Although the manufacture of plates and tires does not come strictly under the head of locomotive building, the forge work would not be complete without some notice, however slight, of such operations. The specification and tests for the plates were given in Section I., the Boiler Shop, and the material stated to he of the best Siemens-Martin mild steel. The method generally adopted in the North of England is to cast ingots from three to five tons in weight, which, after remaining in the moulds from twenty to thirty minutes, are removed into soaking pits, where they radiate their initial heat, or equally distribute it throughout their entire mass for about one and a half hours. They are then cogged down either by hammer or mill, each being equally good, into slabs of suitable size for the mill in which they are to he rolled, or plates they are to form. The cogging mill will either have side rolls of the universal or Belgian type, or an actuating gear, by which the slab is raised or turned about as required, in order to roll it upon the edges. When rolled by the latter arrangement in a vertical position, it is especially beneficial in removing all the scale, it peeling off the flats with the greatest ease. The slabs are then carried forward by means of live rollers to the guillotine, which is actuated either by hydraulic or geared power, having a shearing capacity for slabs up to, say, 3 feet wide by 10 inches thick. The modern arrangements for plate rolling are very complete, the rolls themselves being about 8 feet to 11 feet long by 30 inches and 40 inches in diameter. The labour-saving tools and manipulating gear are so complete in the most modern plate mill in the author's mind, that only four men are required. The reheaters are grouped about the mills in the most convenient manner possible, and every facility is devised for dealing with ingots and slabs, with the most efficient charging and drawing tackle, so that the cost of production is brought down to a minimum.

The section of the engine tire is given in Fig. 218, and the specified deflection for the percussion test is 1« inch per foot of external diameter, for tires up to and including 3 feat, and 2 inches per foot for all larger sizes. Two per cent, are tested in this manner, selected from bulk, and samples cut from these selections must bear an ultimate tensile strain of not les3 than 46 tons per square inch, with a minimum elongation of 20 per cent. on 3 inches for Siemens steel. The deflection for Bessemer steel tires must be 2 inches per foot of external diameter, and the ultimate tensile strength not less than 35 tons per square inch, with a minimum elongation of 20 per cent, on 3 inches.

The usual method of manufacture is to cast suitable ingots, hammer them down, punch, beak and roll. The form of ingot is almost of infinite variety, from the ordinary cheese with oval top and bottom and hexagonal or octagonal sides, to that of a Field's night light, each having for its object a good sound blank. The spongy mass and dirt rises to the top, where, after hammering down, it can be punched out. In hammering down, all fins and flashes are cut away, and care is taken that all cracks and traces of ganister are removed. The ingot moulds are kept in good condition, and of course it follows that the greater care there is bestowed on the metal at the melters and during the punching and beaking, the less percentage loss will take place at the mill. After punching, sometimes the blanks are hammered Upon the edges or periphery upon a mandril attached to a porter bar. They are then beaked, the larger tires by a beaking recess in the hammer block, the small tires by an ordinary smith's fuller. Some firms do not beak, but have roughing-out mills, and others employ both. Afterwards they are weighed, this being essential, otherwise the percentage of wasters after rolling would be unusually high, through errors in weight, the blanks not rolling to the right section when at the required diameter.

Fig. 218.

It is very beneficial to have the speed of the mill sufficiently great, to allow a good heat on the tire when finished. Some firms take the precaution to cover their tires immediately with ashes, which gives an annealing effect and certainly preserves them from chills. After rolling, the tires are carefully examined for inside and outside diameter, those too large being rejected, whereas those too small are blocked, and oval tires made round by hydraulic pressure. The two latter operations are considered to be bad practice, and are seldom required by a good roller, but of course when a mill is hard pressed, it is very probable that a few such tires may be rolled, perhaps to the extent of 10 or 12 per cent. This amount is not excessive, as a dozen oval tires can be rectified in about fifteen minutes.

Great care is bestowed upon the selection of the heats for tires of extra quality to satisfy stringent specifications, sample ingots being taken, rolled and annealed until the required tenacity and ductility is obtained, not that mechanical means should supersede chemical in producing the required result.

Forty-five tons tenacity may be considered sufficient for home use, but for India and other hot climates the tenacity may go up another five or six tons per square inch with perfect safety. The material should be as little liable to molecular changes from a repetition of sudden shocks as possible, which points to a tire having considerable ductility, which would fulfil all requirements, except that it would wear out too soon. The rack and pinion theory is well known, in which is advocated the equality of temper in the material both for tires and rails. The author will be content to leave in the hands of the manufacturers the influence which its chemical composition has upon its physical properties.