The art of making iron castings has been brought to its present state of excellence after many years of labor and experiment. Iron in its natural state exists as ores and to be of use to man it must first be smelted to a form that is known as pig iron. In this state it may be puddled or wrought to convert it into wrought iron or steel. Iron, as steel or wrought iron, was known for many centuries before iron castings were made.
There are records of many attempts to make iron castings but there is nothing of importance recorded until 1544 when a patent for making iron castings was granted in England.
The art of making iron castings first became successful about the beginning of the eighteenth century. At this time the primitive cupola was invented. It consisted of a crucible upon which a stack was set. The whole was filled with coal and iron as is done to-day with the cupolas. A blast of air was forced in over the top of the crucible. After it was tilled with melted iron the stack was knocked away, the debris cleared, and the melted iron from the crucible poured into the moulds.
About the same time a method of leading off gases from the moulds was discovered at the Darby furnace in Colebrookdale. From this time the art of moulding steadily advanced. The introduction of the steam engine in the last half of the century, materially assisted in the development.
The methods employed in the converting of pig iron come under the subject of Metallurgy. After the pig iron has been run it is remelted in a cupola and then is ready to be made into, iron castings. The process of remelting and pouring the melted iron into moulds will be discussed in the following pages.
The Foundry is the name applied to the building, cupola and all the tools and appurtenances that go to form the equipment needed to make iron castings. Of these the cupola is the most important.
The Cupola is the furnace in which the iron is melted. In general construction it resembles the blast furnace in which the pig iron is made. The difference is one of size rather than of principles of construction or operation. There are many patented cupolas but these patents relate, for the most part, to details and do not affect the general construction. The ordinary foundry cupola, as usually built, consists of a shell of wrought iron riveted in the same manner as the shell of a boiler, and lined with firebrick. It is set on end and the bottom is formed of two doors opening downward. A blast of air is admitted above the bottom; fuel and iron are thrown in at the top. Fig. 1 is a vertical section of an ordinary cupola. In this engraving B is the wrought iron shell which contains the firebrick lining and holds the latter in place. The thickness of the plates of the shell varies from ¼ to ½ inch according to its diameter. The thickness for one of 45 inches diameter should be ⅜ inch. This shell and the firebrick lining usually stand upon a strong cast iron base, which in turn is supported by four columns. These are carried on a stone or masonry foundation and are of sufficient length to raise the base three or four feet above the floor.
The lining of the cupola should be made of firebrick. The best method is to use two layers, each 4 inches thick. When the inside layer has burned through it may be renewed without disturbing the one next the shell. Ordinary red brick is sometimes used for the layer next the shell but it is unreliable and used only on account of its cheapness. In lining a cupola the bricks used should be moulded to fit the circle of the shell and should be packed as closely together as possible. The spaces between them should be filled with fire clay but only enough should be used to make an air-tight joint. The smaller the amount of clay used the better. To obtain the best results prepare the clay by making a thin mixture with water. Dip the brick into this mixture before setting, and enough clay will adhere to make the joint. After the brick is put in position rap it sharply with a wooden mallet in order to force all superfluous clay from between the joints. A lining put up in this way will last twice as long as where thick clay is used. The nominal diameter of a cupola is the diameter inside the lining. Hence the inside diameter of the shell should be 16 inches larger than the diameter of the cupola. This may be less for very small cupolas. For such six inches of firebrick will be sufficient thickness of of lining.
Fig. 1. The Cupola.
Sometimes cupolas are so built that the iron and fuel must be thrown in at the top. The ordinary construction, however, is such that the shell and lining are carried high enough to prevent larger of fire from escaping escaping sparks. In this case a charging door is placed in the shell at a at a convenient height above the bottom. The distance from the base to the bottom of the charging hole is known as the height of the cupola.
The height of the cupola varies with the diameter. For small cupola the greater the diameter, the greater the height. An approximate formula for determining the height of a small cupola is.
H = D X (6-D/2)
H = the height of the cupola in feet.
D = the diameter of the cupola in feet.
1. What should be the height of a cupola 24 inches in diameter ? Ans. 10 feet.
2. What should be the height of a cupola 36 inches in diameter ?. Ans. 13 feet 6 in.
3. What should be the height of a cupola 72 inches in diameter ? Ans. 18 feet.
It may be noted that it is unnecessary to pay any attention to the odd inches in such problems and that the nearest even number of fecd may be used. The height may also be varied from those given to suit various conditions.
The height of a cupola is usually a square plate of cast iron having a circular hole in the center of the same diameter as the inside of the lining. It is smooth on the upper surface with the exception of a low ring in which the shell is set. The bottom is provided with povlels for the reception of the columns by which it is carried. Lugs are also cast on the lower side to form the hinge connections for the doors. The doors are also of cast iron and are of sufficient strength to carry the load put upon them. One has a lip cast upon it so that a closed surface on the lower side is presented. There is no latch or other means of holding the doors except a prop C which is knocked away when the bottom is to be dumped.
The Windbox is a belt or pipe surrounding the shell a short distance above the bottom. Air from the blower enters the wind-box before entering the cupola. The openings from the windbox into the cupola are called tuyeres. The location, form and size of the tuyeres have been determined by experiment. They are made square, rectangular, triangular, round and elliptical. Each form has its advantages, although the size and distribution are of more importance than the shape. The distance from the bottom depends upon the work to be done. Low tuyeres have the advantage of effecting some saving in fuel. For ordinary jobbing shops a height of 10 inches from the base to the bottom of the tuyeres will give good results. This applies to all diameters of cupolas. The total area of the tuyeres varies with the size of the cupolas. The following table gives the total tuyere area for cupolas ranging from 24 to 72 inches.
Diameter of Cupola.
Area of Tuyeres.
No. of Tuyeres.
24 sq. in.
75 sq. in.
108 sq. in.
174 sq. in.
204 sq. in.
312 sq. in.
400 sq. in.
440 sq. in.
550 sq. in.
For sizes with a greater diameter than 60 inches it is customary to keep the diameter 60 inches at the windbelt or tuyeres and increase to the full diameter of the cupola above. This is done because a diameter of more than 60 inches at the tuyeres does not permit the air to reach the center of the cupola, the result of which is imperfect combustion. This form of construction is shown in Fig. 2.
The iron is drawn out of a spout, located at the bottom of the cupola as shown in Fig. 1. A peep hole is placed in the wind-box and covered with mica through which the amount of accumulated iron may be noted.