~THINGS TO KNOW ABOUT A BIG GUN~
Before going into a description of the manufacture of a big gun it
would be well to understand the following definitions:
The “breech” of a gun is its rear-end, or that end into which the
projectile and powder charge are loaded.
The “muzzle” of a gun is its forward end.
By “calibre” is meant the inside diameter of the gun in inches. A
5-inch gun is one of “minor calibre,” and one of 14-inches a gun of
“major calibre.”
The length of a gun is never expressed in inches or feet, but in the
_number of times_ that its calibre is divisible into its length; thus,
when we say a 12-inch 50-calibre gun, we mean a gun of 12 inches in
diameter, and 12 times 50, or 600 inches long.
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.
[Illustration: HOW A BIG GUN WOULD LOOK IF YOU WERE TO CUT IT IN TWO
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.
[Illustration:
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.
[Illustration: TAKING THE BORE OF A BIG GUN
Photo by Bethlehem Steel Co.