Part 13

In both operations of “turning” and “boring,” the work revolves while the cutting tools are fed along. Turning is very simple and usually several tools are cutting at the same time, but boring is a more delicate operation, because the workman cannot see what he is doing. And in boring, either a “hog bit” or a “packed bit” is used; a “hog bit” is a half cylinder of cast iron fitted with one cutting tool and used for rough cuts, while a “packed bit” is a full cylinder of wood with metal framing and carrying two tools 180° apart and used for finishing cuts.

The forging, having been rough machined, is now ready to receive its heat treatment in order to give to the steel its required physical characteristics. Every piece of steel used in gun manufacture must conform to certain specifications as regard both its physical and chemical characteristics. The chemical analysis was made at the time the ingot was cast; now for the treatment of the forging, prior to the physical test as to its tensile strength, elastic limit, elongation and contraction.

The “tensile strength” of a metal is the unit-stress required to break that metal into parts. If a round bar ten inches in cross-section area will fracture under a strain of 120 tons, its tensile strength is 120 ÷ 10 or 12 tons per square inch. Tensile strength is usually expressed in pounds per square inch.

The “elastic limit” of a metal is the unit-stress required to first produce a permanent deformation of the metal. If a bar of metal be subjected to an increasing strain, up to a certain point that metal will be perfectly elastic, resuming its normal shape when the strain is removed; at the first permanent set or deformation, however, the elastic limit of that metal has been reached. Elastic limit is expressed in pounds per square inch.

By “elongation” is meant the increase in length in a bar when its tensile strength is reached. If a bar 10 inches long after rupture measures 11.8 inches, its elongation is 18%.

By “contraction” is meant the decrease in cross-section area in a bar when its tensile strength is reached. If a bar 1 square inch in area after rupture is only .75 of a square inch in area, its contraction is 25%.

These definitions being understood, a brief description of the heat treatment can be taken up, because it is after this treatment that standard bars are taken from the forgings to undergo the physical tests. The first step consists in “tempering” or hardening the metal. The piece to be tempered is placed in an upright position in a high furnace and uniformly heated to the required temperature. It is then lifted from the furnace through an opening in the top and carried by a crane to an oil tank of suitable depth and plunged into the oil. This rapid cooling or “tempering in oil” is facilitated by having the oil tank surrounded by a water bath, so arranged that a supply of cold water is constantly in circulation to carry the heat from the mass as quickly as possible. This operation produces exceeding toughness, increases the tensile strength and raises the elastic limit of the metal.

Now the forging is again annealed, so as to relieve any strains set up by tempering and to soften up the metal to the degree required by the specifications. It also increases materially the elongation and contraction. Great care must be exercised in the heat treatment, as the acceptance or rejection of the forging depends upon whether or not the test bars pass the required specifications.

The forging is now submitted for test and the test bars taken. In the manufacture of a big gun, four test bars are taken from the breech end and four from the muzzle end of each forging and these bars sent to the physical laboratory. Quite an elaborate testing machine is provided, and if the bars pass the required tests the forging is accepted and is sent to the machine shop for finish-boring and turning.

~SEARCHING FOR POSSIBLE DEFECTS~

Frequently during finish-boring the work is examined to see that the bit is running true, and great care must be exercised to prevent its running out of alignment.

After finish-boring every forging is “bore-searched,” that is, the bore is carefully examined for any cracks, flaws, streaks or discoloration. A special instrument called a “bore-searcher” is used and consists of a long wooden handle which has a mirror inclined at 45° at one end, together with a light to illuminate the bore, and so shielded as to obscure the light from the observer. (See sketch.)

The bore is also inspected by the foreman after each boring, but the final “bore-searching” is done by an inspector.

Now to measure accurately the inside diameters of long cylinders, such as are used in gun work, a special measuring device called a “star-gauge” is used. Its name is derived from the fact that it has three measuring points set at 120° apart and two measurements are taken, one [Illustration] and the other [Illustration], making a star [Illustration]. Every forging is “star-gauged” after being finish-bored and also the liner of the gun after each assemblage operation.

~PUTTING THE PARTS OF A “BUILT-UP” GUN TOGETHER~

In preparation for the assembling of the different parts, the tube is the forging to be finished. It is bored and turned to exact dimensions and carefully “bore-searched” and “star-gauged.” With the data at hand a sketch is made showing the external diameters of the liner under the tube, due allowance being made for the shrinkage when assembling.

The liner is next bored to within .35 of an inch of the finished diameter, and turned to the dimensions required by the sketch above. This extra metal in the bore is left until the gun is completely assembled and is removed in the finish-boring. The liner is then carefully “bore-searched” and “star-gauged” and liner and tube are ready for assembling.

The liner is now taken to the shrinking pit and carefully aligned in an upright position with the breech end down.

The shrinking pit is merely a well of square section with room enough to permit workmen to move freely about the gun when it is in position, and equipped with a movable table at its bottom upon which the gun rests. In the meantime the tube, with breech end down, is being heated in a hot-air furnace. This furnace is a vertical cylinder built of fire-brick and asbestos and so constructed that air which has been passed in pipes over petroleum burners can enter at the bottom, pass around and through the tube and out through the top to be reheated. This service permits a uniform heat to be transmitted to the tube and when the desired temperature has been attained the tube is lifted from the furnace by a crane, carried to the shrinking pit and carefully lowered over the liner. Great care must be exercised in this operation to prevent the tube from sticking while being lowered into place. Should it happen, the tube should be hoisted off at once, allowed to cool, any roughing of the liner be smoothed off, the tube reheated and a second trial made. When the tube is properly in place a cold spray may be turned upon any particular section where it is desired the tube should first grip the liner. The tube is then left to cool by itself, but cold water is constantly circulating through the liner.

When the gun is sufficiently cool for handling purposes, it is hoisted out of the shrinking pit and taken to the shop for careful measurement, the liner being “star-gauged” to note the compression due to the shrinking on of the tube.

The same procedure is followed in the case of the jackets and hoops, until the entire gun is assembled. The gun is considered completely “built-up” when the last hoop has been shrunk on and is now ready to be finished.

The gun is now finish-bored, as .35 of an inch of metal was left in the liner in the first boring. “Packed bits” are used and the greatest care is exercised to keep the bit properly centered and running true. After this step the gun is finish-turned and the powder chamber is bored.

Following this operation the gun is “bore-searched” for any defects that may have shown up in the finish-boring and chambering, and then carefully “star-gauged.” The gun is then ready to be “rifled.”

Photo by Bethlehem Steel Co.

This photograph shows a gun in the Rifling Machine in the process of being rifled.]

The “rifling” of a gun consists in cutting spiral grooves in the surface of the bore from the powder chamber to the muzzle end, and is done from the muzzle end. Rifling is a very difficult operation, and great care must be exercised that the cutting is uniform. The grooves are separated by raised portions called “lands,” and after “rifling,” these grooves and “lands” are carefully smoothed up to remove the rough edges or burrs caused by the cutting tools of the “rifling” machine.

The necessary holes are now drilled for fitting the breech mechanism and the breech block fitted. This operation usually takes some little time, as quite a bit of hand work is necessary to insure a perfect fit. The “yoke,” really another “hoop,” is now put on at the breech end and the gun is complete.

The centre of gravity of gun and breech mechanism is now determined by balancing on knife edges and the whole then weighed. The breech mechanism is also weighed and the two weights marked on the rear faces of the gun and breech mechanism.