H.E. shell are always made of forged steel; they have coned walls, thicker at the base to give better strength to the shell. The body requires to be as strong as possible so as not to break up too readily and thus lose the value of the pressure set up on detonation; also, unless the best steel is used the body is pulver- ized instead of breaking up into pieces of a size to form effective missiles. Pointed shell, whose general use is for the attack of armour plate, require to be especially tough and strong.
Common shell have been made of cast iron, cast steel, and forged steel; a disadvantage with forged steel is that, with a bursting charge of gunpowder, the shell breaks up into a small number of fragments; the stronger the material, the thinner can the walls be made, and hence the larger the bursting charge.
Shrapnel shell are generally made of forged steel, though in some larger natures they have been made of cast iron. The steel is required to have a high yield point and breaking stress, as this is essential in order that the body, which is made as thin as pos- sible to provide a maximum capacity for the bullets and opening charge, may be able to withstand the pressures set up on the shock of discharge from the gun.
A.P. shell are made of either cast steel or forged steel; the points are made extremely hard, and the bodies softer; great thickness of metal is worked into the head, and the walls are made thicker than in other shell.
The steel for projectiles is made by different methods: (i.) Cru- cible, which is largely used on the continent of Europe, par- ticularly in Germany. With this method there is difficulty in obtaining uniform quality. (2.) Bessemer, which does not lend itself to the careful control necessary for production of the steel suitable, though the method is rapid and cheap. (3 .) Open hearth. The acid process is preferred to the basic as more suited for pro- duction of steel of uniform quality, and more economical. As it does not remove the phosphorus, a purer pig-iron must be used.
The manufacture of H.E. shell (other than solid-pointed) is car- ried out in a hydraulic press. The cast-steel ingot is heated up and punched, care being taken to ensure a central cavity in the forging; for larger shell several punches or drawings, with intermediate heatings, may be necessary to produce the required dimensions. The forging is then oil-hardened by heating up to a specified tem- perature and quenching in oil. Analytical and mechanical tests are next applied to samples and, if satisfactory, the forgings are sent to the machine shop for machining and centring. The shell are then heated to a dull red heat for the purpose of " heading " or " bot- tling " to give the required ogive to the head; this is carried out by forcing the head of the shell into a die by hydraulic pressure, and can be done cold, but cracks are liable to occur at the shoulders on account of the internal stresses. For " bottling " larger shell, it is sometimes necessary to taper the walls of the shell previous to carrying out thrs operation. The head is then bored and screw- threaded to receive the fuze-hole bush, and the exact ogive given in a " radiusing " machine, which is similar to an ordinary lathe, except that the tool-carrier is designed so as to allow the tool to act on the head of the shell at a variable distance. The base is then faced and turned down to the required thickness. x There are also other machining operations necessary, such as recessing the base to permit of the detection of any tendency to weakness at the centre, after which examination a steel disc is inserted in the centre of the base and either screwed in or secured by burring some of the metal of the shell over it by means of a pneumatic hammer. The interior of the shell is sand-blasted, coated with copal varnish, and stoved for six hours. This process gives a very smooth internal surface and it prevents premature explosion from friction, in case of any move- ment of the explosive arising from bad filling; it also prevents chem- ical action of the filling on the metal of the shell.
The groove for the driving band is machined, the sides of the groove being slightly undercut to assist in holding the band in place; the bottom of the groove has three or four waved ribs cut along it to prevent the band from rotating in the groove, and two or three chisel-cuts are made across the ribs to permit of the escape of any air while the band is being pressed into position.
The driving bands themselves are made from discs of copper as free from impurities as possible, the best kind being that which has been electrolytically deposited. The discs are formed into cups and are then annealed and drawn alternately until drawn into a long tube, five draws being the usual number. The copper tube is then parted into rings, which are given a final annealing. For band- ing, the shell are placed in a machine which consists in a circular
1 As the base is the heaviest part of the shell, it is in this opera- tion that the various shell are brought (as nearly as possible) to uniform weight.
holder, of which the periphery is divided into segments to which hydraulic presses are attached; a copper band is placed over the shell opposite the groove, and pressure is applied till the band is firmly wedged into the groove. The driving band is then turned to the required shape and dimensions.
Shrapnel shell are manufactured in two designs, those for larger guns having a separate head while for smaller types the head and body are in one. Except that the operation of " bottling " or " heading " can be dispensed with for the larger sizes, the method of making the body is very similar to that for H.E. shell, but from the nature ' of the design of shrapnel, the body requires some internal machin- ing in addition.
The heads of the larger shrapnel are made of soft steel or malle- able iron, prepared to take the fuze-hole bush (the remaining space being filled by a block of wood), and secured to the body by pins and rivets and soldering firmly enough to ensure that the whole shell rotates in flight as a single body.
The cups for the opening charge are made from tin-plate, and the steel diaphragms from discs sheared from a billet of steel, stamped into shape, 2 and then brought to the required dimensions by grind- ing; a hole is bored in the centre and screw-threaded to receive the central tube, which is made from a butt-ended tube, and is turned and screw-threaded at its lower end to fit the central orifice of the diaphragm. The socket to receive the fuze is a brass stamping, screw-threaded internally to take the fuze and externally to screw into the body of the shell.
Pointed shell may be of three types, which are in Great Britain designated common pointed, common pointed capped, and armour- piercing. The operations in manufacture are very similar in each case. For A.P. shell, more work is put into the steel in order to make it as strong as possible and for this reason these shell are usually forged. The common-pointed and common-pointed-capped shell are punched and drawn in the usual manner. The shell then undergo a heat treatment in order to remove any strains which may have been set up, the temperature of the furnace being raised to about i,looF. After treatment the usual machining operations are car- ried out, and the shell are then heated up again and hardened by being quenched in an oil bath. Since it is required to retain only the head in a hardened condition, 3 the remaining part of the body is then " let down " by being immersed in a heated bath of lead to a short distance below the shoulder; this process removes the hard- ening effect and leaves the body tough instead of brittle.
The hardening of the head is liable to cause the occurrence of spontaneous splits; and shell are therefore stored in the open for a period so as to allow time for any splits to develop before filling. Should a split extend to the cavity of the shell when filled, the sudden fracture might cause the explosion of the bursting charge; consequently the shell are fitted with aluminium containers which, as thin cones, are inserted into the shell and spun into position.
The interior of the body is then bored to its final dimensions and the lower end of the cavity screw-threaded to receive the adapter which carries the base fuze. The shell are then banded and the interiors varnished.
Adapters (which vary in size from a mere fuze-hole lining to what is almost a base in itself) are cut from the billet and screw-threaded externally to fit the shell and internally to take the fuze. In some cases a further organization of the adapter base is required to pre- vent a possible inrush of propellant gas round an ill-fitting fuze into the interior of the shell; this consists of a copper gas-check plate over the fuze, held inside a steel cover which is bound to the adapter base by a locking ring. In such designs the adapter base flange is prepared accordingly during manufacture (fig. l).
FIG. i.
When caps are fitted, the usual method is to make peripheral
notches in the head of the shell before hardening ; the cap is soldered
to the shell and retained in position by indenting the lower edge into
these notches. Other methods of securing the cap are also in use.
Shell Manufacture in War Emergencies. In the adaptation of the engineering industry to the manufacture of shell, the capability and capacity of the plant installed in any one workshop is the ruling factor governing the work to be allotted to that shop. The design of the shell must of course be simplified as much as possible to suit the existing machines. The very rapid output requisite and result- ing from any such adaptation necessitates that the various stages
2 These diaphragms can also be made from drop-forgings.
3 In the case of common pointed, the head is not hardened.
