FigS. 192,193, and 194, are intended to illustrate this process as regards a steam cylinder. Fig. 192 is the entire section of the mould in its first stage; figs. 193 and 194 are the half sections of the second and third stages, preparatory to burying the mould in the pit in which it is to be filled.
The inner part of the loam mould is called the core when small, but the nowel when large; the outer is called the case or the cope. Each part is built upon an iron loam-plate, or a ring cast rough on the face, and with four ears by which it may be d. The mould is occasionally erected upon four shallow pedestals of bricks for the convenience of making a fire beneath it to dry the loam; at other times it is made upon a low truck upon which it may be wheeled into the loam stove, which is heated to about the temperature of 300 to 400 degrees Fahrenheit.
A vertical axis, a, is, a, is mounted in any convenient mauner, frequently in two holes in the truck itself, or as shown in the figure, in a pedestal or socket erected upon the truck; at other times the axis is mounted in a hole in the loam-plate, and in any bearing attached either to the building or its roof.
The first step is to fix upon the spindle the templet b b, at the distance of the radius of the cylinder, either by one or two clutches, with various binding screws. An inner cylinder of brickwork is then built up, plastered by the hands with soft loam, (which is represented black in all the figures,) and scraped into the cylindrical form by the radius board, which is moved round on its axis by a boy. When the surface is smooth and fair it is thoroughly dried, after which it is brushed over with blackwash, and again dried. The charcoal dust in the black-wash serves as a parting, to prevent the succeeding portions of the loam mould from adhering to the first.
The templet c c, fig. 193, cut exactly to the external form of the cylinder, is now attached to the axis at the distance from the core required for the thickness of the metal: some additional loam is thrown on to form the thickness, which is smoothed in the same careful manner as the center, after which the templet and spindle are dismounted, and the thickness, which is represented white in figs. 193 and 194, is also dried and blackwashed.
The ring for the outer case or cope is now laid down, and its position is denoted either by fixed studs or by marks; and the outer case represented in fig. 194, is built up of bricks and loam, with an inner facing of loam worked very accurately to the turned thickness. The new work, or the cope, is also thoroughly dried, and afterwards lifted off very carefully by means of the crane and a cross beam with four chains. This process likewise drags off the thickness, which usually breaks in the removal; its are carefully picked out of the cope; both parts of the mould are repaired, and again black washed and dried. When the cylinder requires ports at the ends, or the short tubes with Manges for attaching the steam passages, models of the tubes are worked into the cope, and are afterwards withdrawn; the cores are made in core boxes, and are partly supported by the outer extremity, and partly upon grains, or two little plates of sheet iron connected by a central wire, the whole being equal to the thickness of the metal at the part. When steam passages are wanted, either along the side, or around the cylinder, they arc worked up in clay upon the thickness, and duly covered in by the cope; their cores are supported, partly by their loose ends, and partly by grains, which become entirely surrounded by, and fixed in the metal, when it is poured.*
The mould is now put together in a pit sunk in the floor of the foundry, and the two iron plates are screwed together; the surrounding space being rammed hard to prevent the mould from bursting open, but the inner part is left much more loose for the escape of the air. The top edges of the mould are covered over with a loam-cake, (which has been previously made and dried,) or a ring three or four inches thick, strengthened with iron bars amidst the clay, the joining being made air-tight by a little cow's hair, and by the pressure of a quantity of iron weights; the loam-cake is generally perforated with many holes as shown at d, for the entry of the metal and the escape of the air. But provision must always be made in casting thin cylinders, boxes, and such like forms, for the breaking up of the core as soon as the metal is set, to prevent the metal scoring or rending from its contraction upon a rigid unyielding center, †
* There is always some uncertainty of the sound union of the grains, or other pieces of iron, with the cast-metal. Some cast them in iron and file them quite bright, others also tin them, apparently to preserve them from rust, as the tin must be instantly dissipated by the hot metal. Grains should always present clean metallic surfaces, and when used for very thin castings, to prevent them from dropping out, the wires are nicked with a file that they may be keyed in the metal.
It is however better to avoid the use of grains, which may be generally done by giving the core sand bearing, and afterwards plugging up the holes in the casting.
† The largest cylinders, such as those of the Cornish pumping engines, of 80,90, and 95 inches bore, and 12 or 14 feet in length, and the blowing cylinders of blast Large pans, and various other circular works, are moulded precisely in the same way as cylinders; except that curved templets are used, and that towards the conclusion, the apertures through which the spindle passed are filled in and worked by hand to the general surface.
Water-pipes are made much in the same mode, but the cores for these are turned upon an iron tube pierced full of holes, which is laid horizontally across two iron trestles with notches, and is kept in rotation by a winch handle at the end; there is also a shaper-board or scraper fixed parallel with the axis; this primitive apparatus is called a founder's lathe.