The inside fire-box shell is made from the best quality copper plates, having a tenacity of from 13 to 14 tons per square inch, 45 per cent, elongation on 8 inches, and 50-60 per cent, contraction of area; the stays of about 14 tons per square inch, 4.5 per cent, elongation on 8 inches, 55-60 per cent, contraction of area, also bearing the test of being bent double, cold, without any signs of cracking. The crown and sides are in one plate similar to the outside casing, and the tube plate is widened out, forming a pocket or "joggle" on the side plates to allow a wide spacing of tubes. The roof is stayed with eight girder stays of good well-annealed steel castings. Figs. 15-17. The copper stays are screwed tightly into the fire-box and shell plates by hand, in order to be absolutely certain as to the tightness of the same. In cutting off the stay ends, care is taken not to injure the threads, and a projection of about « inch is allowed on the inside of the firebox, and ⅜ inch on the outside shell, after which the stays are carefully laid over to a neat job, inside and out, without being snapped, with hammers not exceeding 4 lbs. in weight. Snapping is a bad practice, as it indents the plate, forming receptacles for verdigris and corrosion, which eventually entails the use of stays as much as 1¬ inch in diameter. Basic steel stays are also used, with reputed success.

Fig. 43.

The copper plates are treated in the same manner as the steel plates as regards drilling, planing, etc. They are flanged by hand, as represented by Figs. 44-46, and the following is substantially the universal method of dealing with them. Five heats are required to complete the back or fire-hole plate, viz. two for the flange, two for the fire-hole and one for annealing by quenching. A third heat is very seldom required for either process, save upon rare occasions, and for general practice it is not necessary, but owing perhaps to some delay, or the flange being not quite down upon the block, it is sometimes rendered imperative. The tube plate requires four heats; at the first, the corners are worked to avoid concentrating the metal, which would otherwise pucker up, and as much as possible is done to the top flange. After the second heat further progress is made, usually the flanges being finished as near as possible with mallets. The third heat is devoted to finishing with flatters which are about 4 or 5 inches square, and the fourth to annealing by quenching. After flanging the copper back plate it is bolted to the outside back plate, and the fire-hole rivet holes drilled. A machine has been designed by Mr. Webb for this purpose, Fig. 47. After the inside fire-box crown plate has been bent to radius, the joggle is set in by heating and hammering into a block having the same joggle as required by the plate.

Fig. 44.

Fig. 45.

Fig. 46.

Fig. 47.

The set commences as far as possible from the edge, and near the second row of stays, ensuring that the plate is not drawn, and that the rivet holes will come almost exactly-central with the centre line on the flange of the tube plate. These holes are opened out by a hand drill. The advantages of the joggle or pocket are very doubtful, and in many cases it is being dispensed with. The three plates are then bolted together, the corner rivet holes opened out, the foundation ring put on, and the corners bedded to it. The roof stay bolt holes are opened out with a special tool having the required taper, as at a in Fig. 16, which clears out the hole until a facing cutter just skims up the inside of the crown, to give a good bed for the roofing stay bolts.

The roof bars are then fitted on, the clearance between the top of the box and the bottom of the stay for the ferrule space having been previously planed away. The ends are then marked for machining, to bed on the corners of the box and to bring the ferrule distance right, by using an ordinary forked scriber. The bars are squared up to centre lines, the holes marked for the shackle cotters to template, the holes drilled for the stay bolts through a jacket at a multiple drilling machine, tapped with a lightning tapper, and bolted up with a 6-foot key.

The inside box is then lowered into the outer shell. In order to get it far enough back to enter the fire-hole door flange, the foundation ring is removed. The palm stays and other odd rivets having been put in, it is ready for being tapped by the quadruple tapping machine. This machine, which is shown in Figs. 48-50, consists of a framing, secured by angle irons to the foundation ring, and is entirely rope-driven, each tapping stock being driven separately. The main driving rope from the counter-shaft is 1 inch in diameter, and passes round the pulley A, the power being transmitted by 1¬ inch by « inch belts to the corner pulleys and this in its turn to the tapping stocks by a ¾-inch rope. As the course of the rope for each stock is similar, a description of one will suffice. B is a fast pulley driven by D, C being a loose pulley. The rope leaves B, passes round E to the tapping stock F, thence to the loose pulleys G and H, K being fast, from H to the centre of the machine, over a loose pulley, and round one of the counterweights L to B again. This valuable labour-saving apparatus was designed by Mr. As-pinall. The roof-bar shackles are put in and cottered up underslight tension.