Page:The American Cyclopædia (1879) Volume IV.djvu/86

 78 CASTING erly secured from rising by weights. The cope is then lifted off by a crane, and while suspended its surface is dressed and properly prepared. The pattern is lifted out, and the mould in the bed repaired and dressed, when the cope is replaced, the holes for pouring and for the escape of air and gases having been provided for. Being then securely weighted, to prevent rising from pressure of metal and gases, it is ready for the pouring of the metal. In fig. 3 a greensand floor mould is represented at c. In the casting of metals, especially those having high melting points, there is always more or less production of gases, together with expan- sion of air; and if the operation were per- formed in a mould which was not porous, the bubbles would mar the surface of the casting as well as enter to a certain extent into its in- terior. It is therefore necessary that the mould should possess sufficient porosity to allow of the escape of aeriform matter. Moisture in a mould is only admissible in small castings which cool quickly. Used for large masses of molten iron, the amount of steam formed, to- gether with the expanding gas, would not only endanger the mould, but also the workmen. Dry moulds made of loam are consequently used in heavy castings, partly for the above reasons, and partly because sand could not be properly manipulated or retained in place in large and massive castings. The casting of large cylinders, bed plates, and condensers for steamships is a very intricate process, requir- ing good engineering abilities, skill in draught- ing, and experience in the designer as well as in those who execute the work. The moulds are usually worked from drawings instead of being formed upon patterns. A single piece of machin- ery is often complex in form, and as the art of the moulder consists in forming a hollow cavity where the carpenter or cabinetmaker would make a solid body, it must be seen that he has a much more difficult task before him ; for he has not only to form an inside structure simi- lar to the future cast, but an outside one of a reverse form as well; and these two forms must be perfectly related to each other. He has also to provide channels and gateways for the pouring of the metal, and they must be so arranged as to secure its perfect flowing to every part, and as nearly as possible its simul- taneous cooling. Allowance must also be made for shrinkage, and an almost infinite number of precautions, suited to particular exigencies as they arise, must be observed. The draught- ing requires great forethought and calculation, and the execution not only involves a perfect comprehension of the plan, but a constant vigilance in avoiding errors and causes of mis- carriage. A description of the moulding and casting of a complex piece of machinery would require a very great detail of explanation and numerous illustrations, and then could not be comprehended except by repeated visits to the foundery. The moulding and casting of a simple cylinder will therefore be taken. A loam mould, secured in a pit, and ready for casting, is represented in fig. 13. It is con- structed in the following manner. An iron foundation plate is laid upon the floor of the FIG. 18. Loam Mould, a. Hollow mould, surrounding the core, and surrounded by the cope. f>. Hollow Inside of core, o, c. Bolts holding cope together, d. Air tube for discharging air from core, e,. Air tubes. /, /. Pouring holes. foundery, and levelled. An iron ring, flat and of a breadth equal to the thickness of the walls of the core which is to be built upon it, and of a diameter equal to that of the inside of the future cylinder, is laid down, and the core is built upon it to the height desired. An appa- ratus for describing and sweeping the surface of the core is now erected, which is called a sweep, and consists of a spindle and templet, represented in fig. 14. An arm, a, supported by some portion of the building, holds the up- per end of the spindle ft, while the lower end turns in a hole in the centre of the founda- tion plate. A collar, c, which may be ad- justed at any required height, is provided with an arm, to which again the templet d is firmly held by means of a mortise, which slides over the arm, and may be set at any desired distance from the spindle. From the construction of the ma- chine it will be perceived that it may be used to describe either the inner or outer surface of a cylinder ; therefore it serves to give form both to the surface of the core and to the inner surface of the cope. After the sweep is placed in position the core is commenced by building up a cylinder of brickwork upon the circular plate, its dimensions being governed by the templet, which in sweeping about its axis should leave a small space between itself and the bricks to allow of finishing with loam. The bricks are laid up in loam, and the same FIG. 14. Sweeping the Core.