The 1884 annual report of Brig. Gen. Stephen Vincent Benét, Chief of Ordnance, included an accounting of the construction of converted 8-inch rifle No. 39, breech insertion, and its examination after it was “declared unsatisfactory when examined subsequent to its firing proof.”
Coiled wrought-iron tube No. 39 was manufactured at West Point Foundry as an 8-inch muzzleloading rifle, converted from a 10-inch smoothbore Rodman gun.
A depression 1.5 inches wide and 0.25 to 0.3 inches deep was found in the gun bore 88 inches from the muzzle. “It extends in a general direction at right angles to the axis so as to cover about five grooves.”
In April 1884 General Benét authorized sending the gun to Cold Spring, N.Y., for examination. The following is the report that R. Birnie Jr., Lieutenant of Ordnance, filed about the inspection and the problems that were found:
First after its receipt the gun was entirely dismantled.
The copper vent piece was unscrewed without difficulty and was preserved uninjured.
The securing pin between casing and tube was removed by drilling a hole within it, which hole was afterwards squared and served as a receptacle for a wrench.
The muzzle collar was started by means of a cold chisel held against the outer face and a sledge struck against the head of the chisel. After being started the collar was unscrewed by hand.
The entire tube was unscrewed from the casing. To accomplish this the gun was mounted upon a lathe-bed and arranged to revolve slowly, soft metal supports being arranged, one under chase near the muzzle, the other at the neck of the breech.
An inclosure of brick was then built about the gun and covered to enable it to be thoroughly heated; the portion of the gun exposed to the heat extending along the middle portion to within about 20 inches of the breech and muzzle ends. A strong gearing of cog-wheels was firmly connected with the breech of the tube by which the motive power of the shop was used for the unscrewing. A current of cold water was arranged to circulate through the bore of the tube.
On May 3, 1884, a wood fire was lighted under the gun at 8:30 a.m.; cold water was supplied within the bore and the gun was kept slowly revolving. The fire was drawn about 3 p.m., when the gun was considered sufficiently hot, its temperature being probably between 500? and 600? Fahr., as judged from the action of water when thrown upon it.
The gun was then clamped in its bed and the tube started by applying hydraulic jacks to act upon the spokes of the cog-wheels directly connected with the breech of the tube. After the screw was started the machinery was set in motion and the unscrewing completed.
The gun was then swung out of the lathe and the remainder of the tube withdrawn directly with an ox team and block and tackle. The current of water was maintained in the bore until the last stage of the operation. The above was effected without special difficulty, but the arrangements were very complete and involved a good deal of time and labor.
The securing pin through shoulder of cast-iron breech plug was drilled out and the plug unscrewed.
The jacket was separated from the inner tube as follows: The tube (entire) was mounted in the foundry yard upon two supports, one at breech, the other near muzzle, so that the axis was horizontal and about 30 inches above the ground. A heavy cast-iron collar or crosshead was passed over the chase of tube to abut against the forward end of jacket; another cross-head was placed on the same level about 30 inches in rear of the breech, and these two cross-heads were connected by two heavy iron rods, outside of and parallel to the jacket, and having a thread and nut on each at the rear cross-head.
The threaded opening in breech of tube from which the breech plug had been taken was lined with sheet copper to protect the threads and an iron cylinder passed in to abut against the base of the inner tube and project a few inches from breech of jacket. Between the rear end of this and the rear cross-head an hydraulic jack was placed, so that upon applying the power the tube would be moved forward and out of the jacket.
An open brick structure was built to apply fire (wood) under the jacket and a current of cold water was arranged to circulate through the bore of tube. During the heating the tube was slowly revolved. When the fire was drawn the side rods were passed through the crossheads, and by adding piece by piece to the cylinder between base of tube and the hydraulic jack the inner tube was slowly pushed out.
The operation was performed with great difficulty, owing to the fact that it was scarcely possible to expand the jacket and contract the tube at the same time, but by rebuilding the fire several times and allowing the water current to circulate only when the jack was at work it was accomplished.
The water current would completely cool the inner tube within a few minutes (as shown by the temperature at outlet), and it was at this period that the tube was loosened. The heat applied in this operation was not greater than that ordinarily used for heating the jacket to shrink it originally upon the tube.
The gun casing was examined and star-gauged with a view to deciding upon its use in the reconstruction of the gun. It was found to be in good condition, and the star-gauge measurements compared closely with those made at the South Boston Iron Works during the original construction of the gun, there being an average difference between the two independent sets of measurements of less than 0.001 inch.
The jacket was also carefully examined with the same view, and was found to be in good condition both outside and inside. The muzzle end of the bore was very slightly bell-shaped; but for the remainder of the bore the star-gauge measurements gave almost identically the same result as those made during the original construction of this tube.
The exterior between "screw-head and first shoulder" was found to be slightly out of round, one diameter being about 0.004 inch greater than the other, and the greater diameter would not enter the casing, so that when the jacket was fitted to casing, this eccentricity was corrected.
The following parts were then made new for the gun:
1. Inner tube complete.
2. Muzzle collar.
3. Cast-iron breech plug.
4. Securing pin through casing into tube; and these, with the casing, jacket, and copper vent piece belonging to the old piece, were used in reconstructing the gun.
Cause of Failure
After the inner tube was extracted from the jacket, as previously described, it was cut up for examination. The accompanying drawing shows: Fig. 1, a section and interior elevation of the half tube cut by a plane coincident with the axis of the piece through the defect; and, Fig. 2, a cross-section of the wall of tube cut by a plane through the middle length of the defect.
It is at once apparent that there existed a circular cavity about 1.1 by 0.12 inches in cross-section, extending wholly around the tube at the middle of the wall. This cavity is along the "lap" of the joint of two sections of the tube and was so left by the "press welding."
A limited portion of the thin wall between this cavity and the bore of tube gave way under the powder pressure and caused the defect. The edges of this defect are slightly cracked, but not sufficiently to be marked on the impression which was taken of the defect at the firing ground.
The nature of the original defect in construction, i.e., the existence of the cavity, is sufficiently serious to render highly important a knowledge of the cause of its existence.
Unfortunately the explanation of this is still in doubt. The two remaining section-joints of this tube were also cut through for examination; one of them was perfect, and at the other there existed a cavity very similar to that at the joint where the defect occurred, only that it was much smaller and less dangerous, being about 0.6 by 0.04 inch in cross-section. In all of the joints the "butt welds" were without flaw.
Two different theories may be advanced to account for the existence of the original cavity: (1) That the joint was originally not a fit, that is, a space may have existed along the "lap" between the male and female portion of the joint when put into the "press furnace." This would be due to a mistake of the turner. (2) The cavity may be incident to the operation of "press welding."
The opinions of persons connected with the foundry whose judgment in this matter is entitled to respect are about equally divided between the two theories.
The cross-section of the cavity shown in Fig. 1, with upper and lower lines nearly straight, favors the supposition that the space did exist there before the press welding; but, in that case, the male diameter would have been nearly 0.25 inch less than the female originally, which is not reasonable, for the two sections are held together by shrinkage alone when placed in the press furnace.
The prescribed shrinkage is 0.02 inch on the diameter, and when two sections are first shrunk together a tie-rod is passed through the bore, but this is removed and a porter-bar, much smaller than the bore of sections, is passed through to carry them to the press furnace.
A moderately tight fit even would probably serve to hold the two sections together during the manipulation subsequent to the removal of the tie-rod, but if the joint were loose the sections would almost certainly fall apart. Added to this, the whole purport of evidence on this point is such as almost precludes the supposition that any loose joints were put into the press furnace during construction under the current contract. The lower line of the cavity found at a second joint in this tube shows an inward curve (see Fig. 1) and indicates that the male part of the joint bulged inward during the press welding.
After the weld in the press furnace, under end compression, and with heat applied only in the vicinity of the section joint (extending some 9 inches on either side of its middle), the wall of metal is considerably bulged both inwards and outwards, thus (see Fig. 2).
The exterior of the tube is at once "lap welded" and straightened, at the same heat, under a steam hammer, during which operation the wall is unsupported within, and the inside bulge is left to be removed by the boring. The lip of the joint a is originally made 0.04 inch longer than b, so that a receives that amount of compression before the compression of b begins.
The above facts constitute good grounds for supposing that one or both of the lips may be carried away from the middle line in this bulging process, and offer a reasonable explanation for the existence of cavities in the wall along the lap joint.
The fact that such cavities are found at some joints and not at others must be attributed in some way to more or less defective heats. The interior of the tube being possibly less heated than the outer portion, any tendency of the lip a to bulge inward as a whole would be more decided; at the same time, however, the arch should then offer a greater resistance to this movement.
It is evident, from the position of these cavities, that no means exist for determining whether or not they are present in a tube, unless the tube fails, as in the case of this No. 39.
In any event, however, and whatever may be the cause of these cavities as found at some of the joints, it is evident from this experience that the present method of butt welding the sections of wrought-iron tubes is open to serious objections.