A Treatise On The Principles And Practice Of Dock Engineering

DOCK ENGINEERING. 138 thinner stem remained intact. To remedy such defects in large pièces, without h^ving recourse to a fresh casting, the hole may, under certain circumstances, be bored, tapped, and fitted with a steel screw, or a wrought iion patch may be raised to a white beat and hammered in. A sound casting is, however, always preferable to one that lias been doctored up. Another defect is the presence of extraneous matter, such as loose sand from the mould, or even impurities in the iron itself. These latter should be skimmed off the surface of the casting ladle. But it is difficult to avoid loose sand in a mould which takes some time to close, and light projections are frequently washed away by the influx of metal. Such foreign matter will naturally rise to the top of the casting, and by making the latter a little higher than the nett size required the objectionable material can be removed later by the planing machine. Imperfectly adjusted cores cause the metal to be thicker on one side of a hollow casting than on the other. Wliile perfect adjustment is perhaps not always attainable, yet limits of deviation should be fixed and adhered to. Shortage in the supply of metal to a mould cannot be made good by a second charge. No matter how quickly applied, a shut or flaw will be the inevitable result. Castings which become cracked or twisted are frequently due to defective design. Considérable variation in the thickness of the metal, abrupt changes, and outlying projections cause irregular contraction. The thinner portions cool more quickly than the tliicker portions, and internal stresses, often unsuspected, are set up. Sudden changes in sectional area should, accordingly, be avoided, and projections should be graduated from the main body. Spécification for Castings. “ Castings are to be clean, true, and free from twist, having regular surfaces both inside and outside, with sharp, well-defined angles and lines. They must be sound and free from air or sandholes, cold shuts, and other imperfections. In the case of columns, pipes, <fcc., care should be taken that the lengths are exactly equal to the dimensions given; that the bearing surfaces of flanges are perfectly smooth and regular planes, perpendicular to the centre line; that the bolt holes are of the proper size and in their exact positions, and that the thickness of the metal in the shafts is quite uniform throughout, of which evidence may be taken by drilling holes, if necessary. Any casting will be liable to rejection which deviates more than ^ inch in. thickness and | inch in length from the given dimensions.” Defects in Rolled Plates and Bars________Loose and open fibres, flaws, and signs of lamination are due to imperfect rolling and welding. Coarse crystals or blotches of colour are caused by an insufficiently purified metal, contaminated with scorice and other impurities. A crystalline fracture does not necessarily imply an inferior iron. When