wound” gun is one in which the necessary additional strength is obtained by winding wire around an inner tube of steel, each layer being wound with a different tension of the wire; this type of gun has found great favor with foreign manufacturers. In this country, however, the “built-up” system is used almost exclusively, and so this description will deal with the manufacture of a “built-up” gun.

Sketch Showing Construction of a Modern “Built-up” Gun.

A, HOOP; B, HOOP; C, JACKET; D, TUBE; E, LINER; F, HOOP.]

A modern “built-up” gun is composed of a liner, a tube, a jacket and hoops.

The liner is in one piece and extends the entire length of the bore and carries the “rifling” and the powder chamber.

The tube is in one piece and envelops the liner for its entire length. Formerly the tube carried the “rifling” and powder chamber, but due to the wearing out of the “rifling” with constant firing, a liner was decided on, so that now when the “rifling” becomes worn, the liner can be removed and a new one substituted.

The jacket is usually in two pieces and is shrunk on the tube; it extends the entire length, and its rear end is threaded in the inside for the attachment of the “breech bushing.”

Hoops are shrunk on over the jacket and in a big gun are sometimes as many as six or seven in number.

The liner, tube, jacket and hoops are made of the finest quality of open hearth steel, and the steel must conform to specifications set by the government.

Photo by Bethlehem Steel Co.

This photograph shows a mould for a gun ingot under hydraulic press for fluid compression.]

The chemical composition having been determined, the necessary elements are weighed out and the whole charged into an open hearth furnace. When the furnace is ready to be tapped the molten metal is run into a large ladle, which in turn is taken by a crane to the casting pit, where the mould is filled. The ingots for the large calibre guns run from 42-inch to 48-inch in diameter, and after being poured they are immediately run under a hydraulic press, where they are subjected to a pressure of about six tons per square inch to drive out the gases, and then lowered to about 1500 pounds pressure per square inch for a certain length of time during the cooling. This pressure tends to make the ingot solid, by expelling the gases, which would cause blow-holes, and by preventing “piping” and “segregation.” When a metal cools, the top and sides cool first, and this outer layer shrinks and pulls away from the centre, with the result that a cavity or “pipe” would be formed, but the hydraulic pressure forces fluid metal into this cavity and so prevents the “pipe.” The cooling also causes the various elements to solidify separately, and they tend to break away from the mass and collect at the centre; this is called “segregation,” and is also partially prevented by fluid compression. A solid ingot, however, is obtained, and this is absolutely necessary.

After the ingot has cooled sufficiently it is “stripped,” that is, it is removed from the mould, and then it is sent to the shop to have the “discard,” or extra length, cut off. When the ingot is cast, an extra amount of metal is poured into the mould to permit this discard, the theory being that the poorer metal, together with gases and other impurities, rise to the top. The government specifications require that there shall be a 20% discard from the upper end and a 3% discard from the lower end. The discard having been cut off, the ingot is “cored,” that is, its centre is bored out, the diameter of the hole depending on the size of the ingot.

Photo by Bethlehem Steel Co.

This photograph shows gun ingot in boring mill being cored.]

The ingot is now ready for the “forge,” and on its receipt in the forge shop it is placed in a furnace to be heated; and here great care must be exercised to prevent setting up any additional strains in the ingot. When the ingot was cooling just after casting the metal tended to flow from the centre; the interior is still in a condition of strain, and if the cold ingot is now placed in a hot furnace, cracks are apt to form in the centre, causing the forging to later break in service.

However, the ingot having been properly heated, it is ready for either the forging hammer or the press. The present-day practice, though, is to forge the ingot under a press forge, as the working of the metal causes a certain flow, and as a certain amount of time is necessary for this flow, the continued pressure and slow motion of the press allows the molecules of the metal to adjust themselves more easily, and a better and more homogeneous forged ingot is produced than if the forging had been done with a hammer.

When forging a hollow ingot, a mandrel, merely a cylindrical steel shaft, is placed through the hole in the ingot and the ingot forged on the mandrel, thereby not only is the outside diameter of the ingot decreased, but the length of the ingot is increased. The usual practice is to continue the forging until the original thickness of the walls of the ingot is decreased one-half and until the ingot is within two inches of the required finished diameters. The ingot is now known as a “forging,” and the lower end of each ingot as cast will be the breech end of the forging that is made from it.

The next process is that of “annealing.” This consists in heating the forging to a red heat and then allowing it to cool very slowly, and is usually done by hauling the fires in the furnace after the correct temperature has been attained and permitting both to cool off together. This process is to relieve the strains set up in the metal during forging, and further, it alters the molecular condition of the steel, making a finer and more homogeneous forging.

Photo by Bethlehem Steel Co.

This photograph shows a gun tube ready to be lowered into oil bath for “oil tempering.”]

After annealing, the forging is ready to go to the machine shop to be rough bored and turned. The forging is set in a lathe, the breech end being held by jaws on the face-plate and the muzzle end by a “pot-centre,” a large iron ring having several radial arms screwed through it. The lathe can now be turned and the forging centered by screwing in or out on the jaws of the face-plate or the radial arms of the “pot-centre.” When centered, several surfaces are turned on the forging for “steady rests” and then all is in readiness for the turning and boring.