The production of a larger-caliber gun is
a difficult, expensive, and time-consuming
process. The manufacture, machining, assembly
and testing of a single gun may take as long
as a year, making this process one of the
limiting factors in battleship construction.
There are three different types of guns:
cast, mono-block, and built up.
A cast gun is manufactured by casting metal into
a mold in the form of a gun, or a close approximation
to it, which is then drilled and machined
into the finished product. The US Navy used
only drill guns of cast construction in the
twentieth century, so I will mention them
only in passing.
A mono-block gun is made from a single forging, though
it may or may not have a separate liner.
Small-caliber guns are often mono-block,
but heavy guns (defined as a gun with a bore
over 8") are built up guns. Built-up
guns may be wire-wound, or made of several
forgings, assembled in layers.
A wire-wound gun uses an inner forging, wrapped
with layers of high-strength wire to build
up its thickness, with an outer forging shrunk
over the wire. The wire is steel, with a
square cross section, or sometimes ribbon
shaped, perhaps 0.1" thick. The wire
may have a tensile strength of up to 200,000
pounds per square inch, making these guns
the strongest guns for a given weight. However,
the wire does not give any longitudinal strength
to the gun, and wire-wound guns suffer from
excessive barrel droop. For this reason,
the US Navy did not use wire-wound guns,
although the US Army did deploy a wire-wound
14" gun.
All types of heavy guns start life the same
way, and the forgings are dealt with the
same way whether they are for a mono-block
or built up gun, so I will start with a basic
overview of the process.
- The forging material (nickel steel and
gun steel) is melted and cast into ingots.
- Ingots are machined into hollow tube
- The hollow tube is forged into shape
- Machining, treating, and annealing complete
the raw forging
- Extensive testing verify the quality of
the forging
The various forgings then go to the gun factory.
- The forgings are tested
- Machining brings the forgings to the desired
size
- The various forgings are assembled in the
shrinkage pit
- The gun is finish-machined and riffled
- The completed gun is sent to the proving
grounds for testing
For those of you with a superficial interest
in the subject, what follows is a very long
and detailed post, so you might want to stop
reading now! ;)
The process begins in a basic acid open-hearth
furnace, or sometimes an acid electric furnace,
which is heated by electric arcs rather than
gas or oil. An 'acid' furnace is one that
is lined with a refractory lining high in
silica, which requires a low-phosphorous
pig iron to be added to furnace first. This
silica lining reduces the amount of oxygen
in the melted material, called a furnace
charge.
The furnace charge is composed of 20-30%
pig iron, with the rest being composed of
plain or nickel-steel scrap, the percentage
depending on the quality of the scrap. This
scrap is composed primarily of parts of old
ingots, plus the cuttings and turnings removed
from other guns in the machining process,
but any scrap with 2-2.5% nickel is acceptable.
The furnace charge takes 6 to 8 hours to
melt completely. Samples are then taken every
half hour, and tested for the percentage
of carbon, manganese, silicon, sulphur, phosphorous,
and nickel. The nickel content is increased
as needed by adding blocks of pure nickel
to the mix, and adding hematite, a type of
refined iron ore, decreases the carbon content.
For nickel steel, .35-.42 % carbon is desired,
while gun steel will run between .42 and
.50 %. Other material may be added to the
furnace charge, like ferromanganese or spiegeleisen
to increase carbon and manganese, ferrochrome
to increase chromium, loam to increase the
slag, or limestone to thin the slag. This
process takes anywhere from 10 to 20 hours,
not counting the 6-8 hours to melt the charge.
Once the furnace charge is ready, it is drown
off from a tap hole at the bottom of the
furnace to avoid slag. Ferrosilicon is added
to the ladle as the mix is drawn off, to
bring the silicon content up to .27%. The
mix is then drawn from the bottom of the
ladle to further reduce the amount of slag.
Three more samples are taken at this stage,
to test for the proper percentages of trace
materials, to determine the proper forging
temperature, and for forging into test bars
for destructive testing.
The molten metal is now cast as ingots by
pouring into molds. These ingots are narrower
at the top than at the bottom, and have a
tong hold at the top for easier handling.
The size of the ingot depends on the gun
being manufactured: it will be much wider
than the outside diameter of the gun barrel.
While the metal is still molten in the mold,
the Whitworth process of fluid compression
is applied to the ingot. The pressure is
slowly brought up to 2,300 pounds per square
inch, which forces the gas content out of
the metal for a stronger casting. The pressure
is maintained for 4-5 hours, until the metal
has solidified but not cooled.
As soon as the ingot has solidified and cooled
enough to handle, it is removed from the
mold and stamped with an identification number.
It is then immediately taken to an annealing
furnace, where it is slowly and evenly heated
by burning coal. Baffles protect the ingot,
so that the flame does not actually touch
the metal. The ingot is held at 1400-degrees
for 5 hours, then the fires are allowed to
die out. The ingot cools slowly with the
furnace, which takes 3-4 days for a large
casting.
The ingot is now sent to the machine shop,
where it is turned on a lathe into a circle.
A boring mill may be used to rough-bore the
ingot into a hollow tube, and for smaller
caliber guns, this tube may be cut into several
pieces to make several forgings. The inner
tubes of a 14" gun are not bored before
forging, but the outer tubes are. After extensive
inspection, the hollow tubes or blocks are
sent to the forge.
A silica-lined gas furnace is used to very
slowly heat the block to 2100-degrees over
the course of several days. For small caliber
guns, the entire block goes in the furnace,
but for large guns only one end goes in.
The other end is not heated, but used to
handle the piece. The ends are alternately
heated and forged until the entire piece
is completed. Several heats and working are
required for each end.
The hot piece is taken by overhead crane
to a hydraulic forging press. A hollow block
is fit over a solid mandrel to avoid disfiguring
it in the press. The press has a V-shaped
anvil and a concave die head, which is used
to press down gradually for three seconds,
then released for one second. The material
is repeatedly worked, drawing it out until
it has cooled to 1500 degrees, then the mandrel
is removed and it goes back in the furnace.
Eight reheats are usually needed. Once the
metal has been drawn out to the required
length and the walls thinned to the correct
size, the forging process is completed.
After more inspection, and the generation
of a forging report on the item, the forging
is once again annealed. A very high heat
is used in an oil furnace, and the annealing
is continued for an extended period of time,
until microscopic examination of a sample
shows that strains are removed and the previous
structure has been broken down. The forging
then cools for several days, and is sent
to the machine shop again.
The rough ends of the forging are cut off,
but the piece is still left long to allow
for samples to be cut off at every stage
of manufacture. The forging is turned on
a lathe to remove scale, and then the forging
is bored to within 1 inch of the final diameter.
This process may take many weeks or even
months, as material must be removed slowly
over the course of many passes. After each
pass the piece is extensively inspected.
On small forgings, plenty of material is
left both outside and inside the piece so
that any warping from the next process can
be machined out.
The forging must now be tempered. It is suspended
down into a pit furnace, the largest of which
is 70-inches in diameter and 60-feet deep.
Baffles are again used to protect the forging
from the direct action of the gas flames,
and the piece is brought up to a temperature
determined by its size and carbon content.
This takes 10-12 hours. Once the desired
temperature is reached, the forging is lifted
out of the pit and immersed into a similar
pit filled with circulating oil. The piece
sits in the oil for about 12 minutes, and
then it is taken out and put in a horizontal
annealing gas furnace. Over the course of
6-8 hours, the forging is brought up to the
desired temperature, then the flames are
gradually reduced and the piece is allowed
to cool to 300-degrees. It is then removed
from the furnace, and allowed to cool completely.
The piece is checked for warping, and large
forgings are heated to 850-degrees and straightened
in a hydraulic press. Smaller pieces are
machined, and then both are annealed again.
The forging is now considered complete, and
samples are taken for extensive testing by
the Navy and the gun manufacturer. If all
tests and inspections are passed, the forging
is sent to the gun factory for manufacturing
into a gun.
One a forging has been received and inspected
at the gun factory (The Naval Gun Factory,
Bethlehem Steel, Midvale Steel, or the United
States Army Arsenal), it is centered on a
lathe and lightly turned to remove scale.
No further machine work is done to the outside
of the forging until the forgings that are
assembled over it have been finish-machined
to their final internal tolerance, and shrinkage
sheets have been computed for them. The shrinkage
is the difference in diameter between the
inside of the outer tube and the outside
of the inner tube, as measured at room temperature
before assembly. This amount usually varied
over the length of the barrel, and between
the different layers of forgings.
Once ready for machining, the forging is
turned in the lathe to a tolerance of 0.001
inch, using multiple cutting tools on two
lathe carriages to speed up the work. The
cutting tools are up to 2-inched wide.
The forging is then bored, in several stages.
During each pass, after every couple of inches,
the cutting head is removed and the bore
exhaustively gauged and inspected, using
a 45-degree mirror and a telescope.
Once all tests and inspections are passed,
the forging is ready for assembly.
The innermost layer, the liner, is assembled
last. But the other layers are assembled
from the inside out, shrinking each layer
over the ones below it. The gun parts are
assembled in a deep shrinkage pit, with the
largest being over 100 feet deep. At the
bottom of the pit is a platform used to adjust
the depth of the pit for each gun caliber.
The floor has a centering mandrel, which
has holes to allow the circulation of cold
water inside of the inner forging. Rings
of nozzles are fitted at the floor, and moveable
rings of spray nozzles travel up and down
to cool the outside of the assembled forging.
Next to the shrinking pit, an electric heating
furnace, built in sections to allow for any
size forging, heats the outer forging to
800 degrees.
The inner tube is placed breach end down
into the pit, supported only by the mandrel,
and chilled by the cold water circulating
through it. The outer forging has expanded
from the heat of the furnace, and is lowered
over the inner forging. When properly aligned,
cold water is sprayed from the ring of jets
at the bottom, assuring that the outer forging
will shrink and 'grab' the breach end first.
The traveling ring of jets then starts at
the bottom, cooling the assembly slowly towards
the muzzle. The cooled assembly is checked
and measured in every way possible, and if
it conforms to the expected values then the
process is repeated for the additional layers.
After the final layer is assembled, the gun
is removed from the pit and the outer layer
is threaded on a lathe for the locking rings.
The assembled gun barrel is now ready for
a liner. The initial lining of the gun is
different from the relining procedure once
the gun is in service. The barrel is bored
with a series of conical bits, so that it
is tapered from the breach end to the muzzle
end. After careful gauging and inspection,
it is heated in a furnace muzzle end down.
Meanwhile, the liner is bored and machined
like the other parts were, with the exception
that the exterior is tapered to match the
gun barrel. It is capped and filled with
water, and lowered down into the gun barrel.
Hydraulic pressure from a yoke and jack hold
the liner in place until it cools.
This cooling is accomplished by gradually
draining out the water, which allows the
liner to absorb heat and expand, starting
at the breach end. As it engages the gun
barrel progressively towards the muzzle end,
the entire assembly is allowed to cool. The
gun is now ready for final finishing.
When the gun is a room temperature, it is
carefully measured, then turned on a lathe
to within 0.2 inch of final diameter. The
bore is then given two passes on the finish-boring
machine, to a tolerance of 0.002" oversize
and zero undersize. The chamber is finish
bored, the gun barrel goes back on the lathe
for final turning, and the muzzle is flared
out to the distinctive bell shape. The ends
are faced off to the correct length, with
the liner projecting a quarter inch beyond
the barrel.
The final machining process is the riffling
of the liner, from the muzzle end in. One
half of the groves are cut (every other one),
then the other half are cut. It takes 100-150
passes with the riffling head to cut each
set of groves.
Properly riffled, the gun is set in a jig
and rotated. As it is rotated, the amount
of barrel droop is measured, and the position
at which the least amount is recorded it
marked as the top of the gun. The threads
are then cut in the breach end for the box-screw
liner. The different layers of forgings are
machined as required, and the screw-box liner
is fitted to the gun, so that the gun always
had the right orientation in service.
The bore is given a final cleaning and polishing
called lapping, with the gun being stationary
and a rotating wooden head covered with emery
cloth being drawn in and out of the barrel.
The necessary holes are drilled for the breach
block, hinge lugs are installed, and the
gun is hand fitted to its breach. The gas
check seat is reamed to specks, a keyway
is milled to keep the gun from turning in
its slide, the yoke is installed, and a final
inspection is done. If everything checks
out, the gun is painted and sent off for
test firing, then returned to the factory
for re-lapping and inspection.
Sources:
Officers of the United States Navy, "Naval
Ordnance, A textbook prepared for the use
of the midshipmen of the United States Naval
Academy", Naval Institute Press, 1937
Robertson, F.L., "The Evolution of Naval
Armament", Constable and Company Limited,
1921
Sumrall, R.F., "The Iowa Class Battleships",
Naval Institute Press, 1988
Jones, Frank, "The Machine Shop Training
Course", Industrial Press, 1964
"Dictionary of Metal Terminology",
MTB Technology Incorporated, date and author
unknown